Populations and Gene Pools
Peccaries are small, tough relatives of the modern pig, whose lineage diverged about 40 million years ago. They live in southern Texas, Arizona, and New Mexico.
A herd of peccaries consists of about 15 individuals, half males and half females. They stay close together and use scent as a cohesive force, moving, feeding, playing and resting as a peaceable social unit.
If individuals become separated from the herd, they get upset and utter loud wailing cries. It is rare, therefore, for a peccary to move from one herd to another.
Together, these socially and behaviorally egalitarian animals form a population. It is at the population level that the phenomenon of evolution is observed, studied and measured.
Individual peccaries cannot evolve, but the little herd, or population, of peccaries is subject to all the forces of variation and natural selection. They can undergo all the changes in genotypes and phenotypes we associate with evolutionary change.
A population is the smallest unit of living organisms that can undergo evolution.
Within populations are combinations of genes and different gene types. A peccary is a diploid organism, which means that there are two copies of each gene in every cell in its body. A mutation can produce a subtle variation in either, or both, of these genes, some lethal and some that simply alter a trait slightly.
Suppose that bristle length on the bodies of the peccaries is controlled by a single gene (written as B), and that a mutation in this gene results in shorter bristles (written as b).
These two varieties of the bristle-length gene are called alleles that, in various combinations, produce the peccary phenotype.
A single peccary may have any one of three different genotypes:
BB (homozygous dominant)
Bb (heterozygous), and
bb (homozygous recessive)
These genotypes produce one of two phenotypes:
long bristles (BB and Bb), or
short bristles (bb).
Thus, individual peccaries can have only 0, 1, or 2 copies of any one gene variant (- -, - b, or bb, respectively), giving a frequency of 0, 0.5 or 1.0 (0%, 50%, or 100%) for that specific gene in an individual peccary.
With 15 peccaries in the population, each with two copies of the bristle-length gene, there are 30 copies, or alleles, in all. When scientists wish to study evolution (a population phenomenon), they have to measure and follow what happens to all these genes, in the whole population of peccaries, all at the same time.
A gene pool is the sum of all the individual genes in a given population.
Within a gene pool, every allele or gene variant has a particular ratio or frequency.
The frequency of an allele is the number of occurrences of that allele in that population
15 individual peccaries in the population, thus 30 alleles -
if 6 alleles in this population are of the b variety,
and 24 are of the B variety,
then the frequencies of these alleles are:
6/30 of the genes in the gene pool are b - a frequency of 0.2
6/24 of the gene in the gene pool are B - a frequency of 0.8.