1. A population shares a common gene pool
Populations, not individuals, evolve

2. Genetic variation leads to phenotypic variation.
Genetic variation in a population increases the chance that some individuals will survive.
Genetic variation leads to phenotypic variation.
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
made up of all alleles in a population
allele combinations form when organisms have offspring

3. Allele frequencies measure genetic variation.
measures how common allele is in population
can be calculated for each allele in gene pool

4. Genetic variation comes from several sources.
Mutation is a random change in the DNA of a gene.
can form new allele
can be passed on to offspring if in reproductive cells
Recombination forms new combinations of alleles.
usually occurs during meiosis
parents’ alleles arranged in new ways in gametes

5. Mutations produce the genetic variation needed for evolution.

6. Natural selection acts on distributions of traits.
A normal distribution graphs as a bell-shaped curve.
highest frequency near mean value
frequencies decrease toward each extreme value
Traits not undergoing natural selection have a normal distribution.

7. Natural selection can change the distribution of a trait in one of three ways.

8. Stabilizing selection—favors the average form

9. Directional—one extreme is favored

10. Disruptive selection-either extreme variation is favored

11. Natural selection is not the only mechanism through which populations evolve
Gene flow is the movement of alleles between populations due to immigration and emmigration
Gene flow keeps neighboring populations similar.
Low gene flow increases the chance that two populations will evolve into different species

12. Gene flow moves alleles from one population to another.

13. Natural selection is not the only mechanism through which populations evolve
Genetic drift—allele frequency changes due to random event—much more important in small populations

14. Genetic drift changes allele frequencies due to chance alone.

15. Hardy-Weinberg equilibrium describes populations that are not evolving
1. No mutations occur
Individuals do not enter or leave the population (no immigration or emigration)
The population is large (no genetic drift)
Individuals mate randomly (no sexual selection) Sexual selection—in general females choosing males—peacocks with large feathers, lions with largest mane—extreme traits attract female
All traits are equally successful so natural selection does not occur

16. If these conditions are met, the frequency of the alleles does not change
However, these conditions rarely occur, so the gene pool changes (equilibrium is disrupted)

17. Divergence—the accumulation differences between groups
New species can arise when populations are isolated.
Divergence—the accumulation differences between groups
Geographic isolation—condition in which 2 populations of the same species do not breed with each other due to geographic separation
As natural selection occurs, members of a species become so different over time that they no longer can interbreed and become separate species

18. Behavioral barriers can cause isolation. called behavioral isolation
includes differences in courtship or mating behaviors
Hybridization is the crossing of two different species.
occurs when individuals of closely related species mate—rarely occurs in nature
Example—Mule (cross between mare and donkey)
Many are sterile and cannot reproduce

19. The effects of natural selection add up over time.

20. Patterns of evolution
Coevolution—species in close association (predators and prey, parasite and host)
Convergent evolution—the environment selects similar phenotypes, even though the ancestors were very different
Divergent evolution—species become more and more different, usually due to differing habitat

21. Convergent evolution
Sharks and porpoises have similar appearance but very different genetic background

22. Divergent evolution
Example--Darwin’s finches—beaks differ by diet

23. Factors that drive natural selection
All populations have genetic variation-due to mutations, crossing over, independent assortment, random fertilization.
2. The environment presents challenges to successful reproduction

24. Individuals tend to produce more offspring than the environment can support
Individuals better able to cope with the challenges presented by their environment tend to leave more offspring

25. Tempo of evolution
Gradualism—evolution is a gradual process that occurs continuously
Punctuated equilibrium (catastrophism)—major environmental changes cause evolution to occur rapidly followed by periods in which successful species change little