460 likes | 568 Views
The Mechanisms of Evolution. Life’s History: Seen in Fossils & Relics. Broad patterns Change through time communities change habitats change Each of Earth’s biotas replaced a similar, but distinct biota. Modern Life: Seen by Direct Observation.
E N D
Life’s History: Seen in Fossils & Relics • Broad patterns • Change through time • communities change • habitats change • Each of Earth’s biotas replaced a similar, but distinct biota
Modern Life: Seen by Direct Observation • Modern life exhibits characteristic patterns • species are variable • in the short term • species are stable • environments are stable
Biological Evolution • encompasses the changes in Earth’s biotas • detected in remnants of the changes found in the modern biota
The Contribution of Charles Darwin • Darwin proposed a mechanism by which evolution may have occurred • based on observations in South America • SA flora & fauna differed from European • temperate SA forms resembled tropical SA forms more than temperate European forms
The Contribution of Charles Darwin • Darwin proposed a mechanism by which evolution may have occurred • based on observations in South America, especially the Galápagos Islands • ~600 miles west of Ecuador • each with dramatically different conditions and communities
Darwin’s map Darwin’s Travels Figure 23.1
The Contribution of Charles Darwin • Darwin observed stable, variable populations • each possessed adaptations (n) to its environment • Darwin imagined the different island populations came from a founding population • populations underwent adaptation (v) and now thrive under different conditions
The Contribution of Charles Darwin • In 1859 Darwin proposed a mechanism by which adaptation may have occurred • species change over time (are not immutable) • some changes enable species to more effectively inhabit their environments • adaptive changes occur by natural selection
The Concept of Natural Selection • artificial selection of domesticated species mimics natural selection • artificial selection • breeders retain desirable individuals and remove undesirable individuals
The Concept of Natural Selection • artificial selection of domesticated species mimics natural selection • natural selection occurs • when some individuals produce more offspring than other individuals • because more individuals are produced than the environment can support • those best-suited to the conditions survive & reproduce, others don’t
The Concept of Natural Selection • natural selection is a conservative process • in a stable environment, “average” individuals will survive and reproduce • in a changing environment changes, those best-suited to new conditions will survive and reproduce
The Contribution of Charles Darwin • Darwin did not know the mechanisms of heredity
Evolution: change in the genetic composition of a population over time • evolution is population-based • phenotypic variation in a population is due to genotypic differences in individuals • evolution results from differential success of individuals with different heritable phenotypes
Evolution: change in the genetic composition of a population over time • at one genetic locus • an individual has two alleles • a population may have many alleles • the sum of all alleles for all loci in a population is its gene pool
a population’s gene pool for the X locus:Figure 23.3allele frequenciesX1 = 0.2X2 = 0.5X3 = 0.3
a population’s gene pool for the X locus:Figure 23.3genotypefrequenciesX1X1 = 0.1 X1X2 = 0.1 X1X3 = 0.1 X2X2 = 0.3 X2X3 = 0.3 X3X3 = 0.1
Evolution: change in the genetic composition of a population over time • a population’s genetic structure • allele frequencies • genotype frequencies
Evolution: change in the genetic composition of a population over time • a genotype’s or phenotype’srelative contribution to the next generation = fitness • depends on thesurvivaland reproductive success of individuals with it
the mathematics of population genetics • for a population with only two alleles, A & a, at a locus • the frequency of allele A is p and • the frequency of a is q = 1- p • allele frequencies can be calculated from genotype frequencies p = (2NAA + NAa)/2N and q = (2Naa + NAa)/2N
the mathematics of population genetics • equal allele frequencies do not imply equal genotype frequencie • Figure 23.6
the mathematics of population genetics • undisrupted, a population’s genetic structure remains the same over time
undisrupted, a population’s genetic structure remains the same over timeFigure 23.7 p = 0.55 p = 0.55 q = 0.45 q = 0.45
Hardy and Weinberg did the math • a population in Hardy-Weinberg equilibrium • has allele frequencies p & q • has genotype frequencies p2, q2 and 2pq and • succeeding generations will have the same genetic structure
Hardy and Weinberg did the math • Hardy-Weinberg equilibrium requires • random mating • a large population size • no migration • negligible mutation • stabilizing natural selection
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • mutation • spontaneous, random changes • usually detrimental or neutral • may be pre-adaptive • natural rates are very low • rates of accumulation vary
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • gene flow • migration incorporates new alleles or changes allele frequencies • migration is typical among populations of the same species
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • random genetic drift: • chance events that alter allele frequencies • most likely in small populations • bottlenecks shrink populations abruptly • the founder effect occurs when a small sub-population is displaced
founder effect occurs when a smallsub-population isdisplacedFigure 23.10
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • random genetic drift: • chance events that alter allele frequencies • bottlenecks & the founder effect produce low allelic variation compared to the parent population
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • assortative mating • one genotype prefers another genotype • results in changed genotype frequencies
Hardy and Weinberg did the math • a population in Hardy-Weinberg equilibrium • has allele frequencies p & q • has genotype frequencies p2, q2 and 2pq and • succeeding generations with have the same genetic structure IF…
Hardy and Weinberg did the math • Hardy-Weinberg equilibrium requires • random mating • a large population size • no migration • negligible mutation • stabilizing natural selection
Hardy-Weinberg agents of evolution • changes in a population’s genetic structure occur because of agents of evolution • natural selection • enhanced reproductive success by individuals with particular genotypes • may lead to a change in allele frequency • leads to adaptation (v.)
Natural Selection May Have Different Effects Under Different CircumstancesFigure 23.12
two food sources that differ significantly in hardness produce a bimodal distribution in beak sizesFigure 23.15