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

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

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?

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

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?

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

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

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…

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.

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

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.

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?

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

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

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?

Chemical defense/toxicity/ warning coloration

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

Cryptic coloration Poorwill and frog

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

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

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

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

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