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
frequencies."
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+.