Diversification

Diversity is a measure of variety. It is usually assessed at a taxonomic level, for example, the number of species or families living at a particular time. It could also be applied to individual variation, or even to variations in DNA, but variation at this level is not generally used in the interpretation of the fossil record.

All organisms share a single common ancestor that probably lived around 3.8 billion years ago. At this stage in evolution, diversity would have been extremely low by any measure. Its increase since that time must ultimately be a function of evolution, which is the means by which new species appear.

How does diversity increase? It may be that at some level this is always accomplished by Darwinian selection. In this model, selection pressures acting on individuals result in changes in the phenotype. If these changes occur in a single lineage of animals, then there is no net change in diversity, but if the lineage splits then diversity is increased. It is believed that most lineage splits occur due to the physical separation of populations from the rest of the gene pool. This may be as a result of migration, or as a result of the erection of physical barriers by geological processes.

The break-up of a major continent into smaller elements is an obvious way to physically separate populations of the same species. It is notable that such continental break-ups do seem to relate to episodes of diversification. However, such separations must often happen at smaller scales, for instance when falling water levels split a single lake into two, or when changes in drainage pattern separate two river systems.

There is also an important biological instrument for increasing diversity. This is the origin of new potential, either in the form of new anatomical characteristics, or in the form of new modes of behavior. The origin of a burrowing habit in members of the Modern fauna, or the acquisition of flight by birds, or sight in trilobites, for example, all led to increases in diversity.

The rate of diversity change appears to be linked to the causes of change, and even more clearly to the degree of competition experienced by organisms. Broadly, the most rapid diversification occurs when ecospace is empty, and slower diversification occurs when the environment is already stocked with organisms. Working in the other direction, the higher the standing diversity is, the greater the competition for resources, and hence selection pressure. At these times there are also more, and more varied, genomes from which innovation may come.

There are times in the past when diversity seems to have increased slowly, and times when diversification has been rapid. Slow increases, such as those seen during the Jurassic

Jurassic

Cretaceous

Diversity Extinction Graph

Cretaceous

Cenozoic

Fig. 16.5 Graphs of diversity through time for (a) Jurassic and Cretaceous echinoids, and (b) Cretaceous and Cenozoic vertebrates. Echinoid diversity increased slowly as a result of biological innovations related to burrowing. Mammalian diversity increased dramatically as the group radiated to fill empty ecospace left by the extinction of the dinosaurs.

Cretaceous

Cenozoic

Fig. 16.5 Graphs of diversity through time for (a) Jurassic and Cretaceous echinoids, and (b) Cretaceous and Cenozoic vertebrates. Echinoid diversity increased slowly as a result of biological innovations related to burrowing. Mammalian diversity increased dramatically as the group radiated to fill empty ecospace left by the extinction of the dinosaurs.

and Cretaceous, seem to be a function of biological innovation resulting in niche diversification. As organisms evolve novel solutions to environmental problems, they increase potential and actual biodiversity. A good example ofthis is the evolution of a burrowing habit in echinoids, which led to an overall increase in diversity (Fig. 16.5a).

Times of rapid diversification usually appear to follow mass extinctions. In these cases, ecospace has been emptied by the huge, non-selective cull that has just been applied to the system and the survivors evolve quickly to fill the niche space. In the 10 million years after the dinosaurs went extinct, over 20 new families of mammals evolved, filling all the main empty niches from tree dwellers to marine predators (Fig. 16.5b).

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