On pages 122-124, Coyne discusses the difference between evolution by genetic drift and evolution by natural selection. He says that certain features, like blood type, "especially in small populations...can change over time entirely by chance".
Think of a neutral ("neither useful nor injurious") trait, such as an animal's external feature, that seems to have evolved by genetic drift, and argue that it has a selective advantage. Suggest features for other responders to argue for.
Are there any features that are known to have evolved by genetic drift that are obviously helpful, (or that used to be injurious or neutral but are now helpful under different circumstances)? Why is it hard to tell whether helpful features evolved by natural selection or genetic drift?
We also know that "genetic drift is...powerless to create adaptations, but can actually overpower natural selection". Are there any small populations that seem to have unhelpful features that evolved by genetic drift? Name the population, the feature they have, and what makes the feature unhelpful.
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Great question! Definitely took a lot of effort and research to come up with the answers.
ReplyDeleteFirstly, a neutral trait that has evolved by genetic drift, and might have a selective advantage. This would be the eye color of the human being. It seems that the variation in colors have emerged through genetic drift instead of natural selection as the difference in eye colors do not provide for the elimination of any injurious trait. However, a selective advantage for this could be simply the attraction of mates which promotes the survival and reproduction of the species (for example, we’ve all seen the beautiful girl with bright blue eyes :-) ).
At the same time, the eye color can have injurious effects. For example, those with lighter iris color have been found to be at a higher risk for age-related macular degeneration (ARMD) (http://en.wikipedia.org/wiki/Eye_color#Medical_implications). This medical condition usually affects older adults; symptoms include a loss of vision in the center of vision because of damage to the retina. There is an obvious disadvantage to losing one’s sight as it is much more difficult to find food and to avoid danger on a daily basis – the necessities to survival (http://en.wikipedia.org/wiki/Macular_degeneration).
It is very difficult however, to determine if some feature evolved by natural selection or genetic drift. For example, the diverse shapes of leaves were once suggested to be “neutral” traits that evolved by genetic drift. The difficulty is that “it’s hard to prove that a trait has absolutely no selective advantage” (124). Just because it is difficult for scientists to uncover the function for some feature, does not make it “neutral.” This is shown when Coyne says “features whose evolution we don’t understand may reflect only our ignorance rather than genetic drift” (124).
To answer the lat part of the question, yes there are small populations who have developed unhelpful features by genetic drift. On page 124 coyne mentions that the “sampling effect” can be so large in a small population that the frequency of harmful genes rises, thus working against natural selection. This is precisely the cause for a high incidence of genetically based diseases in isolated communities. Examples of these are the high percentage of Gaucher’s disease – in which a fatty lipid substance accumulates in cells and certain organs preventing them to function properly - in northern Sweden (http://en.wikipedia.org/wiki/Macular_degeneration). Another example of this is the heavy concentration of Tay-Sachs disease in the Cajuns of Louisiana. This disease is an autosomal recessive genetic disorder which causes the deterioration of mental and physical abilities. Usually symptoms are first seen at 6 months and death occurs at age 4 or 5 (http://www.mayoclinic.org/tay-sachs-disease/symptoms.html).