1. Mechanisms of Evolution and Speciation

2. 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

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

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

5. 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

6. 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

7. Before the Predators Strike
Fifteen total fish in the population – five of each color (blue, red, and yellow)

8. 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%)

9. 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%)

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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

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

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

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