Whoa! It's been over two months since I made my last post! And even longer since I made my last meaningful one... Anyway, this post is about evolution and what it is. I will try to keep to the facts in this post, and talk about "intelligent design" and creationism (two alternative "theories" for the origin of life) in a later one.
Boiled down to it's most basic level, evolution is a theory that postulates that new species arise, not from the ether, as it were, but by modifications to already existing organisms. Evolution is not a theory for the origin of life. It is merely a theory for how the amazing diversity we see in the natural world arose (and continues to arise). There are of course several possible explanations for how this happens, but Darwin's theory of natural selection and it's modern modifications (collectively called Neodarwinism) are the most widely accepted theories. The amount of evidence that has been collected for Darwinian evolution is staggering, with thousands, if not tens of thousands of papers published on the topic. So what does this theory say?
If the theory is simplified, it can be broken down into three bits:
- Organisms tend to overbreed. This basically means that organisms will attempt to produce offspring at the highest level that is practical for them. This might be once every few weeks for rats, or once every few years for humans. Note that this rate isn't necessarily a reflection of the biological capacity of the organism to breed! Humans can produce (in theory) a child every nine or so months, but we don't because it is impractical for us, since the additional child would probably reduce the survival probability of both the children. In other words, organisms will tend to reproduce at the rate that represents the best trade-off between ability to for the parent(s) and offspring to survive (and thrive) and the maximum biological ability to reproduce.
- Because populations tend to increase rapidly, the only those individuals that are healthy and have some kind of advantage over the others will survive. Even in times of plenty, this manifests itself when the time comes to mate. Typically, only the "fittest" individuals will be chosen to mate by individuals who are themselves fit. "Fitness" is of course a subjective term that changes according to species. The offspring of such a union are of course likely to be "fit" themselves, thereby increasing the probability that they will survive and so on. Over the generations, those "fitter" individuals will come to dominate the population.
- If there is a change in the prevailing conditions, or if part of the population migrates to a new area (maybe due to lack of food or other resources, maybe due to some other change), what was previously considered "fit" for the old population might no longer apply to the new population. For example, if there was a population of large carnivores (say tigers) in a particular place where large game was abundant, they would have no problem obtaining prey of the right size. However, if some of that population (maybe due to overcrowding) moved to a new area where large game wasn't readily available, they would have to make do by killing smaller game. However, this would mean that they had to hunt more frequently, something that large carnivores don't often do. In a situation like this, obviously, individuals needing less food (i.e., smaller individuals) would be favoured. Therefore, these individuals would now have a selective advantage over the old "large" breed. In such a case, a new "small" breed of tiger would evolve. Another way this could go is that the tigers would eventually become faster or stealthier, allowing them to sustain the new requirement to hunt more often. This requirement to adapt to the conditions to survive is called "selection pressure".
This is pretty much the essence of the theory of natural selection as postulated by Darwin. Our current understanding of genetics and molecular biology fits right into this theory and the resultant theory is known as Neodarwinism. But I think I've fried enough brains for the time being, and so I'll leave that for the next post.