What are the conditions for natural selection?

Answer 1

"The excess fecundity, and consequent competition to survive, in every species provides the precondition for the process Darwin called natural selection. Natural selection is easiest to understand as a logical argument, leading from premises to conclusion. The argument requires the four conditions listed below.

1. Reproduction. Entities must reproduce to form a new generation.

2. Heredity. The offspring must tend to resemble their parents: roughly speaking, 'like must produce like'. This condition is provided by Mendelian inheritance.

3. Variation in characteristics of the members of the population.

4. Variation in the fitness of organisms associated with these characteristics.

Provided these conditions are met for any property of a species, natural selection automatically results, as in the case of the peppered moth. If any are not, it does not. Thus entities, like planets, that do not reproduce cannot evolve by natural selection. But when the four conditions apply, the organisms with the property conferring higher fitness will leave more offspring, and the frequency of that type of organism will increase in the population."

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Answer 2

Selection, natural or artificial, is one part of evolution. The others are: radiation (not nuclear radiation; the spreading out kind of radiation) and mutation (change).

An organism comes into the world just slightly different than its parents, lets say a giraffe with a longer neck. This organism gains a slight advantage (in the case of the giraffe, access to taller trees), lives longer/makes more offspring. Those offspring then spread out, outcompeting the "old" models until they reach different environments. Again, with the giraffe, tall-necks proliferate until the vegetation isn't so tall that they have an advantage, or it's too cold, or too wet, or there's little other game, so lions increase their predation on giraffes.