Review of the concept of evolution

Since the 18th century scientists have increasingly found evidence challenging the static traditional theory of diversity:

• Distribution of species on earth.
• The existence of unexplainable similarities between species.
• An apparent progressive series of fossils in the geological record.

These observations suggested that species could change, challenging the previous assumptions of immutability. Various theories were advanced to explain the evidence:

• Catastrophism suggested that the geological record was misleading.
• Lamarkianism suggested that change occurred by inheritance of acquired characteristics.

Both theories are contradicted by evidence. Darwin proposed a theory in which theories change as a result of "natural selection".

In populations competing for limiting resources some traits present in some individuals may allow them to compete more effectively. Those individuals, and so there genetic traits, will tend to reproduce more than others. As a consequence, populations can change over time as result of this selection.

This unit looks at the mechanisms which can cause populations to change.

Variation among individuals

We have already seen how individuals in a population might differ.

Difference can arise among individuals as a result of mutation. The differences can propagate through a population as a result of sexual reproduction. Because of meitoic processes (chromosome segregation and recombination) alleles can be shuffled in a population, creating new combinations of alleles of multiple genes.

Therefore, in a large population of individuals there can be great variation in alleles among individuals. This allelic variation would provide the material for the process described by Darwin as natural selection.