13_Natural_Selection.pdf

Lecture 13: Natural Selection BIO 4083: Evolution Today’s Lecture Review of selection and fitness Overview of natural selection Examples of natural selection Repeated evolution as natural experiments How often does evolution repeat itself? Three main requirements for natural selection to occur: 1) V...

Lecture 13: Natural Selection BIO 4083: Evolution Today’s Lecture Review of selection and fitness Overview of natural selection Examples of natural selection Repeated evolution as natural experiments How often does evolution repeat itself? Three main requirements for natural selection to occur: 1) Variation 2) Heritability 3) Differences in fitness/survival Recall: Darwin’s and Wallace’s logic of natural selection Limited resources Variation of a trait Competition for resources Heritable variation Change in future generation Thomas Malthus (1766-1834) Violations of HardyWeinberg Recall: the concept of fitness Fitness (W): the survival and reproductive success of an individual with a particular phenotype or genotype Components of fitness: Survival to reproductive age Mating success Fecundity Relative fitness (ω): contribution of individuals with one genotype compared with the fitness of the fittest genotype in the population Recall: contribution of alleles to fitness Conceptually, fitness is a product of an organism’s entire phenotype, but this is difficult to assess Average excess of fitness: difference between relative contribution of individuals with one genotype and the average fitness of the population as a whole: Δp = p x (aA1/ϖ) Δp = change in allele frequency due to selection p = frequency of the A1 allele ϖ = average fitness of the population aA1 = average excess of fitness for the A1 allele Δp = p x (aA1/ϖ) Alleles that lower fitness experience negative selection aA1 < 0 Alleles that increase fitness experience positive selection aA1 > 0 Recall: selection in quantitative traits R = h2 x S Recall: selection in quantitative traits Breeder’s equation R = h2 x S Modes of selection Mutations are random Selection is not random Selection acts on existing genetic and/or phenotypic variation in a population Examples of Natural Selection Diversity in Darwin’s finches Diversity in Darwin’s finches 1) 2) 3) Variation Heritability Differences in fitness/survival Medium ground finch (Geospiza fortis) 1) Variation 2) Heritability 3) Differences in fitness/survival Variation in beak size influences efficiency at eating different types of seeds Rosemary Grant Peter Grant Medium ground finch (Geospiza fortis) 1) Variation 2) Heritability 3) Differences in fitness/survival Variation in beak size influences efficiency at eating different types of seeds Rosemary Grant Peter Grant Beak size evolution Beak size evolution Beak size is heritable 1) Variation 2) Heritability 3) Differences in fitness/survival Beak size evolution Severe drought resulted in only harder, woody seeds available to birds Result: larger-beaked birds favored Carvajal-Endara et al. 2019 Beak size evolution Severe drought resulted in only harder, woody seeds available to birds Result: larger-beaked birds favored 1) Variation 2) Heritability 3) Differences in fitness/survival Carvajal-Endara et al. 2019 Beak size evolution Severe drought resulted in only harder, woody seeds available to birds Result: larger-beaked birds favored 1) Variation 2) Heritability 3) Differences in fitness/survival Carvajal-Endara et al. 2019 What type of evolutionary response? Beak size evolution Severe drought resulted in only harder, woody seeds available to birds Result: larger-beaked birds favored Carvajal-Endara et al. 2019 Directional selection Natural selection can lead to rapid changes Natural selection is variable over time Coat color variation affects fitness in oldfield mice Hopi Hoekstra Coat color variation affects fitness in oldfield mice Hopi Hoekstra 1) Variation 2) Heritability Coat color variation affects fitness in oldfield mice 1) Variation 2) Heritability Clay models Hopi Hoekstra Coat color variation affects fitness in oldfield mice Hopi Hoekstra 1) Variation 2) Heritability 3) Differences in fitness/survival Natural selection of crypsis in feather lice 1) Variation 2) Heritability Painted lice Impaired preening ability Bush et al. 2019 Sarah Bush Scott Villa Dale Clayton 1) Variation 2) Heritability 3) Differences in fitness/survival Sarah Bush Scott Villa Dale Clayton More conspicuous lice were removed by pigeons Bush et al. 2019 Experimentallyinduced evolution toward lighter coloration in pigeon lice 4 years of experimental manipulation Bush et al. 2019 Rapid, climate-driven selection Campbell-Staton et al. 2017 Shane Campbell-Staton Anolis carolinensis Physiological response to cold winter temperatures in southern-most Anolis populations Campbell-Staton et al. 2017 Genetic differences between lizards before and after cold winter of 2013-2014 Campbell-Staton et al. 2017 Gall fly larvae induce plants to produce galls, an extended phenotype Extended phenotype: a structure constructed by an organism that can influence its performance or success. Although it is not part of the organism itself, its properties nevertheless reflect the genotype of each individual. Gall diameter is variable and heritable Pressure from predators and parasitoids leads to selection in opposite directions Intermediate size favored What type of selective response? Intermediate size favored Stabilizing selection Replicated changes as natural experiments Populations that independently experience parallel environmental changes represent replicated natural experiments Such conditions are ideal for evolutionary studies How quickly does selection act in a population? Remember: the Breeder’s Equation Eda signaling involved in production of variable lateral plates in stickleback High-Eda Low-Eda Marine ancestral form Freshwater derived form Low-Eda allele favored in freshwater populations Production of armor is energetically costly Little predation pressure to counterbalance Without predation pressure, low armor fish are more fit Low-Eda allele present at low frequency in marine environment Favored when introduced to freshwater Rapid loss of armor in Loberg Lake Repeated evolution of freshwater phenotype (low plate) over 50 years Lescak et al. 2015 Repeated evolution of Anole “ecomorphs” on different Caribbean islands Jonathan Losos Tom Dunne Repeated evolution of bird louse “ecomorphs” Johnson et al. 2012 Multicolored tanager (Chlorochrysa nitidissima) Adaptive radiation of African cichlid fish: repeated evolution of similar ecomorphs Brawand et al. 2014 Is evolution repeatable? Is evolution repeatable? Stephen Jay Gould No! Simon Conway Morris Yes! Assignments Activity on Friday Exam 2 on Friday, March 8th