Variation and Selection

Natural Selection

The process by which organisms adapt to changes in the environment to continue their survival. It selects for organisms that has the most advantageous gene.

Species

A group of organisms that are similar in morphology and physiology that can interbreed to produce fertile offspring

Darwin's 1st Observation

All organisms over-reproduce

Darwin's 2nd observation

Population numbers generally remain constant over long periods of time

Darwin's 3rd observation

There are variations within a species

Darwin's 4th observation

Some of these variations are inherited

Darwin's 1st deduction

There is competition for survival

Darwin's 2nd deduction

Individuals that survive and reproduce possess the best
characteristics allowing them to adapt to their environment

Darwin's 3rd deduction

If these characteristics can be inherited, then the organisms
will pass them on to their offspring.

Stabilizing Selection

As long as environmental conditions stay constant, then the existing alleles will be selected for in each generation

Directional Variation

If a change in the environment was to occur, selection pressures would change. Other alleles latent in the population that were insignificant before may now offer a better survival and would now be selected for

Disruptive Selection

Sometimes the variation in the middle of the range is at a selective disadvantage compared to the extremes. Natural selection will select for alleles' resulting in the extremes producing two different forms