With complexity can come diversity. Although organisms of greater complexity are more susceptible to extinction, their defense seems to lie in numbers. We are just beginning to study their forms in detail, but it does appear that the number of bacterial species is far lower than the number of insect species. Yet if complexity brings with it the price of greater vulnerability to extinction, how have complex animals and plants survived as long as they have on this planet through the various mass extinctions? There must be some aspect of complex animals and plants that helps protect them from extinction—not as species, but as higher taxa—categories above the species level.
The history of animal phyla during and after the Cambrian Explosion described in Chapter 7 is an example of this. Some paleontologists have suggested that as many as 100 animal phyla may have evolved during the Cambrian period (although the consensus seems to be far fewer than this). Some of these phyla went extinct during the Cambrian or at its end. Since that time, not a single phylum has gone extinct. It is probably not a simple case of weeding out the bad from the good, where the survivors were those body plans best suited for our world. Rather, it appears that the surviving phyla have endured subsequent planetary disasters by having large numbers of species. As long as a single species survives, the phylum survives and is in a position to re-diversify. In the Cambrian, on the other hand, all phyla contained just a few species each; the Cambrian disasters eliminated whole phyla because there was such low species-level diversity within the various phyla. As far as ani mals are concerned, the Cambrian (or just before) was the most dangerous period in the long history of Earth. Since that time, millions of species have evolved in the various phyla, making them far more "extinction-proof." Diversity—the stocking of body plans through the evolution of numerous species—may be the best protection against extinction.
How do higher animals and plants create and then maintain this diversity? First, they have evolved rapid (compared to bacteria) speciation rates. Because higher organisms use sexual reproduction as the dominant means of reproducing, they create a great deal of variability within populations, upon which natural selection can act. Speciation (the creation of new species) occurs when small populations split off from larger parent populations and can no longer exchange genes. Gradually, adaptation of the smaller population makes it sufficiently distinct from the parent population to prevent successful interbreeding if the two populations come in contact again.
This process is critical to maintaining diversity. Constant formation of new species is needed if diversity is to persist, for there is a relatively high rate of species extinction in all groups of animals and higher plants. Since the Cambrian Explosion, the engine driving the creation of new species has been causing the diversity of complex life on Earth to increase, though long-term gains have been periodically—and temporarily—reversed by the various mass extinctions. During the mass extinction events, it is diversity that saves higher taxa. If a given phylum or major body plan is sufficiently diversified— is represented in many forms living in many different environments—it has a high likelihood of withstanding the extinction event.
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