Natural Selection

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Descent with Modification:

1. Claimed that all biodiversity came from one common ancestor & that ancestor changed and adapted over time.

2. Proposed a mechanism for adaptive modification and biodiversity; called Natural Selection.

3. Implied that humans evolved the same way as other life forms.

Mechanisms for Evolution Include...

1. Mutation

2. Natural Selection****

3. Genetic Drift

4. Gene Flow

Natural Selection Occurs when...

1. Individual phenotypes vary within species.

2. Some variations are passed onto offspring.

3. More offspring are produced than can survive (only the fittest survive).

4. Survival + Reproduction is not random; individuals that have favorable variations survive to reproduce.

^^ These are called Darwin's 4 Postulates.

Postulates 1 & 2: Phenotypic variation exists, and some variations are passed on to offspring (ex: demonstrating heritability of human height).

Postulate 3: More offspring are produced than can survive (there is a struggle for existence in a world with limited resources).

^^What factors contribute to the struggle for existence??

Biotic: predators, disease, foot, etc.

Abiotic: Day length, water availability, sunlight, etc.

Postulate 4: Survival + reproduction is not random (individuals that have favorable variations survive to reproduce)

**If all four postulates are satisfied, then evolution by natural selection will occur**

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