Transcript G. fortis
• For natural selection to work – traits must vary • Individuals & pop’ns do vary extensively – Beaks of G. fortis – Color spots on guppies • Existing variation of pop’n is subject to selection – Individuals live or die based on trait and environmental challenge – Adaptations may develop in pop’n – New species can evolve • Phenotypic variation in a population: • Appearance of trait = environment + allele combinations • For evolution to occur, variation in a trait must be partly genetic (nature rather than just nurture) – We can test the contribution of nature and nurture to a given trait. • Breeding animals and following offspring over time • Identical twin studies in humans • Common garden experiments in plants Simple traits vs. continuous traits • Simple traits = one gene with 2 alleles – If dominance, how many possible phenotypes? • Continuous traits (review pgs. 120-121) > 1 gene and many alleles determine the trait How many possible phenotypes? LOTS! • Most traits are continuous – Pop’n of individuals depicted as a hump – many intermediates, few extremes • Nat’l selection shapes existing variation in pop’ns • Individuals are selected, but populations evolve • What is a population? – Group of individuals belonging to the same species – Gene pool = collection of alleles – Evolution happens when allele frequencies change over time Ways to change allele frequencies & cause evolution: 1. Natural selection (and sexual selection) 2. Limit exchange of genes between populations (i.e. limit gene flow); may result in: a. Each separate pop’n may diverge over time, esp if reinforced by environmental differences (nat’l selex reinforces pop’n differences) b. Non-random mating may reinforce differences within pop’ns: Gene flow - tends to reduce genetic differences between populations Could have a + or - effect on overall fitness, depending on the situation 2b. Inbreeding is one form of non-random mating Inbreeding can occur in small populations. Small populations are formed in two ways: Founder effect Bottleneck Ways to change allele frequencies & cause evolution: 3. Random change b/c pop’n is small (genetic drift) Ways to change allele frequencies & cause evolution: 4. Mutation to DNA (deleted or duplicated base makes different protein) 1. Only source of new alleles of genes, introduces new variation 2. Mutations are random – can be bad, neutral or good 1. 3. 4. Non-lethal mutations are source of new, potentially beneficial proteins that can be selected for Mutations caused by environment are relatively common; but random 1. Human mutation rate ~ 10-8 (1 in 100 million) per nucleotide per generation, Average person has 1.6 new alleles created by mutation, and new combinations are created by crossing-over and independent assortment, bacteria 10-5 to 10-7 per gene per generation 2. where it happens in genome is random 3. Mutation must be present in gametes to be inherited Small mutations can produce enormous change in body plan and form Hox genes Evo Devo Chimps and humans genetically differ by 1-2%, can developmental switches generate different appearance? • Microevolution – change over short time and within same species. – Directional selection (1) – Stabilizing (balancing) selection (2) – Disruptive (diversifying) selection (3) • Macroevolution – large-scale changes that occur over a long time within a lineage – Speciation – new species arise. How? – Reproductive isolation must occur – Reproductive isolating mechanisms: genetic isolation (4), habitat isolation (5), behavioral isolation (6), temporal isolation (7), mechanical isolation (8), reduced hybrid fertility (9)