population genetics and evolution what is evolution?

Population genetics and evolution

• What is evolution?


• What is evolution?– Descent with modification


• What is evolution?– Descent with modification– A change in the characteristics of a population

over time


• What is evolution?– Descent with modification– A change in the characteristics of a population

over time– A change in the frequency of genes in a

population over time


• Some genetics terminology– Chromosomes…


• Some genetics terminology– Chromosomes…– Homologous chromosomes…


• Some genetics terminology– Chromosomes…– Homologous chromosomes…– Locus…


• Some genetics terminology– Chromosomes…– Homologous chromosomes…– Locus…– Alleles…


• Some genetics terminology– Alleles may be dominant and recessive


• Some genetics terminology– Alleles may be dominant and recessive– In Biston betularia, the gene for melanism is do

minant (M)



minant (M)– The gene for typical color is recessive (m)



minant (M)– The gene for typical color is recessive (m)– Each individual moth has two alleles:– MM and Mm are melanic, mm are typical


• Gene frequencies– The frequency of all of the alleles at a particular

locus in a population = 100% or 1.0



locus in a population = 100% or 1.0– If there are two alleles, the frequency of one = p

and the frequency of the other = q




and the frequency of the other = q– p + q = 1.0




and the frequency of the other = q– p + q = 1.0 – With sexual reproduction, p + q alleles in the

eggs are combined with p + q alleles in the sperm


• Gene frequencies– (p + q) x (p + q) = (p + q)2 = p2 + 2pq + q2



– In Biston betularia:– p2 = MM, 2pq = 2Mm, q2 = mm



– In Biston betularia:– p2 = MM, 2pq = 2Mm, q2 = mm– The population is composed of p2 homozygous

melanic individuals, 2pq heterozygous individuals (which are melanic), and q2 homozygous typical individuals



– In Biston betularia:– p2 = MM, 2pq = 2Mm, q2 = mm

– Phenotype frequencies:– Let’s say for argument’s sake that the populatio

n consists of 81% typical individuals and 19% melanic individuals…


• Gene frequencies– If p2 = MM, 2pq = 2Mm, and q2 = mm– And if q2 = .81, what is q, the frequency of m?



– q = 0.9 (0.92 = 0.81)



– q = 0.9 (0.92 = 0.81)

– What is p, the frequency of M?



– q = 0.9 (0.92 = 0.81)

– What is p, the frequency of M?– p = 0.1 (p + q = 1.0)

Population genetics and evolution• Gene frequencies

– If p2 = MM, 2pq = 2Mm, and q2 = mm– And if q2 = .81, what is q, the frequency of m?

– q = 0.9 (0.92 = 0.81)

– What is p, the frequency of M?– p = 0.1– p2 = 0.01 (MM), 2pq = 0.18 (Mm)

Population genetics and evolution• Gene frequencies

– If p2 = MM, 2pq = 2Mm, and q2 = mm– And if q2 = .81, what is q, the frequency of m?

– q = 0.9 (0.92 = 0.81)

– What is p, the frequency of M?– p = 0.1– p2 = 0.01 (MM), 2pq = 0.18 (Mm) – 19% of individuals are melanic


• To summarize:– Homozygous dominants = MM = p2

– Heterozygotes = Mm = 2pq– Homozygous recessives = mm = q2


• What happens during reproduction?– Meiosis separates alleles into gametes


• What happens during reproduction?– Meiosis separates alleles into gametes– Gametes bear alleles in proportion to their

frequency in the population…


• What happens during reproduction?– Meiosis separates alleles into gametes– Gametes bear alleles in proportion to their frequ

ency in the population…– there are p sperm with M and q sperm with m


• What happens during reproduction?– Meiosis separates alleles into gametes– Gametes bear alleles in proportion to their frequ

ency in the population…– there are p sperm with M and q sperm with m– there are p eggs with M and q eggs with m


• What happens during reproduction?– Meiosis separates alleles into gametes– Gametes bear alleles in proportion to their

frequency in the population…– there are p sperm with M and q sperm with m– there are p eggs with M and q eggs with m– Eggs and sperm combine to form zygotes…


• What happens during reproduction?

EGGS

SPERM M (p = 0.1) m (q = 0.9)

M (p = 0.1) MM (p2 = 0.01) Mm (pq = 0.09)

m (q = 0.9) Mm (pq = 0.09) mm (q2 = 0.81)


• After reproduction:– Homozygous dominants = MM = p2 = 0.01– Heterozygotes = Mm = 2pq = 0.18– Homozygous recessives = mm = q2 = 0.81

– Melanics = 0.19 (0.01 + 0.18)– Typicals = 0.81


• After reproduction:– Homozygous dominants = MM = p2 = 0.01– Heterozygotes = Mm = 2pq = 0.18– Homozygous recessives = mm = q2 = 0.81

– Melanics = 0.19 (0.01 + 0.18)– Typicals = 0.81

– THIS IS WHAT WE STARTED WITH!!


• What is evolution? A change in gene frequency within a population


• What is evolution? A change in gene frequency within a population

• What processes lead to changes in gene frequency?


• processes that lead to changes in gene frequency:


• processes that lead to changes in gene frequency:– Mutation (the ultimate source of all genetic

variation)


• processes that lead to changes in gene frequency:– Mutation– Gene flow (usually accomplished by migration

of individuals from one population to another)


• processes that lead to changes in gene frequency:– Mutation– Gene flow – Non-random mating (inbreeding may increase t

he production of homozygotes with recessive alleles, which may be selected against)


• processes that lead to changes in gene frequency:– Mutation– Gene flow – Non-random mating– Selection (the most powerful agent of

evolutionary change)


• processes that lead to changes in gene frequency:– Mutation– Migration – Non-random mating– Selection– Genetic drift (changes in gene frequency in

small populations due to random sampling error)


• Genetic drift:– Changes in gene frequency in small populations

due to random sampling error


• Genetic drift:– Population bottlenecks - reduction of

population size results in loss of genetic variation and potentially in the loss of alleles from the population

– Reduction of population size must be catastrophic and non-selective


• Genetic drift: Population bottleneck


• Genetic drift:– The Founder Effect - a small population

disperses from a larger population, and founds a new population in another geographic location. The gene frequencies in the founding population are not representative of the larger population.

– Especially important in speciation on archipelagoes

population genetics and evolution what is evolution?

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