1. EVOLUTION
15.3 NOTES

2. Mechanisms of Evolution
Genetic Drift
A random change in the frequency of an allele in a population NOT due to natural selection.
More likely to occur in small populations.

3. Mechanisms of Evolution
Genetic Drift
Founder effect: a small group separates from the population and lives somewhere else
they carry a subset of the population’s gene
Ex: Amish community

4. Mechanisms of Evolution
Genetic Drift
Bottleneck: population declines to a very low number and then rebounds
the gene pool of the rebound population is similar to the original population

5. Northern Elephant Seals

6. Ex: Northern Elephant Seals
Bottleneck Effect
Ex: Northern Elephant Seals
Large population  overhunting  20 Total
Population eventually recovered in size
Loss of genetic variation
Could hurt long-term survival of species

7. Bottleneck Effect - Cheetahs

8. Mechanisms of Evolution
Remember mutations?
A random change in a sequence of DNA.
A small population with a lot of mutations has the greatest potential for evolution

9. Mechanisms of Evolution
Natural Selection
How does it alter phenotypes?

10. Mechanisms of Evolution
Stabilizing selection: selection against extreme expressions of a trait, selection for the average
Ex: birth weight in human babies

11. Mechanisms of Evolution
Directional selection: selection toward one extreme of a trait
Ex: Galapagos finches - large beaks during drought
Peppered moths - color

12. Mechanisms of Evolution
Disruptive selection: selection against the average, selection for both extremes
Ex: Cichlid fish size - males

13. Mechanisms of Evolution
Sexual selection: selection of a trait in males to attract a mate and intimidate other males
Ex: peacocks

14. Isolation Reproductive Isolation
Some members of a population change so much that they can no longer produce offspring with members of the original population.

15. Different mating patterns
Eastern meadowlark
and
Western meadowlark
Different mating patterns
Liger
Sterile

16. Isolation Geographic Isolation A separation of population by a barrier
Ex: mountains, rivers, the Grand Canyon

17. Speciation
For speciation to happen, a population has to move away and then be reproductively isolated.

18. Speciation Allopatric Speciation
A physical barrier divides one population into 2 or more populations
Abert squirrel
South Rim
Kaibab Squirrel
North Rim

19. Speciation Sympatric Speciation
A species evolves into a new species without a physical barrier
The old species and new species live side by side during speciation

20. Patterns of Evolution Adaptive Radiation (Divergent Evolution)
The evolution of a new species in a relatively short period of time
one species evolves into several different forms that live in different habitats
homologous structures

21. Adaptive Radiation

22. Patterns of Evolution Coevolution
A close relationship between 2 species
The evolution of one species influences the evolution of the other

23. Coevolution
Datura Plant & Hawk Moth

24. Patterns of Evolution Convergent Evolution
Unrelated species evolve similar traits even though they live in environments that are really far apart
similar ecology and climate
analogous structures

25. 3. Convergent Evolution
Unrelated species evolve similar adaptations, due to environmental pressures (natural selection)
These adaptations may look similar from the outside, but actually evolve independently from each other
Ex: sharks, dolphins, seals, penguins

26. Analogous structures
Similar in appearance and function, but are developed from anatomically different parts
Evidence for convergent evolution
Ex: octopus eye versus vertebrate eye (both complex eyes

30. South American North American Pliocene Pleistocene
Natural Selection  Common Adaptation
**But common adaptations do not necessarily imply common ancestor!

32. How fast does speciation happen?
Gradualism:
Evolutionary change occurs gradually, over long periods of time

33. How fast does speciation happen?
Punctuated Equilibrium
Patterns of long periods of stability (no change) interrupted by episodes of rapid change

34. How fast does speciation happen?

35. Evolution can follow both patterns, depending on the situation and the time in evolutionary history