The Role of Predators in Evolution

By Branden Holmes (Surroundx) on September 6, 2010

The evolutionary arms race between predator and prey is virtually universal. As such, it is vital to have a grasp of how predators can drive evolution in prey species. Of course, the arms race is run in evolutionary time rather than real time; adaptation and counter-adaptation, separated by a lag of several or many generations in genetic response. Adaptation is a long and steady process of replacement: maladapted individuals give way to adapted individuals, and adapted individuals give way to better adapted individuals. But the adaptations that are beneficial to an individual which lives free from predators are, as we shall see, diametrically opposed to those that individuals which experience the constant threat of being eaten, find to help aid survival.

Australia's Pleistocene megafauna experienced much higher levels of speciation than their North American counterparts (35 species in 19 genera, compared with 44 species in 42 genera). This difference in the rate of speciation can be attributed to the relative absence of significant large predators in Australia at the time. Whereas the prolificacy of predators in North America kept speciation in check there (the vast majority of genera being monotypic).

In a newly created environment, such as Lake Victoria was 12,400 years ago, there are no predators because there needs to be prey species before predators can sustain themselves in that environment. And so prey species are able to proliferate exponentially compared with their predator habitat inhabiting cousins. Therefore absence of predators is an important aspect of adaptive radiation. And we can make an educated guess that wherever adaptive radiation has occurred, there has been a contemporaneous relative absence of predators.

But the problem with this evolution in the absence of predators scenario is that if/when predators do occupy the same geographical area in due time, the species which have gone through this adaptive radiation have not evolved adaptations and/or ESS's to cope with the constant threat of being consumed. A perfect example of this is again Lake Victoria in equatorial east Africa, and the deliberate introduction of the exotic Nile Perch (Lates niloticus) to that lake in 1954. Which ironically was supposed to 'improve' the fishery. Much of the adaptive radiation that had occured in the lakes endemic haplochromine cichlid species during the period since Lake Victoria filled up again 12,400 years ago, has been undone. The Nile Perch eats the fry of cichlid species, and is thus a typical predator, but it also actively destroys their habitat too. And being such an efficient exterminator of cichlids it is no surprise then that dozens upon dozens of Lake Victoria's prized cichlid fish have been driven to extinction.

Because animals which share their environment with predators have to adapt to that threat, it is obvious that animals of the same species which inhabited predator-free environments would be free from this constraint. They would be free to evolve colours more conducive to female preference, because females prefer males with bright colours, which are phenotypic manifestations of the males' genetic quality. Whereas prey species which inhabit predator inhabited habitats tend to be dull and camouflaged against the background, which makes it harder for predators to see them. But this is a double-edged sword because it also means that females are less likely to choose them as mates, instead preferring the brightest males. In a predator infested environment, males have a hard time attracting females because of their dull colours, because the selective pressure from predators to be camouflaged is greater than the selective pressure to be more brightly or conspicuously coloured in order to attract females. But they also can't use their voices to attract females because it attracts predators even more efficiently. Females are discerning, whereas hungry tiger isn't. Thus males are forced to be relatively dull and quiet.

The evolutionary direction that species with no natural predators are free to take has been particularly well documented by Dr. John Endler of the University of Exeter. He carried out a number of experiments on guppies (Poecilia reticulata) from Trinidad, to quantify the difference that predators make to the evolution of species. Amongst the many correlations that he noted, were a group of characteristics all inextricably linked with each other: larger size, later maturing, smaller litters, larger offspring etc. But at the other end of the adaptive spectrum were the individuals which had to live with the unrelenting threat of predators. The traits that they manifested were the diametric opposite, as the selective pressure from predators outweighs the selective pressure from females to be bright.

What About Multiple Predator Species? In the case of Lake Victoria we can see how even a single predatory species can have profound effects on the ecological balance of the environment (native species can also be disrupted by competition from other species which fill a similar ecological niche). But more than one type of predator presents even more challenges. Different predators hunt differently; some rely on camouflage and stalking their prey, while others simply overwhelm their prey with numbers. The existence of two totally different predators in an environment requires equally different counter-strategies. But it is virtually impossible to be counter-adapted to both predators equally, as their strategies are likely to be very different. What will evolve in the prey species will be the best compromise possible under the circumstances. A combination of traits designed to evade both predators, usually devoted to evading/escaping each predator 50:50, or thereabouts.

Camouflage Both predator and prey species utilize camouflage as part of their tactics; staying unseen is the aim for both. And I’m surprised that there are not any predator species which have adopted mimicking the phenotype of their prey. Surely that is the best strategy possible, in theory or practice? Maybe it is harder to mimic prey species than it is, say, flowers? The individuals of both species which are the best camouflaged survive and reproduce, and the offspring of those fittest individuals themselves mate and reproduce, so that even if both species weren’t very well camouflaged to start with, within a few dozen generations their phenotypes can change quite dramatically towards replicating their surroundings.

I am unsatisfied with this article, believing it to be unfinished. But I can't tie down exactly what is incomplete about it, so I have decided to publish it 'as is', hoping that criticisms and suggestions from others will allow me to see which part/s require attention.

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About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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