The "Cambrian Explosion"

Although the fossil record shows that the first multicellular animals lived about 640m years ago, the diversity of species was low until about 530m years ago. At that time there was a sudden explosion of many diverse marine species, including the first appearance of molluscs, arthropods, echinoderms and vertebrates. "Sudden" here is used in the geological sense; the "explosion" occurred over a period of 10m to 30m years, which is, after all, comparable to the time taken to evolve most of the great radiations of mammals. This rapid diversification raises fascinating questions; explanations include the evolution of organisms with hard parts (which aid fossilisation), the evolutionary "discovery" of eyes, and the development of new genes that allowed parts of organisms to evolve independently.

The evolutionary basis of human behaviour

The field of evolutionary psychology (once called "sociobiology") maintains that many universal traits of human behaviour (especially sexual behaviour), as well as differences between individuals and between ethnic groups, have a genetic basis. These traits and differences are said to have evolved in our ancestors via natural selection. There is much controversy about these claims, largely because it is hard to reconstruct the evolutionary forces that acted on our ancestors, and it is unethical to do genetic experiments on modern humans.

Sexual versus natural selection

Although evolutionists agree that adaptations invariably result from natural selection, there are many traits, such as the elaborate plumage of male birds and size differences between the sexes in many species, that are better explained by "sexual selection": selection based on members of one sex (usually females) preferring to mate with members of the other sex that show certain desirable traits. Evolutionists debate how many features of animals have resulted from sexual as opposed to natural selection; some, like Darwin himself, feel that many physical features differentiating human "races" resulted from sexual selection.

The target of natural selection

Evolutionists agree that natural selection usually acts on genes in organisms - individuals carrying genes that give them a reproductive or survival advantage over others will leave more descendants, gradually changing the genetic composition of a species. This is called "individual selection". But some evolutionists have proposed that selection can act at higher levels as well: on populations (group selection), or even on species themselves (species selection). The relative importance of individual versus these higher order forms of selection is a topic of lively debate.

Natural selection versus genetic drift

Natural selection is a process that leads to the replacement of one gene by another in a predictable way. But there is also a "random" evolutionary process called genetic drift, which is the genetic equivalent of coin-tossing. Genetic drift leads to unpredictable changes in the frequencies of genes that don't make much difference to the adaptation of their carriers, and can cause evolution by changing the genetic composition of populations. Many features of DNA are said to have evolved by genetic drift. Evolutionary geneticists disagree about the importance of selection versus drift in explaining features of organisms and their DNA. All evolutionists agree that genetic drift can't explain adaptive evolution. But not all evolution is adaptive.