Biology Reference
In-Depth Information
chances are quite good that you will get 4 reds (32 percent chance).
When you reconstitute your population of marbles for the next bowl
generation, you will no longer have blue marbles. h ey will have been
lost because of sampling error, that is, drit .
h is same process goes on in real honey bee populations from gen-
eration to generation. h e proportional representation of the sex alleles
drit s because there is not an exactly equal representation of all of them
from one generation to the next. Some colonies contribute more gam-
etes to the mating pool by making more reproductive of spring (queens
and drones); others fail to survive because of chance occurrences, like
disease or predation—events that may have nothing to do with which
sex alleles they have. h e more marbles you sample from the bowl each
generation to constitute the next, the longer it will take to lose one of
the colors, and the same holds for losing sex alleles in populations. h e
larger the breeding population (queens and successful drones), the lon-
ger sex alleles stay in the population, and, therefore, more sex alleles
persist. With some assumptions about the size of the breeding popula-
tion and mutation rates (how ot en a mutation occurs at the
csd
locus),
population geneticists can predict how many alleles are expected in
dif erent populations.
Another prediction from population genetics is that the sex alleles
will be nearly equal in frequency in the population. Natural selection
maintains alleles in populations. h is is a consequence of their com-
plementarity. Only diploid heterozygotes can survive. Because eggs
and sperm gametes fuse at fertilization to make a zygote, gametes
with rare sex alleles are less likely to pair with alleles that are like
them than are gametes with common alleles. h erefore, rare alleles
will be favored (fewer homozygous combinations), and common al-
leles will be selected against (more homozygous combinations). At
equilibrium, if a very large population is assumed, all alleles will be
equally frequent. If there are 10 alleles in the population, each will
represent 10 percent of the genes in diploid females as a consequence
of pairing of sperm and egg gametes, and also in the males as a con-
sequence of being derived directly from the gametes of the heterozy-
gous queens.
