Evolutionary Trade-offs

This Evol 101 post is by MSU grad student Tyler Derr

Before even jumping into the evolutionary biology material, what is a trade-off? Well, a trade-off is when a choice must be made between multiple things that are either incompatible or an increase in one thing might lead to a decrease in another. Theodore Garland Jr. gave the perfect example, “…money spent on rent is not available to buy food.”, makes sense. In a past post we discussed correlated traits, one can think of a biological trade-off as simply negatively correlated traits (i.e. if one trait goes up, the other goes down).

A one-trait trade off is the result of an opposing selection on that one trait via a different environment or through perhaps a limiting resource (Agrawal, Conner, & Rasmann 2010). An example of a single trait trade-off in humans is the weight of a new born baby. A higher birth weight provides a higher chance of survival in the first few weeks, but babies that are too large have higher mortality rate (Karn & Penrose 1951). Thus there is a trade-off in the birth weight that greatly impacts the chance of survival.

When two or more traits are involved a multiple-trait trade-off can be thought of as having a single resource where it is impossible to use this resource to increase more than one trait at once. If it sounds a bit confusing, for now just think of the aforementioned example of taking your money (i.e. the resource) and either paying rent or buying food (which can be thought of as the two traits).

Fig. 1: Howler monkeys seen roaring. [Image Credit: Mariana Raño]

An example of a multiple-trait evolutionary trade-off can bee seen in different species of howler monkeys (shown in Fig. 1). Dunn et al. just recently had their paper “Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys” published in Current Biology. In their research they discovered that the bigger a male’s vocal organ and louder the roar, the smaller their testes resulting in less sperm production.

The University of Cambridge made quite an interesting few minute clip, Calls vs. balls: An evolutionary trade-off, in which they discuss the findings of this paper and you get to hear Dr. Jacob Dunn imitate the roar of a howler monkey. Although it is not exactly a clear cut answer as to why this trade-off exists, it was observed that the loud species tend to live in a social model where there is only one male dominating a set of females, as compared to the species that instead have bigger testes (and weaker roar) live in groups of multiple males where the females mate with all the males. The first scenario would lead rise to selecting for a louder roar to claim territory and to ensure having females to reproduce with, while the second scenario would give rise to selecting for larger quantities of sperm since this would be the way to ensure offspring when the females are mating with all the males in their group. As mentioned before about the single resource and multiple traits to invest in, Dunn et. al suggests that the reason for this trade-off is simply the howler monkeys do not have sufficient energy to invest in the acts of loud roaring combined with large sperm production.

It is quite clear from the examples that although we might observe some evolutionary trade-offs in nature that we certainly do not always fully understand the reasoning behind such trade-offs. In the words of Dr. Jacob Dunn, “In evolutionary terms, all males strive to have as many offspring as they can, but when it comes to reproduction you can’t have everything.”

References:

1. Agrawal, Anurag A., Jeffrey K. Conner, and Sergio Rasmann. (2010). “Tradeoffs and negative correlations in evolutionary ecology.” Evolution since Darwin: The First 150 Years. 243-268.

2. Dunn, Jacob C., Lauren B. Halenar, Thomas G. Davies, Jurgi Cristobal-Azkarate, David Reby, Dan Sykes, Sabine Dengg, W. Tecumseh Fitch, Leslie A. Knapp. (2015). “Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys”. Current Biology. 25(21): 2839-2844.

3. Garland Jr., Theodore. (2014) “Trade-offs”. Current Biology. 24(2): R60-R61.
4. Karn, M. N. and L. S. Penrose. (1951). “Birth weight and gestation time in relation to maternal age, parity and infant survival. Ann. Eugenic. 16: 147-164.

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Science and Social Justice

This post is by NCAT faculty Joe Graves

Yes, that’s me holding up the fist, next to the fist in the Science for the People banner stating: “Scientists Against Reagan’s War”. This picture was taken at a national march against US intervention in El Salvador, in Washington, DC on May 3rd 1981. Next to me in the picture are my life-long friends Dr. Brian Schultz, now a biologist and Cruz Phillips, now a rancher and social activist for the Farm Workers movement in California. We were graduate students then in the Department of Ecology and Evolution at the University of Michigan.

You didn’t have to be a scientist to march for this cause. Just like those who marched across the bridge in Selma, Alabama on March 7 (Bloody Sunday) were not scientists. They were simply good and courageous people demonstrating against social injustice. They were met with police, dogs, horses, tear gas, clubs, and whips. Many were severely beaten, including John Lewis (now Democratic Congressman for the 5th District, Georgia).

Definitions

However this post is not about social justice in general, but science and social justice. To start a discussion of the relationship between the two it is necessary to define what is meant by the term social justice. One definition of social justice is promoting a just society by challenging injustice and valuing diversity. In this definition diversity refers to various aspects of human identity (socially defined race, ancestry, gender, sexual orientation, national origin and others) that have been the subjects of social subordination. Accepting this definition suggests that you believe that all people have a right to equitable treatment, support for their human rights, and a fair allocation of community resources. In addition this definition suggests that individuals should not be discriminated against, nor their welfare and wellbeing constrained or prejudiced on the basis of any of these identity features or any other characteristic of background or group membership1.

With this definition in mind, we might want to ask to what degree historically or currently has science as an enterprise, or the actions of individual scientists been associated with this goal. First, scientists employ a supposedly neutral and objective method (the scientific method) to understand the workings of nature. This method, employed at its highest standards, should allow us to work out how various processes in nature work. A child might run along a field flapping their wings vigorously in an effort to fly, but the principles of physics tell us that this simply cannot happen. However, our ability to unravel the laws or rules of nature begins to diminish as we move from the physical to the biological and social sciences.

This is further complicated by the fact that science as an enterprise has always been more than working out the various principles of nature; it has always been imbedded in a social milieu, and therefore has always been tasked to addressing particular socially defined priorities. Some of these priorities have been neutral to or even in rare cases supported notions of social justice. For example general scientific principles such as the conservation of matter, organic evolution, or ecological food webs in themselves need not have had any specific impact on human society. Yet, on the other hand, Newtonian physics allowed improved methods of producing projectile weapons, thermodynamic principles helped to produce better explosives, the computer made possible better inscription and code breaking, ecological principles have been utilized to conduct biological attacks on crops, and microbiology has been deployed to produce better bacteriological and viral weapons2. Even one of the core principles of natural selection, the struggle of existence, was developed not in attempt to understand population growth in general, but specifically as an attempt by Reverend Thomas Malthus (1766—1834) to explain the poverty of the English poor. The gist of the essay was devoted to his analysis of the English Poor Law and the impact of the transition of the poor from agricultural to industrial labor3. While Malthus correctly reasoned that geometrical sequences increase more rapidly than arithmetic ones, he had no reason to assume that the food resources of human societies necessarily increased only arithmetically.

Science as power

What is clear then is that precisely because science allows us to understand the workings of nature, it has inherent power. Power than can be used for good or evil. Here I will make the controversial statement that for the most part, the scientific enterprise has aided and abetted social injustice. This statement should not be taken to mean that scientists are the only profession that can be tarred with this ugly moniker, as Karl Marx once said:

“The bourgeoisie has stripped of its halo every occupation hitherto honored and looked up to in reverent awe. It has converted the physician, the lawyer, the priest, the poet, the man of science, into its paid wage laborers4.”

In this passage, Marx was alluding to two things, first the obvious interpretation that the new economic system had converted occupations that had in the past been held mainly by persons of leisure into wage labor jobs, but also that this new economic system had won the ideological allegiance of these professions. Furthermore he thought that this new economic system was initially a great improvement over feudal relations. For example it had been responsible for the industrial revolution that had greatly improved the living conditions for the majority of Europe’s population. Scientists and engineers had played an important role in this transition, but unfortunately what had been good for Europeans had not been good for the rest of the world’s population. For example, the Guyanese economist Walter Rodney (1942—1980) explained how Europe’s primitive accumulation (the wealth required to drive the industrial revolution) had depended upon both the Trans-Atlantic slave trade and the colonization of Africa5.

Ironically, the colonization of the tropics by European powers had important impacts of the development of scientific disciplines such as anthropology, agriculture, botany, ecology, and zoology6. It also had important influences on the demography and the development of scientific infrastructure in Africa. It is not accidental that African still lag behind European nations in the production of scientists or the building of scientific facilities. Similarly, persons of African-descent and other socially subordinated ethnic populations living in nations dominated by European populations (e.g. USA, England) still suffer from the hegemony of Eurocentric culture in science. This does not refer to the scientific method, but rather to the choice of research problems that the scientific community is engaged in and the way in which the scientific community repopulates itself7. In the 19th century, the majority of anthropologists were concerned with validating the overall superiority of persons of European descent. This was associated with social and legal programs that denied fundamental human and civil rights to non-Europeans. For example, the Supreme Court’s decision in the Dred Scott v. Sanford Case (1857) was premised on the scientific consensus that Africans/Negroes were members of a separate and inferior species8. These views dominated the field until the 1960’s. It is important to recognize that evolutionary biologists played a significant role in debunking this myth. This process began with Darwin and culminated with the thinking of evolutionary geneticists like Th. Dobzhansky, Richard C. Lewontin, and their intellectual progeny (of which I am one)9. However at the same time, there were just as many scientists who had played a role in developing the myths of racial superiority and actively worked to help implement racist policy. Examples of such individuals include Louis Agassiz, Samuel Morton, Josiah Nott, and George R. Gliddon (the four horsemen of American polygeny), John Jeffries, Herbert Spencer, Arthur Comte de Gobineau, Francis Galton, Charles B. Davenport, Eugen Fischer, Henry Garrett, Arthur Jensen, J. Phillipe Rushton, just to name a few10.

Science and social injustice: An Ongoing Problem

It this point the reader may argue that I have chosen scientific racism as an example of science and social injustice as a “low hanging” fruit. Clearly it was scientifically unfounded and morally reprehensible. My response to that claim would be that the scientists themselves never felt that they were unscientific in their analysis, and all of them felt that their work was morally defensible (and in some cases imperative), particularly from the point of view of improving human society. I would also argue that there are currently groups of scientists working in arenas that they feel adhere to the scientific method and that will ultimately be beneficial to human society. Yet to these individuals I lay down the challenge that a little more thinking is required before anyone can be satisfied with that position. One such example is the modern push for personalized medicine. While the readers of this post may question the degree to which medicine is “scientific” it certainly relies on the principles of biological science to develop its approaches and treatments. So in that regard, let’s briefly examine the scientific ideas behind personalized medicine.

Modern medicine is really only now beginning to fully appreciate the significance of individual and population variation11. With this in mind, physicians are beginning to recognize that there is no such thing as one treatment that is suitable for all patients suffering from a specific disease. Again part of the previous failures in this regard resulted from the past racist/sexist practices of primarily testing drugs in males of European descent. As more populations began to be tested for drug efficacy, it was found that there was significant variation in individuals that influenced the way drugs were metabolized12. In addition, individual variation can now be determined with accuracy not possible before the age of next generation sequencing (NGS). Now, physicians can genotype individuals quickly and cheaply. Furthermore, the use of NGS has revealed hitherto unrealized variation not just in patients, but also in some of their diseases (such as infectious disease and somatic tumors). All of these realizations have come together in a perfect storm, leading inexorably (or it would seem) to the necessity to develop personalized medicine.

Clearly, the scientific questions engaged in this effort are intellectually appealing. However, I would argue that whether something is of intrinsic interest to the scientist, should not be the sole criterion determining whether we pursue it. In the modern world, the vast majority of scientists depend upon public funding to support their work. Given this fact, the priorities of the scientific enterprise should be the subject for public discussion. On the surface of this, everyone should want to see the development of personalized medicine. However, the resources that we have to spend on scientific research are not unlimited. Also the resources that are available to implement societal interventions based upon research programs are not unlimited. So the question becomes, what do we really wish biomedical research to achieve? If one takes the view that biomedical research should improve the overall wellbeing and health of our society, than it turns out that the personalized medicine approach is a poor use of our limited funding. In a recent discussion with a cancer physician at Duke Medical School I learned that the cost of these next generation cancer drugs are well beyond what even well-employed persons can afford via insurance and their personal wealth. Furthermore, at the end of the day, these drugs do not cure cancer; they simply increase the patient’s life by several months. The cost of these drugs means that these treatments will be beyond the reach of the underemployed and poor in US society. This group is disproportionately ethnic minority, meaning that should we succeed with developing personalized medicine, its initial impact will be to widen the health disparity gap that already exists in American society.

On the other hand, we could reduce the health disparity gap by implementing good science that does not rely on NGS and complex drug regimes. The first would be to examine the causes of complex disease (e.g. heart disease, diabetes, cancer) and the causes of the differentials. In fact, this is something that ecologists, environmental scientists, nutritionists, and social scientists have understood for a long time. Poor people shoulder the burden of morbidity and mortality in all societies because they are the ones who are differentially exposed to the causes of disease (infectious, chemical, radiation, poor diet, etc.) For example, one of the most effective approaches to malarial disease in the tropics is not the development of new anti-malarial drugs, but reducing mosquito breeding habitat and provisioning people with sufficient anti-mosquito netting. Similarly with water-borne bacterial disease the most effective treatments are providing people with clean drinking water13. In the United States, ethnic minorities are differentially exposed to chemical toxins (lead, PCBs, organophosphate pesticides, socially-induced stress, and poor diet14.) Thus I have argued repeatedly that if our goal is to reduce morbidity and mortality in the United States our money would be better spent on research that helps us implement programs to reduce toxic waste exposure, racism, and provide people with nutritious food options15.

Again, these are not mutually exclusive options (NGS v. health/nutrition) however we should consider how social justice is served by these two approaches. In the former, high technology treatments may be developed by cutting edge genomic science. Who benefits from this? Clearly, people with sufficient wealth to afford the treatments, but also the genomic/biomedical scientific community, sequencer technology companies, drug companies, and health care companies. We can see the logic of this approach already impacting how government funding agency mechanisms (NSF, NIH, etc.) are already being structured. This is also seen in the new emphasis that universities have for “translational research” and “university-industry collaboration.” To be clear, it is not that these mechanisms are inherently wrong, but tied into an over-arching paradigm of “fixing the patient after the fact” there is much to be concerned about. In addition, at present, the demography of this consortium of interests still favors persons of European descent. This means that should this program achieve its goals it will only reify the social dominance of European Americans, and support the ongoing subordination of racial-ethnic minority groups.

On the other hand, the environmental approach will benefit a wider variety of persons, since the goal is to reduce and prevent complex disease, as opposed to treating it after you acquire it. Also a serious approach to environmental remediation would benefit a different demography of scientists and social scientists. Also to achieve such remediation would empower the communities most impacted by the pollution.   Again, I have argued that repairing the toxic environment experienced by racial/ethnic minorities is absolutely crucial to alleviating the opportunity gap16.

Conclusion: What is to be done?

It has been my experience that most scientists are quite comfortable living with or simply unaware of social injustice17. There are also many who have decried the unfairness of the world, but have simply felt powerless to make any significant impact on the state of affairs. Most scientists do not see how their science careers are connected to greater social issues. This last statement is less true of evolutionary biologists. Our discipline by its very nature has always been engaged in social controversy18.

On the other hand, ethnic minority scientists have always understood the relationship between science and social injustice. Our very existence stood as living example of the fallacies of racist thinking in science. For example, Charles R. Davenport, director of the Eugenics Record Office at Cold Spring Harbor said of Ernest Everett Just (an early African American biologist) that he did not display any extraordinary talent19. The Howard University Anthropologist William Montague Cobb (b. 1904) spent much of his career debunking the racist claims of physical anthropology. Similarly I have spent much of my career debunking racist claims in genetics, anthropology, medicine, and psychology.

However, racism, sexism, or anti-gay bigotry are not the only topics in which science impacts social injustice. I would argue that all scientific research has the potential to contribute to or alleviate injustice. Therefore, all science falls within ethical discourse. Here we should always evaluate how our science impacts others (obligations), how it is influenced by our values (moral ideals), and what are the beneficial or detrimental consequences of our work. In addition, as science is a social enterprise, we must always be concerned with which persons have the opportunity to engage in it. All of us to some degree have the power to address injustice within the science community (as Richard Lenski did in my case).

If you choose to walk the path of social justice in science, understand that you are not alone. There have always been networks of individuals who have united to address specific justice issues in science. In the 1980’s I was a member of a group called “Science for the People” which grew out of an earlier iteration called “Scientists against the Vietnam War.” Some of you may have heard of the “Union of Concerned Scientists” whose purpose it was to alert society of the growing danger of nuclear annihilation. I am currently involved on the Race and Genetics List Serve that addresses ongoing racial misconceptions in genomic research and also the Social Justice and Science Think Tank, associated with Kalamazoo College20.

Finally, I end with one precautionary note. The pursuit of social justice can be a rabbit hole. There is so much to do, and it can seem like all you ever do, accomplishes nothing. This can be a real danger for graduate students. So I always caution students that their most important goal during their graduate training is to complete their training and earn their degree. You will have much more ability to accomplish your life and societal goals after you have the title of PhD after your name (as I explain in my “Belly of the Beast” essay). There are also more responsible ways of contributing to social justice initiatives without at least initially placing yourself on the “black list.” Of course I used to say, “…if it’s a black list, than I want my name on the top of it!” All, kidding aside, you cannot engage in this work without consequences, and you should decide if you are willing to accept them. This is the path I chose for my life and career, and quite frankly I don’t think I could have done otherwise.

Notes

  1. For a useful definition of social justice consider John Rawls, Justice as Fairness: A Restatement (Cambridge, MA: Harvard University Press, 2001).
  2. Bousquet, A. The Scientific Way of Warfare: Order and Chaos on the Battlefields of Modernity, (London: Hurst and Co), 2009.
  3. Gilbert, G. Economic growth and the poor in Malthus’ Essay on Population, History of Political Economy 12(1): 83—96, 1980.
  4. Marx, K. and Engels, F. Manifesto of the Communist Party, 1848.
  5. Rodney, W., How Europe Underdeveloped Africa, (London: Bogle-L’Ouverture Publications), 1972.
  6. Tilley, H., Africa as a Living Laboratory: Empire, Development, and the Problem of Scientific Knowledge, 1870—1950, (Chicago and London: U. Chicago Press), 2011.
  7. Mead et al. Mead, L.S., Forcino, F.L., Brown Clarke, J., and Graves J.L., Factors influencing the career pursuit of underrepresented minorities with an interest in biology, Evolution: Education and Outreach 8:6 DOI: 10.1186/s12052-015-0034-7, 2015.
  8. Graves, JL. The Emperor’s New Clothes: Biological Theories of Race at the Millennium, (New Brunswick, NJ: Rutgers U. Press), 2005. See Table 3.2 summarizing 19th century anthropologist’s views on the Negro as an inferior and separate species.
  9. Darwin outlined his views on race in the human species in The Descent of Man and Selection in Relation to Sex, 1871. Desmond and Moore’s Darwin’s Sacred Cause: How a Hatred of Slavery Shaped Darwin’s Views on Human Evolution (Boston/New York: Houghton, Mifflin, Harcourt), 2009 is an excellent read illustrating how social perspectives shaped Darwin’s reasoning. My intellectual descent from Dobzhansky and Lewontin can be seen on the Evolution Tree at http://academictree.org/evolution/ .
  10. I spend a great deal of time on these individuals and their work in The Emperor’s New Clothes, 2005.
  11. Hidaka BH, Asghar A, Aktipis CA, Nesse RM, Wolpaw TM, Skursky NK, Bennett KJ, Beyrouty MW, and Schwartz MD. The status of evolutionary medicine education in North American medical schools. BMC Med Educ. 2015 15:38. doi: 10.1186/s12909-015-0322-5.
  12. See excellent discussion of “What is a patient” in Stearns, S. and Medzhitov, R. Evolutionary Medicine, (Sinauer Associates) 2016.
  13. Ntonifor NH, Veyufambom S. Assessing the effective use of mosquito nets in the prevention of malaria in some parts of Mezam division, Northwest Region Cameroon. Malar J. 2016 15(1):390. doi: 10.1186/s12936-016-1419-y.
  14. Bullard, R. et al. Toxic Wastes and Race at Twenty, United Churches of Christ Justice and Witness Ministries, 2007.
  15. Graves, L., Evolutionary versus Racial Medicine: Why it Matters? In Race and the Genetic Revolution: Science, Myth and Culture, edited by Sheldon Krimsky and Kathleen Sloan, Columbia University Press, 2011.
  16. Graves, L, Looking at the World through ‘Race’ Colored Glasses: The Influence of Ascertainment Bias on Biomedical Research and Practice, in Laura Gomez and Nancy Lopez, eds. Mapping “Race”: A Critical Reader on Health Disparities Research, Rutgers University Press, 2013.
  17. Graves, L., Science in the Belly of the Beast: A Look Back at My Career in the Academy, in Voices of Historical and Contemporary Black American Pioneers, V.L. Farmer and E. Shepherd-Wynn Eds., (Westport, CT: Praeger Publishers), 2012.
  18. Berkman, M and Plutzer, E. Evolution, Creation, and the Battle to Control America’s Classrooms, (New York: Cambridge University Press), 2010.
  19. Manning, K. Black Apollo of Science: The Life of Ernest Everett Just, (New York and London: Oxford University Press), 1985.
  20. The link to the Social Justice and Science Think Tank, inaugural lectures can be found on the Science and Social Justice Praxis site of the ACSJL (https://reason.kzoo.edu/csjl/sciencesj/) .
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Exaptation

This Evol 101 post is by MSU grad student Alex Lalejini

Figure 1. Microwave oven

A Conceptual Analogy

Technological innovations throughout the history of human invention are often the result of co-­opting an existing technology to develop something new. The invention of the microwave depended on the development of the magnetron, a device initially developed for World War II era radar technology. The magnetron was invented and adapted for its radar application; the inventors certainly did not have the use-­case of reheating food in mind during its development. However, once it was discovered that the device could be used to reheat food, the magnetron was co­-opted and adapted for use in microwave ovens like the one shown in Fig. 1 (Fernández, 2014).

The laser has a similar story of unforeseen co-­option of functionality for a variety of domains. Since its invention, the laser has seen use in microsurgery, precision measurement, bomb guidance systems, barcode identification, holography, cutting, drilling, welding, printing, telecommunications, and many other domains (Bonifati, 2010). The inventors of the laser did not develop the laser with these domains in mind. The laser was initially developed as a proof of concept (first laser: Fig. 2). At first, people referred to the laser as a solution without a problem (Townes, 2003), but eventually, the laser was co-­opted and adapted to a huge variety of domains.

This process of co-­option is widespread and influential in innovation across many technological domains. The idea of co­-opting an invention for a totally different purpose – using the magnetron for microwaves, or lasers for microsurgery – is analogous to an important concept in evolutionary biology: exaptation.

The term exaptation was first coined by Stephen Jay Gould and Elisabeth S. Vrba in 1982. Gould and Vrba wanted a way to distinguish between two different ways evolution could produce adaptive or fitness­-enhancing features (Gould & Vrba, 1982):

  1. adaptation – the process by which natural selection shapes a feature for a particular use. Features produced by this process are often referred to as adaptations.
  2. exaptation – the process by which a feature previously shaped by natural selection for a particular function or a feature that is entirely non-­adaptive (i.e. a feature that came about through neutral mutations) is co-­opted for an entirely new use. Features produced by this process are often referred to as exaptations.

For example, Gould and Vrba would say that the magnetron was an adaptation for radar technology, but it was an exaptation for reheating food. The difference between an adaptation and an exaptation helps us to understand and explain many fitness­-enhancing characteristics we see in nature.

Figure 3. An artist’s impression of Zhenyuanlong suni. This velociraptor cousin posessed large, complex wings and looked bird-like. Image by Chuang Zhao

Exaptation in Nature

The evolution of complex features can seem unintuitive to those just considering the evolutionary process by which natural selection shapes a feature for a particular use. By Gould and Vrba’s definition, adaptation alone is not sufficient for the evolution of many complex traits we see in species today because what good is only part of a wing for flight? Feathered wings used by birds for flight provide a good example of exaptation in action.

Figure 4. Zhenyuanlong suni fossil by Junchang Lü & Stephen L. Brusatte

The earliest known examples of feathered appendages belong to dinosaurs incapable of flight. A recently found and well preserved fossil of the Zhenyuanlong suni species of dinosaur (artist’s impression: Fig. 3; fossil: Fig. 4) shows that this dinosaur species possessed feathered wings; however, it was unlikely to be capable of flight (Gill, 2015; Lü & Brusatte, 2015). If Zhenyuanlong suni had wings but could not fly, its feathered wings must have evolved for some reason other than flight, which implies that birds’ ability to fly via feathered wings is an exaptation.

It is thought that feathers may have initially evolved for insulation, for mating displays, or for social displays (Zimmer, 2011). Wings are hypothesized to have been initially beneficial for capturing small prey, for aiding bipedal animals in leaping, or for sexual displays (Hutchinson, 1996). Because it is unlikely that wings and feathers evolved initially and specifically for flight, the feathered wings we see on today’s flight-­capable birds are a clear example of an exaptation. This is just one example of an exaptation we see in modern species; evolutionary history is filled with many more!

Together: Exaptation and Adaptation

Gould and Vrba’s versions of adaptation and exaptation do not occur in isolation from one another. An exaptation like feathered wings for flight can be further enhanced through natural selection for more efficient flight. Any exaptation can undergo further adaptations for its particular purpose, and any adaptation may be subject to exaptation to a totally different function. Gould and Vrba’s concept of exaptation is meant to underline the incredible importance of the co­-option of existing traits, behaviors, or characteristics for new purposes in evolution. Exaptation enables us to see vestigial structures in modern species from a new perspective. Perhaps a given organ or structure has no current function today (e.g. the appendix), but through exaptation, it could one day take on a totally new and fitness-­enhancing function in the future!

References

Bonifati, Giovanni. “‘More is different’, exaptation and uncertainty: three foundational concepts for a complexity theory of innovation.” Economics of Innovation and New Technology 19.8 (2010): 743­760.

Fernández, Eliseo. “Evolution of organisms and invention of artifacts: Analogies and contrasts.” 57th Annual Meeting of the Midwest Junto for the History of Science (2014).

Gill, Victoria. “Dinosaur find: Velociraptor ancestor was ‘winged dragon'”, BBC News, (2015).

Gould, Stephen Jay, and Elisabeth S. Vrba. “Exaptation­a missing term in the science of form.” Paleobiology (1982): 4­15.

Hutchinson, John R. “Vertebrate Flight: The Evolution of Flight (a.k.a. How to Wing It)”. (1996).

Lü, Junchang, and Stephen L. Brusatte. “A large, short­armed, winged dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous of China and its implications for feather evolution.” Scientific reports 5 (2015).

Townes, Charles H.”The first laser”. A Century of Nature: Twenty­One Discoveries that Changed Science and the World. University of Chicago Press (2003): pp. 107–12.

Zimmer, Carl. “The Evolution of Feathers: Their origin may have had nothing to do with flight. Photographs by Robert Clark. Art by Xing Lida.” National Geographic 219.2 (2011): 32.

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Getting to the Stem of Central Nervous System Regeneration

This post is by UW grad student Shawn Luttrell

Why do some animals regenerate missing or damaged tissue and organs while others do not? How are animals able to regenerate new tissue and do they all use the same genetic program? Was the common ancestor of all animals able to regenerate and that trait was passed down to multiple animal lineages while it was lost in others, or has this remarkable trait evolved independently numerous times throughout the animal kingdom? Many animals are able to regrow missing or damaged body parts and some can even regrow an entirely new animal from just a small segment of tissue. If you cut sponges, planarian worms, and hydra into pieces, they will regrow complete and normal animals from most, if not all, of the pieces (Alvarado, 2000). Chop certain kinds of sea stars or hemichordates in two and both halves will grow into complete animals (Carnevali, 2006; Humphreys et al., 2010; Rychel and Swalla, 2008). Nearly all annelid worms can regenerate tail segments and some can even regenerate their head. Some species of ctenophores, also known as comb jellies, can heal wounds and regrow new structures without generating any scar tissue. Some crickets and spiders can regrow missing legs and even some hard-shelled invertebrates, like crabs and lobsters will regenerate missing appendages during molting. Nearly every animal phyla has members that are able to regenerate to some degree (Alvarado, 2000). The vast regenerative abilities found throughout the different animal lineages suggest an ancient and ancestral mechanism for this trait. It is likely that the common ancestor of all animals was able to regenerate and this remarkable characteristic has been highly conserved in many groups. The answer to this question, as well as how and why certain animals regenerate, remains a mystery, but through the concentrated efforts of scientists studying this process, like myself, we are getting closer to finding the answers.

I am a fourth year Ph.D. student in the Biology Department at the University of Washington. Over the years, my research has been driven by these fundamental questions and has evolved into focused areas of regenerative science, including stem cells and central nervous system (CNS) regeneration. Humans have limited regenerative abilities, particularly in the CNS. When nerves are damaged or destroyed in the brain and spinal cord, they typically will not regrow. It is imperative to understand the barriers preventing this process and develop approaches to overcome these obstacles as millions of people suffer from brain and spinal cord injuries, as well as neurodegenerative diseases, like Parkinson’s, Alzheimer’s, and Huntington’s to name a few (Brown et al., 2005; Mahabaleshwarkar and Khanna, 2014). One way to gain a better understanding of CNS regeneration is to study animals with extensive regenerative abilities. This is the path my research has taken.

Figure 1. Ptychodera flava, a hemichordate, from Honolulu, Hawaii.

Figure 2. Ptychodera flava collection sites. A) Tetiaroa, French Polynesia. B) Paiko Bay, Honolulu, Hawaii.

Figure 3. Deuterostome phylogeny. Humans are vertebrates, to the right. Hemichordates are a sister group to the well known echinoderms.

I study CNS regeneration in the solitary hemichordate, Ptychodera flava. This animal is also known as an acorn worm and is closely related to echinoderms, like sea stars and sea urchins. Hemichordates are strictly marine animals and all have a tripartite body plan with anterior proboscis that is used for digging and burrowing in the sand and mud, a middle collar region, a ventral mouth between the proboscis and collar, and a long posterior trunk (Figure 1). This particular species of hemichordates lives in the tropical waters of the Indo-Pacific and we collect them in Hawaii and Tahiti (Figure 2). Hemichordates are in the same group of animals as chordates, including humans, and as such, share numerous developmental and morphological features (Figure 3). Most notably for my research, P. flava has a hollow, dorsal neural tube in the collar region that our lab has shown develops in a very similar fashion to the chordate neural tube. In humans, the neural tube becomes the brain and spinal cord. More impressive is the fact that P. flava can regenerate its neural tube after complete ablation. In fact, they can regenerate all of their body structures. I bisected adult worms in the trunk region (Figure 4) and after two weeks of regenerating at 26°C, the posterior half of the animal has regrown a new proboscis, collar and neural tube, and anterior region of the trunk (Figure 5). As humans, we have lost most of our regenerative abilities, and our lab is interested in whether we still have the genes that are important for regeneration, as we share many developmental genes with hemichordates.

Figure 4. Bisected Ptychodera flava. The boxed area indicates the regeneration site of the posterior half of the animal.

Figure 5. Regenerating Ptychodera flava. A) The open wound of the cut site on day zero of regeneration. B) Day 1 of regeneration showing the wound has healed. C) Day 7 of regeneration showing the proboscis and partial collar. D) Day 14 of regeneration showing complete proboscis and collar regeneration.

We have recently submitted a manuscript for publication that details the regeneration transcriptome for Ptychodera flava. A transcriptome shows all of the genes that are being turned on and off at a particular time when that tissue was collected. We sequenced and analyzed nine different time points during early hemichordate regeneration and identified hundreds of genes regulated during this process. This was part of an overarching transcriptome project that is looking at numerous animals that are able to regenerate to see if there is a common gene or set of genes that starts the regeneration program. If the common ancestor of all animals was able to regenerate and that trait was passed down, it’s likely that the regeneration program will be similar in all animals. Importantly, humans will likely possess many, if not all, of the genes required for the regeneration program. If that is the case, it may be possible to reactivate those genetic pathways using the regeneration program from other animals.

Aside from activating the correct genes to start the regeneration program, another possible way to regrow new CNS neurons is to use stem cells. These special cells have the ability to become any type of cell in the body. Hemichordates may use stem cells to regrow missing or damaged tissue. The other possibility is that they use existing cells in their body and reprogram those cells in regenerating structures. For example, a muscle cell in the existing tissue might be reprogrammed to become a nerve cell in the new tissue. To help answer this question, I was awarded a grant though the National Science Foundation as part of the East Asia and Pacific Summer Institute (EAPSI) program to work on a project with one of our collaborators in Taiwan. I am currently at Academia Sinica, the premier Science and Technology Research Institute in Taiwan, using molecular methods with stem cell markers to try and identify what type of cells hemichordates use to generate new tissue. In humans, when a cell differentiates into a particular cell type the cell fate can usually not be reversed or altered. If hemichordates employ this method during regeneration, it will give us a model to study how cell fates can be reassigned. If, on the other hand, hemichordates use stem cells to populate new tissue, this will give us a model to study how stem cells are activated and recruited during regeneration. Hemichordates may use both models to regenerate new tissue and this possibility is exciting because it will give us two models in one system to develop methods for regenerating missing or damaged tissue, including CNS neurons, in humans.

References

Brown R.C., Lockwood A.H., Sonawane B.R. 2005. Neurodegenerative diseases: an overview of environmental risk factors. Environ Health Perspect. 113(9): 1250-1256.

Mahabaleshwarkar R, Khanna R. 2014. National hospitalization burden associated with spinal cord injuries in the United States. Spinal Cord 52(2): 139-44.

Candia Carnevali, M.D. (2006). Regeneration in echinoderms: Repair, regrowth, cloning. Invert Surv J. 3, 64-76.

Humphreys, T., Sasaki, A., Uenishi, G., Taparra, K., Arimoto, A,. Tagawa, K. (2010). Regeneration in the hemichordate Ptychodera flava. Zoolog Sci. 27(2), 91-95.

Rychel, A.L. and Swalla, B.J. (2008). Anterior regeneration in the hemichordate Ptychodera flava. Dev. Dyn. 237(11), 3222-3232.

Sánchez Alvarado, A. (2000). Regeneration in the metazoans: Why does it happen? Bioessays. 22, 578-590.

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BEACON at GECCO 2016

We are very excited to acknowledge the success of some fellow BEACONites. Kalyanmoy Deb and students of the COIN (Computational Optimization and Innovation) lab this year received multiple awards and nominations at the GECCO conference (The Genetic and Evolutionary Computation Conference).

For the Real-World Applications (RWA) track, Zhichao Lu, Kalyanmoy Deb (Michigan State University) and Ankur Sinha (Aalto University) were nominated for the Best Paper Award for their paper: Finding Reliable Solutions in Bilevel Optimization Problems Under Uncertainties.

In the Evolutionary Multi-objective Optimization (EMO) track, Rayan Hussein and Kalyanmoy Deb (Michigan State University) were nominated for the Best Paper Award for their paper A Generative Kriging Surrogate Model for Constrained and Unconstrained Multi-objective Optimization.

In the Genetic Algorithms track, Kalyanmoy Deb (Michigan State University) and Christie Myburgh (MapTek) won the Best Paper Award voted by the GECCO participants for their paper Breaking the Billion Variable Barrier in Real-World Optimization Using a Customized Evolutionary Algorithm.

20160724_130959 IMG_3625The Niching Methods for Multi-modal Optimization Competition Award was awarded to Ali Ahrari (ME PhD student, Michigan State University), Kalyanmoy Deb (Michigan State University), and Mike Preuss (University of Munster, Germany) for their paper Benchmarking Covariance Matrix Self Adaption Evolution Strategy with Repelling Subpopulations.

Lastly ACM’s SIGEVO Impact Award, recognizing papers previously published 10 years earlier in the GECCO conference that are highly cited and deemed to be seminal by the SIGEVO Executive Committee, was awarded to Kalyanmoy Deb, and J. Sundar for their paper Reference point based multi-objective optimization using evolutionary algorithms in Proceedings of the 8th annual conference on Genetic and evolutionary computation. ACM, 2006.

Congratulations!

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BEACON now funds video games?

This is a repost from Terry Soule (Computer Science, UI) and Barrie Robison (Biological Sciences, UI)

Also visit Telliamed Revisited (Richard Lenski’s blog) for another write-up. 

Hello BEACONites,

Thanks to BEACON’s support Polymorphic Games has created the evolutionary video game Darwin’s Demons, and placed it on the Steam website as part of the green light process.

Darwin’s Demons adds an evolutionary component and modern flair to an arcade classic. Darwin’s Demons models biological evolution using enemies with digital genomes. Enemies acquire fitness by being the most aggressive, accurate, and longest lived, and only the most fit enemies pass their genomes to the next generation. The result? The creatures you found hardest to kill have all the babies, making each generation more challenging than the last!

The game includes in-game graphs for tracking evolution, displays the most fit enemies from each wave, and has an experiment mode where you can set parameters like the mutation rate, fitness function, etc. It also dumps all of the evolutionary data to a file. So, there are opportunities for experiments on user driven evolution if anyone is interested. (We are more than happy to share the code and/or make simple modifications for controlled experiments.)

If you get the opportunity please try out the demo (downloadable at either of the sites listed above, with Windows, MAC, and Linux versions), vote for us on Steam, and send us comments, suggestions, or ideas for future directions and collaborations.

Thanks,
Terry Soule (tsoule@cs.uidaho.edu), Computer Science, UI
Barrie Robison (brobison@uidaho.edu), Biological Sciences, UI

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Culture, Sociality, & Evolution

This Evolution 101 post is by MSU grad student Alex Lalejini

Culture and Chimpanzees

Our species is incredibly social, and one of the major products of our sociality is culture. People typically imagine culture to be exclusive to humans. The idea of culture brings to mind examples that are uniquely human phenomena: blockbuster movies like The Fast and the Furious franchise, great arthouse crime films like Drive, the art exhibited at the Louvre in Paris, or sharing a traditional meal with one’s family. These phenomena are perfectly valid associations with the idea of culture, but it may come as a surprise that culture is not exclusive to our species.

Dr. Andrew Whiten, a professor at the University of St. Andrews and an expert on non­human cultures (yes, non-­human!), has done extensive work on chimpanzee culture. Chimpanzee culture! But we don’t see chimpanzees publishing novels or displaying art in galleries. This observation is likely correct; however, one must consider the definition of culture. Whiten defines culture as “all we learn from others that endures for long enough to create socially transmitted traditions” (Whiten, Vince, & Mace, 2014). With this definition, cultural behavior is well established in species other than humans: dialects in song­-birds, sweet-­potato washing and stone-­handling by Japanese Macaques, and tool use and various other social behaviors in African chimpanzees (Whiten et. al., 1999). By the way, you can hear about some of Whiten’s work on chimpanzee culture in a presentation here.

Culture is a deeply intriguing social phenomenon. How and why does cultural behavior emerge from evolutionary processes, and what advantages does cultural behavior bestow upon its practitioners? The journey to understanding the evolution of cultural behavior meanders through the evolution of sociality. Basic forms of social interactions form the building blocks for cultural behavior. Without sociality, individuals lack the capacity to socially transmit information among one another, and without the capacity to socially transmit information, cultural behavior cannot exist. So, what exactly does it mean to be social? What other forms of social behaviors are found in nature? And, why might social behavior evolve in the first place?

What does it mean to be social?

For the purpose of this discussion, we are going to consider sociality as a tactic employed by an individual or group of individuals that increases the fitness of group members (Armitage, 1999). Evolution has produced many examples of sociality in organisms; aside from cultural behavior, this blog post will give an overview of two: social grouping and parental investment. Social grouping can be seen in herds of African elephants, schools of surgeonfish, or flocks of seagulls. Parental investment is defined as “any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (Campbell, 1972). Parental investment can be seen in mother grizzly bears taking care of their cubs. Rather than abandoning her offspring at birth in favor of having more offspring, the mother grizzly takes care of her cubs until they reach sufficient maturity to take care of themselves.

Why be Social?

All of these examples are great, but if we think back to natural selection, an individual’s fitness is a function of their ability to reproduce relative to their competitors. If one individual is acting socially to increase the reproductive success of other group members, doesn’t sociality decrease an individual’s fitness relative to other group members, and shouldn’t natural selection oppose social behavior? At a glance, this seems like a reasonable hypothesis; however, with a closer look, one can see the evolutionary benefits of organismal sociality.

Social grouping can decrease an individual’s susceptibility to predation through increased vigilance (example: shared responsibility for watching for predators in herds), aggressive group defense behavior, increased ability to find and obtain food sources, or just through the fact that larger groups lower an individual member’s probability of being preyed upon (Swedell, 2012; Alexander, 1974).

Parental investment is a deeply interesting area of research, and is much more complicated in actuality than what I will mention here (see the parental investment section of Alexander 1974 for a nice introductory overview). Kin selection offers an insightful view into some of the advantages of parental investment. Kin selection is “the evolution of characteristics which favour the survival of close relatives” (Smith, 1964), which is potentially at the cost of the individual. In other words, if an individual’s close relatives are reproductively successful with the aid of the individual – even at the cost of the individual – the individual’s genes are likely to persist into the next generation because the individual and their close relatives are likely to share many genes. In the context of parental investment, parents may invest heavily in their offspring in order to increase the possibility of grand-­offspring.

We can clearly see how social behaviors like social grouping and parental investment facilitate cultural behaviors by looking to our own species. The social transmission of traditions from parent to offspring and from individual to social group is widespread in human societies; this transmission can be thought of as an instance of parental investment. We also often share socially acquired knowledge and traditions with other people in our local communities (e.g. your grandmother’s famous recipe for gumbo); these communities act as our social groups and provide a forum to socially transmit information to others. Social grouping and parental investment might be potential building blocks for the evolution of cultural behavior, but what possible selective advantages are conferred to practitioners of cultural behavior?

Why be Cultural?

If we consider culture and organisms’ genomes as information storage mechanisms, culture provides an alternative method of encoding and passing information among individuals in a population as compared to gene propagation through reproduction. As an information storage mechanism, culture is much more fluid and quick to change than information stored at the gene level (there is no need to wait around for mutation and natural selection to act); however, this is at the cost of being more fragile (traditions must be taught to offspring, there is no way to pass them down genetically).

Insights into the evolution of sociality may lead to a deeper understanding of evolutionary history because many species behave socially. Our own species is incredibly social; you can imagine how the human species displays all of the examples I’ve talked about here (cultural behavior, parental investment, social grouping). Progress in our understanding of the evolution of sociality may offer us an even deeper understanding of our own species.

References

Alexander, R. D. (1974). The evolution of social behavior. Annual review of ecology and systematics, 325­383.

Armitage, K. B. (1999). Evolution of sociality in marmots. Journal of Mammalogy, 80(1), 1­10.

Campbell, B. G. (Ed.). (1972). Sexual selection and the descent of man, 1871­1971. Heinemann.

Smith, J. M. (1964). Group selection and kin selection. Nature, 201, 1145­1147.

Swedell, L. (2012). Primate sociality and social systems. Nature Education Knowledge, 3(10), 84.

Whiten, A., Goodall, J., McGrew, W. C., Nishida, T., Reynolds, V., Sugiyama, Y., … & Boesch, C. (1999). Cultures in Chimpanzees. Nature, 399(6737), 682­685.

Whiten, A., Vince, G., Mace, R, (2014). Human and Other Animals: Cultural Evolution and Social Learning. Lecture given at The Royal Institute

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Genetic Drift

This Evolution 101 post is by MSU grad student Ali Tehrani

Darwinian evolution suggests that the pattern of changes in populations is driven by natural selection, i.e. those individuals that are more fit survive and reproduce. Does that mean populations of organisms are constantly gaining beneficial genes and losing detrimental genes through natural selection? So, is it not possible that some genes that are actually beneficial or at least neutral to the organism’s fitness have disappeared in evolutionary history only by chance? Or alternatively that random events could cause slightly deleterious genes to increase in frequency?

There are several examples in nature in which beneficial or neutral traits have disappeared from a population. Consider an island with a population of ten rabbits. This island can only hold a small population of rabbits (about 10) due to resource limits. Five rabbits out of ten are white and the other five are brown (figure 2). Also assume that the coat color does not have any effect on their reproductive success. Coat color is a genetically inherited trait with two different alleles1, one for white coat color and the other for brown. If both parents have the same coat color (i.e. both of them are white or both are brown), their offspring will also have that coat color (same as their parents). If one of the parents is white and the other is brown, their offspring will be either white or brown with equal probabilities (see figure 1). Rabbits in this population mate randomly; thus, the probability of mating two white rabbits is the same as the probability of mating between two brown rabbits. So, there is a fifty percent chance that each individual in the next generation will be white or brown. Does this mean that the population of rabbits will always have five white rabbits and five brown ones?

Figure 1. Coat color in offspring of rabbit couples. If both parent 1 and parent 2 are white, all their offspring will be white (top left). If both parents are brown, all their offspring will be brown (bottom right). If one parent is white and the other is brown there is a 50% chance that their offspring is white or brown (bottom left and top right).

In reality, there is no guarantee that such system will always have five rabbits of each coat color. It is similar to flipping a coin ten times and getting different numbers from five heads and five tails. This phenomenon, which is called “sampling error”, results in fluctuations in number of rabbits with white/brown coat color. Sampling error is more likely to happen in small populations because in these populations, obviously, the sample size is smaller. The random fluctuations in allele frequency in a population which occurs due to sampling error is called “genetic drift”. The effect of genetic drift is stronger in small populations. Sampling error becomes smaller as the population size grows for the same reason that if we flip a coin one million times the deviation from 50%-50% becomes much smaller.

Figure 2. Initial population of rabbits consisting of five white-colored and five brown-colored rabbits.

Let’s go back to our rabbit population. Given the initial population in figure 2, and the mating pattern illustrated in figure 1, each individual in the second generation would be brown or white with equal probability of 50%. So how many white rabbits would be in the second generation? Well, this is similar to ask: how many heads do we get if we flip a coin ten times? In reality, it is possible that we get any number of heads. So assume we flipped the coin ten times and we got six heads and four tails.

In a similar manner, suppose we have six white rabbits and four brown rabbits in the second generation (figure 3). Now, the probability of a particular rabbit being white in next generation is not 50% anymore (it is higher than 50%). This deviation can become either larger or smaller each generation, randomly, and it is possible that eventually the population becomes uniformly white- colored (figure 3). When all individuals in the population have the white coat color allele, this allele is said to be “fixed” in the population. Because there is no rabbit with a brown coat color allele in the population anymore, it is therefore impossible for this allele to appear in the population again in the absence of mutation and migration. As a result, the frequency of the white coat color allele becomes 100% and remains the same thereafter (figure 4 shows how coat color allele frequency changes through generations).

Figure 3. Fluctuations in the number of white/brown rabbits through generations. Genetic drift in this scenario led to fixation of white coat color allele in the population.

Genetic drift and consequent random fixation of alleles in populations can occur as a result of abrupt reduction in population size. The abrupt decline in population size which may occur due to environmental effects (e.g. drought, fire, flood, etc.) is called “population bottleneck”. Genetic drift is a significant evolutionary process in such small populations and usually leads to fixation of alleles to one or the other. Hence, the genetic diversity within such populations decreases or is totally lost as a result of genetic drift operating on small number of individuals founding the population.

Figure 4. Coat color frequency of rabbits through generations.

[1] https://en.wikipedia.org/wiki/Genetic_drift
[2] Conner, Jeffrey K., and Daniel L. Hartl. A primer of ecological genetics. Sinauer Associates Incorporated, 2004.
[3] https://en.wikipedia.org/wiki/Population_bottleneck

1 An allele is a variant form of a gene. In this example, the gene for white coat color and the gene for brown coat color are two alleles of the coat color gene. For the purposes of this example, we will ignore the fact that rabbits are diploid — containing two alleles of every gene, one inherited from each parent — to simplify the math.

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Evolution of Reliable Signals

This Evolution 101 post is by MSU grad student Thassyo Pinto

The ownership of goods such as luxury cars, expensive boats and conspicuous consumption, and showing it off to others, transmits a signal informing that owner is capable of bearing expenses. The wealth and status are used to advertise “fitness”, which demonstrates higher quality to potential mates. In the Martin Scorsese’s film “The Wolf of Wall Street”, Leonardo DiCaprio (Figure 1) takes the role of a crooked stock trader who has a great talent for conspicuous consumption. As said by his character, Jordan Belfort: “Their money was better off in my pocket – I knew how to spend it better”. Not only did his character spend a fortune on extravagant goods and illegal drugs, but he also used his status to mate with multiple partners. Since the cost of conspicuous consumption is high, in theory there should be few cheaters (underprivileged men) who attempt to mimic these “honest” signallers in order to attract women.

Figure 1. A scene from the movie “The Wolf of Wall Street”

In the animal kingdom, signals are important characters since them convey various information from signallers to receivers. The reliability of a signal can change depending on the receiver responding to reliable signals and ignoring non-reliable ones. Moreover, the cost of a signal could affect its reliability since the investment in it could be worthwhile to honest signallers and not worthwhile to cheaters. We can take a peacock’s tail for example (Figure 2), which provides positive value to the female. If this signal is really costly (a long and heavy tail), a male not only could look attractive to a female, but the signal could also tell that he has a better chance for survival. As long as a certain quality is not impacting a male’s health, he must be well-adapted in comparison to less adorned ones. This idea was first proposed by Amotz Zahavi (Zahavi, 1975) and it is called the handicap principle.

Figure 2. Male Indian peacock (Pavo cristatus), displaying colorful and vibrant feathers

In Zahavi’s handicap principle, the handicap not only is an indicator of genetic quality, but it also needs to be costly in order to ensure that signalling is reliable. Otherwise, low quality males would be able to equally advertise and females would not be able to identify the honest signaller. Although this original idea was largely rejected by many scientists in its earlier days, Alan Grafen’s (Grafen, 1990) paper provided the first full game-theoretical formalization, confirming the plausibility of the handicap principle mathematically. Figure 3, shows how tail length relates to costs and benefits. A costly tail (very long) will be more affordable to high quality (HQ) males than low quality males (LQ). If there is a benefit in tail length due to mating success, the optimal tail length will be higher for a high quality males than for low quality males. Furthermore, if low quality males try to grow a much longer tail than its optimal value, his benefit will decrease, showing that the tail length becomes an honest signal. Therefore, the cost creates a correlation between male quality and tails cost, making a cheating behavior maladaptive.

Figure 3. Optimal tail length for a low quality male (LQ) and for a high quality male (HQ) when a long tail is a handicap (Lotem, 1993).

Signals can also be directed from prey to predator. Stotting gazelles send signals to predators conveying that they have a lot of energy and they are in good health condition. As described by Zahavi (Zahavi, 1997), “it shows off its strength and fitness by jumping straight up. Only a gazelle certain of its ability to outrun a predator dares squander its strength is this way”. These hard-to-catch prey benefit by differentiating themselves from the overall population, and consequently deter predation. If there was no cost in this signal, there would be a growing number of cheaters, and predators would learn to ignore these false signals.

As suggested by Zahavi (Zahavi, 2008) “the signal encodes neither threat nor invitation, but rather dimensions of a quality, i.e. strength, which produces different reactions in different receivers”. Zahavi also infer that signalling systems are by nature collaborations. In order to a signal to be effective, there must be a cooperation between the receiver and the signaller. Therefore, Zahavi implies that the handicap principle guarantee the reliability of signals and it is an essential component in all signals.

References:
Grafen, Alan. “Biological signals as handicaps.” Journal of theoretical biology 144.4 (1990): 517-546.

Lotem, A. “Secondary sexual ornaments as signals: the handicap approach and three potential problems.” Etologia 3.209-18 (1993).

Zahavi, Amotz. “Mate selection—a selection for a handicap.” Journal of theoretical Biology 53.1 (1975): 205-214.

Zahavi, Amotz, and Avishag Zahavi. The handicap principle: a missing piece of Darwin’s puzzle. Oxford University Press, 1997.

Zahavi, Amotz. “The handicap principle and signalling in collaborative systems.” Sociobiology of communication. Oxford University Press, Oxford (2008): 1-11.

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Artificial selection and correlated traits

This Evolution 101 post is by MSU grad student Tyler Derr

One of the basic mechanisms of evolutionary change is natural selection. It was in Charles Darwin’s famous book, On the Origin of Species, where he defined natural selection to be the “principle by which each slight variation [of a trait], if useful, is preserved.” (Darwin 1859). Knowing that many people would be skeptical of what he presented in his book, the first chapter is structured to first discuss selection in terms of breeding. He presented examples of where humans have selectively bred both animals and plants.

Artificial selection (also called selective breeding) is a process in which humans interfere with natural selection to obtain certain traits we desire an organism to have. This process is performed by choosing which animals or plants are allowed to mate with each other in the hopes if both of the parents have a certain observable trait then their offspring will as well. Shown in Fig. 1 are common vegetables that have all been cultivated from wild mustard by past farmers artificially selecting traits in the plant.

Fig. 1. Vegetables that arose from the wild mustard due to selecting for different traits.

Selective breeding has been done on many animals. Examples of such artificial selection being performed can be seen in dogs. Due to this selective breeding, there are now hundreds of different breeds. It was only just recently, thanks to whole genome sequencing, that we have discovered that gray wolves and dogs started to diverge from a common ancestor at the same time roughly 27,000- 40,000 years ago (Skoglund 2015). Although today we breed dogs for certain traits such as cute floppy ears or a fluffy coat, we can pretty safely say that these would have probably not been at the top of the list with humans that long ago. We know that based on when dogs first arose this would have been the time when humans were still hunter-gathers (Freedman 2014). Fig. 2 below shows a few dog breeds representing were they were selectively breed, based on geographical location, and also representing from what previous dog breeds they were breed from.

So if humans tens of thousands of years ago did not select for all the same traits that we select for today, what did they desire? Well, it can be hypothesized that they simply desired the dogs to be non-aggressive. Although we normally think of evolution as a very long process (which it is with natural selection), the game changes when we as humans start getting involved. It was in 1959 that a Russian geneticist Kmitry K. Belyaev (shown in Fig. 3) began a study in the hopes of breeding a population of tame foxes (Trut 1999). Belyaev solely selected for tameness and strictly against aggression when breeding. After just ten generations, 18% of the pups were not only tame, but showing signs of affection such as whimpering for attention and even licking the experimenters (Trut 1999). Evolution in Action: The Silver Fox Experiment, is a short clip from the BBC’s documentary [The Secret Life of Dogs] in which they discuss the early stages of the fox experiment and then also show the progress that has been made over the last 50 years.

Fig. 2: Examples of how selective dog breeding has branched from the common ancestor based on geographical location. Cain, Michael L., Damman, Hans, Lue, Robert A. and Carol Kaesuk Yoon. Discover Biology Second Edition. New York: W. W. Norton, 2002.

As mentioned before, the only trait that was selected for in the fox experiment was tameness, however interestingly enough quite a number of other traits came with it. Other physical traits such as a curly tail instead of straight, floppy ears, and shorter limbs began to appear in the tame foxes, which are traits commonly shared among other domesticated animals (Trut 1999). What was just described is commonly known as the correlation of traits, which is another interesting topic to mention when discussing artificial selection. This is when selecting for a specific trait not only allows the offspring to have the trait selected for, but also a set of other traits that are genetically correlated.

Fig. 3: Belyaev shown with some of the tame foxes that were bred in his experiment.

 

As seen from both the plant and animal examples, humans can have a large impact on a species when performing selective breeding. It will be interesting to see how future organisms will be changed to fit the wants and or needs of humans in the future.

References:

Darwin, C. (1859). “On the origin of species by means of natural selection, or by preservation of favoured races in the struggle for life”. London: John Murray.

Freedman, A., et al. (2014). “Genome sequencing highlights the dynamic early history of dogs”. PLoS genetics 10(1): e1004016.

Skoglund, P., et al. (2015). “Ancient wolf genome reveals an early divergence of domestic dog ancestors and admixture into high-latitude breeds”. Current Biology 25(11): 1515–9.

Trut, L. (1999). “Early Canid Domestication: The Farm-Fox Experiment Foxes bred for tamability in a 40-year experiment exhibit remarkable transformations that suggest an interplay between behavioral genetics and development”. American Scientist 87(2): 160-169.

 

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