Current theories about the origin of life include the two competing ideas (1) that the primitive earth environment led to the spontaneousorganization of organic chemicals into primitive cells and (2) that primitive forms of life arrived on the earth from space. Basic units of life were probably similar to present-day prokaryotes. These primitive cells could have changed through time as a result of mutation and in responseto a changing environment.

All sexually reproducing organisms naturally exhibit genetic variety among the individuals in the population as a result of mutations and the genetic recombination resulting from sexual reproduction. The geneticdifferences are reflected in differences among individuals. These genetic differences are important for the survival of the species because natural selection must have genetic variety to select from. Natural selection by the environment results in better-suited individual organisms that have greater numbers of offspring than those that are less well-off genetically.

Organisms with wide geographic distribution often show different gene frequencies in different parts of their range. A species is a group of organisms that can interbreed to produce fertile offspring. This definition of a species is also known as the biological species concept. People often distinguish species from one another based on structural differences. This is known as a morphological species concept. The process of speciation usually involves the geographic separation of the species into two or more isolated populations. While they are separated, natural selection operates to adapt each population to its environment.

At one time, people thought that all organisms had remained unchanged from the time of their creation. Lamarck suggested that changedid occur and thought that acquired characteristics could be passed fromgeneration to generation. Darwin and Wallace proposed the theory ofnatural selection as the mechanism that drives evolution.