Hardy-Weinberg equilibrium

if p = frequency of allele A q = frequency of allele a

p + q = 1, ( p + q )2 = 1 p2 + 2pq + q2 = 1

if only law of probability affects the frequency w/ which gametes combine to form new individuals

Bisexual population

Large population

Random mating

No mutation

Migration ~ 0

Natural selection does not affect the locus

A population that is in Hardy-Weinberg equilibrium will experience no change in either genotype frequency or allele frequency

If one or more of the conditions is violated, genotype frequency and allele frequency will change

Example If only 6% of the population displays

pale eyes (recessive gene e). What is the frequency of genotype Ee in this population?

q2 = 0.06 ---> q = 0.24

p + q = 1 ---> p = 0.76

Ee = 2pq = 2(0.76)(0.24) = 0.36

Genetic variation Cline – a measurable, gradual change

over a geographical region in the average of some phenotype character

Ecotype – abrupt changes in the phenotype characters within a species, which often reflect abrupt changes in local environment

Geographic isolates – semi-isolated populations prevented by some extrinsic barriers from a free flow of genes

Genetic polymorphism – the existence within a species or population of different forms of individuals

Maintenance of balanced polymorphism

(vs. transitional or directional polymorphism) heterosis diversifying evolution frequency-dependent selection selective forces operating in different

directions within different patches of a fine mosaic in the population

Stabilizing, directional, diversifying or disruptive evolution

Speciation

Allopatric speciation

Sympatric speciation

Isolation mechanism

Pre-zygotic: habitat, temporal, ethological, mechanical

Post-zygotic

hybrid inviability or weakness hybrid sterility F2 breakdown

Reduction in variation

inbreeding, bottlenecks, founder effect, genetic drift

genetic drift - random shifts in allele frequencies

Effect of small populations More demographic variation, inbreeding

depression, genetic drift → higher risk of extinction

Minimum viable population size the threshold # of individuals that will

ensure the persistence of subpopulation in a viable state for a given time interval

Effective population size (Ne) the size of a genetically idealized population

with which an actual population can be equated genetically, Ne = N , if

equal sex ratio

equal probability of mating

constant dispersal rate

progeny per family randomly distributed

unequal sex ratio

Ne = 4 Nm˙Nf / (Nm + Nf )

population fluctuation

1 / Ne = (1 / t )(1/N1 + 1/N2 + … + 1/Nt)

non-random progeny distribution

Nk

Ne = -----------------------------------------(N/N-1)˙Vk/k˙(1+F) + (1-F)

Effect of continental drift

Distribution/radiation of monotremes and marsupials

Extinction and its causes

Natural causes: climatic changes and stochastic event

Human disturbance

habitat alteration over-exploitation exotic species diseases and other factors