Pressure

So I briefly described natural selection and promised to write more about selective pressures. Basically, any element of an organism's surrounding can provide a selective "pressure". By "pressure" I mean something in the environment which provides a reason for change in an organism that would be selected for via natural selection. Now just because there's a selective pressure doesn't mean the change is going to happen. It may be that there is constraint on what type of changes can occur, and of course, just because there's a need for a change to happen, doesn't mean the change is going to happen. The initial change is random, and it could take years, hundreds of years, thousands of years, millions of years etc before a particular random change occurs that actually benefits the organism. In fact, most mutations are deleterious - they actually impair an organism's ability to make the most of its environment. But then again, mutations that could be deleterious in one environment aren't in another environment. Let me explain.

In Mexico, there are blind cavefish. One of my professors at University of Maryland, William Jeffery, studied the blind cavefish, as well as their non-blind relatives. In fact, the blind fish can breed with the seeing relatives - they are the same species. However, at some point a mutation occurred in one of the regulatory genes that controls the development of eyes. Now if this mutation happened in the non-cave dwelling fish, it would be deleterious, and natural selection would most likely result in the elimination of that mutation from the population. But in the cave-dwelling fish, sight isn't very important and therefore the mutation remained in the population because it did not affect the fitness of the fish carrying that mutation. You could say that for the cave-dwelling fish, there was a selective pressure for loss of sight (especially if loss of sight is complemented by an increase in another sensory organ).

Darwin suggested that the cavefish became blind because of disuse, and that elimination of the unnecessary organ increased their fitness. Darwin was wrong on a variety of topics, and this is one of them. Disuse doesn't result in elimination of anything. However, disuse can be a type of selective pressure, or more accurately, a release from a selective pressure. Mutations in genes that are responsible for a particular function, organ, etc can render them useless. Then if they are useless, those mutations will persist in the population, either through genetic drift or natural selection (if those mutations are beneficial in some other way). But just because something isn't used, doesn't mean it will go away. Think of the appendix. Modern humans don't use their appendix because we tend to not eat raw meat and rocks. But we're still born with appendices and our bodies go through all the trouble of making them. Now if someday a mutation occurred that resulted in the appendix not being formed, that mutation would have the chance to spread through the population. It could be that not having an appendix is neither good or bad for you, and the mutation would spread through genetic drift (very slowly given the human population size). Alternatively, not having an appendix could be beneficial (since you remove the risk of dying from appendicitis) and the mutation would be selected for, and it would spread through the population a little faster than if it was just spreading via genetic drift. But eventually, humans might no longer have appendices. Or we might split into two species, humans with appendices (because some group still needed them and not having them was detrimental) and humans without appendices.

I think the hardest part of understanding evolution is understanding how small events, over time, could result in great diversity of life on this planet. I'll work on some examples of this.