The most important thing an animal can do in its lifetime, at least in terms of evolution, is reproduce. An individual, regardless of how long it lives, contributes nothing to the evolution of its species unless it passes its genes on to the next generation.

Organisms reproduce in a wide variety of ways. Some reproduce asexually, producing offspring that are genetically identical to themselves. Many plants, including aspen trees and strawberry plants, reproduce this way at least some of the time. They do this by sending out roots or stems that grow horizontally underground or along the surface, depending on the type of plant. These offshoots ultimately give rise to an entirely new plant that is a carbon copy of its parent. Many single-celled organisms also reproduce identical copies of themselves by replicating their DNA and then dividing in half -- a process called binary fission. Only when a mutation occurs in the replication process does an offspring differ genetically from its parent.

Other organisms, including humans and the creatures featured in this activity, produce offspring by a process called sexual reproduction. During sexual reproduction, organisms divide their genetic material in half and then combine their half-set of genes with the half-set of another individual of the same species. This method, however, requires some effort on the part of the individual. After all, finding a suitable mate can be a time-consuming process. Furthermore, where the objective is to pass the maximum number of genes on to the next generation, it seems that sexual organisms get a smaller return from their investment than do asexual organisms. So if sexual reproduction is such an arduous and inefficient process, why do organisms bother?

Scientists often refer to sex as a biological tradeoff. On one hand, the genes of a sexually reproducing organism are, in a sense, diluted: Each parent passes on only half of its genes to its offspring. On the other hand, the genetic mixing that results from sex ensures that each generation will be different from the previous generation and that individuals of the same generation will be sufficiently different from one another.

This genetic variation provides an important evolutionary advantage: It gives species (not individuals) the ability to evolve more quickly in response to challenging and constantly changing environmental conditions. This is because natural selection, the process that drives evolution, acts on genetic variation within a population. Stated simply, in a highly varied population under a given set of conditions, some individuals will have an advantage relative to others in the population. They will pass more of their genes on to the next generation and will have a greater effect on shaping the genome of the species than will others in the population. In populations with little genetic variation, no individual has much survival or reproductive advantage over any other individual. Thus, evolution moves much more slowly.