Populations. Population genetics. Modern Evolutionary Synthesis. Outline. Blending Inheritance Example. Lesson: Evolution in more detail

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1 Population genetics Populations Group of individuals of the same species in a given area All of the genes in a population = their gene pool All alleles at all loci Evolution in more detail Outline 1. Modern Evolutionary Synthesis 2. Individuals live or die, but POPULATIONS evolve 3. Mechanisms of Evolution Modern Evolutionary Synthesis In Darwin s time, only one person had figured out how inheritance works Mendel s work was not widely known Darwin knew less about the mechanisms of genetics than you do! This gap caused difficulties for understanding Evolution by N.S. In particular, if you assume blending inheritance, N.S. seems not to work.?!!! Blending Inheritance Example Large population of 20mph bunnies Lesson: If inheritance happens by blending, rare variants will get diluted over time - useful variation cannot be preserved for long. 1

2 Particulate (Mendelian) inheritance 30mph bunny x 20mph bunny Lesson 30 mph bunnies can become more common if they survive or reproduce better (without losing speed through blending). Mendelian inheritance allows N.S. to work. What happens if rare variant is due to a new allele? Common allele: b 20 Rare allele: b 30 (assume it s dominant for this example) Now you re talking! Modern Evolutionary Synthesis or Neodarwinism Modern Evolutionary Synthesis or Neodarwinism, summarized Darwin gathered extensive evidence on geographical patterns of biodiversity, on variation in domestic and wild animals, and on artificial selection. But it s a big planet out there. In the 50 years after the publication of the Origin of Species, many paleontologists, biogeographers, population geneticists and others made more intensive studies. Those studies, together with the rediscovery of Mendel s papers on genetics, led to a new, more complete understanding of evolution Darwin, Wallace (Natural Selection) + Mendel (Genetics) + Wright, Fisher (Population Genetics) + Mayr, others (Biogeography, Paleontology) = Modern Evolutionary Synthesis - a more complete understanding of how evolution proceeds in the wild. Macro- vs. Micro- Evolution Macroevolution - change that results in the formation of new species, new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction. Microevolution: the generation-togeneration change in the frequency of alleles in a population Evolution at its smallest scale The core definition of evolution Ë Microevolutionary changes give rise to macroevolution Mechanisms of Evolution: How do allele frequencies change? I am convinced that Natural Selection has been the most important, but not the exclusive, means of modification - Charles Darwin, On the Origin of Species 2

3 Mechanisms of Evolution Why does the allele frequency in a population change? Genetic drift Natural selection Gene flow Mutation Mech. of Evol. (1) Genetic Drift Changes in a population s allele frequencies due to chance Is genetic drift a form of evolution? Drift can cause rapid change in small populations A problem when populations get too small Related issues with small populations Bottleneck effect Founder effect Bottleneck effect Survivors after large reductions in populations do not represent the gene pool as well as before the reduction Reduced genetic variability + drift lead to changes in allele frequencies that are not necessarily adaptive Fig Founder effect When a few individuals colonize a new area, they represent the entire gene pool Reduced genetic variability + drift lead to changes in allele frequencies that are not necessarily adaptive Cheetah Polydactyly due to Ellis-van Creveld syndrome Mech. Of Evol. (2) Natural Selection Natural selection: differential success in survival and reproduction N.S. leads to populations becoming better adapted to their environments (adaptation) N.S. works most powerfully in large populations (effect of drift is small, and changes due to N.S. are preserved) Fitness No, not the body-builder kind Darwinian fitness: an individual s contribution to the gene pool of the next generation compared to the contribution by others This is what biologists really mean when they talk about survival of the fittest 3

4 Natural selection acts on The population? The individual? The phenotype? The genotype? Modes of selection Three ways that natural selection affects the frequency of heritable traits Directional selection Diversifying (disruptive) selection Stabilizing selection Modes of selection (2) It is Possible to Measure Natural Selection in the Wild. Fig Darwin s term Sexual selection a form of Natural Selection Why is there sexual dimorphism? If natural selection is so powerful, how come we re not perfect already? Evolution is limited by history or ancestry Adaptations are often compromises Not all evolution is adaptive Selection can only edit existing variation 4

5 Mechanisms of Evol. (3): Mutation Mech. Of Evol. (4): Gene Flow Mutations are the origin of all differences between alleles But mutations are rare So mutations must still spread by drift or selection if they are going to impact allele frequencies in a population Sickle cell anemia is the result of a single point mutation Migration of individuals can change allele frequencies in a population Mechanisms of Evolution: Summary Drift and Natural Selection seem to be the most powerful forces in natural populations Drift is important in very small populations Natural Selection is important in all other populations, and is especially powerful over long time periods. Hardy-Weinberg theorem What would the gene pool (and thus the gene frequency) of a NON-evolving population look like? In 1908, Hardy and Weinberg independently developed this idea The frequencies of alleles in gene pool remain constant over generations unless they are acted upon by things other than segregation in meiosis and random fertilization In a state of Hardy-Weinberg equilibrium Allele frequency in a gene pool Evolution is measured as changes in gene frequency Allele frequency in a non-evolving gene pool Using the same example as before, what do the offspring look like? Fig. 23.3a Fig. 23.3b 5

6 Hardy-Weinberg Equation p 2 + 2pq + q 2 = 1 p = frequency of allele 1 q = frequency of allele 2 Using this equation, you can calculate frequencies of alleles in a non-evolving gene pool Hardy-Weinberg assumptions Very large population size No migration No mutations Random mating No natural selection How realistic are these conditions? Assumptions of HWE 1. Large population size 2. No migration 3. No mutation 4. Random mating 5. No selection Deviation from HW assumptions usually means allele frequencies are changing between generations - Evolution is happening HWE is more interesting when absent! 6