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Darwin's Observations and Inferences

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Observation #1
The capacity to overproduce is a characteristic of all species but only a small fraction of offspring produced complete their development and reproduce successfully to leave offspring of their own.
Observation #2
Populations tend to remain stable in size, except for seasonal fluctuations.
Observation #3
environmental resources are limited
Observation #4
individuals of a population vary extensively in their characteristics; no two individuals are exactly alike
Observation #5
much of this variation is heritable.
Inference #1
(Observe. 1-3) production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population with only a fraction of the offspring surviving each generation.
Inference #2
(Observ.4-5) differential reproductive success- in which organisms with traits favored by the environment produce more offspring than do organisms without those traits= result in the favored traits being disproportionately represented in the next generation. This is what is meant by survival of the fittest- it had nothing to do with size or strength.
Inference #3
(Observ.4-5) this unequal ability of individuals to survive and reproduce will lead to a gradual change in a population with favorable characteristics accumulating over generations.