Mechanisms of evolution-I
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This document discusses the mechanisms of evolution, including natural selection and genetic variation. It explains how Darwin and Wallace proposed that evolution occurs through natural selection, where individuals with traits best suited to the environment leave more offspring, changing allele frequencies over time. The modern synthesis in the mid-1900s combined Mendelian genetics with Darwin's theory of evolution by natural selection. The document also introduces key concepts like fitness, gene pools, allele frequencies, and Hardy-Weinberg equilibrium.
Mechanisms of evolution-I
- 2. Last week.... Heredity • How traits are passed from one generation to the next • Considered individual organisms & families Today.... Mechanisms of Evolution • Changes in the frequency of traits over time • Causes of differences among populations in traits • Scaling up to populations
- 3. Origin of species & diversity of form Species are fixed • ~2.500 years ago • Aristotle & the ancient Greeks • Judeo-Christian culture • Earth is ~6,000 years old (very young!) Life evolves, descent from common ancestors • Buffon, mid-1700’s, earth older than 6,000 years • Lamarck, early 1800’s, Inheritance of acquired characteristics • Darwin & Wallace, 1858 • Descent with modification, or, natural selection Lamarck Wallace
- 4. 1. Individuals compete for resources 2. There is phenotypic variation among individuals 3. Some of this variation is heritable 4. Those individuals with traits best suited to current conditions leave more progeny than others (natural selection) *Leads to changes in allele frequencies over time, or, EVOLUTION Evolution by Natural Selection The Origin of Species - Darwin 1859 “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.”
- 5. • Overproduction & competition • Phenotypic variation • Some variation is heritable • Differential survival & reproduction *Leads to changes in allele frequencies over time
- 6. The Modern Synthesis • Mid-1900’s • Fusion of the work of Mendel & Darwin Mendelian genetics can help us understand the patterns of phenotypic variation observed by Darwin. Gregor Mendel Charles Darwin
- 7. Terminology Evolution – changes in allele frequencies over time Natural selection – a mechanism for evolution, differential fitness among phenotypes present in a given environment Fitness – survival & reproduction, the contribution an individual makes to the gene pool of the next generation Gene pool – all gametes produced by a population in a generation Population – a group of individuals of the same species living in the same area at the same time Allele frequency – # of particular allele/total # of alleles in a population Genotype frequency – # of particular genotype/total # of individuals in a population
- 8. The Equilibrium Population: Assumptions of Hardy-Weinberg • No mutation (no new genetic variation) • No genetic drift (infinitely large population) • No migration (no individuals entering or leaving the pop) • No selection (genotypes have equal fitness) • Random mating (dealing with a single population)
- 9. p2 +2pq + q2 = 1 Hardy-Weinberg Equilibrium p + q = 1 For a single locus with two alleles: • p = frequency of the recessive allele (a) • q = frequency of the dominant allele (A) • Allele & genotype frequencies sum to 1 within a population Genotypic and allelic frequencies are stable & predictable freq. aa = p2 , freq. AA = q2 , freq. Aa = 2pq
- 10. p2 +2pq + q2 = 1 Hardy-Weinberg Equilibrium p + q = 1 For a single locus with two alleles: • p = frequency of the recessive allele (a) • q = frequency of the dominant allele (A) • Allele & genotype frequencies sum to 1 within a population Genotypic and allelic frequencies are stable & predictable freq. aa = p2 , freq. AA = q2 , freq. Aa = 2pq
Editor's Notes
- #2: TL- marine iguana of the Galapagos Bottom-examples of camouflage as evolutionary adaptation, related mantid species L = leaf mantid in Costa Rica, C = flower mantid in Malaysia, R = trinidad tree mantid mimics dead leaves
- #3: Scaling up to talk about populations now – not just individuals Because how an individuals traits interact with the environment determines if they are selected for or against, But evolution can only occur in populations!
- #4: Before we can talk about what causes evolution, we first have to discuss what evolution is, where it came from. View persisted until early 1800’s when French naturalist Lamarck suggested that present species are similar but different from fossil forms and that evolution must have occurred, had his mechanism wrong though Until Darwin, Wallace and Mendel come along...
- #5: In 1859, Darwin publishes the Origin of Species with two bold statements Species on earth today descended from ancestral species this descent is accompanied by modification due to natural selection *So he proposes natural selection as the mechanism for descent with modification (evolution) 4 primary arguements to his reasoning.... Remember: selection happens to individuals, but evolution happens to populations!!!!
- #6: Natural selection does not create variation! It just works with what is there already NS favors characteristics for a certain time/place – what it favors changes as conditions change!
- #7: But Darwin had no understanding of genetics – this was filled in with Mendel’s work Mendel and Darwin were contemporaries (mid 1800’s) but their work wasn’t sythesized and appreciated until the mid 1900’s (decades later!) The fact that Mendel’s genetics nicely fit the hole in Darwin’s reasoning (which lacked an understanding of heritability of traits) meant we could use Mendel’s rules to understand the phenotypic variation among and within species that Darwin had observed. This is POPULATION GENETICS, and what we’ll spend the rest of this talk discussing.
- #8: Need some new terms.... Selection happens to individuals! Evolution happens to populations!!!
- #9: To explore the forces that changes allele frequencies over time, we’re going to start with an idealized population at equilibrium.... This means that the population is not experiencing any changes in allele frequ over time – which alleles the pop has and how rare/frequent they are is the same in every generation (no evolution) A population in this condition is said to be in Hardy-Weinberg equilibrium (named after two guys developed it in 1908) A population that is in H-W Equ (experiencing no evolution) meets these 5 assumptions
- #10: We can understand H-W by using what we learned of basic Mendelian genetics & probability Where do the p2, 2pq & q2 come from???? Recall 2 basic principles – 1) alleles at same locus segregate independently at meiosis and fuse randomly at fertilization, 2) alleles at different loci segregate independently