Evolution – Take 2 Why do monarch butterflies have orange and black wings? • Proximate answer: “Because of special pigments in their wings” (functional process) • Ultimate answer: “Because bright colors provide an advantage by advertising toxicity” (evolutionary process) Genetics review Individuals have 2 alleles for each gene Variation in phenotype can be due to genes AND environment: A population has a frequency of alelles and genotypes Hardy Weinberg Equation • Original proportions of genotypes in a population will remain constant from generation to generation Hardy Weinberg Equilibrium • Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2 • p = individuals homozygous for first allele • 2pq = individuals heterozygous for alleles • q = individuals homozygous for second allele Agents of evolutionary change Mutations • Mutations are random and spontaneous • Provide raw material for evolutionary change Gene flow • Introduces new alleles into a population or changes allele frequencies • Decreases differences between populations and usually counteracts natural selection Small population size • Genetic drift – Change in a gene pool happens by chance due to small size • Founder effect – Small population not representative of original population • Population bottleneck – Restriction in population size Non-random mating • Decreases the frequency of the heterozygote • Inbreeding and self-pollination are examples Inbreeding • Nonrandom mating between related individuals leading to increased homozygosity and a lack of genetic variation • Can lower fitness when deleterious recessive alleles are expressed Ellis-van Creveld Syndrome Inbreeding Evolution = Change in allelle frequencies over time What are the necessary conditions for evolution due to natural selection to occur? Evolution by natural selection happens… • … and leads to adaptation! Phenotypic variation due to environment Ecotypes and common garden experiments Species evolve together Speciation • Process of creating new species or “kind” of organism Species are created through genetic divergence • Gene flow reduced in isolated populations • Gradual accumulation of differences in the gene pools of populations through natural selection, genetic drift, and mutation • Reproductive isolation occurs as a by-product of genetic change Genetic Divergence Biological species concept • Species are groups of interbreeding natural populations that are reproductively isolated from other such groups – Ernst Mayr What is a Species? • A species is one or more populations of individuals that … – Interbreed under natural conditions – Produce fertile offspring – Are reproductively isolated from other such populations Separate species may hybridize Mechanisms of Speciation – Allopatric speciation – Sympatric speciation – Parapatric speciation Mechanisms of Speciation Allopatric Speciation • Populations are first geographically isolated, then become reproductively isolated • Species separated by geographic barriers will diverge genetically • If divergence is great enough it will prevent inbreeding even if the barrier later disappears • Effectiveness of barrier varies with species Geographic Isolation • With the members (and their gene pools) isolated from one another, they become TWO UNIQUE populations Mechanisms of Speciation Parapatric Speciation Adjacent populations evolve into distinct species while maintaining contact along a common border Mechanisms of Speciation Sympatric speciation • Speciation by polyploidy is a common event in plants • Polyploids have more than two sets of chromosomes Speciation by Polyploidy • Change in chromosome number (3n, 4n, etc.) • Offspring with altered chromosome number cannot breed with parent population – In autopolyploids, offspring have a doubling of chromosome number from parents – Allopolyploids are interspecific hybrids Evolution of wheat Sterile hybrids Rates of speciation • Gradual model – Species emerge through many small morphological changes that accumulate over a long time period • Punctuation model – Most changes in morphology are compressed into brief period near onset of divergence – Adaptive radiation Evolutionary Trees