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March 10, 2006

Coevolution: Let's Mutate Together

Biologically speaking, evolution is the process by genetic variations or mutations in individuals become dominant through natural selection, thereby passing those traits to future generations and ultimately resulting in the development of new species. But while many people see evolutionary change as something that takes thousands, millions or billions of years, we often forget that, in reality, change is happening constantly.

When I think of evolution and the genetic mutations, I often envision the process as completely random. Some little bit in the genes gets flipped and if the resulting trait works and helps the host live, then the trait is likely to stick around. If not, the host has no added advantage and therefore its mutation is not likely to stick around. But what is interesting is that while the biochemistry of genetic mutation may very well be random, the selection of advantageous traits is anything but random. Put another way, the how is random (new traits through chance and mutation), but the what (those smart traits that survive) are not random at all.

One should also consider that evolutionary traits compound by nature over time. Once a trait exists, its fair game for further mutations in the future. For example, coloring might become less subtle or venom might become more potent over successive iterations of genetic mutation.

For example, let's look at snow hares - you know, the ones that are brown in summer months and white in the winter? This trait is considered an adaptation for camouflage against predators. Now, it's not outside the realm of possibility that some very fortunate brown hare just happened to get some weird mutation that made her brown in summer and white in winter, all in one go. Perhaps she wasn't eaten and made lots of baby bunnies just like her, solidifiying her trait in future generations. I'm sure that this kind of extreme mutation has happened, but how likely is it, really? Instead, what if the mutation was just a subtle lightening in color in winter, which helped the hare survive. Now the "lighter fur in winter" trait exists... and future cycles of mutation may adjust it as needed. As the hare's prey become accustomed to the coloring change (perhaps by their own mutations for better eyesight or whatever), the hare in turn must adapt again, perhaps its fur lightens further to help it blend in to its environment.

Two species interacting in this fashion, causing mutual evolutionary changes, is called co-evolution. The two species put selective pressures on the other, thereby affecting each others' evolution. One great example of this was covered in an article in Spring 2006's UC Santa Cruz Review Magazine:

In the creeks and woodlands around San Francisco Bay, garter snakes and newts are engaged in a biological arms race—the snakes eat newts, the newts produce a potent neurotoxin in their skin, the snakes evolve resistance to the toxin, the newts evolve to produce more toxin, and so on. While the newts still fall prey to toxin-resistant snakes, even a resistant garter snake may be incapacitated for hours after eating a highly toxic newt.

This situation has evolved over time as a result of “tit for tat” evolutionary changes driven by natural selection—a classic example of the coevolution of two interacting species.

Coevolution shapes all kinds of interactions between species—not only the antagonistic interactions of predators and prey or parasites and their hosts, but also mutually beneficial partnerships like those of flowering plants and their pollinators. In fact, most plants and animals depend on coevolved interactions with other species in order to survive, says John Thompson, an internationally recognized authority on the subject.

“Much of evolution turns out to be coevolution, and ecological communities are based on these deeply coevolved relationships between species,” says Thompson, a professor of ecology and evolutionary biology at UC Santa Cruz.

More At: UCSC Review: Coevolution by Tim Stephens.

It shouldn't come as a surprise that coevolution happens at a very local level and can happen over a surprisingly short period of time. Individual populations of the same overall species may coevolve into totally different ways. This is interesting when you start to consider how many similar living things exist which are somewhat genetically different, but all came from the same root species. It helps explain why, at a certain point, some mammals entered the water, while others developed more efficient land abilities. These traits could have occured as a result of the same species, living in different pockets of the world and interacting with different predators, prey and other living things.

But this kind of thinking also has serious ramifications when one considers conservation concerns. It seems like when we going about our conservation, we often set aside a single solitary area for the perpetuation of that species. We fence it off and try to keep it as pristine as possible. We might even ship in other populations to this sanctuary. But this subtle but constant evolutionary process is not often taken into account - after all, a grizzly bear is a grizzly bear, regardless of where it was born. They're more or less interchangable within their own species, right? But if you consider that coevolution makes a species more finely tuned for its very specific environment, we could be weakening the genetic strength of the overall species more than we know.

Here's an example of how things might go awry. Yes, it is a hypothetical one. Let's take the orca or killer whale. I've written about the orcas in the past, as they have recently been added to the Endangered Species Listing. If you were to take a resident killer whale and stick him in with a bunch of transient killer whales, you likely wouldn't get very positive results. Despite being the same species, these guys look, act, speak, and eat completely differently. It wouldn't even surprise me if the transients ate the resident killer whale. Not the kind of thing you'd like to see happen with an endangered species.

But it's these kinds of problems you can encounter when you do not take the subtlies of these genetically different populations into account. This could also help explain why relocation of species is often a very high risk enterprise. You might think you're giving the species exactly what it needs to survive, but without taking into account the minute differences in environment, the subject might not be capable of adjusting sufficiently. Research into species must also to take into account diverse populations in order to understand its mysteries.

Posted by sorsha at March 10, 2006 5:28 PM