Mechanisms of Evolution and Speciation

Mechanisms of Evolution and SpeciationObjectivesRecognize mutation and sexual reproduction are the underlying causes of all genetic variability within organismsIdentify natural selection, genetic drift, and gene flow as the mechanisms of divergent evolution Distinguish the differences between these three mechanisms by knowing how each operatesRecognize reproductive isolation has an important role in the formation of new speciesCompare and contrast allopatric and sympatric speciation, identifying potential factors in each that may contribute to reproductive isolation among new species ReviewGenetic variation is due to: 1) Mutation 2) Sexual reproduction

This heritable variation is the underlying factor that makes microevolution possibleChanges in Allelic Frequencies of Populations Can Lead to Evolutionary DivergenceThree mechanisms can lead to microevolution:

Natural SelectionGenetic DriftGene FlowNatural SelectionSelective pressure that promotes survival of organisms with heritable trait(s) that provide an advantageOver time, advantageous trait(s) is/are passed on, potentially leading to greater relative fitnessResult is selection of those most well-suited to their environment so that they can better survive (obtain food, evade predators, etc.), reproduce (acquire mates, attract pollinators, etc.), and pass on beneficial allelesModeling Natural Selection

Number cube = predator*Dots = fish (yellow, red, and blue)Square of colored paper = habitat background

* - Rolling a 1 represents predator eating a blue fish; rolling a 2 or 3 represents predator eating a red fish; rolling a4, 5, or 6 represents predator eating a yellow fishBefore the Predators Strike

Fifteen total fish in the population five of each color (blue, red, and yellow)After Five Predators Strike

Predator 1 (roll of 3) red fish eatenPredator 2 (roll of 6) yellow fish eatenPredator 3 (roll of 5) yellow fish eatenPredator 4 (roll of 1) blue fish eatenPredator 5 (roll of 2) red fish eaten

10 fish remain 4 blue (40%), 3 red (30%), & 3 yellow (30%) After another Five Predators Attack

Predator 6 (roll of 2) red fish eatenPredator 7 (roll of 4) yellow fish eatenPredator 8 (roll of 3) red fish eatenPredator 9 (roll of 2) red fish eatenPredator 10 (roll of 5) yellow fish eaten

5 fish remain 4 blue (80%) & 1 yellow (20%)Natural Selection at Work:

Ok, were no longer eating cows!Survival of the CutestGenetic DriftOperates on chance events causing unpredictable fluctuations in allele frequencyHas a greater impact on smaller populations Two examples of how genetic drift can significantly affect a population include: bottleneck effect (catastrophic reduction) and founder effect (colonization event)

Bottleneck EffectAllele frequency of skittle gene before catastrophic event:Strawberry allele = 34/66 (52%)Grape allele = 22/66 (33%)Lime allele = 10/66 (15%)Allele frequency of skittle gene after catastrophic event:Strawberry allele = 6/8 (75%)Grape allele = 2/8 (25%)Lime allele = 0/8 (0%)The genetic drift resulting from this bottleneck has reduced the genetic variation of the population

Founder Effect

Genetic drift resulting when a small group of individuals colonizes a new habitatChanges in allelic frequency will depend on the alleles carried by colonistsThe genetic makeup of a small colony is unlikely to be representative of the larger gene pool from which colonists came

Gene FlowCauses gain or loss of alleles when fertile individuals move into or out of a population (e.g. migration)Introduces new alleles with gametic transfer between populations (e.g. plant pollination)Tends to reduce genetic differences between populations over timeAbsence of gene flow can result from reproductive isolation, leading to greater genetic difference between populations

Reproductive IsolationExistence of biological factors that impede members of two species from producing viable, fertile offspring (hybrids)Two classes of barriers: 1) Prezygotic impede mating and fertilizationEx: Different mating calls, times of activity and reproductive structures2) Postzygotic prevent hybrid offspring from becoming viable, fertile adultsEx: Weak or sterile hybrids that dont reproduce

matinghybridviablefertileSpeciationProcess by which one species* splits into two or more species:

Occurs in two ways:1) Allopatric speciation resulting when populations become geographically isolated2) Sympatric speciation resulting within the same geographic area as parent speciesReproductive BarriersSTOPgeneflowhybridviablefertileNew Species Arise* = biological species conceptAllopatric SpeciationGeographic BarrierPopulations IsolatedMutation, Genetic Drift, Natural SelectionReproductive Barriers Arise/Reproductive IsolationNew SpeciesSympatric SpeciationNO Geographic BarrierPopulations in Same AreaMutation, Genetic Drift, Natural Selection, PolyploidyReproductive Barriers Arise/Reproductive IsolationNew SpeciesKey VocabularyNatural selectionGenetic driftGene flowBottleneck effectFounder effectReproductive isolationPrezygotic barrierPostzygotic barrierSpeciationAllopatric speciationSympatric speciation