Predicting Future Evolution (Spring 2017)

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 one section of this semester’s class…

Prediction: Globalization will reduce variation in skin color among humans

Of course humans are a very popular species when it comes to predicting future evolutionary change, and students tend to gravitate towards traits that are most obvious to them. Humans are incredibly aware of variations in the density of skin melatonin, perhaps because such variations are among the few clear traits that differentiate human populations from different geographic regions. I think that Americans are particularly aware of the fact that the racial boundaries that used to separate people reproductively have come down in some rather dramatic ways. So this prediction is not particularly surprising. And it is clearly feasible, but for reasons that students don’t fully identify. This semester I got a couple of very similar versions of this prediction, and both focused on the social and cultural factors that are blending away a lot of the previous variation in skin color: migration of people away from their geographical region of ancestry, and changes in cultural values about who a person should and should not make babies with. These are certainly valid reasons, but I also wonder why students ignore the adaptive story here: because of the invention of sunscreen (which allows lighter-skinned people to protect their skin in very sunny regions) and vitamin D supplements (which allow darker-skinned people — or pretty much anyone who spends too much time indoors — to get sufficient vitamin D in low-sun regions), humans have been completely released from the selective pressures that gave us variation in skin color in the first place. Obviously you need both factors — opportunity for moving and mating plus release from selection pressure — in order for skin color variation to go down, but it is interesting that students focused more on the social/cultural factors. It should be noted that some students did contest whether racial segregation has been sufficiently minimized to allow this prediction to be feasible. This is an interesting question — have racialized barriers to broad human inter-breeding come down significantly? — that I don’t really have an answer to.

Prediction: Climate change will cause a global increase in the density of human skin melanin

Ugh, apparently for every thought-provoking prediction about human skin color variation there has to be a head-scratching prediction. I guess that I can forgive my students for not fully understanding what kinds of climate changes we are in for, as this was not covered in this class. But it is still a bummer to see students confuse the causes of climate change, and all I can think here is that students fundamentally don’t understand the roles of sunlight and heat on the earth. Given that we know that human skin color has evolved in response to the intensity of sunlight and that climate change is not caused by increased intensity of sunlight, we can pretty much dismiss this prediction. Although I guess that if you stretch it really far, you could argue that darker skin does provide the advantage of more rapidly dissipating heat. But something tells me that even in the most extreme climate change scenarios this sort of minor advantage is not likely to change our global skin color.

Prediction: Humans will get taller

This is an interesting prediction because it gets at the question of whether changes in traits are caused by changes in environment or by some form of selection. There have been numerous studies that have shown the effect of changes in diet on height, but there have also been studies that demonstrate that height might be under selective pressure (for example, Zhang et al. 2013 or Stulp et al. 2015). My students who made feasible versions of this prediction were focused on a rather obvious source of selection: mate choice. While it seems at least reasonable to suggest that sexual selection for taller mates might increase human height, I don’t know of any studies that show that this is the reason why height seems to be under positive selection.

Prediction: Human traits will change due to our use of technology

I am going to bin a bunch of wacky predictions — including one that I like to use to seed this activity — into this one general prediction. To start off this activity I need a “first prediction”, so I always like to predict that human thumb dexterity will increase due to the use of touchscreen devices. To my own silly prediction you can add the following analogous predictions about how the human body will evolve in response to technology:

• Humans will become more slumped over due to sitting at desks using computers;
• Human head-to-body-size ratio will increase due to the use of technologies that require more of our brains and less of our bodies; and
• Humans will lose their pinky toes because they are unnecessary and get in the way of squishing our feet into fashionable shoes.

These various predictions about changes to the human body all seem unfeasible to me, each to a varying degree for the following reasons:

1. Many of the changes that students predict are not heritable. Might we get more slumped over from our exposure to all the sitting we do? Sure. But will that lead us to pass on our posture to our offspring? No way. This is a Lamarckian prediction, and should be saved for culture, which does evolve in a Lamarckian manner.
2. Human technology is evolving too fast for us to respond to biologically. Human generation time is hovering around twenty years (at least!), and in the course of twenty years technologies change so rapidly that there’s no way that any selection on one generation due to its use of technologies is going to be adaptive in future generations.
3. It’s not clear that any of these changes confer advantages. Even if a larger brain size led a person to be better at using technology, would that increased ability lead to better survival and reproduction? Probably not, at least if you look at the global trends in reproductive success. Those populations with the greatest population growth rates are those with the least access to technology, so it is hard to argue that our current environment globally is selecting for facility with computers and other forms of technology.

The one version of the prediction above that I do find potentially feasible is the idea that our bodies might be getting less massive. Like other feasible predictions I have outlined above, this one makes more sense because of release from selection rather than selection for the trait. Technology has released many people from the need to do physical labor in order to survive, attract a mate, and reproduce. That means that there may be reduced selection for muscle mass and perhaps overall size (although see above for some papers that do show selection on height). So maybe humans could become less muscular due to our technologies… maybe…

Prediction: The immune system of humans will weaken due to advances in pharmaceuticals and the human rights movements

This was perhaps the most provocative prediction I received this semester. The idea that pharmaceuticals such as antibiotics might release us from selection for parasite resistance certainly makes sense. But what about the human rights movement? Although it is not entirely clear what this pair of students meant, I think that there is something to take seriously here: if we have a society that is more inclusive of people who are more prone to illness — and that inclusiveness allows for improved survival and reproduction of these people with lowered immunity — then it is feasible that the overall immunity of the human population might decrease. There are certainly people who might suggest that this is a bad outcome. But I would ask these same people to also consider whether our tolerance of people who wear corrective lenses is problematic. Although we haven’t made global human rights campaign out of the move to include people who wear corrective lenses, clearly the presence of corrective lenses has allowed uncorrected human vision to deteriorate (I am among the maladapted!). This prediction about how our cultural technologies might change our resistance to parasites is just another in a slew of feasible predictions that we might make about how culture has released us from selection. So long as we maintain technological cultures that protect us from the adverse effects of our environment, none of these “deteriorations” in our traits will have evolutionary consequences.

Prediction: Humans will engage in more non-reproductive sex due to changes in cultural expectations

I thought that this was an interesting prediction because of what it did not mention. First, it did not mention that many anthropologists have argued that “recreational sex” is an ancestral human trait. So if we have been having sex that’s not for making babies for a long time, what’s there to change? Second, the presence of birth control was not mentioned, which is strange given that it seems like the most obvious recent cultural innovation that facilitates the choice to have sex-for-fun rather than sex-for-babies.

Whether or not this prediction is feasible is a bit hard to suss out. On the one hand you can make the argument that humans now have the option via birth control to only have sex-for-reproduction when they choose, which by definition would make more sex the sex-for-fun variety. But what’s not clear is how much sex-for-fun actually competes with sex-for-reproduction. If some people choose to exclusively have sex-for-fun, any genetic predisposition they have to avoid pregnancy is not going to be passed on to future generations. So in order to really assess whether this prediction is feasible, we would need to know more about how sex-for-fun is affecting the reproductive output of people.

It should also be noted that “cultural expectations” can themselves evolve. If it is true that there is an increase in the ratio of recreational sex to procreational sex today, can we be sure that such a cultural norm will be maintained. That depends a lot on what cultural traits the parents of today pass on to their offspring (vertical transmission of culture), but also on how much those offspring are influenced by society (horizontal and oblique transmission).

Prediction: Human gender roles will become obsolete due to globalization and economic shifts

I did not tell students that their focal traits had to be biological! This one is clearly cultural, and I find it fascinating. To really argue that conceptions of human gender roles will break down — perhaps completely — you have to explain why traditional gender roles in particular regions might be selected against… or at least no longer be selected for. I guess that you can make a release from selection argument based on the availability of technologies that no longer require men and women to provide physical labor in particular ways. But the problem with this argument is that it exists on the individual level, and gender roles are a cultural phenomenon that exists at the level of a group. So why might one group abandon its strict gender roles, and what might be the advantage of doing so for that group? One possibility is that societies that allow males and females to perform any social role might make better use of their “talent pool”, most likely by making better use of the variety of skills that different individuals bring to society. I find this to be a pretty feasible prediction, especially given how culturally dominant large, technologically-complex industrial societies have become. Although it is dangerous to assume that we can guess where our economies or their technologies will go, it does seem feasible that societies that divorce social roles from biological sex might outcompete those still stuck on limiting each sex to particular roles.

Prediction: Elephant tusks will decrease in size due to poaching

Humans weren’t the only focus of my students’ predictions, and this has been a popular prediction through the years. It’s not just feasible because it logically deduces the evolutionary implications of poaching elephants for their tusks: it is also supported by empirical evidence (Chiyo, Obanda, & Korir 2015). As predators humans exert pressure whenever our predation is selective, and in the case of elephants human poachers prefer the elephants with the largest tusks. This reduces the survival and therefore reproductive prospects of elephants who have inherited the potential to grow very large tusks.

Prediction: Domesticated dogs will lose their acute sense of smell due to human care

This is an interesting prediction, and again one based on release from selection. In general, canids have really well-developed senses of smell, which are used to both optimize foraging (smelling your prey) and social identification (know whose scent markings you are dealing with). Early in their coevolution with humans, some domesticated dogs were clearly valued for their ability to smell things that humans cannot, and to this day many breeds of dogs are bred in part for their ability to use their sense of smell to find prey (or, in more recent times, contraband). But might some dogs lose their acute sense of smell due to domestication? Surely the vast majority of dogs no longer use their sense of smell to survive, and many breeders may not care whether they select for a keen sense of smell. This makes it reasonable to assume that released from selection on their sense of smell, the general dog population may drift away from the olfactory acuteness of their ancestors.

Prediction: Polar bears will evolve larger paws and darker fur in response to climate change

Polar bears are a popular species for this exercise, perhaps because they are the most well-known symbol of a species in crisis as the climate warms. A couple of student teams predicted that the polar bear’s fur will become less white as its snow-covered habitat melts. This might be an oversimplification of how climate change is altering the habitat of the polar bear, but there is some evidence that polar bears are moving inland and even hybridizing with grizzly bears. Although polar bears have adapted rather specialized lifestyles in arctic regions, they are still bears and bears can be pretty resourceful about how they obtain resources, so perhaps the species will survive the loss of sea ice and adapt accordingly to a darker environment.

The idea that polar bears’ paws will get larger in order to allow them to swim further distances in the melting arctic also has a certain logic to it, but I think that this prediction is a bit more dubious. I wonder whether bears have sufficient variation in their paws to allow them to rapidly become endurant enough swimmers to swim the large open ocean gaps that climate change will eventually produce in the arctic. This prediction seems more informed by the iconic — and perhaps misleading — image of a polar bear stranded on an ice floe than on any real selective pressure that polar bears will face in the future.

As you can see from the predictions above, my students are still — after a full semester with me — making some very questionable predictions. Some are the cringers, predictions that really make me wonder whether my students have learned the basic process of evolution. Others make more subtle errors, ones that result not from misunderstanding the evolutionary process but from misjudging its pace and constraints. It would be tempting for me to set the goal of having all these predictions be feasible in this activity, but I think that such a goal would be unrealistic. While the game we play in this activity incentivizes students to make valid predictions, many of them are probably making predictions that they know are a little crazy, perhaps to trip up the other students who have to analyze their predictions and perhaps out of the inability to make a valid predictions. As I said from the outset, evolutionary prediction is a fool’s errand, so making even feasible predictions is hard. What I focus on is what can be learned by analyzing each prediction.

To read about Predicting Future Evolution posts from other semesters, navigate here.

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!