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How Populations Evolve

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How Populations Evolve. Gene pool. All genes present in population. microevolution. Change in relative frequencies of alleles in a population over time - PowerPoint PPT Presentation

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Page 1: How Populations Evolve

How Populations Evolve

Page 2: How Populations Evolve

Gene pool All genes present in population

Page 3: How Populations Evolve

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

Page 4: How Populations Evolve

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

Page 5: How Populations Evolve

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

P2 + 2pq + q2 = 1 P from mom p from dad p2

P from mom q from dad pq P from dad q from mom pq 2pq Q from mom q from dad q2

Page 6: How Populations Evolve

Example 1 If p = .7 (allele A) then q = .3

(allele a) Then P2 + 2pq + q2 = 1 P2 = AA =.49 2pq = 2Aa = .42 q2 = aa = .09 P = frequency of dominant allele A

Page 7: How Populations Evolve

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

Page 8: How Populations Evolve

Example 2 Solution Frequency of A = .42 + 1/2 (.46) =

.65 Frequency of a = .121/2 = .35 OR Frequency of a = 1-.65 = .35Answer is “C”

Page 9: How Populations Evolve

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

Page 10: How Populations Evolve

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

Page 11: How Populations Evolve

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%

Page 12: How Populations Evolve

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

Page 13: How Populations Evolve

Example 5 In a random sample of a population of

Shorthorn cattle, 73 animals were red (CRCR),

63 were roan (CRCW –a mixture of red and white), and 13 were white (CWCW). Estimate the allele frequencies of CR and CW and determine whether the population is in Hardy-Weinberg equilibrium.

Page 14: How Populations Evolve

Example 5 Solution Frequency of CW = [13/(73+63+13)]1/2

CW=(0.09)1/2 = .3 Frequency of CR = 1-.3 = .7 This genotypic ratio is what would be

predicted from these frequencies if the population were in Hardy-Weinberg equilibrium.

Page 15: How Populations Evolve

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

Page 16: How Populations Evolve

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

Page 17: How Populations Evolve

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

Page 18: How Populations Evolve

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

Page 19: How Populations Evolve

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

Page 20: How Populations Evolve

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 105 or 106 gametes

Mutation is the original source of genetic variation, consequently, it is central to evolution

Page 21: How Populations Evolve

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

Page 22: How Populations Evolve

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

Page 23: How Populations Evolve

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

Page 24: How Populations Evolve

species Reproductive and genetically

isolated group of individuals Limitation of this concept is it can’t

apply to asexually reproducing organisms

Page 25: How Populations Evolve

Reproductive barriers Prezygotic barriers: before formation of

zygote Postzygotic barriers: prevention of

development of fertile adult

Page 26: How Populations Evolve

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

Page 27: How Populations Evolve

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

Page 28: How Populations Evolve

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)

Page 29: How Populations Evolve

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

Page 30: How Populations Evolve

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