How Populations EvolveClass DiscussionIn 17th, 18th century Europe—almost everyone believed in Creationism and that the earth was 6,000 years old (which was worked out as an estimate by Archbishop James Ussher in 1650). I. Natural Selection (NS) a.k.a “Descent with modification” A. Observations by Darwin:1. Species tend to: over reproduce2. Individuals in a population: varyB. Conclusion – the varied traits that an individual inherits affects this individual’s ability to:Secure limited resources to better survive and reproduceTraits that are “better” than others for a particular environment become: More prevalent in the populationC. Natural Selection (NS) = differential reproductive success of organisms. In other words, individuals whose characteristics best adapt them to their CURRENT environment are most likely to survive and reproduce. Successful or “fit” individual = an individual that is well adapted to their environment, able to pass on their genesD. Three key points about NS1. NS is an editing process, not a creative process. What does this REALLY mean?2. NS is contingent on: time and location3. Significant evolutionary change can occur in a short time. 1II. Population Genetics Charles Darwin – In 1859, published theory of NS, without knowing about inherited traits/genes (Gregor Mendel – 1860’s his work was not recognized until 40 years after the Origin of Species was published). Darwin, therefore, did not understand WHERE the variation of individuals came from. Interestingly, Darwin’s theory was attacked because of the lack of understanding of genes—it was argued that if two animals were blended (like horse and zebra) traits would be diluted and offspring would never have clear advantages.) Darwin was so unsure that he even stopped arguing his theory!A. = Modern synthesis (1940’s)- The ideas of Darwin and Mendel together. How is it different from Darwin’s theory of NS?1. It recognizes 4 processes of evolution, not just natural selection (gene flow, mutation, genetic drift)2. It recognizes that “traits” = inherited genesB. NS acts on individuals but the effect is a change in the population C. population: group of organisms of the same species living at the same time/place D. Microevolution – a change in allele frequency over timeIII. Hardy-Weinberg model of a non-evolving population (a.k.a a population that’s in genetic equilibrium)A. Do non-evolving populations exist? Purely theoreticalWhat 5 conditions would have to be met? Large population, no mutations can occur, no immigrationand emigration, random mating, reproductive success (no natural selection).Mutations?Size?Immigration/Emmigration?Mating?Reproductive success?B. Hardy-Weinberg Principle (1908): sexual reproduction alone does not cause or lead to evolution2Butterfly example: an imaginary butterfly population is in genetic equilibrium (a.k.a. “Hardy-Weinberg equilibrium”). 1,000 individualsPopulation: 490 A/A –dark blue (490, .49, 49%) 420 A/a- medium blue (420, .42, 42%) 90a/a- white (90, .09, 9%)What are the individual genotype frequencies in the population?A/A- 49%A/a- 42%a/a- 9%What are the allele frequencies in the population? Total Alleles – 2,000A = 1400/2000 = 70%a = 600/2000 = 30% What will the individual genotype frequencies be in this population in the next generation if nothingother than sexual reproduction is occurring?A(.7) a (.3)A (.7) AA(.49) Aa (.21)a (.3) Aa (.21) aa (.09)C. HW equations to know: p = dominant trait; q = recessive trait 1. p+q=1 (use to find allele frequencies)2. p^2 + 2pq + q^2 = 1D. Try an example problem:An imaginary blue-footed booby population in the Galapagos Islands is in Hardy-Weinberg equilibrium. The frequency of the recessive allele for webbed feet is 0.4. What is the frequency of individuals that have non-webbed feet?q= .4; p=1-q=.6; punnet square .84 for dominant trait p^2 =2pq= .84 (non-webbed!) 3The Hardy-Weinberg principle assumes that only sexual reproduction occurs in a population and that no other forces are at play…resulting in genetic equilibrium and no evolution of a population. THIS IS NOT REALITY!!! ALL POPULATIONS ARE EVOLVING. So, what types of forces lead to microevolution (i.e. changes in allele frequency)?Working in groups of three, use the words below to compose a sentence that explains your view of co-evolution– You can use additional words– You can duplicate words– Don’t need to use all the words provided– Make a list of the unused wordsCo-evolution Frequency Need Selective forceTry Design Intend DecideSpecies Evolve Seek ChangeAllele Response Want AdaptEvolution DemandSentence:4The Origin of Species Class DiscussionI. Concepts of SpeciesA. Ernst Mayr’s Biological Species Concept: a group of populations whose individuals have the potential to interbreed.B. Alternative definitions are needed. Why? Fossils and prokaryotes. We cannot know how fossils for example reproduce. Prokaryotes have asexual reproduction. C. Reproductive Barriers – why can’t related species mate? List some ideas:1. Five pre-zygotic Barriers and examples:a. behavioral – different courtships prevent matingb. habitat – species live in different placesc. temporal – breeding occurs at different time of year or dayd. gemetic isolation – when male and female gametes fail to unitee. mechanical isolation – differences in physical structures prevents successful mating2. Three post-zygotic Barriers and examples:a. reduced hybrid fertility – hybrids fail to reproduce functional gametesb. reduced hybrid viability – the development or survival of hybrids is impairedc. hybrid breakdown – offspring of a hybrid is weak5II. How do reproductive barriers form?Form when gene flow is blocked between two or more populations. Within each population, naturalselection, mutation, and genetic drift can occur.A. Speciation-1. Two species from one ancestral species = ancestor >Divergence2. A burst of divergences from a single ancestor that leads to many new species each adapted to a new habitat or using new resources = ancestor >>Adaptive radiationB. Allopatric SpeciationGeographical barrier impeding gene flowC. Sympatric Speciation1. Animals – new species arise within the same geographic location as the parent species; rare2. Plants- can become reproductively isolated in one generation due to: polyploidyEx: A 2n plant (“species 1”): if nondisjunction of all chromosomes occurs in meiosis
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