but a brief slice of geological time. However, some segments of the fossil record represent useful reality checks for the mathematical models that examine such relationships.

Biotic Interactions

Ecological relationships between groups can be positive or negative. On the positive side, diversification in one group can promote speciation in another. Furthermore, ecologic interactions between two groups can promote the diversification of both through a kind of evolutionary synergism—combined actions that produce enhanced results. For example, during the past 150-million-years the diversification of flowering plants went hand in hand with an expansion of insect pollinators. New varieties of plants offered new food resources for insects, while at the same time new varieties of pollinators promoted reproductive isolation, and hence speciation, in flowering plants. Unfortunately, this relationship has rendered both groups more vulnerable today. The two groups are so interdependent that extinction of species within either will often lead to extinction of species within the other.

Negative interactions between groups have included both competition and predation. As one example, during the period dominated by the dinosaurs, the progressive decline of several major groups of seafloor life is attributed to the expansion of predatory groups that remain prominent in modern seas: crabs, bony fishes, and carnivorous snails. The declining groups include certain kinds of bivalves, snails, and calcareous algae that were especially vulnerable to attack according to studies of functional morphology. The long-term result was a wholesale transformation of seafloor life.

Changes in the Physico-Chemical Environment

Nonbiological aspects of the environment also change in ways that can have positive or negative effects on particular forms of life. Some of these changes influence evolution and extinction by removing barriers to migration, thus allowing species to move to new regions. Tectonic events and changes in global sea level have had this effect by connecting landmasses or oceans that had previously been separated. The tectonic origin of the Isthmus of Panama, for example, served as a natural experiment for testing faunal equilibrium. Migration of mammals at first gave the appearance of maintaining equilibrium, but this has broken down in the past few hundred thousand years. Numerous northern forms infiltrated the South American system without having drastic effects on existing elements, until South American carnivores experienced heavy extinction. Apparently, the northern immigrants had some ability to migrate not possessed by their southern counterparts.

Environmental changes can also result in new habitats that tend to produce diversification of the groups that first gain access to them. The origins of islands and lakes epitomize this phenomenon. Just as modern mouth-breeding fishes proliferate rampantly in the lakes that have formed recently in rift valleys of Africa, primitive fish groups underwent spectacular diversifications in the rift valley lakes that developed as North America separated from Europe and Africa. Today, their 200-million-year-old fossils are preserved in lake sediments of eastern North America.

Other changes in the physical environment have had global effects. During the past 35-million-years or so, a decrease in mean annual temperature at the surface and an increase in seasonality and aridity promoted evolutionary changes in plants that propagated up the food chain. A proliferation of new species of seed-producing herbs and grasses contributed to a rampant diversification of seed-eating rodents and song birds, which in turn fostered a great increase in the diversity of predatory snakes (Figure 3.14).

Ironically, deterioration of a species' habitat can also promote evolutionary diversification if its effect is to fragment the habitat, producing isolation and eventual speciation. This occurred when the fragmentation of tropical rain forests in Africa and South America during dry intervals of the current ice age led to the origin of many new species in the small remnants of forest that survived until the return of better times.

Adaptive Radiation

Rapid proliferation of species within a group constitutes adaptive radiation. This process accounts for most evolutionary change. Typically, numerous distinctive new taxa arise during relatively brief intervals in the early stages of adaptive radiation. Fossil discoveries that date from 600-million-years ago, when organisms first developed hard skeletons, record the initial explosive radiation of animal life. But more recent, and more modest, radiations present special research opportunities when they can be studied with high-quality analytical tools that test diversification as well as spatial and temporal distri

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement