CH 22 POPULATION GENETICS
Population genetics: the study of genes and genotypes in a population
22.1 Genes in Populations
- All the genes in a population make up its gene pool where each member of a population receives its genes from its parents
which are members of the gene pool.
A population is a group of interbreeding individuals
- Large mountain or geographic barrier may separate two or more populations on the same continent.
- Populations are dynamic: change from one generation to the next in number, location, and genetic composition.
- Polymorphism: phenomenon that many traits display variation in phenotypes – i.e. colour and pattern.
o Ex. In elder-flowered orchid, both yellow- and red-flowered individuals are prevalent
o Caused by 2 or more alleles that influence the phenotype of the individual.
- Polymorphic gene: gene that commonly exists as two or more alleles in a population.
- Monomorphic gene: exist as a single allele in a population. 99% or more are identical.
- Polymorphism can involve various changes: duplication of region or change in single nucleotide, SNPs (single-
nucleotide polymorphism) – smallest type of genetic change that can occur within a given gene and are most common.
Population Genetics is concerned with allele and genotype frequencies
- Allele frequency = number of copies of a specific allele at particular locus in a population / total number of all alleles
for that gene in a population
- Genotype frequency = number of individuals with a particular genotype in a population/total number of individuals in a
- Allele and genotype frequencies are always less than or equal to one.
o Monomorphic genes will have allele frequency close to 1.
Hardy Weinberg Equation relates allele and genotype frequencies in a population
- Relates frequencies when they’re not changing (not affected by evolution)
- P+q=1 for genes that exist in two alleles
- P + 2pq + q = 1 for one allele where p is the genotype frequency of RR, pq is genotype frequency of Rr, and q is 2
genotype of rr.
- Predicts an equilibrium of unchanging allele and genotype frequencies in a population.
- Equilibrium is when the population is not adapting and evolution is not occurring under certain conditions:
o Population is so large that allele frequencies do not change through random sampling error
o Members of population mate with one another without regard to their phenotypes and genotypes. o No migration occurs between different populations.
o No survival or reproductive advantage exists for any of the genotypes – no natural selection occurs.
o No new mutations occur.
22.2 Evolutionary Mechanisms and their effects on populations
- Microevolution: term used to describe changes in a population’s gene pool from generation to generation.
- Rooted in 2 related phenomena:
- Introduction of new genetic variation into a population is one essential aspect of microevolution.
o New alleles of pre-existing genes can arise by random mutation, and new genes can be introduced into a
population by gene duplication, exon shuffling, and horizontal gene transfer.
o Action of evolutionary mechanisms that alter the prevalence of a given allele or genotype in a population –
natural selection, random genetic drift, migration, and non-random mating.
- To consider the effects of evolutionary mechanisms, we will examine how they can affect the type of genetic variation
that occurs when a gene exists in two alleles in a population.
Natural Selection in Differential Reproductive Success of Individuals in a Population
- Reproductive success: certain genotypes that result in selective survival of individuals who inherited them.
- Natural selection favours individuals with adaptations that provide a survival advantage.
- Natural selection favours individuals who produce viable offspring.
*Natural Selection Individuals.
* Evolution population
- Darwinian fitness: the relative likelihood that a genotype will contribute to the gene pool of the next generation as
compared with other genotypes; measure of reproductive success.
- Variation in fitness occurs because individuals with certain genotypes have greater reproductive success.
- Mean fitness of the population: average reproductive success of members of a population. Over many generations, as
individuals with higher fitness values become more prevalent, natural selection increases the mean fitness of the population.
Natural Selection can Follow Different Patterns
1. Directional Selection
- Favours individuals consistently above or below the mean or median of a phenotypic distribution that have greater
reproductive success in a particular environment.
- Can arise when new allele is introduced into a population by mutation and the new allele confers a higher fitness in
individuals that carry it. - The population may be exposed to a prolonged change in its living environment and fitness values can change to favour
o Finches have a genetic variation that affects beak size and a small number of birds migrate to an island where
the seeds are larger.
o In this new environment, birds with larger beaks would have a higher fitness because they would be better
able to crack open the larger seeds and survive to reproductive age.
2. Stabilizing Selection
- Favours the survival of individuals with intermediate phenotypes and the extreme values of a trait are selected against.
- Decreases genetic diversity
o Ex. Clutch size (# of eggs laid): Birds that lay too many or too few eggs per nest have lower fitness values
than those that lay an intermediate number of eggs.
o Laying too many eggs offspring die because if inadequate parental care and food and the strain on parents
decrease their likelihood of survival and ability to produce more offspring.
o Laying too few doesn’t contribute many individuals to next generation.
3. Disruptive Selection (aka Diversifying selection)
- Favours the survival of 2 or more different genotypes that produce different phenotypes
- Fitness values of a particular genotype are higher in one environment and lower in another.
- Likely to occur in populations that occupy diverse environments so that some members of the species will survive in
each type of environmental condition.
o Ex. Colonial bentgrass can survive in soil contaminated with high levels of heavy metals. This contamination
has selected for the proliferation of mutant strains of grass that show growth and tolerance to heavy metals
but showed inhibited growth on uncontaminated soil.
4. Balancing Selection
- A type of natural selection that maintains genetic diversity in a population and can create a situation called balanced
polymorphism in which 2 or more alleles are kept in balance and maintained over many generations.
- No one allele is favoured, this occurs for 2 reasons:
- For genetic variation involving a single gene, balancing selection favours the heterozygote rather than e