Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Sexual selection wikipedia , lookup
Hologenome theory of evolution wikipedia , lookup
The Selfish Gene wikipedia , lookup
Microbial cooperation wikipedia , lookup
Gene expression programming wikipedia , lookup
Natural selection wikipedia , lookup
Saltation (biology) wikipedia , lookup
Genetics and the Origin of Species wikipedia , lookup
The eclipse of Darwinism wikipedia , lookup
Population Genetics Reconciling Darwin & Mendel Darwin • Darwin’s main idea (evolution), was accepted • But not the mechanism (natural selection) – Scientists did not understand Darwin’s mechanism because there was no understanding of genetics • Even once scientists grasped Mendel, genetics was viewed as an either/or – didn’t understand many traits are polygenic • So how do you get the variation on which selection works? Ideas About Evolution • Orthogenesis – 1920’s – saw evolution as a predictable progression to more & more elite forms of life • Population Genetics – 1930’s – reconciled Darwin & Mendel Genetics of Populations • Population – a localized group of individuals belonging to same species – The definition of a species not always clear • Gene pool = The total genes in a population • Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution Genetics of a Non-evolving Population • The gene pool is in stasis • This is described by Hardy-Weinberg Theorem: • The frequencies of alleles in a population’s gene pool remain constant over the generations unless acted on by agents other than sexual recombination • i.e. shuffling the deck has no effect on the overall genetic make-up of the population The Hardy-Weinberg Theorem • Example • In pink flowers (A), is dominant over white flowers (a) • 2 alleles for at this locus • Sample 500 plants: • 20 white flowers (aa) • 480 pink [320 (AA); 160 (Aa)] • Therefore there are 1000 genes for flower color in the population • Example (Continued) • The dominant allele accounts for 800 of these: – [(320 x 2) + (160 x 1)] • Therefore: – the frequency of A in the population = 80% – the frequency of a = 20% Predicting Change • How will genetic recombination during sexual reproduction affect the frequencies in the next generation? • If mating is random: – the probability of picking 2 AA = (0.8 x 0.8) = .64 – the probability of picking 2 aa = (0.2 x 0.2) = .04 – and of heterozygotes = 2(0.8 x 0.2) = 0.32 • There are 2 heterozygote combinations: aA & Aa • sperm or egg Hardy-Weinberg Equilibrium • This shows that the alleles are present in the gene pool in the same frequencies as they were in the previous generation A: [0.64 + (0.32 2)] = 0.8 a: [0.04 + (0.32 2)] = 0.2 • The gene pool is at equilibrium • This is called Hardy-Weinberg equilibrium The Hardy-Weinberg Equation • This example is the simplest case: – 2 alleles, one is dominant • For this case: – if p = frequency of one allele – q the frequency of the other • Then: p + q = 1 probability of AA = p2 probability of aa = q2 probability of Aa = 2pq • Therefore: p2 + 2pq + q2 = 1 Uses of Hardy-Weinberg • Thus you can calculate the frequency of a gene in a population if you know the frequency of the genotypes • This is important in genetic disease counseling Relevance to Evolution • A population at genetic equilibrium does not evolve • Hardy-Weinberg tells us what to expect in non-evolving populations • Therefore it is a baseline for comparing actual populations where gene pools may be changing. • Can determine if the population is evolving Genetic Equilibrium • Hardy-Weinberg equilibrium is maintained only if the population meets all 5 of the following criteria: – Very large population size – Isolation from other populations • migration can effect the gene pool – No net mutations – Random matings – No natural selection • no difference in reproductive success) • Describes an ideal that never exists in nature Altering Genetic Equilibrium • For evolution to take place something must upset the genetic equilibrium of the population: • Factors that change genetic equilibrium are: – Genetic drift – Migration (Gene flow) – Non-randon mating (Isolation) – Mutation – Natural selection Genetic Drift • Changes in gene frequency of a very small population due to chance • Controlled by the laws of probability & chance • Bottleneck effect – Chance sampling error due to small population • Founder’s effect – a few individuals colonize a remote spot – causes drift Illustrating Genetic Drift The Bottleneck Effect Gene Flow (Migration) • Movement of organisms into or out of a population • Takes their genes out of the gene pool • Most populations are not completely closed – gain & lose alleles Non-random Mating • More apt to mate with close neighbors • Promotes inbreeding • Assortive mating – seek mate like self (i.e. size) Isolation Mutation • A change in a gene • An alteration of DNA • The original source of variation • Raw material on which natural selection works Natural Selection • If one type produces more offspring than another, upsets the balance of equilibrium • There are three types of natural selection: – Stabilizing Selection – Disruptive Selection – Directional Selection