POPULATION GENETICS WITH EVOLUTIONARY CHANGE
As your textbook states on page 740, "Genetic variation and natural selection are population phenomena, so they are conveniently discussed in terms of allele
1. How can alleles and allele frequencies change over long periods of time?
As we have seen earlier, inheritable mutations (i.e., mutations in gametes or in cells that are precursors of gametes) result in new alleles.
Since natural selection favors genotypes that are better able to survive and reproduce, a new "favored" (i.e., beneficial) allele will increase in frequency over a number of generations. The rate of increase in frequency of the favored allele will depend on whether the allele is dominant or recessive. This is shown in Figure 17.26 (page 744), which shows allele frequency plotted versus number of generations. In general, a new favored dominant allele will increase rapidly in the population, because even the heterozygous individuals have the "improved" phenotype (produce more surviving offspring). If this new favored allele shows only incomplete dominance ("additive case" in Figure 14.24), the allele frequency will increase somewhat slower at first, but eventually will actually cross above the dominant allele case. A new favored recessive allele will increase very, very slowly for many generations until the allele becomes quite common (and thus there are some significant numbers of homozygous recessive individuals), and then it will increase much more rapidly.
The curve for incomplete dominance is particularly informative. This curve at first increases slower than the case for the favored dominant allele because "early on" most of the genotypes containing the allele are heterozygotes, and for incomplete dominance these heterozygotes are not as favored as they would be if the allele was completely dominant. Eventually, however, the incomplete dominance curve crosses above the "dominant" curve because for the case of incomplete dominance with A being a favored allele, AA organisms do even better than Aa organisms.
2. Why don't favored alleles totally take over in populations; that is, why don't harmful alleles (such as the f allele of the CFTR gene in humans) get eliminated totally (to 0%) by natural selection?
Quoting from page 745. "Natural selection usually acts to minimize the frequency of harmful alleles in a population. However, the harmful alleles can never be totally eliminated, because mutation of wildtype alleles continually creates new harmful mutations. With the forces of selection and mutation acting in opposite directions, the population eventually attains a state of equilibrium, or selection-mutation balance, at which new mutations exactly offset selective eliminations."
3. Why can evolutionary change appear to happen so much faster as a result of "artificial selection" compared to natural selection?
Artifical selection differs from natural selection in that only organisms with certain genotypes or phenotypes are allowed to reproduce. This makes it possible for very rapid alterations in allele frequencies in the population to occur over a fairly small number of generations. Because the overall phenotype of the organism is determined by many different genes, after some limited number of generations organisms will appear that have a certain combination of alleles that NEVER showed up back in the original population. Thus, much of what can be obtained quite "quickly" by artificial selection is due not to the creation of new alleles, but rather is due to new COMBINATIONS of alleles of a variety of genes.
We can run through a very rough idealized scenario that shows the effect described above. Imagine a species that has 10 chromosomes, and consider ten genes (one on each chromosome, so we get independent assortment) that determine 10 details of what the organism looks like. Assume that each of these genes has two alleles, a dominant one present at 90% (A,B,C,D,E,F,G,H,I,J) and a recessive one at 10% (a,b,c,d,e,f,g,h,i,j) in the population. In the population, there will almost certainly be no individuals showing all of the "rare" traits, but there will be a few individuals (1% or so) showing any ONE rare trait (recessive pheotype). If we allow only THESE organisms to reproduce, the next generation will be made up of organisms mostly showing the first trait, and a small fraction (1% or so) of these will show one OTHER rare trait. We allow only these organisms to mate, and we get some offspring showing TWO of the formerly rare traits. We continue on, and by the time of the tenth generation, we very well might have some organisms that are showing all ten of the formerly rare traits. We have not created any new alleles, but we now have organisms with an allele composition (i.e., genotype aa,bb,cc,dd,ee,ff,gg,hh,ii,jj) that did not exist in any individual in the original population.
4. What is an example of the long-term evolutionary history of a human gene?
The B-globin gene provides a good example of slow change over a long evolutionary time-frame. The divergence of this gene within mammals and between mammals and birds is shown in Figure 17.1.
5. What is an example of the short-term evolutionary history of a human gene?
The allele frequencies of gene CCR5 are currently changing in some parts of the world. CCR5 codes for a protein on our cell surfaces which HIV virus particles bind to as part of the process of getting into our cells. The "delta-32" allele of this gene is due to a 32 base pair deletion mutation in an exon region, as shown in Figure 17.5. This mutation results in a truncated protein that is non-functional.
During the current three decades of the global spread of HIV, the delta-32 allele frequency has started to increase in populations where a high percentage of people are HIV+.