In biological terms sexual reproduction involves the union of gametes - the sperm and the ovum - produced by two parents. Each gamete is formed by meiosis. This means each contains only half the chromosomes of the body cells (haploid). Fertilization results in the joining of the male and female gametes to form a zygote which contains the full number of chromosomes (diploid). The zygote then starts to divide by mitosis to form a new animal with all its body cells containing chromosomes that are identical to those of the original zygote (see diagram 13.1).
Diagram 13.1 - Sexual reproduction
The offspring formed by sexual reproduction contain genes from both parents and show considerable variation. For example, kittens in a litter are all different although they (usually) have the same mother and father. In the wild this variation is important because it means that when the environment changes some individuals may be better adapted to survive than others. These survivors pass their “superior” genes on to their offspring. In this way the characteristics of a group of animals can gradually change over time to keep pace with the changing environment. This “survival of the fittest” or “natural selection” is the mechanism behind the theory of evolution.