200 years later, we still don't get it

yesterday was the 200th anniversary of Charles Darwin's birth, so naturally the news was aflutter with stories of celebrations, anti-celebrations, and evolution factoids. but one of the most astonishing things that I read was that only 39% of Americans believe in evolution, only 15% believe in the evolution of humans from apes, and at least 25% firmly do not believe in evolution.

staggering. really.

it's true, I grew up in a relatively secular family. While I was exposed to religion and religious ideas, I was not indoctrinated into religious theory from a young age. By the time I was in high school, I had made up my mind that my science textbooks made a heck of a lot more sense than my bible. so maybe I can understand why people who grew up differently from me would be less than willing to give up their beliefs.

many religious leaders lead their parishioners to believe falseties in the theory of evolution. they question how a perfectly formed human eye could possibly have evolved by chance. they question how a [insert interesting animal here] can be so perfectly suited to its environment without the guidance of a higher being. and they falsely lead people to believe that the theory of evolution involves a series of "leaps" caused by random, beneficial genetic mutations from one species to another.

so most people have a vague and often flat out wrong idea of what evolution is and how it occurs. at this point, I'd love to recommend that everyone out there read "The Selfish Gene" by Richard Dawkins, but I doubt that they would. so I'm going to describe evolution as simply as I can.

imagine, if you will, a population of horse-like mammals living in a forest. they happily munch on the leaves of trees. Now the horses live like this for thousands, possibly millions of years. But at some point, the trees that they feed on start to die off (perhaps it's climate change, or perhaps a weird tree disease, or maybe a fire) and another tree species takes its place. The new tree is taller than the last one, so only the tallest of the horses can reach the leaves. obviously, if you happen to be a tall horse, you'll probably do ok. but if you're short, then you may die. Over many generations, the entire population of horses gets bigger because they are more likely to survive. This is natural selection - nature confers a survival benefit to individuals with one trait over individuals with another. Eventually, the short horses will die out altogether. and given thousands of generations, the tall horses may not look anything like their old ancestors.

speciation like what I just described can happen even faster when there is an element of geographical isolation. perhaps during a drought, some of the horses manage to cross a river that is normally impassable. They continue to feed on the short trees on the other side of the river, while back in the original habitat, the horses are forced to eat the tall trees. In this case, you get geoegraphical separation over several years. The horses on the tall-tree side are under selective pressure to breed taller offspring (i.e. the taller horses will survive better and will produce more offspring than the shorter horses, which may die before they reproduce), while the horses on the short-tree side of the river face no selective pressure. Over thousands of generations, the populations may diverge enough that even if you were to bring them back into contact with one another, they would not be able to mate and produce viable offspring. At this point, you have 2 distinct species.

this is only an example of how different species can evolve, but the principle of evolution remains the same across species (including ours). There must be selective pressure that confers a survival benefit to an individual with one trait over an individual with another. It doesn't happen overnight and it doesn't require a massive genetic mutation (i.e. one horse miraculously grows an extra mouth). evolution happens in minute increments that gradually, over multiple generations result in speciation. geographical isolation speeds up the process by preventing (for example) the short-horse genes from re-entering the tall-gene horse gene pool before speciation is complete.

finally, i want to illustrate that evolution is still happening, even today. For this, I'll need to describe another selection pressure - sexual selection. In this case, rather than the environment conferring a survival benefit, mate preference confers the survival benefit. The Paxton Lake sticklebacks - small fish on the west coast of Canada - illustrate this perfectly. Paxton Lake contains threespine sticklebacks (Gasterosteus aculeatus) that use 2 distinct habitat types and have therefore developed 2 distinct phenotypes (in this case, body shape). Some of the sticklebacks like to live near the bottom of the lake, and have gradually evolved to be rather stout and sedentary, preferring to blend into the background to escape predators. Some of the sticklebacks like to live in the middle of the lake and have gradually evolved to be thin and agile to escape predators in the open water.

Sexual selection comes into this because the sticklebacks that live in the middle of the lake select mates that are skinny and agile like themselves, whereas sticklebacks that live on the bottom of the lake select mates that are stout and well camouflaged. Gradual diversion like what I described with the horses likely led to the sticklebacks being able to take advantage of the 2 habitat types, but sexual selection keeps the populations isolated from one another (like the river did to the horses).

So at this point, it sounds like we've got 2 separate species, but in fact speciation is not yet complete, because when you muddy the water and prevent the sticklebacks from being able to see one another, they will interbreed and they can still produce viable offspring. And this is exactly what is happening in nature.

Crayfish were introduced to several lakes where these 2 phenotypes of sticklebacks are present. The crayfish muddy the water near the shoreline, making it impossible for the sticklebacks to see one another well enough to choose a mate. So they've started to interbreed, and the result is that the evolution that occurred up to now is being undone because the selection pressure that kept the phenotypes apart has disappeared. Evolution in reverse, you might say. The Paxton Lake sticklebacks are now considered to be a species of concern because the "evolution in action" that we were once able to see and document has been reversed by the careless introduction of a new species.

So, next time you doubt the theory of evolution, think about horses and sticklebacks and try (try!!!) to wrap your brain around the enormous timelines involved.. not hundreds of years... not even a thousand years! and definitely not in giant leaps of mutations!

No comments: