Genes and Evolution: Genetics of Populations

Genes and Evolution:
Genetics of Populations
28 November, 2005 Text Chapter 23
A species is a group of individuals with the potential to interbreed to
produce fertile offspring.
A population is a localized group of individuals of the same
The collection of all existing alleles for all genes is the gene pool
for the population. Remember that each gene can exist as a
number of different alleles.
For each gene, the proportion of the total alleles represented by
any given allele is the allele frequency for that allele.
Allele frequency example.
Population: 500 diploid individuals (1000 total alleles for gene A)
Gene A : Two possible alleles
In our population,
A (dominant, pink flowers)
a (recessive, white flowers)
20 individuals are aa
160 individuals are Aa
320 individuals are AA
The allele frequency for A is ((320)(2)+160)/1000 = 800/1000 = .8
The allele frequency for a is ((20)(2)+160)/1000 = 200/1000 = 0.2
For an idealized,
sexually reproducing
population, these allele
frequencies will remain
constant over successive
If all individuals reproduce to give
the same number of offspring, and
mating is random, then the allele
frequencies in the F1 generation will
be the same as the allele frequencies
in the parental generation.
This is like taking all of
the alleles in the
population and drawing
them two at a time to
determine the genotype
for each F1 individual.
p2 + 2pq + q2 = 1
Hardy-Weinberg Criteria
This situation where succeeding generations have the same allele
frequencies is called Hardy-Weinberg equilibrium. In order to have no
genetic change over time, five conditions must be met.
The population must be large.
The population must be isolated.
Mutations may not occur.
Mating must be random.
All individuals must have equal reproductive success.
If all of these criteria are not met, then the genetic structure of the
population will change over time. Microevolution is occurring.
The five agents of microevolution, genetic drift, gene flow, mutation,
sexual selection, and natural selection represent departure from the five
conditions for equilibrium.
Genetic drift is microevolution caused by changes in the gene
pool of a small population due to chance.
Population bottlenecks are an example of genetic drift.
The founder effect is a related agent of genetic change.
Gene flow occurs when a population gains or loses alleles in
exchange with another population.
This effect tends to reduce genetic differences between
populations that were previously isolated.
Mutation (heritable changes to DNA occur constantly in every
cell of every individual. In humans, a mutation rate of about 1 per
locus per 1,000,000 gametes is typical.
Mutation cannot lead to large changes in allele frequency unless it
is accompanied by selection.
Selection can take the form of nonrandom mating, or Unequal
reproductive success (natural selection). Of all of the agents of
microevolution, only natural selection is adaptive.
Genetic variation, the substrate for selection, occurs both within and
between populations. In Drosophila populations, about 30% of loci have
more than one allele, and each fly is heterozygous at about 12% of its
genes. Variation between populations is even greater.
Diploidy and Balanced Polymorphism
Preserve Variation.
Frequency-dependent Selection
Selection may be stabilizing, directional, or disruptive.
Sexual Reproduction is