population genetics and evolution what is evolution?

Population genetics and evolution

• What is evolution?

Population genetics and evolution

• What is evolution?– Descent with modification

Population genetics and evolution

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

over time

Population genetics and evolution

• 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

Population genetics and evolution

• Some genetics terminology– Chromosomes…

Population genetics and evolution

Population genetics and evolution

• Some genetics terminology– Chromosomes…– Homologous chromosomes…

Population genetics and evolution

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

Population genetics and evolution

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

Population genetics and evolution

• Some genetics terminology– Alleles may be dominant and recessive

Population genetics and evolution

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

minant (M)

Population genetics and evolution

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

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

Population genetics and evolution

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

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

Population genetics and evolution

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

locus in a population = 100% or 1.0

Population genetics and evolution

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

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

Population genetics and evolution

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

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– p + q = 1.0

Population genetics and evolution

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

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– p + q = 1.0 – With sexual reproduction, p + q alleles in the

eggs are combined with p + q alleles in the sperm

Population genetics and evolution

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

Population genetics and evolution

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

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

Population genetics and evolution

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

– 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

Population genetics and evolution

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

– 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…

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?

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)

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?

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 (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

Population genetics and evolution

• To summarize:– Homozygous dominants = MM = p2

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

Population genetics and evolution

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

Population genetics and evolution

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

frequency in the population…

Population genetics and evolution

• 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

Population genetics and evolution

• 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

Population genetics and evolution

• 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…

Population genetics and evolution

• 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)

Population genetics and evolution

• 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

Population genetics and evolution

• 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!!

Population genetics and evolution

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

Population genetics and evolution

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

• What processes lead to changes in gene frequency?

Population genetics and evolution

• processes that lead to changes in gene frequency:

Population genetics and evolution

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

variation)

Population genetics and evolution

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

of individuals from one population to another)

Population genetics and evolution

Population genetics and evolution

• 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)

Population genetics and evolution

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

evolutionary change)

Population genetics and evolution

• 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)

Population genetics and evolution

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

due to random sampling error

Population genetics and evolution

• 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

Population genetics and evolution

• Genetic drift: Population bottleneck

Population genetics and evolution

• 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