18: Population Genetics

population genetics

genetic differences within and among populations

• adaption

• speciation

• genetic drift

• population structure

population

group of interbreeding individuals of the same species and that live in a defined geographic area

questions asked by population geneticists

• how much genetic variation is present in a population?

• are genotypes randomly distributed in time and space, or do discernible patterns exist?

• what processes affect the composition of a population’s gene pool?

• do these processes produce genetic divergence among populations that may lead to the formation of new species?

microevolution

evolutionary change within populations of a species

macroevolution

evolutionary events leading to the emergency of new species/taxons

selective force

any environmental factor that influences the reproductive success of individuals in a population

• determines which traits are passed into successive populations

  • predation

  • disease

  • climate

  • competition

selective forces act on

variation in fitness

• over time, alleles associated with higher fitness tend to increase in frequency due to selection

fitness

the ability for an organism to reproduce/contribute offspring to a population

• assigned value of 1

evolution

change in heritable traits in a population over successive generations

• at genetic level it corresponds to a change in genotype frequencies

selection

non-random differential survival or reproduction of individuals within different phenotypes

mutation

error in the replication of a nucleotide sequence or any alteration of the genome

drift

random changes in the frequency of two or more alleles (or genotypes) within a population

assortive mating

non-random mating based on a specific phenotype (i.e., the tendency to mate with individuals of similar or dissimilar phenotype) but does not necessarily change allele frequency

Hardy-Weinberg equilibrium

describes what happens to allele and genotype frequencies in an ideal population

• no evolutionary force is acting

• all genotypes contribute equally to next generation

• allele frequency do not change over time

ideal population

• infinitely large

• randomly mating

• no mutation

• no selection

• no migration

• no drift

assumptions of HW

1. individuals of all genotypes have equal rates of survival and equal rates of reproductive success

2. no new alleles are created or converted through mutation

3. individuals do not migrate into or out of the population

4. population is infinitely large (no sampling errors/random effects)

5. individuals in population mate randomly

forces of evolution

1. mutation

2. selection

3. migration

4. drift

p

frequency of allele A

• dominant

q

frequency of allele aa

• recessive

2pq

frequency of Aa

• heterozygous

consequences of HW

1. dominant traits do not necessarily increase from one generation to the next

2. genetic variability can be maintained in a population since, once established in an ideal population, allele frequency remains unchanged

3. knowing the freq of just one genotype enables us to calculate the frequency of all other genotypes at that locus

CCR5

gene coding for a chemokine receptor

• plays a role in CD4 (T-helper) migration

• point of entry for HIV

CCR51

wild type

1/1

CCR5-∆32

homozygous mutant of allele of CCR5

• loss of function mutation

• beneficial as HIV cannot bind due to ∆32