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.
Populations tend to remain stable in size, except for seasonal fluctuations.
environmental resources are limited
individuals of a population vary extensively in their characteristics; no two individuals are exactly alike
much of this variation is heritable.
(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.
(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.
(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.