How Populations Evolve

Gene pool All genes present in population

microevolution Change in relative frequencies of alleles in a population over time Hardy-Weinberg Theorum: in absence of selection, the allele frequencies within a population will remain constant from one generation to the next

Hardy-Weinberg Theory 5 conditions Large population No migration No net changes in gene pool due to mutation Random mating Equal reproductive success of each genotype

Hardy Weinberg equation P and q represent proportions of the two alleles within a population Combined frequencies of the alleles must equal 100% of the genes for that locus within a population p + q = 1 P 2 + 2pq + q 2 = 1 P from mom p from dad p 2 P from mom q from dad pq P from dad q from mom pq 2pq Q from mom q from dad q 2

Example 1 If p =.7 (allele A) then q =.3 (allele a) Then P 2 + 2pq + q 2 = 1 P 2 = AA =.49 2pq = 2Aa =.42 q 2 = aa =.09 P = frequency of dominant allele A

Example 2 If a pop has the folloing genotype frequencies, AA =.42, Aa =.46, aa=.012, what are the allele frequencies? A) A = 0.42, a=0.12 B) A=0.88, a = 0.12 C) A=0.65, a = 0.35 D) A= 0.6, a = 0.4

Example 2 Solution Frequency of A = /2 (.46) =.65 Frequency of a =.12 1/2 =.35 OR Frequency of a = =.35 Answer is “C”

Example 3 In a population with two alleles, B and b, then allele frequency of B is 0.8. What would be the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium? A).8 B).16 C).32 D).64

Example 3 Solution Population of Bb = 2(.8)(.2) =.32 Answer is “C”

Example 4 In a population that is Hardy-Weinberg equilibrium, 16% of the population shows a recessive trait. What percent is homozygous dominant for the trait? A) 6% B) 36% C) 48% D) 84%

Example 4 Solution aa =.16 a =.4; then A =.6 AA =.36 (and Aa = 2*.4*.6 =.48) Answer is “B”

Example 5 In a random sample of a population of Shorthorn cattle, 73 animals were red (C R C R ), 63 were roan (C R C W –a mixture of red and white), and 13 were white (C W C W ). Estimate the allele frequencies of C R and C W and determine whether the population is in Hardy- Weinberg equilibrium.

Example 5 Solution Frequency of C W = [13/( )] 1/2 C W =(0.09) 1/2 =.3 Frequency of C R = 1-.3 =.7 This genotypic ratio is what would be predicted from these frequencies if the population were in Hardy-Weinberg equilibrium.

Example 6 In a study of population of field mice, you find that 48% of the mice have a coat color that indicates that they are heterozygous for a particular gene. What would be the frequency of the dominant allele in this population? A).4 B).5 C).7 D) you cannot estimate allele frequency from this information

Example 6 Solution Frequency of heterozygous = 2pq, therefore it would not be possible to estimate the frequency of either p or q without more information

Causes of Microevolution 5 potential agents of microevolution Small populations Migration or emigration Spontaneous mutations-point mutations Nonrandom mating Some genotypes are not equally successful reproductively

Genetic Drift Chance change in a gene pool of a small population; it is not related to the fitness of the individuals Bottleneck effect occurs if a catastrophic event reduces the population size and the survivors are not representative of the original population Founder effect is when a few individuals colonize a new area; unlikely to be representative of parent population

Gene flow Migration of individuals or transfer of gametes between populations may result in gain or loss of alleles Eg. Pilot whale populations – pods intermingle and mate at upwellings; transfer of gametes

Mutation Mutations are the main method of diversity in prokaryotes, but of little importance in microevolution of eukaryotes Mutation rates for most gene loci is one mutation in every 10 5 or 10 6 gametes Mutation is the original source of genetic variation, consequently, it is central to evolution

Variation within populations Individual variation is what natural selection acts on-on the phenotype Polygenic traits that vary provide variation Polymorphism provide variation (blood types) 2 flies in a Drosophila pop may vary at 25% of their loci-individual differences

Geographic variations Regional differences in allele frequencies among the populations of a species Variations may be due to differing environmental selection factors or genetic drift If parameter changes gradually across a distance then a cline may develop

Origin of Species Speciation is the basis of evolution of biological diversity Anagenesis (phyletic evolution) is the transformation of an entire population into a different enough form that it is renamed a new species Cladogenesis, branching evolution, new species arise from a parent species that continues to exist

species Reproductive and genetically isolated group of individuals Limitation of this concept is it can’t apply to asexually reproducing organisms

Reproductive barriers Prezygotic barriers: before formation of zygote Postzygotic barriers: prevention of development of fertile adult

Prezygotic barriers Mechanical isolation- parts don’t fit Geographical isolation – never meet Temporal isolation – breed at different times Behavioral isolation – wrong courtship dance, wrong pheromones Gametic isolation – gametes will not fuse to form zygote-can’t line up or wrong molecular recognition mechanism of egg and sperm

Postzygotic barriers Hybrid inviability – hybrid zygote fails to survive embryonic development Hybrid sterility – viable hybrid is sterile (usually gametic problem) Hybrid breakdown – hybrids are viable and fertile but their offspring are defective or sterile Exception may be introgression when offspring may be able to mate w parent species variation in gene pool without sacrificing species

Biogeography of speciation Allopatric speciation: gene pool of population is segregated geographically from other populations (opposite sides of river) Parapatric speciation: genes pools of both populations diverge without the dilution of genes from their neighbors (theoretical) Sympatric speciation: subpopulation becomes reproductively isolated within parent population (plants,wasps)

Adaptive radiation vs convergent evolution Adaptive radiation is the formation of numerous species from one parent population – like Darwin’s Galapagos finches Convergent evolution is the formation of homologous structures due to environmental conditions

Gradual evolution vs punctuated evolution Gradual divergence of populations by microevolution species continue to evolve over long periods of time Punctuated evolution (Gould & Eldredge) long period of stasis are punctuated by episodes of relative rapid change and speciation in a few thousand years vs millions of years – Cambrian explosion of species