The popular media often presents evolution as being a predictable process with a

Question

The popular media often presents evolution as being a predictable process with a definite goal. For instance, in one “Star Trek: Voyager” episode, the captain instructs the ship’s computer to extrapolate the “probable course” of evolution of hadrosaurs (a bipedal dinosaur), if hadrosaurs had been removed from Earth before the Cretaceous-Tertiary (K-T) Extinction and allowed to evolve on another planet. What information about the hadrosaurs’ new environment would have been useful in developing the best possible prediction? Given what you know on genetic drift and selective agents, is it possible to accurately predict the long-term course of evolution?

Explanation / Answer

Ans

Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to "sampling error" in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations.

Genetic drift example

Let's make the idea of drift more concrete by looking at an example. As shown in the diagram below, we have a very small rabbit population that's made up of 888brown individuals (genotype BB or Bb) and 222 white individuals (genotype bb). Initially, the frequencies of the B and balleles are equal.

What if, purely by chance, only the 555circled individuals in the rabbit population reproduce? (Maybe the other rabbits died for reasons unrelated to their coat color, e.g., they happened to get caught in a hunter’s snares.) In the surviving group, the frequency of the B allele is 0.70.70, point, 7, and the frequency of the b allele is 0.30.30, point, 3.

In our example, the allele frequencies of the five lucky rabbits are perfectly represented in the second generation, as shown at right. Because the 555-rabbit "sample" in the previous generation had different allele frequencies than the population as a whole, frequencies of Band b in the population have shifted to 0.70.70, point, 7 and 0.30.30, point, 3, respectively.

b allele is completely lost from the population.

Population size matters

Larger populations are unlikely to change this quickly as a result of genetic drift. For instance, if we followed a population of 100010001000 rabbits (instead of 101010), it's much less likely that the b allele would be lost (and that the B allele would reach 100%100%100, percentfrequency, or fixation) after such a short period of time. If only half of the 100010001000-rabbit population survived to reproduce, as in the first generation of the example above, the surviving rabbits (500500500 of them) would tend to be a much more accurate representation of the allele frequencies of the original population – simply because the sample would be so much larger.

Why would be this case ?

This is a lot like flipping a coin a small vs. a large number of times. If you flip a coin just a few times, you might easily get a heads-tails ratio that's different from 505050 -505050. If you flip a coin a few hundred times, on the other hand, you had better get something quite close to 505050 - 505050 (or else you might suspect you have a doctored coin)!

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