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Transcript
Darwin’s Second Idea – Natural Selection • Evolution: controversial? • Natural selection – Darwin’s postulates – Controversial? – Two examples of NS Darwin’s Ideas 1. Species evolve over time: “descent with modification” Pattern Charles Darwin Darwin’s Ideas 1. Species evolve over time: “descent with modification” Charles Darwin – Darwin compiled abundant data – Accepted over 100 years ago Still Controversial? “...some scientists doubt the idea that all organisms have evolved from a single common ancestor.” -- Steve Meyer & John Campbell* San Francisco Chronicle, December 10, 2004 (after all these years) *Discovery Institute, Seattle, WA % who think evolution is a scientific theory well supported by the evidence This class 68% Postgrad Ed 65% 52% College Grad High School 20% Darwin’s Ideas “a naturalists…might come to the conclusion that each species … had descended…from other species. Nevertheless, such a conclusion… would be unsatisfactory until it could be shown how the innumerable species inhabiting this world have been modified….” Darwin’s Ideas 1. Species evolve over time: “descent with modification” 2. Adaptations arise by natural selection Charles Darwin – Major evolutionary mechanism. – Controversial until Modern Synthesis (1940s). Adaptation Traits that increases fitness of an individual (ability to survive or reproduce) compared to individuals without the trait NS explains: • The shape of life • • • • • Chisel Bill Rotated toe Stiffened tail Padded brain Long tongue Natural selection: 4 Postulates variability heritability “competition” Fitness differences Freq. of long necks increases 1. Variability • Discrete • Continuous Phenotype The properties of an organism (morphology, behavior, physiology, etc.) Variability – Size, color • Continuous Frequency • Discrete – Size Size 2. Heritability Genotype Alleles of an individual inherited from parents Phenotype The properties of an organism (morphology, behavior, physiology, etc.) Heritability Genetic basis: Genotype The set of alleles possessed by an individual • Single locus: – discrete variation • Polygenic: – continuous variation Heritability • The proportion of phenotypic variation due to alleles inherited from parents Genotype Environment Phenotype 3. “Competition” • More individuals are born than can survive – “Struggle for existence” • Differential reproductive success between individuals – Survival,reproduction • Relationship between phenotype and fitness Selection for largersize size Selection for intermediate Fitness • Fitness varies among phenotypes Frequency 4. Fitness nonrandom Size Still controversial? (after all these years) • Given 4 postulates, natural selection is inevitable • Accepted since the Modern Synthesis of genetics and evolutionary theory Artificial Selection Still controversial? (after all these years) “Intelligent design holds that certain features of the universe and of living things are best explained by an intelligent cause, not by an undirected process such as natural selection.” Phillip Johnson Michael Behe Darwin’s Finches • Galapagos Islands • 15 species • Differences in beak size, shape Darwin’s Finches Can natural selection account for adaptation of beak size? • Peter and Rosemary Grant • Medium ground finch • Study began in 1973 1. Variability: beak depth • Continuous variation, a quantitative trait, many loci • 2. Heritability – Parent-offspring resemblance 3. “Competition”: during drought – Fewer seeds, larger seeds – Not all survive Key Point 1 • Natural selection: not forward looking • Many selective deaths: lacked the right bill for the drought conditions 4. Non-random success Key Point 2 • Natural selection is not a random, chance, undirected process • Reproductive success is associated a nonrandom set of trait values Darwin’s Finches Adaptation via natural selection occurred! Examples of Selection: 2 Flower color in Snapdragons: (Jones & Reithel 2001) • • Discrete character variation single locus genetic control: S locus – SS homozygote & Ss heterozygote are white, ss homozygote is yellow. Example of Selection Snapdragon example: Variation & Heritability Fig. 3.3 Example of Selection Snapdragon example: Competition & Non-random fitness Fig. 3.3 Example of Selection Snapdragon example: Evolution occurred! Frequency of S allele Before: 0.50 After: 0.57 Fig. 3.3 Key point 3 • Individuals experience natural selection • Evolutionary change occurs in the population