From January, 2009. This video is lecture 3 of 36 from Principles of Evolution, Ecology and Behavior with Professor Stephen C. Stearns
OBSERVATION #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully.
OBSERVATION #2: Nonetheless, populations tend to remain stable in size, except for seasonal fluctuations.
OBSERVATION #3: Resources are limited.
INFERENCE #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving each generation.
OBSERVATION #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike
OBSERVATION #5: Much of this variation is heritable.
INFERENCE #2: Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment have higher fitness and are likely to leave more offspring than less fit individuals.
INFERENCE #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations.
Just remember these 5 letters - OVCSR
Survival of the best adapted