Mechanisms of Evolution and Speciation

Objectives Recognize mutation and sexual reproduction are the underlying causes of all genetic variability within organisms Identify natural selection, genetic drift, and gene flow as the mechanisms of divergent evolution Distinguish the differences between these three mechanisms by knowing how each operates Recognize reproductive isolation has an important role in the formation of new species Compare and contrast allopatric and sympatric speciation, identifying potential factors in each that may contribute to reproductive isolation among new species

Review Genetic variation is due to: 1) Mutation 2) Sexual reproduction This heritable variation is the underlying factor that makes microevolution possible

Changes in Allelic Frequencies of Populations Can Lead to Evolutionary Divergence Three mechanisms can lead to microevolution: Natural Selection Genetic Drift Gene Flow

Natural Selection Selective pressure that promotes survival of organisms with heritable trait(s) that provide an advantage Over time, advantageous trait(s) is/are passed on, potentially leading to greater relative fitness Result 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 alleles

Modeling 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 a“4,” “5,” or “6” represents predator eating a yellow fish

Before 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 eaten Predator 2 (roll of 6) – yellow fish eaten Predator 3 (roll of 5) – yellow fish eaten Predator 4 (roll of 1) – blue fish eaten Predator 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 eaten Predator 7 (roll of 4) – yellow fish eaten Predator 8 (roll of 3) – red fish eaten Predator 9 (roll of 2) – red fish eaten Predator 10 (roll of 5)– yellow fish eaten 5 fish remain 4 blue (80%) & 1 yellow (20%)

Natural Selection at Work: Ok, we’re no longer eating cows! “Survival of the Cutest”

Genetic Drift Operates on chance events causing unpredictable fluctuations in allele frequency Has 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 Effect Allele 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 habitat Changes in allelic frequency will depend on the alleles carried by colonists The genetic makeup of a small colony is unlikely to be representative of the larger gene pool from which colonists came

Gene Flow Causes 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 time Absence of gene flow can result from reproductive isolation, leading to greater genetic difference between populations

Reproductive Isolation viable mating fertile hybrid Existence of biological factors that impede members of two species from producing viable, fertile offspring (hybrids) Two classes of barriers: 1) Prezygotic – impede mating and fertilization Ex: Different mating calls, times of activity and reproductive structures 2) Postzygotic – prevent hybrid offspring from becoming viable, fertile adults Ex: Weak or sterile hybrids that don’t reproduce

Speciation Process by which one species* splits into two or more species: Occurs in two ways: 1) Allopatric – speciation resulting when populations become geographically isolated 2) Sympatric – speciation resulting within the same geographic area as parent species Reproductive Barriers viable STOP gene flow New Species Arise fertile    hybrid * = biological species concept

Allopatric Speciation Geographic Barrier Populations Isolated Mutation, Genetic Drift, Natural Selection Reproductive Barriers Arise/ Reproductive Isolation New Species

Sympatric Speciation NO Geographic Barrier New Species Populations in Same Area Mutation, Genetic Drift, Natural Selection, Polyploidy Reproductive Barriers Arise/ Reproductive Isolation New Species

Key Vocabulary Natural selection Genetic drift Gene flow Bottleneck effect Founder effect Reproductive isolation Prezygotic barrier Postzygotic barrier Speciation Allopatric speciation Sympatric speciation