Predicting Future Evolution (Fall 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. This semester the cap for my Evolution course was lower, so fewer students produced fewer predictions. But there were still some really good ones! Below are some highlights from one section of this semester’s class…

Prediction: Pigeons will lose their ability to fly due to abundant food supplies on the ground and fewer predators

This is an appealing prediction because it is in that very interesting realm of “release from selection”. The premise of this prediction is that flying evolved for a couple of reasons: to avoid being caught by predators and to get to food. If there are no longer predators on pigeons and they can get food without moving around too much, perhaps pigeons who aren’t good flyers will survive and reproduce, eventually causing the loss of flight in the species. The problem with “release from selection” explantations is that they only work if a particular trait is completely released from selection. And in this case, this prediction ignores a very significant role of flight in pigeon reproduction. Pigeons have evolved to make nests in rocky cliffsides — and of course a variety of building ledges will do the trick as well — and you can’t make such a nest if you can’t fly. So the loss of flight would have to evolve alongside a change in reproductive habits… and it’s hard to see how a pigeon could reasonably be “released from selection” when it comes their nesting behavior. There are significant risks associated with nesting on the ground. And perhaps that also indicates that the idea that pigeons no longer need to fly in order to avoid predators is a bit unlikely as well… not only would cats, rats, and dogs all pose a potential risk to ground-nesting pigeons, but cats are an existing predation risk. Oh, and then there are all those people walking, people on bikes, and people driving cars: flying also helps escape these risks.

Prediction: Whales will evolve to have gills rather than lungs due to the energetic cost of diving and surfacing and hunting risk on the surface

Part of the environment proposed to cause this future evolution makes sense, and part of it does not. Whales have been doing quite well in spite of whatever costs they pay for having to surface for air, so it is not clear what has changed about this selective pressure that would suddenly make this behavior too energetically-costly to survive. So scratch that cause of whale gills off the list. But hunting is at least a source of selection: anything that whales could do to avoid hunting would be valuable (although I do wonder how intense hunting is as a selective pressure these days… whale hunting is a pretty marginal activity compared to other things we do to impact whales). Wouldn’t having gills make sense then? If you don’t surface, you can’t be killed by a whaling boat! The problem here is, of course, evolutionary constraint. Whales are mammals, long-since diverged from their common ancestor with gill-breathing fish, and that means that their body plan would take massive reorganization in order to leave behind the costs associated with having lungs. It’s hard to imagine any path back to having lungs, much less one that could provide an incremental benefit from a mutation that would be the “first step towards gills”. Whales are stuck with lung breathing, and whatever they do to survive human impacts, it will probably have to be a modification — rather than an abandonment — of lung breathing.

Prediction: Whales will be able to digest plastic due to the prevalence of plastic waste in marine environments

This prediction gets one thing right: the marine environment contains a disturbing amount of plastic, and that plastic can be a risk to marine animals. So wouldn’t a whale that could digest plastic, turning it into a source of food rather than a hazard, be at a great evolutionary advantage? The problem with this prediction, like the other whale prediction above, is that we are asking too much of the whale’s current phenotype: plastic is such a novel material that it seems really unlikely that a whale would be able to develop mutations that would allow it to actually break down, digest, and absorb plastics as a food resource. Does this mean that no organism can do this? Are plastics so novel as part of the environment that no organism has managed to evolve to exploit them? Imagine the potential food source that plastic could represent! This is an open niche! Well, as it turns out whales are just the wrong candidate for evolving plastic digesting abilities: to really respond to this new environmental resource, we need a short-lived, rapidly-evolving species that is really flexible in what it can evolve to metabolize. Can you imagine a large vertebrate that evolved the ability to consume crude oil? Probably not, but microbes have had the ability to feed off of deep-sea crude-oil leaks long before humans started pumping the fossil fuels out of the ground. So it probably won’t come as a surprise that microbes have also been found that can digest certain kinds of plastic. Perhaps the evolutionary process could hand us a solution to the very trashy way that we have handled our plastic waste thus far.

Prediction: Bears will evolve bulletproof fur due to the risk of being hunted by humans

Can you imagine? A bear is a scary enough creature as it is… imagine a bulletproof bear! Instead just carrying around bear-bangers, hikers would have to carry hand grenades! It won’t be safe to go into the woods with all those bullet-proof bears… thank goodness there are bear hunts to keep us safe! Wait a second… it is those very bear hunts that are going to make the bears bulletproof. Call off the hunt! Okay, let’s slow this down a little. Hunting is definitely a potent source of selection on bears, particularly in areas where bears are hunted because they are considered a nuisance. So the idea that hunting might cause bears to evolve makes sense. But is the evolution of bulletproof fur the most likely way that bears will evolve to avoid being hunted? Probably not. The most obvious traits that would evolve in response would be behavioral: bears that are more scared of humans are going to be less likely to be shot and killed (although I suppose that there is also the argument that bears who stay closer to human settlements will also be afforded some protection from hunting, and most residents don’t want people firing shotguns on their property). Like other predictions above, the idea that bears will develop bullet-proof fur is also called into question by evolutionary constraint: the material that fur is made of (keratin) is not going to be easily modified to become resistant to the very acute force created by a bullet.

Prediction: African Grey Parrots will develop symbolic and figurative language due to living in large social groups and habitat destruction

Wouldn’t this be cool? We already know that African Grey Parrots are capable of developing an extensive vocabulary and solving reasonably-complex problems. Aren’t they good candidates for slightly modifying this behavior to develop the full-blown symbolic and figurative language that humans use? Perhaps, but the question is what selective pressure would push parrots in this direction. They already live in large social groups, so whatever language they employ in that context seems to be working well enough. But what about habitat destruction? Could they use their language to somehow better survive the threats they face from human encroachment? And that’s the tough question to answer: What are these parrots doing with their language to better survive habitat destruction? Could they be conspiring to start a rebellion against the humans? Would this extra capacity to conspire be enough to overcome us as a threat? (Not!) Maybe could they demand that their right to exist be respected? That would be cool — and probably pretty effective — but would require a massive leap not just in language ability but in cognitive capacity. The ability to understand the threat one faces and how to argue for one’s own liberation from that threat is pretty advanced, and too many evolutionary steps away from where these birds currently exist.

Prediction: Rats will speciate due to specialization on different food sources

I have to hand it to the students who made this prediction: they were making some very creative connections between concepts in the course and facts on the ground. What they are suggesting is that rats will sympatrically speciate, and they use the idea that rats have a relatively small home range and the fact that they exploit a variety of different food supplies to justify the idea that sympatric rat speciation could be feasible. But, there are some problems with this very creative argument. The first is that having a small home range is not enough to interrupt gene flow between rat populations: so long as rats are coming into contact with each other at the boundaries of their home range and mating, it doesn’t matter if each individual rat doesn’t move very far afield. The second and bigger problem is that it is not clear how specialization on particular foods would promote speciation: when we see speciation due to ecological niche specialization, there are very clear trade-offs associated with specialization. These trade-offs make genetic isolation of two populations advantageous, and this would reward any changes in mating behavior or other reproductive barriers that allow each specializing population to avoid hybridizing with the other. But where’s the big trade-off in resource exploitation for rats? As generalist, highly-flexible foragers, rats don’t seem to face any real strong trade-offs associated with the different foods they eat.

Prediction: Humans will evolve to lose their pinky toe due to continued bipedalism

This is just silly. Have you ever injured your pinkie toe? If you have, you know it helps maintain your balance as you walk. And we have been bipedal for a long, long time… why would the pinkie toe suddenly become a disadvantage?

Prediction: Human retinal cells will get better at distinguishing variation in the blue light spectrum due to the abundance of “blue light” screens

I can’t evaluate the assumption of this prediction (that the screens we use have more variation in the blue part of the light spectrum), but let’s first assume that is true. A large part of the human population is using these screens (even in regions where people don’t live in industrialized societies, cell phones are becoming ubiquitous), so there certainly is the potential for selection. But what advantage might a person with better color discrimination gain in this screen-filled environment? To imagine a benefit, we need to make some pretty big leaps. Do people with better “screen eyes” do better in the workplace? Is there really an advantage associated with fine-scale color discrimination that leads to better job success? If so, does that job success lead to higher pay and greater prestige? And does that higher pay and greater prestige lead to higher reproductive output? Of all these questionable steps between “screen eyes” and reproductive success, it is this last one that is most questionable. Humans are downright weird. In any other species, an individual who accumulates more resources and sits higher in a social hierarchy is probably going to have a higher average reproductive fitness. But in humans, this just doesn’t seem to always be the case (although this is a complicated story). The other problem with this prediction is that it ignores the problem of human generation time: in order for the advantage of “screen eyes” to lead to selective advantage, those who now are more successful in this environment full of screens have to pass on their genes at a higher frequency to the next generation. And that takes time, a long time (in the range of 22 to 32 years). By the time that the next generation of humans are capable of reproduction, it is hard to know whether screens of the type that we now use will even exist. For evolution to happen, selection needs to push in a particular “direction” for a long enough period of time to cause change in the population, which means that the environment needs to be relatively consistent for many generations. Like a lot of other parts of our cultural environment, it is just hard to imagine these screens remaining static for that long a period of time.

Prediction: Humans hearing will get better at higher frequencies due to the abundant use of headphones

Like the screen prediction above, this one suffers from the common issues associated with biological evolution in response to culture: it is not clear how adaptation to this part of the cultural environment would translate to reproductive fitness, and it is not clear that the cultural environment is going to remain static enough to be a sustained source of selection. For this example, it is even less clear that being able to hear particular frequencies in one’s headphones would be advantageous.

Looking at all the “wrong” predictions above, you would think that it is pretty
hard to make a feasible evolutionary prediction. And it is, but below
are a few feasible predictions created by this semester’s class.

Prediction: River fish will develop tolerance to mercury due to increased levels of mercury in waterways

The premise of this prediction, that fish are exposed to a lot of mercury, makes total sense (although levels of mercury in waterways vary greatly across the globe). The question is whether or not fish can (or will) become more tolerant than they already are. Most studies of mercury tolerance have been conducted in microbes, but there’s less scholarship in fish. Obviously fish can tolerate enough mercury to be dangerous as food sources to humans, suggesting that they have developed some tolerance. But whether or not they can evolve more tolerance will depend on whether there’s a mutation in their populations that allows them to better de-toxify mercury… and whether or not we continue to increase aquatic mercury levels through our industrial activities.

Prediction: Locusts will become more resistant to pesticides due to the use of pesticides

It is always a pretty safe bet that any organism that we try to get rid of with a new synthetic chemical is likely to eventually evolve resistance to this chemical; there’s an extensive history of this sort of resistance evolution. The only question with this prediction is the choice of focal organism. Locusts can be agricultural pests, but the question is whether or not they are the target of newly-developed pesticides. The fact that they aren’t a consistent agricultural pest that we specifically target makes this feasible prediction a little less likely.

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

This is a prediction that I have gotten before [1, 2, 3], and it is a good one. I prefer 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. Nonetheless, scientific evidence suggests that this evolution of smaller tusk size has already begun.