1. Examples of Evolution
How does evolution occur? Natural selection is the mechanism that drives evolution and the environment dictates the direction and amount of change.

2. Elements in Natural Selection:
1. All populations have genetic variation. (Inherited variations)
2. Individuals tend to produce more offspring than the environment can support. (Overproduction)
3. The environment presents challenges to successful reproduction. (Competition)
4. Individuals that are better able to cope with the challenges presented by their environment tend to leave more offspring than those individuals less suited to the environment do. (Selection)

3. Darwin’s Natural Selection:
There are 2 types of rabbits: those that strictly eat grass and those that strictly eat berries and flowers. A drought occurs one year, and the plants have difficulty producing any extras (flowers, berries, etc.). They can only try and keep themselves green. The rabbits have had babies all year long and many have starved to death.
berries/
flower
What rabbit will natural selection select AGAINST? _______
FOR it? _______
Identify Darwin’s elements of natural selection.
Populations have variations. _______________________
Some variations are favorable. _____________________
More offspring are produced than survive. ____________
Those that survive have favorable traits. _____________
A population will change over time. _________________
grass
Rabbits that eat grass & berries/flower
Rabbits that eat grass
Babies all year long
Grass rabbits survive
More rabbits that eat grass

4. Examples of Natural Selection:
1. Antibiotic-resistant strains of bacteria: Bacteria can become resistant through natural selection. For example, the drug rifampin acts by binding to the RNA polymerase preventing transcription and therefore killing the bacterial cell. Through natural selection, a single base change from cytosine to thymine in a gene called rpoB makes the bacteria resistant by preventing rifampin from binding to the polymerase.

5. 2. Darwin’s Finches: Darwin collected 31 specimens (9 species) of finches while on the Galapagos Island. All were very similar, except for their bills. He suggested that the 9 species evolved from an original ancestral species and that changes occurred as different populations accumulated adaptations to different food sources.

6. Darwin’s Finches

7. 3. Peter and Rosemary Grant (1973) also showed that natural selection influences evolution. Their study showed a relationship between the environment (dry years vs. wet years) and how the availability of food was related to beak size.

8. Grant Studies

9. Patterns of Evolution:
Divergent evolution – the accumulation of differences between related groups
Convergent evolution – the accumulation of similarities between unrelated groups
Coevolution – the process by which 2 or more species evolve in response to changes in each other.

10. Divergent vs. Convergent
Similar Characteristics

11. Coevolution

12. Barriers that may isolate two or more closely related groups:
Speciation – the formation of new species as a result of evolution by natural selection; they become so different they can no longer interbreed (reproductive isolation)
Geographical isolation – physical barriers such as rivers or mountains separate a population
Temporal Isolation – timing which prevents reproduction between a population
Behavioral Isolation – differences in courtship or mating behaviors