1. How organisms evolve: Part 2
Chapter 15:
Sections 15.2 (finish) & 15.3

2. What causes evolution? 3. Small population size
Genetic drift
Chance events change allele frequencies
Much more likely to impact small populations
Example: What if only half the individuals in a population, chosen randomly, reproduce?
What happens to the frequency of a particular allele, just by chance?
Key: Examine difference between large vs. small population

3. Changes in allele A (started at 0.5)
Remember, for each generation, half the population is chosen at random to reproduce!
Based on our definition of evolution, did evolution occur for either population? If so, which one?

4. What causes evolution? 3. Small population size (cont.)
Population bottlenecks
By chance, very few individuals survive the particular “bottleneck” event.

5. Population bottlenecks: real example
Northern elephant seal
What was the bottleneck event for this species?
From how many elephant seals is the current population descended?

6. What causes evolution? 3. Small population size (cont.)
Founder effect
Isolated colonies founded by a small number of organisms
A subset of the alleles in the original population make it to the new population
Example: Ellis-van Creveld syndrome
Short arms and legs, multiple fingers, heart defects.
Common among the Amish of Lancaster County, PA
One of the few founders of this colony had this syndrome

7. What causes evolution? 4. Non-random mating
Assortative mating
Example: snow geese prefer mates with the same color plumage

8. What causes evolution? 4. Non-random mating…
NOTE: The examples of male-male competition (bighorn sheep) and female choice (peahens and peacocks) belong in the section on natural selection, not in the section on non-random mating!
These are examples of sexual selection, a type of natural selection…

9. What causes evolution? 5. Natural selection!
Natural selection occurs because not all phenotypes are equally beneficial in a particular environment
Individuals with the best-adapted phenotypes will, on average, leave the most offspring.
To the extent that the phenotype was determined by genotype, the most beneficial alleles will increase in the population.
Thus, natural selection acts on individuals, but causes changes in populations over generations.

10. Traits that are selected often present a compromise of various selective pressures.
Example: Length of male giraffe’s neck

11. What causes evolution? 5. Natural selection!
Types of selective pressures
Abiotic factors
Example: Animals in the intertidal have adaptations to keep from drying out.
This burrowing anemone can withdraw into the sand and close up during low tide.

12. What causes evolution? 5. Natural selection!
Types of selective pressures
Competition between organisms
Example: Some coral reef fish compete for algae growing on the reef. How are they adapted to do so?

13. Reef Herbivores: Damselfish
Damselfish “farm” algae and defend their territories.
Dusky damselfish (EcoDives, Key West)

14. Reef Herbivores: Tang gangs!
Blue tangs raid damselfish territories in “gangs”
Photo: Karen Haberman

15. What causes evolution? 5. Natural selection!
Types of selective pressures
Predation!
Example: Animals have various methods of dealing with their predators.
Do all animals do this in the same way?

16. Chemical defense/toxicity/ warning coloration

17. Giant petrel chick: “gacking” behavior
Photo: Karen Haberman

18. Cryptic coloration
Poorwill and frog

19. What causes evolution? 5. Natural selection!
Types of selective pressures
Sexual selection
Male-male competition
Female choice

20. What causes evolution? 5. Natural selection!
Selection can influence populations in three ways…

21. Directional selection
Selects for one extreme of the characteristic
Selection of antibiotic resistance in bacteria
Range of particular characteristic

22. Stabilizing selection
Selects for average values of the characteristic
Often a balance between two selective pressures
Example: coloration in male guppies
Bright coloration: attract mate, but also predators
Too dull  no predation, but no mate either
So coloration may be intermediate
Range of particular characteristic

23. Disruptive selection
Selects for extremes, and against individuals with intermediate values
Example: Beak size of black-bellied seedcrackers
Large beaked: Feed on hard seeds
Smaller, pointed beak: Feed on soft seeds.
Intermediate beaks not very good at eitherone.
Range of particular characteristic