Predicting Future Evolution (Spring 2016)

One of the activities that I regularly have my students complete in my Evolution course is called “Future Evolution“. The activity sends students on what most evolutionary biologists consider a fool’s errand: to try to predict the future evolution of some particular trait in some particular species. Making such predictions is really difficult for these basic reasons:

• So much of evolutionary change relies on random mutations, and predicting where or how new mutations might arise is nearly impossible;
• All future genetic changes will occur in the context of the existing genetic architecture of each organism, and although we are getting better at understanding this architecture we are a long ways off from being able to predict what’s possible to change in traits by making genetic changes; and
• Although there are some very clear environmental changes occurring now — most of them caused by human activities — it is difficult to know if these changes will be sustained for long enough to lead to changes in the traits of particular species.

Why send students off to make predictions when most evolutionary biologists would be loathe to make such predictions themselves? Well, as a thought exercise making these predictions is actually really valuable.

A skill that I try to teach to all of my students is to “tell an evolutionary story”. I want students to be able to explain what kinds of changes to existing traits would be required for a new trait to evolve, how the resulting trait variant might provide advantage in a particular environment and therefore increase in prevalence due to some form of selection, and explain how that overall evolutionary process would produce this new trait as an adaptation of a particular species. When we look retrospectively at traits that have already evolved, the goal is to verbalize a reasonable scenario under which this trait evolved. When we make predictions about the evolution of novel traits, the goal is to predict the evolution of a trait in a particular species that feasibly might occur given what we know about recent changes to some aspect of that species’ environment. The prediction doesn’t necessarily have to be likely to come true — even most reasonable evolutionary predictions are still low-probability outcomes — it just has to be feasible. Making these feasible predictions requires a strong understanding of how evolution works, which is why we do this exercise in our final class session.

Every semester I get some really fun, interesting, provocative, and off-the-wall predictions. Below are some highlights from this semester’s class…

Prediction: Humans will develop increased thumb dexterity due to the ubiquitous use of smart phones.

Although students are predominantly analyzing the predictions of their peers, I have to seed this activity with a first prediction to be analyzed by the first group to finish their first prediction… and this is a common seed that I use. This semester the group that analyzed this prediction decided that it was not feasible, which seems like the most reasonable conclusion to draw. Their explanation for why this was not feasible was that “it doesn’t help in a matter of life and death”, which makes a bit of sense. They also claimed that thumb dexterity is not heritable, which probably isn’t true (although I understand that much of the difference in “smart phone thumb skill” is learned rather than inherited). I like this example because you can understand how absurd this prediction is by considering absurd scenarios: would a person really be more attractive to a potential mate because they respond by text faster? But perhaps the most important issue here is about rate of change: the demands of smart phones on our thumb dexterity are changing rapidly as these phones evolve as a cultural technology. And like a lot of other technologies, phones are probably evolving way too fast to be a major selective force: unless the demands on our thumbs made by smart phones are sustained for something resembling human generation time, it is unlikely that any advantage gained by having a dextrous thumb would be passed on to future generations.

Prediction: Toy-sized dogs will become even smaller in response to human preferences.

This predictions brings to light all sorts of dilemmas associated with human artificial selection. One might criticize this as being a prediction related to a subpopulation of a species rather than a species, and factually this criticism is correct. But the way that we have artificially selected different dog breeds has produced such extreme traits that effectively toy dogs are genetically isolated from much of the rest of their species. We generally don’t allow them to breed with bigger breeds or mutts, and if they did there certainly are some logistical barriers to successful reproduction. So I guess this is a feasible prediction if people really still desire an even tinier dog. The big assumption of this prediction is that there still remains in the toy breed genome sufficient genetic variation to allow artificial selection for even tinier dogs. It’s possible we have bred all the genetic variation for size out of these breeds already, which would make further artificial selection for size ineffective. You have to have heritable variation in order for selection of any kind to act!

Prediction: Crows will develop increased dexterity in their talons in response to increased commensalism with humans in urban environments.

Unlike the prediction that human thumb dexterity will increase in response to the environment that we have created, this prediction makes some sense because crows do rely increasingly on human food sources that might require some dexterity to access. Crow survival might be related to their ability to manipulate human environments with their beaks and feet: their cognitive abilities are already well-developed, so perhaps the more successful crows will combine intelligence with a slightly-greater ability to break open human food sources. It is hard to predict how our commensal species will evolve in response to the ever-expanding human-dominated environment, but the environment is certainly there for longer-term evolution of traits that are generally-valuable in human settlements.

Prediction: Subway rats will become hairless and more lithe in response to climate change.

This is an interesting prediction because it is doubly-tied to us as humans: rats are a commensal species, and perhaps due to climate change they might have to adapt to survive in the human-created environment. The problem with this prediction is that it supposes that rats don’t already have behavioral and physiological adaptations that allow them to deal with high temperatures. As anyone who has spent a little time in NYC subway stations during the summertime knows, they get pretty hot already. And while they might get even hotter due to climate change, the question is whether rats already have the physiological and behavioral adaptations needed to contend with this heat. The problem with hairlessness is that in a temperate region the rats would still have to contend with colder months, months in which hairlessness would be a major liability (those NYC subway tracks are also really cold in the winter!).

Prediction: Humans will become more hairless in response to climate change.

Humans are already pretty hairless: would it advantage us to be less hairy in a warming world? The data suggests otherwise. Humans vary slightly in their density of body hair and arguably some of that variation is geographical and therefore related to the temperature regime of the local environment. But so many humans now have access to cooling technologies — from low-tech evaporative cooling to high-tech air conditioning units — that it is hard to imagine that much survival will be dictated by the amount of body hair we have. The only thing that makes this prediction feasible is that it could happen over a long period of time: the earth is warming slowly and consistently and wherever the climate stops it is going to stay that warm for a long period of time. So strip us of enough technology and there certainly will be enough time in a warmed environment to produce either anatomical or physiological adaptations in future humans (whether further hairlessness would help us deal with heat is at question, but perhaps physiological adaptations are more likely).

Prediction: Elephants will evolve to have no tusks due to poaching.

I like the less ambitious version of this prediction — elephants will evolve to have smaller tusks — because this kind of evolved response to human predation has already been observed in other species. Our quest to catch the biggest fish in the oceans has led to the evolution of smaller and slower-growing fish (Conover & Baumann 2009), so presumably if poachers take the elephants with the largest tusks then tusk size should evolve to decrease. As with other predictions listed here, two major concerns mitigate the feasibility of this prediction. First, there is the question of whether mature elephants whose tusks might be poached are likely to have already done most of their reproducing; if so, being killed for having the largest tusks is actually not selected against. Second, the question is where culture will go; with so many international and national laws already making trade in tusks illegal, it is unclear if any selective pressure exerted by poaching will be sustained for long enough to lead to biological evolution.

Prediction: Walruses will evolve to have slimmer bodies (maintaining less blubber) in response to climate change.

This is an interesting prediction that’s kind of difficult to classify. If blubber is for food storage, one might wonder whether a decreased body size might result from lowered access to food in a warming environment: this would be an environmentally-mediated change in traits that might signal serious extinction risk. But if climate change persists and walruses can adapt in other ways that allow them to survive in a warming world, it is not entirely crazy to think that they might eventually need to amass less blubber as a means of insulating their bodies. We need a walrus expert to help mediate the issues surrounding this prediction… do walruses primarily retain blubber for insulation or as a stored food source?

Prediction: Camels will develop a third hump in response to climate change.

The idea here makes some basic sense: if camels have fat stored in their humps in order to have a stored source of both energy and water, wouldn’t a warming climate produce additional humps? The problem with this line of thinking is that it is hard to imagine how modification of existing variation might produce an additional hump, at least as the next change. Clearly the number of humps does evolve, as dromedaries have one hump and bactrian camels have two. But would a new hump be the evolutionary response of these species to climate change? A more conservative (and near-term) prediction would be that camels would increase their fat storage in existing humps. Perhaps eventually that change would become unsustainable and lead to redistribution of this fat to a third hump. But it also seems equally likely that camel species might just migrate away from places that are so dry that they require further development of an already-evolved trait that helps them deal pretty well with low-water environments.

Prediction: Homosexuality will become more prevalent due to overpopulation.

This is an interesting prediction because it exposes a kind of “naive for the good of the species” thinking that I discourage at various junctures in the course. As the instructions for this activity point out, I am looking for traits that evolve biologically, so this prediction has to be taken to mean that homosexuality will become more prevalent due to genetic changes. This is a very complex topic that I tackle in my Evolution of Sex course but don’t get into in this Evolution course, so it is understandable that students don’t appreciate all the nuances of the evolution of homosexuality challenge. But what’s clear from this prediction is that homosexuality is supposed to evolve as a response to having too many babies, and that only seems feasible if cultural evolution is involved. Neither kin selection nor group selection — the two mechanisms we discussed in class in relationship to altruism — is likely to produce more prevalent homosexual behavior in response to increased population size. This might actually be a logic that could work if the genetics of homosexuality were sufficiently complex (which they appear to be based on what we know so far) and humans lived in discrete populations that could either over-populate or lower birth rates in response to population density. But the problem with human populations is that we are so inter-connected and therefore our over-population problems are rarely local enough to have kin- or group-selective mechanisms serve as a solution. I am interested in whether societies that are more tolerant of a variety of gender identities and romantic partner preferences are more successful than those that are less tolerant, but if that dynamic is at play any rise in homosexuality would be due to cultural rather than biological evolution. We have clearly evolved to be a little queer, but any increase in queerness is probably going to be cultural in nature… and hey, maybe that might — as a secondary byproduct — lower our birth rates a bit.

Prediction: Humans will evolve more efficient livers in response to an environment containing more novel toxins.

This is an interesting prediction, because it seems entirely feasible if these toxins threaten our pre-reproductive survival. If we are likely to die young because our livers don’t efficiently cleanse our bodies of these toxins, any individual with a mutation that allowed for more efficient toxin breakdown in the liver might enjoy a benefit. What’s aiding this prediction is that so many of the toxins that we are now exposed to are novel: synthetic chemistry has created components of our environment that humans never encountered in the past. The problem is how sustained that environment is, because synthetic chemistry is another cultural product that might be changing our environment at such rapid pace that our slow biological evolution might not be able to keep up with changes in the chemicals we experience. The other problem with this prediction is that it relies heavily on a significant number of humans with “contemporary livers” to die before reproducing in order for the “liver of the future” to evolve; if the environmental toxins we now encounter are affecting us, it probably is mostly in diseases of older (in other words, post-reproductive) people… cancer, auto-immune disorders. These diseases are unlikely to serve as the engine of evolution unless they appear early in life.

Prediction: Humans will become more obese in response to an environment that requires less activity and provides higher-calorie food.

This kind of prediction is the classic, one that mistakes changes in environmental influence on existing genetic diversity for an environment with the potential to change that genetic diversity. Sure, you see more obese people today, especially in economically-developed countries. But is that evolution? Probably not, as what you are seeing is how changes in the environment interact with existing genetic diversity to produce changes in phenotype. Unless a heritable susceptibility to over-consuming high-calorie foods leads to lowered prospects for survival and reproduction, our propensity to become obese is unlikely to evolve biologically. And if there was any effect of our evolutionary mismatch to the cultural food environment we have created, one would imagine that it would be to select against those who are more likely to fall prey to the diseases of obesity.

Prediction: Humans will evolve to be more altruistic in response to the collective challenge posed by global warming.

I love this prediction because as a biological prognostication it is so clearly very wrong. We only deal very briefly with altruism, so I am not terribly worried that my Evolution students are making this sort of prediction; I sure hope that graduates of my Evolution of Cooperation course would see the problematic logic of this prediction. Climate change is a global problem and therefore can’t be solved by changes in our genetic propensity towards altruism. We all will suffer from the problems that climate change creates: there’s no way to belong to a particularly altruistic group that insulates itself from climate change, and therefore no way that either group or kin selection can cause the evolution of altruism in this domain. If we had several earths, perhaps we might have the potential for this kind of biological evolution! The other problem here is one of rate: both the problem of and the solutions to global climate change are driven by technological changes, and that means that climate change is a change related to our cultural — rather than biological — evolution. If we evolve the altruism needed to save future generations from the potential horrors of massive climate change, we will have to do so by cultural means.

Prediction: Humans will evolve to have fewer racial differences due to migration and expanded range of mating options.

During our week on human evolution we talked quite a bit about race and the fact that there’s no biological means of creating categories that match up with human cultural conceptions of race. So this prediction is doomed to fail because it invokes race, an inherently non-biological categorization. But if I blur my eyes a bit on this prediction and replace “race” with “diversity due to regional origin”, it could make some sense. It is easy in a place like the United States — where people whose ancestors came from completely different continents routinely have children (and have done so here for something like five centuries) — to imagine that some day the differences in human phenotype that originated regionally will disappear. How fast this will happen will depend on how rapidly our populations migrate and mix, which is as much a question of cultural evolution as it is a question of biological change. What’s also interesting about this form of change is that it might not really involve any change in allele frequencies: presuming that many of the regional differences between humans no longer have any bearing on success at surviving and reproducing, we might just end up breeding away our previous geographic genetic structure rather than evolving in response to any particular feature of our environment. In this sense, globalization and the invention of a variety of technologies that allow us to survive in a great diversity of climates are releasing us from both constraint and selection. If allele frequencies don’t change but regional differences begin to disappear, is that even evolution? I guess that depends on whether these regional adaptations ever end up mattering for future survival: currently, our cultural technologies appear to render most of these regional adaptations irrelevant. As with so many of the human examples above, it is our cultural environment that’s exerting the biggest influence on how humans evolve.

You can check out the instructions for this activity here and the playsheets for this activity here. Feel free to contact me if you want help in implementing this activity in your classroom!