A. Organisms do not adapt new traits over their lifetimes.

I. Population Genetics and Evolution

1. Natural selectionNatural selection acts on ALL organisms in a population.

2. As a population’s genes change, the characteristics of that population also change.

3. All of a population’s genes is collectively known as a gene pool.

a. If a populations genes don’t change over many generations, the population is in genetic genetic

equilibrium.equilibrium.

b. Populations in genetic equilibrium are not changing.

15.2 Notes

B. MutationsMutations can sometimes be responsible for changing the genes in a population.

1. Mutations are usually fatal.

C. Genetic driftGenetic drift can also cause a population’s genes to change.

1. Small populations can experience genetic drift.

D. Mutations and Genetic drift influence mostly smaller populations.

1. Natural selection affects mostly large, less isolated populations.

Selection for average size

spiders

Normal variation

1. StabilizingStabilizing selectionselection – favors average organisms.

Normal variation

Selection for longer beaks

2. Directional selection - favors one of the extreme variations of a trait.

Selection for light limpets

Normal variation

Selection for dark limpets

3. Disruptive selectionDisruptive selection – individuals with either extreme are selected for.

II. The Evolution of Species

A. The evolution of a new species – SpeciationSpeciation.

B. Species is defined as:

1. A group of living things.

2. Can breed with others in that group.

3. Breeding results in FERTILE offspring.

C. Fertile-able to reproduce by forming egg and sperm cells.

D. There are three major events that lead to species formation:

1. A barrier forms that separates members of a species (Geographic IsolationGeographic Isolation) .

Loxodonta africana

Elephas maximus

Mammuthusprimigenius

Mammuthus

Elephas

Loxodonta

Primelephas

about 55 million years agoAncestral species

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4

3

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Speciation Rates

The fur of an Arctic fox turns white in the winter. Is this an example of natural selection? Why or why not?

Question 1

The answer is no. An individual cannot evolve a new phenotype (in this case, changing the color of its fur) within its lifetime in response to its environment.

Which type of natural selection does NOT favor the evolution of new species?

Question 2

D. directional

C. stabilizing

B. disruptive

A. divergent

The answer is C. Stabilizing selection reduces variation in a population.

Which of the following rarely affects a population’s genetic equilibrium?

Question 3

D. disruptive selection

C. gene flow

B. lethal mutations

A. genetic drift

The answer is B. Organisms with lethal mutations do not survive. Therefore, organisms with lethal mutations cannot produce enough offspring to affect a population’s genetic equilibrium.

Why are the Galapagos Islands rich in unique species of organisms?

Question 4

D. The island species have been subjected to stabilizing selection.

C. The island species have been subjected to increased gene flow.

B. The islands are geographically isolated.

A. The islands are an area exhibiting an abnormal number of mutations.

The answer is B. Geographic isolation has helped to keep the islands’ species unique.