Transcript q 2 = 0.81
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 dominant (M) Population genetics and evolution • Some genetics terminology – Alleles may be dominant and recessive – In Biston betularia, the gene for melanism is dominant (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 dominant (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 population 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 frequency 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 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 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 the 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