Tuesday, May 10, 2011

This Post's Title Keeps Evolving, So I'll Just Call It This

I just scrolled through my Twitter stream, and just over the most recent tweets, I saw multiple tweets devoted to just one idea:

#evolution

Although that may not sound too impressive--after all, I follow almost all science teachers--the fact that this one idea is still sparking debates and controversy after centuries of existence is actually pretty impressive when you stop to consider it. Simply clicking on the #evolution hashtag reveals a community that is updated at least once every five to ten minutes.

So what's the real idea behind evolutionary theory?

Let's go back to the beginning...although not the very beginning. Let's start in 1801, when John-Baptiste Lamarck published his Theory of Inheritance of Acquired Characteristics. Right away, if you have a background in evolutionary theory, an alarm bell will go off in your head. If you have that red flag, you've caught the big thing Lamarck got wrong. Lamarck thought that organisms could inherit acquired characteristics. For example, if a snake, throughout the course of its life, develops muscles that allow it to wriggle faster, Lamarck thought that it would pass those muscles on to its offspring.

Darwin didn't think that. He said that organisms pass on mutations, not acquired characteristics. And because organisms pass on mutations, if those mutations give an organism a certain competitive advantage, they have a better chance of producing more offspring. Those offspring may then have the same mutation, which gives them the same advantage over other organisms, giving them a better chance of producing more offspring. These offspring carry the mutation, which gives them an advantage...and the cycle continues.

Now, that's the basic mechanism of the theory of evolution--and it's known as natural selection. It's where phrases like "survival of the fittest" come from. However, that phrase isn't really accurate. Natural selection depends more upon reproduction than survival. Of course, survival helps to reproduce, but it does not guarantee offspring.

The classic, almost cliched, case of evidence for natural selection is the story of the peppered moths in Britain. There are two main phenotypes for this type of moth: light and dark. Before 1848, dark moths made up less than 2% of the total population. However, by 1898, over 95% of moths in industrialized areas were dark.

Why?

The mid-1800s was the time of the Industrial Revolution in Britain. All of those factories were emitting soot, so the landscape itself became somewhat darker. This meant that birds could see the lighter moths more frequently, and so the lighter moths were the ones that were eaten. The darker moths were able to reproduce, which produced more dark moths. Over a period of fifty years, the gene pool of the population changed, which is evolution by definition.

That is an example of microevolution--change within a species--and there are other ways in which this can occur. For example, let's spend some time talking about genetic drift. Genetic drift has to do with the change of the frequency of alleles due to random chance. Genetic drift holds more power in smaller populations, simply because even a slight change in the frequency can have an impact on the overall pool. Essentially, that means:




Genetic drift basically means that some random things will happen to a population to change its gene pool. It boils down to this: If a population is relatively small, then there is an increased chance that rare genotypes will not make it to the next generation.

There's another mechanism for microevolution--gene flow. (Don't worry--this one's much easier to understand than genetic drift!) All that gene flow says is that the migration of organisms has an effect on the gene pool. If two populations share an general boundary, there's nothing to stop them from interbreeding, introducing genes into the gene pool of both populations.



So, that's a little overview of microevolution--change within a species. Now, I realize that I've made some pretty big claims over the last few paragraphs. What evidence is there for those?

It turns out that there's a lot. First off, let's look at the fossil record. I've often been told that there simply aren't any intermediary fossils (fossils of animals that were "in between" species--they have features of both). And the more I research that claim, it's simply not true. For example, look at these fossils:





In these fossils, over a period of 50 million years, we can watch the nostrils move from the front of the skull to their current location at the top of the skull.

Another important branch of evidence for evolution is the idea of homologies. Homologous structures are structures that are shared across many species. For example, even though a cactus and a rose have very different structures that serve as leaves, they both have the same ultimate function. Another example of this is forelimbs among four-legged creatures. Even though, say, a rabbit and a lizard are clearly quite different, the bones within their forelimbs are constructed in the same manner.

Homologies can also be found at the cellular and molecular levels. For example, plant and animal cells are essentially identical--there are only two organelles in a plant cell that are not in an animal cell, and one animal organelle not in a plant. Additionally, a significant percentage of genes are shared across species. Just look at a roundworm and a human. Between those two phenotypically different species, 25% of genes are identical.

Finally, let's look at one more support column for the theory of evolution: the field of embryology. Basically, it says that, in their early stages, embryos of different vertebrates have strong similarities. This is taken to suggest that these vertebrates have a common ancestor.

So, now, I've thrown out a lot of information. I've shown the proposed mechanisms for microevolution and we've looked at evidence for macroevolution. But it doesn't mean anything if I don't accept it. Now, I do accept the theory of evolution as valid, simply because the ideas simply seem to line up. I don't see why, if there was not a common ancestor, there would be so many similarities at the molecular and cellular level of organisms that are in different kingdoms. The fossil record, to me, serves to back that belief up.

As someone with a mathematical background, I tend to accept things based primarily on the reasoning behind them. Now, I recognize that that's probably not the best idea in the other sciences, and it's something I probably need to work on. But that's another reason I think I support the theory: the reasoning behind it simply seems to, for me, line up. I don't see why it shouldn't be true.

Now...off to a #TeachIn11 post! 

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