HardyWeinberg equilibrium n if p frequency of allele
Hardy-Weinberg equilibrium n if p = frequency of allele A q = frequency of allele a n p + q = 1, ( p + q )2 = 1 p 2 + 2 pq + q 2 = 1 n n if only law of probability affects the frequency w/ which gametes combine to form new individuals
n Bisexual population n Large population n Random mating n No mutation n Migration ~ 0 n Natural selection does not affect the locus
n A population that is in Hardy-Weinberg equilibrium will experience no change in either genotype frequency or allele frequency n If one or more of the conditions is violated, genotype frequency and allele frequency will change
Example n If only 6% of the population displays pale eyes (recessive gene e). What is the frequency of genotype Ee in this population? q 2 = 0. 06 ---> q = 0. 24 p + q = 1 ---> p = 0. 76 Ee = 2 pq = 2(0. 76)(0. 24) = 0. 36
Genetic variation n Cline – a measurable, gradual change over a geographical region in the average of some phenotype character n Ecotype – abrupt changes in the phenotype characters within a species, which often reflect abrupt changes in local environment
n Geographic isolates – semi-isolated populations prevented by some extrinsic barriers from a free flow of genes n Genetic polymorphism – the existence within a species or population of different forms of individuals
Maintenance of balanced polymorphism (vs. transitional or directional polymorphism) n n heterosis diversifying evolution frequency-dependent selection selective forces operating in different directions within different patches of a fine mosaic in the population
n Stabilizing, directional, diversifying or disruptive evolution n Speciation n Allopatric speciation n Sympatric speciation
Isolation mechanism n Pre-zygotic: habitat, temporal, ethological, mechanical n Post-zygotic n n n hybrid inviability or weakness hybrid sterility F 2 breakdown
Reduction in variation n inbreeding, bottlenecks, founder effect, genetic drift n genetic drift - random shifts in allele frequencies
Effect of small populations n More demographic variation, inbreeding depression, genetic drift → higher risk of extinction Minimum viable population size n the threshold # of individuals that will ensure the persistence of subpopulation in a viable state for a given time interval
Effective population size (Ne) n the size of a genetically idealized population with which an actual population can be equated genetically, Ne = N , if n equal sex ratio n equal probability of mating n constant dispersal rate n progeny per family randomly distributed
n unequal sex ratio Ne = 4 Nm˙Nf / (Nm + Nf ) n population fluctuation 1 / Ne = (1 / t )(1/N 1 + 1/N 2 + … + 1/Nt) n non-random progeny distribution Nk Ne = --------------------(N/N-1)˙Vk/k˙(1+F) + (1 -F)
Effect of continental drift n Distribution/radiation of monotremes and marsupials
Extinction and its causes n Natural causes: climatic changes and stochastic event n Human disturbance n habitat alteration n over-exploitation n exotic species n diseases and other factors
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