1. 15.3 Mechanisms of Evolution
THERORY OF EVOLUTION
15.3 Mechanisms of Evolution

2. I. Populations Genetics
A. Populations evolve, not individuals
1. Gene pool – all the alleles in a population
2. Percentage of alleles in the gene pool is called allelic frequency

3. B. Hardy- Weinberg Principle
1. Evolution will not occur in a population unless allelic frequencies are acted upon by environmental forces that cause change
2. Hardy- Weinberg Principle states that when allelic frequencies remain constant a population is in genetic equilibrium

4. C. Mechanisms of Evolution
1. For a population to be in genetic equilibrium it must meet five conditions:
A. No genetic drift
= Any change in the allelic frequencies in a population that results from
chance
B. No gene flow
= Uncommon since organisms migrate between populations and reproduce
C. No mutation of DNA/genes
= This could shift allele frequencies and majority of mutations are harmful
D. Mating must be random
= Rare since matting occurs between individuals that live in the same habitat
E. No natural selection occurs
= Rare since species are always being selected for/against in an environment for survival

5. D. Natural selection acts on variations with in a population to select the individuals that are best adapted for survival and reproduction
1. Stabilizing selection favors average individuals and eliminates extreme expressions of a trait in a population.
Selection for
average size
spiders
Normal variation

6. 2. Directional selection occurs when natural selection favors one of the extreme variations of a trait.
Selection for longer beaks
Normal
variation

7. 3. In disruptive selection individuals with either extreme of a trait’s variation are selected for and the average traits are removed
Selection for
dark limpets
Selection for
light limpets
Normal variation

8. II. Evolution of a new species
A. Evolution of a new species when populations diverge and become either reproductively or geographically is called speciation
Examples:
- Behavioral differences in mating pairs or climate
shift to begin ovulation earlier
(Reproductive isolation)
- Earthquake occurred separating a population or
building a road in a forest
(Geographic isolation)

9. Reproductive Isolation
Geographic Isolation

10. B. Rates of evolution
1. Gradualism – species originate through small, gradual steps
2. Punctuated equilibrium – species occur rather quickly, in rapid bursts, with long periods of genetic equilibrium. Usually results from rapid climate changes

11. C. Patterns of Evolution
1. Divergent evolution or Adaptive Radiation – species once similar become increasingly different a. populations adapt to different environmental conditions b. results in new species

12. 2. Convergent evolution – unrelated species evolve similar traits even though they occupy similar environments in different parts of the world
a. share similar pressures of natural selection b. results in very similar species in different locations

13. 3. Co-Evolution
A. Occurs when relationships between species are so close they influence each other’s evolution
IE One species changes in response to another species
* Recall Mutualistic, Parasitic or Predatory Relationships