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Mechanisms of evolution
Right! Let's get down to business. It is the most important and interesting of all - how do creatures actually evolve? It goes right down into the core - it occurs at the level of alleles. What is responsible for the minor changes seen in organisms? And what is natural selection? This topic will help you learn about all of the above and more.
There are 3 main factors which cause evolution:
As time passes, the primary genetic material - the DNA is affected by some errors.As you have read,it consists of 4 letters - A, G, C, and T. Suppose a part of the DNA consists of the letters AAG. Due to radiation, or unknown causes, one of the letters change. The genetic information now is different. It might be for the good or bad for the organism. But the fact is that there has been a change in the DNA. This sort of a change is called a mutation.
Mutation produces variety in genes over time, and these different forms produced are called variants. This effect is usually negligible, however, because the rate of mutation is very low. You would need a population of about 500,000 people in order to sample one mutation event. If a mutation did happen, its effect would be on the order of about 1 in a million, which is not detectable under normal conditions.
While flipping a coin 1000 times, we might mostly get a ratio of 500:500 = 1:1. But what about flipping it 10 times? We might get 7 heads and 3 tails and the ratio may not be 1:1 frequently. This is what happens in genetic drift.The heads and tails are the alleles,coin is the individual, and the person who flips the coin is nature.
Genetic drift is the most basic of the evolutionary mechanisms, but is among the most difficult to understand because it has neither purpose nor true cause. It is just a random change - increase and decrease in the frequency of an allele. There is no control over this at all.
Suppose a population of 10 has two alleles for hair colour - B & b, you might find a lot of people having a certain allele, and in the next generation having more of the other. This sort of variation might help or prove to be a problem to the species, and natural selection will decide which variation will remain.
If you have a small population reproducing, you are, so to speak, limiting the number of times you flip the coin. The ratio between two alleles can shift dramatically from one generation to the next in a small population: this shift in frequency is genetic drift.
Genetic drift can begin a trend that results in one allele vanishing completely from the gene pool of a small group, thus decreasing or eliminating variation in that population with regard to that gene. But, since the change is undirected--nothing more than random chance is causing an allele to increase or decrease in frequency--the changes of allele frequencies in several different small groups of a species are likely to be different. Remember that our earliest ancestors probably lived in small groups.
