men observed that the individual members of a particular species are not identical but can differ in many ways. For example, some will be able to run a little faster, have a different color, or respond to the same circumstance in different ways. (Humans—including any class of high school students—have many such differences.) Both men further observed that many of these differences are inherited and can be passed on to offspring. This conclusion was evident from the experiences of plant and animal breeders.

Darwin and Wallace were both deeply influenced by the realization that, even though most species produce an abundance of offspring, the size of the overall population usually remains about the same. Thus, an oak tree might produce many thousands of acorns each year, but few, if any, will survive to become full-grown trees.

Darwin—who conceived of his ideas in the 1830s but did not publish them until Wallace came to similar conclusions—presented the case for evolution in detail in his 1859 book On the Origin of Species by Natural Selection. Darwin proposed that there will be differences between offspring that survive and reproduce and those that do not. In particular, individuals that have heritable characteristics making them more likely to survive and reproduce in their particular environment will, on average, have a better chance of passing those characteristics on to their own offspring. In this way, as many generations pass, nature would select those individuals best suited to particular environments, a process Darwin called natural selection. Over very long times, Darwin argued, natural selection acting on varying individuals within a population of organisms could account for all of the great variety of organisms we see today, as well as for the species found as fossils.

If the central requirement of natural selection is variation within populations, what is the ultimate source of this variation? This problem plagued Darwin, and he never

From top left, Charles Darwin (1809-1882), Alfred Russel Wallace (1823-1913), and Gregor Mendel (1822-1884) laid the foundations of modern evolutionary theory.

Glossary of Terms Used in Teaching About Evolution

Evolution: Change in the hereditary characteristics of groups of organisms over the course of generations. (Darwin referred to this process as "descent with modification.")

Species: In general, a group of organisms that can potentially breed with each other to produce fertile offspring and cannot breed with the members of other such groups.

Variation: Genetically determined differences in the characteristics of members of the same species.

Natural selection: Greater reproductive success among particular members of a species arising from genetically determined characteristics that confer an advantage in a particular environment.

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