2. Biologist now know that natural selection is not the only mechanism of evolution

3. Inheritable variation may come from any of the following:
1. Genetic drift
2. Gene flow
3. Mutation
4. Natural selection

4. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
1. Genetic Drift: when the same genes are combined in different ways to produce different results
Due to chance with independent assortment

5. In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.

6. Chapter 15
Evolution
Extreme Examples of Genetic Drift
Founder Effect
Occurs when a small sample of a population settles in a location separated from the rest of the population
Alleles that were uncommon in the original population might be common in the new population.

7. Ex. Blue People of the Appalachian Mountains

8. Chapter 15
Evolution
Extreme Examples of Genetic Drift
2. Bottleneck
Occurs when a population declines to a very low number and then rebounds

9. The rebound group has traits that are most similar to the smallest group

10. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Gene Flow:
Increases genetic variation as individuals move in and out of a population

11. 3. Mutations: Random changes in the genetic code

12. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
4. Natural Selection
Acts to select the individuals that are best adapted for survival and reproduction

13. Chapter 15
Evolution
15.3 Three Types of Natural Selection
Stabilizing selection eliminates extreme expressions of a trait when the average expression leads to higher fitness.

14. Ex. Babies who have too low birth weights are less likely to survive

15. Chapter 15
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15.3 Shaping Evolutionary Theory
Directional selection when an extreme trait makes organisms more fit it becomes more common.

16. Ex. Body color of the peppered moth found in England
Originally, most were white, now most are brown

17. Why?

18. Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Disruptive selection a process that splits a population into two groups.

19. Tends to remove individuals with average traits but retains the extreme traits to better suit them for their environment
Ex. Same snake species found in two areas

22. Speciation
Speciation: The development of new species through evolution
A population must diverge and then be reproductively isolated

23. Two Types of Speciation:
1. Allopatric speciation
2. Sympatric speciation

24. Allopatric Speciation
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Allopatric Speciation
A physical barrier divides one population into two or more populations.
Abert squirrel
Kaibab squirrel

25. A species evolves into a new species without a physical barrier.
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Sympatric Speciation
A species evolves into a new species without a physical barrier.
The ancestor species and the new species live side by side during the speciation process but cannot reproduce.

27. Patterns of Evolution
Speciation takes a very long time (compared to a human life) but there is evidence that it occurs:
Divergent Evolution (Adaptive Radiation)
Coevolution
Convergent Evolution

28. Divergent Evolution (Adaptive Radiation):
when two different species share the same ancestral origins but have evolved differently 

29. 15.3 Shaping Evolutionary Theory
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory
Divergent Evolution (Adaptive Radiation)
Can occur in a relatively short time when one species
gives rise to
many different
species
Follows large-scale extinction events

30. Chapter 15
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15.3 Shaping Evolutionary Theory
Coevolution
The relationship between two species might be so close that the evolution of one species affects the evolution of the other species.
Mutualism

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15.3 Shaping Evolutionary Theory
Convergent Evolution
Unrelated species evolve similar traits even though they live in different parts of the world.

32. Chapter 15
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15.3 Shaping Evolutionary Theory
Rate of Speciation
Evolution proceeds in small, gradual steps according to a theory called gradualism.
Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.

33. 15.3 Shaping Evolutionary Theory
Chapter 15
Evolution
15.3 Shaping Evolutionary Theory