An adaptive zone is a set of ecological niches that may be occupied by a group of species that exploits the same resources in a similar manner. (After Root, 1967)

After a new species appears, it continues to adapt to new environmental niches.

Let's look at the dynamics of adaptive radiation. First, a lot of speciation takes place within a short period of time. Remember that natural selection, acting on a single allele, causes a rapid change in gene frequency, followed by a period of very little change. Following the speciation events, there is rapid adaptation and radiation into diverse adaptive zones.

There is some good information from the fossil record on the number and duration on earth of various lungfish genera. The first lungfishes appeared during the Devonian, more than 300 million years ago. There was a rapid increase in the number of new characters, species, and genera but most of these species and genera didn't last long. Later, the rate of new characters being added slowed, and there were fewer, longer-lasting genera. This pattern of rapid change followed by a period of slower activity fits the model we have proposed for adaptive radiation.

If we look at present-day taxa, we find that within any level of taxa, many members of the next lower taxa are concentrated in just a few of the lower taxa, whereas there are many lower taxa with just a few members. In the case of animal phyla, the arthropods have a very large number of species. A very few other animal phyla have a large number of species (but none have even 1/10 as many as in the arthropods) but many phyla have a very small number of species. We would expect that diversification into many taxa at any given level would be a result of adaptive radiation. However, the presence of many species in just a few taxa indicates a role for progressive occupation.

1. General adaptation. A new type of adaptation allows a group to exploit a new adaptive zone. Bird flight was such an adaptation. Once the ability to fly was developed, a whole new adaptive zone was opened for exploitation and radiation into that zone was rapid. How else could an animal like a finch have traveled to the Galapagos Islands from South America? Or, what about arctic birds which live on cliff faces? The arctic tundra doesn't offer much protection and birds which lay eggs on the tundra have to expend a lot of energy guarding them and attacking predators. A bird which lives on a cliff face is protected from predators such as bear and fox. Price points out that there are more than 1,500 species of bird lice, which "colonized" birds, then became parasitic, and radiated into the different microhabitats on bird's bodies.

2. Environmental change. There have been several changes in sea level during the Earth's history. During the Cambrian, more than 500 million years ago, the sea level rose, flooding continental shelves. This provided an opportunity for radiation into this new environment.

3. Archipelagoes. Islands and island groups are isolated habitats - a type of archipelago. (Another type of archipelago would be a mountain isolated in the center of a barren dessert.) Because they are isolated from other habitats, movement into them is a rare event. Typical of these environments is a rare colonization event, followed by rapid divergent evolution. This occurs, because like exploitation of the air by birds, there is a lack of predators and competing individuals and lots of vacant ecological niches. Radiation into archipelagoes requires a diverse habitat to provide the ecological niches - It is unlikely that much adaptive radiation would occur onto a barren island. Examples of adaptive radiation of archipelagoes includes Darwin's finches, Hawaiian honeycreepers and Hawaiian silverswords.

1. Ammonites are a group of shelled molluscs, which were very successful during beginning in the Cambrian, with the explosion in the number and diversity of invertebrates. The present-day nautilus is a relative of this formerly successful, and now extinct group. Since the ammonites were shelled, a good fossil record was left behind. This record shows that many taxa which consisted of very many species became extinct early on while taxa which lasted for a long time often arose from minor branches in the evolutionary tree.

2. Steven J. Gould has discussed the Burgess Shale fauna. The Burgess Shale is a site in Canada which contains soft-bodied fossils dating back to the Cambrian. Soft-bodied organisms sometimes form fossils which look very much like a squashed blackened replica of the original organism. Some people originally thought that the black color was the result of the carbon in the biological structure but it is now believed that the carbon has been replaced by salts in a process not yet fully understood. The Burgess Shale shows many invertebrate taxa which were very common but are now extinct. It seems that some of the animals which were very successful in the Cambrian turned out not to be those which gave rise to the present groups. Among the weird animals found in the Burgess Shale is Wiwaxia a flattened worm-like creature originally mistakenly placed together with the segmented worms but now known to not fit in any currently recognized phyla. Another weird creature is Hallucigenia (The name says it all!) which does not fit into any present phyla. In the Cambrian, there were about 6 genera closely related to Wiwaxia but only two of small clams. However, the clams survived and all of the Wiwaxiids became extinct.

The conclusion from these examples is that one cannot seem to predict eventual success of an adaptive radiation based upon the number of taxa in that group. Some taxa with lots of genera or species become extinct and other taxa with few genera or species survive.

It would seem that organisms become more complex with time. The average amount of complexity increases. Gould has argued that this does not evolution is directed towards increased complexity but rather is just the consequence of a simple principle of mathematics: Imagine that one has a simple organism which then radiates into different niches or adaptive zones. Random variation alone will produce more complex and less complex organisms. But, since the complexity of the "ancestral" organism is already very low, there is a limited range of organisms which could exist which would be less complex. There are many variations which would be more complex! So the average complexity of descents increases, even just considering random variation. One does not have to propose some exotic mechanism involving adaptation to account for increasing average complexity.

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