Transcript Evolution
Evolution Chapters 13 & 14 DARWINISM AND DESCENT WITH MODIFICATION Chapter 13 Pre-Darwinian Thought • Aristotle – All members of a species identical – All species are the work of a Creator – Species never went extinct or changed • New species were additions by creator • George Cuvier – Studied strata and fossil record – Catastrophism caused most geological formations • Fossils were the extinct organisms that resulted • Repopulation from migration of nearby species Jean-Baptiste de Lamark • Proposed 1st mechanism for evolution – Remembered for being wrong • Principles – Use and disuse – Inheritance of acquired characteristics • Genetics now can refute – Phenotypes not affecting genotype aren’t inherited • Bonsai trees exemplify Charles Darwin • Round the world voyage on the H.M.S Beagle – Observed similarities and differences b/w organisms • Fossils and current species were similar • Same environments had different organisms • Different parts of world had similar organisms – Now known as convergent evolution – Wrote his works upon returning, but waited to publish • Father of evolution – Genetic change in an organism over time • Adaptation to better fit a habitat – He termed as descent with modification • Decades of backlash and denial Darwin’s Conclusions • Organisms have variations – Certain inherited traits provided for more offspring – Similarities and differences in organisms • Organisms struggle to exist – More individuals produced then can be supported • Influenced by work of Thomas Malthus resources limited population size • Organisms differ in fitness – Inherited characteristics allow unequal reproduction • Those favorable traits accumulate • Organisms become adapted – Enhanced ability to survive and reproduce Darwin’s Published Work • On the Origin of the Species by Means of Natural Selection – Descent with modification explains life’s unity & diversity – Natural selection matches organisms to their environment • Heritable traits make it more likely an organism will survive to reproduce • Exclusions – Avoided reference to humans – Term ‘evolution’ not mentioned • Relied on familiar examples of artificial selection • Explains succession of current organisms from ancestors • Alfred Russel Wallace published similar idea first though Artificial Selection at Work • All species Canis familiaris, but different breeds – Exemplary specimens of bree – Poor genetic specimens • Inbreeding causes small gene pools increase harmful alleles • Basset hound – Ear infections, arthritis, and back problems • Great Danes – Heart weakness and joint problems • Common vegetables from wild mustard Natural Selection at Work • Peter and Rosemary Grant’s finch work – Beak size changed with climate and seed abundance • Insects and pesticides – Initially kill 99% – Each successive application decreases in effectiveness • Resistant individuals survive to reproduce and pass on resistant allele • Peppered moth (first lecture) Key Points of Natural Selection • Individuals do not evolve • Only heritable traits are diminished or amplified – Not encoded by genes = acquired characteristics • Not goal directed – Is not a perfect process – Often trade-off of needs (mating colors/calls) • Editing, not a creative, mechanism Evolutionary Evidence • Artificial and natural selection • Fossils – Organisms trapped in sediment so can provide a fossil record based on layer of strata (rock) that they appear • Biogeography is the study of plant and animal distribution – Closely related species in same geographic region – Similar appearances in different parts of world due to ecological niche Evolutionary Evidence (cont.) • Comparative anatomy – Homologous structures have different functions, but are structurally similar due to common descent • Post-anal tail and pharyngeal slits in chordates, forearm bones in mammals – Vestigial organs such as appendix in humans, femur in some snakes • Molecular Biology – Homologous genes matching closely suggest recent relation – Universality of DNA, RNA, and genetic code Chapter 13 POPULATIONS Populations • Group of individuals, in the same place at same time • Can be isolated which prevents interbreeding and limits gene exchange – Uncommon now in humans, but mates generally chosen locally • Gene pool is total collection of genes in a population at one time – Includes all alleles in all individuals • Often more than 2 alleles per gene – Relative frequencies (proportion) change over generations = microevolution • Hardy-Weinberg equation to verify it isn’t Genetic Variation • Darwin and Mendel’s contribution – Contemporaries, ideas unified in 30’s and 40’s = broad acceptance • Alleles are key – Polygenic (height) or single (blood type) – Mutation produces new alleles – Sexual reproduction • Independent assortment, random fertilization, and crossing over • Necessary for evolution, but doesn’t guarantee – Allelic frequency constant w/o outside factors Evolutionary Change • Natural selection – Certain variations leave more offspring then others – Improves match between individual and environment • Phenotype exposed to environment changes genotype • Genetic drift – Chance events that alter allele frequencies • Gene flow (migration) – Transfer of alleles between populations Reduces differences between populations Modes of Natural Selection • Stabilizing: favors intermediate (birth weight); stable environment reduces phenotypic variation • Directional: acts against one extreme phenotype (peppered moths); with environmental change or migration • Disruptive: favors either extreme (less common); with varied environment Genetic Drift – Reduces variation through random loss • Founder effect – Isolated individuals form a new population • Bottleneck effect – Sudden event drastically reduces the population – Recovery can’t replace genetic diversity loss – E.g. Cheetahs • Two isolated populations remain – Skin grafts readily accepted b/w unrelated animals – Low sperm count • Recovery shows low variation levels – Can’t flourish, difficult to breed in captivity – Single environmental change could cause extinction Fitness and the Environment • Natural selection blends chance (random assortment) with sorting (allele preferences) – Enhances fit between organisms and environment • Environment changes faster than evolution can occur • Fitness is not ‘survival of the fittest’ – Survival doesn’t guarantee reproductive success – Organism’s contribution to next generation’s gene pool • Offspring must also survive to reproduce • Lead to trade offs (praying mantis) • Selection favors individuals with phenotypic traits that provide higher reproductive success than others Sexual Selection • Individudals with certain inherited characteristics are more likely than others to obtain mate • Creates sexual dimorphism, marked differences between the 2 sexes that don’t have direct effect on survival or reproduction – Include size, color, ornamentation, and behavior • Intrasexual selection (within same sex) – Males defends status through force or displays • Intersexual selection (between sexes) – Female choice depends on showiness of male • Not always beneficial, pose risks by making more visible = tradeoff – Females want mates with ‘good genes • Midshipman Fish – Males become singers or sneakers – Singing induces females to lay eggs in male’s nest – Female leaves and male resumes singing • Attract more mates – Sneakers hangout by singers • Sneak in to fertilize eggs • Resemble females Natural Selection Isn’t Perfect • Can only act on existing variations – Aren’t always ideal • Evolution limited by ancestry – Doesn’t scrap existing structure – Adapts/modifies to something new • Adaptations often compromises • Interaction of chance, natural selection, and the environment – Change not necessarily best fit environment initially • Species evolve to be ‘better than’ Chapter 14 ORIGINATION OF SPECIES Speciation • Emergence of a new species – Biological Species Concept • Species are organisms that can potentially breed and produce fertile offspring • Reproductive isolation maintains species separation – Two groups can’t interbreed which prevents gene flow Reproductive Barriers • Biological features of an organism that can prevent interbreeding and maintain a species • Prezygotic barriers (prevents mating or fertilization between species) • Postzygotic barriers (affects after hybrid zygote forms)