Goals: Review some of the processes of microevolution that we have studied this term by seeing how they apply to humans.
Related Textbook Material: Freeman and Herron (2001) Chapter 16
Lab Manual Questions over this material are in Lab Manual Chapter XXII
The Lecture:
In these last lectures of the term, we are reviewing a number of the principles we've discussed during the semester by applying them to humans. In this lecture, we'll consider primarily microevolution.
In your first computer assignment, you learned how single-gene traits can evolve through natural selection and genetic drift. The differences in how traits where the highest fitness form is dominant or recessive, or whether we see heterosis, underdominance, or fitness codominance, can be used to understand some traits in humans. In particular, they can be used to understand genetic diseases.
Remember that when the dominant has highest fitness, the dominant allele will evolve until it is extremely common, but not quite fixed. This is because when the dominant allele is very common, the recessive allele will occur almost entirely in heterozygotes, which have fitness that is as high as that of the dominant homozygote, so there is no selection against the recessive allele in this form. Very rarely, a recessive homozygote will be produced -- such individuals have low fitness.
The predicted result of this form of evolution, where the dominant has the highest fitness, is that the dominant allele will be very common in a population, but the recessive allele will still exist, but be rare. Many human genetic diseases fit this prediction. Many genetic diseases in humans are recessive. Individuals with these diseases are born only rarely (perhaps one in ten thousand individuals would be born with such a disease.) The alleles for these diseases occur in heterozygous form in some individuals, but do not decrease their fitness. Early estimates were that each person has on average one recessive allele for some serious genetic disease. More recent estimates are higher -- they suggest we may each have 3-5 alleles, at different genes, for different genetic diseases. They don't harm us as long as we have the dominant allele to mask their effects.
In contrast, remember that when the recessive has the highest fitness, the recessive allele becomes fixed and the dominant allele becomes lost. Genetic diseases in humans also fit what we would expect in this case. Serious genetic diseases that are harmful early in life for which the allele that codes for them is dominant are extremely rare -- they occur in about one in a million people; this is the rate we would expect from mutation. So apparently these diseases are lost from the population and only recur as rare mutations.
There are some dominant genetic diseases that are more common than this. These are diseases that are expressed late in life. If diseases do not take effect until after the typical age for reproduction, there is no selection against them -- they do not affect reproduction, so they do not affect fitness, even though they may cause diseases in older ages.
Now let's consider heterosis. Remember that in heterosis, when the heterozygote has highest fitness, both alleles are maintained in the population because both are reproduced by the heterozygote. As a result, homozygotes (with lower fitness) are also produced. Some genetic diseases fit the predictions of heterosis. An example that has been known for many years is sickle-cell anemia. At the sickle-cell gene, SS individuals produce normal red blood cells bur are susceptible to malaria. ss individuals have the serious genetic disease sickle-cell anemia -- their red blood cells are an abnormal shape and do not carry oxygen well. The heterogyous Ss individuals produce red blood cells that do carry oxygen well but that apparently change to an abnormal shape, and are destroyed by the body, when they are invaded by the parasite that causes malaria. As a result, heterozygotes are resistant to malaria. In areas where malaria is a common disease, it causes high levels of mortality, and being heterozygous at the sickle-cell gene has high fitness. This is likely to explain why human populations in the parts of Africa where malaria is common, and human populations descended from those populations, suffer from high levels of sickle-cell anemia; the sickle-cell allele has been maintained in the population through heterosis. Another example that has been discovered more recently is cystic fibrosis. Based on studies of mice it appears that heterozygotes at the cystic fibrosis gene may resist the worst effects of cholera -- they differ in fluid secretion properties and as a result do not get the diarrhea that can cause severe dehydration and death in people with cholera. This could explain why in populations in Europe and of European descent suffer from high levels of cystic fibrosis. Both sickle-cell anemia and cystic fibrosis are much more common, as genetic diseases, in the populations where they occur, than are the recessive genetic diseases discussed above. This is predicted because heterosis results in much higher frequencies of the harmful alleles than does the situation in which the dominant has highest fitness.
Genetic diseases can also evolve through genetic drift. Various genetic diseases and abnormalities turn out to be more common in small, relatively isolated populations than they are in large populations. This is most likely because they have increased, by chance, through drift.
Another phenomenon that we studied when we were considering microevolution was local adaptation. Click here to review the lecture notes on local adaptation. Remember that local adaptation is most likely if there is a little gene flow, at least one individual per generation, to maintain some genetic variation, but not so much gene flow that there are no differences among populations. Studies of allozymes in human populations from different parts of the world and different ethnic groups most often indicate high levels of genetic variation within populations and populations that are similar to each other. Remember that this pattern is predicted if there is a lot of gene flow. So we would expect for most traits that human populations would be similar to each other -- there is not too much chance for local adaptation.
There are some traits of humans that do show some local adaptation. Skin pigmentation functions in ways that are clearly predicted if skin pigmentation evolved through local adaptation. The typical geographic pattern for skin pigmentation is that dark skinned people occur in hot climates in and around the tropics and light skinned people occur in cold northern climates, especially Northern Europe. The difference is likely to be adaptive. Dark skin has an advantage: it helps protect the body from ultraviolet radiation, so it is associated with lower rates of skin cancer than is light skin. However, light skin also has an advantage: it allows the body to absorb more sunlight, which results in more vitamin D in the body and therefore better calcium metabolism and stronger bones. In tropical areas, the advantage to dark skin is most important -- skin cancer is a serious risk because of the intensity of the sunlight, and because of the intensity of the sunlight even dark skin allows enough light absorption to allow adequate vitamin D for normal bone development. In northern areas, in contrast, the light intensity is much lower. Skin cancer is much less of a risk, and the advantage to light skin is more important -- light skin allows adequate light absorption to prevent vitamin D deficiencies. The exception to this pattern of light skin occurring in northern areas occurs in the Native Americans of the far north -- Alaska and Canada. One possible explanation for this is that their diets are typically extremely high in fish oils that are high in vitamin D, so they get so much vitamin D from their diet that the cost to dark skin in terms of vitamin D deficiency is not important, and they benefit from dark skin in preventing skin cancer.
In this lecture, we've reviewed some areas of study in microevolution as they relate to humans. In the next lecture, we'll review macroevolution as it applies to humans.