5.4 Evolutionary radiations

Processes that drive evolutionary radiations

Many different processes can cause groups to undergo dramatic proliferations that result from an elevated speciation rate. One of these is the phenomenon of species selection (see section 5.2 of this chapter). Species selection may result in an evolutionary radiation not because organisms within a given group are more fit (due to natural selection), but instead because the species they belong to are more likely to speciate. Another process that can result in a higher speciation rate has been discussed in the previous section where we described the Turnover Pulse Hypothesis. In Turnover Pulse events, episodes of climate change cause populations of species that belong to multiple groups to become geographically isolated, leading to an increase in rates of speciation. Therefore, Turnover Pulses produce “climatic radiations.”

Adaptive radiation is the most frequently discussed cause of evolutionary radiation in the scientific literature. The increased rate of diversification in adaptive radiations is triggered by biotic factors and primarily involves sympatric speciation (i.e., speciation in geographic proximity). In such cases, species diverge in their morphology and preferred habitat, such that they come to compete less with one another over time. Darwin's finches, which were introduced in the introductory section of this chapter, provide a classic example of an adaptive radiation: the descendants of an ancestral tanager (not, as it turns out, a type of finch) diversified into different ecological niches on the Galapagos Islands in order to reduce competition for limited resources.

Another type of evolutionary radiation frequently encountered in the scientific literature is a geographic radiation. In a geographic radiation, diversification is triggered by the existence of numerous opportunities for geographic isolation that lead to speciation. These are especially apt to occur in regions that are geologically complex, such as tectonically active island chains or mountainous regions, and are thus fueled by increased opportunities for geographic isolation.

Left, the Hawaiian island chain, photographed by an astronaut aboard the International Space Station (NASA; public domain). Right, the central Andes Mountains, photographed facing eastward (towards Argentina) by an astronaut aboard the International Space Station (NASA; public domain).

The process of geographic isolation may be why island chains often serve as sites of evolutionary radiation. Note that phenomena that might lead to geographic radiation in one type of organism might not necessarily lead to radiation in a different type. This is because certain types of ecologies and dispersal abilities will make populations of species more or less likely to become isolated in general and thus more or less prone to speciate.

One way of thinking about the different processes that drive evolutionary radiations involves distinguishing between the relative role(s) that biotic as opposed to abiotic factors play in causing the increase in speciation rate. For instance, the existence of an adaptive radiation implies a prominent role for biotic factors such as competition and sympatric speciation. By contrast, a geographic radiation implies a prominent role for abiotic factors, such as tectonic forces, that lead to increased opportunities for geographic isolation.

It is important to recognize that both abiotic AND biotic factors are often involved in evolutionary radiations. This is because several different processes may interact to produce the radiation. A likely example involves African cichlid fishes (see image at the top of this page). These fishes occur in geographically complex lakes in a geologically dynamic rift valley that creates numerous opportunities for the isolation of aquatic species.

The East African Rift Valley; some low-lying areas within the rift are filled with water, forming freshwater lakes (the bright-white region is sunlight reflecting off of Lake Magadi. Photograph captured by an astronaut aboard the International Space Station (NASA; public domain).

One process fueling this evolutionary radiation is geographic in nature and is ultimately caused by abiotic tectonic forces that create basins that can fill with water, forming lakes. Furthermore, climate in this region has cycliced repeatedly between wet and dry conditions. During drier times, larger lakes would fragment into many small, isolated lakes, which would become cauldrons for geographic isolation and speciation. During wetter periods, these lakes reconnected and the newly diverged species expanded their ranges into the larger, continuous lakes. The cycle would then repeat as the climate dried again. Thus, African cichlids might also comprise a climatic radiation.

That might not, however, be the end of the story. There is also likely a biotic factor that makes cichlids more likely to speciate. Cichlids have complex behaviors associated with their breeding systems that might favor the formation of reproductive groups that are apt to evolve rapidly. Thus, the African cichlid radiation may also involve an example of species selection. This is supported by the fact that there are other groups of fishes in the region that are far less diverse than the cichlids, despite living in a similar environmental setting.

We see from this example that complex factors may interact in many different ways to produce an evolutionary radiation. This is one of the exciting aspects of evolutionary radiations, and likely one of the reasons why so many scientists are interested in studying them. Scientific study of evolutionary radiations is not, of course, restricted to the living biota. Paleontologists have uncovered many episodes of dramatic diversification across the history of life, from the Cambrian radiation of animals to the diversification of mammals following the end-Cretaceous mass extinction. Intriguingly, the phenomenon of adaptive radiation was first introduced by the famous paleontologist Henry Fairfield Osborn to explain the proliferation of certain Cenozoic mammal groups. And some of the most notable instances of diversification in the history of life can be seen in the evolutionary radiations of fossil groups like Cambrian trilobites and Mesozoic ammonoids.

Left, the Cambrian trilobite Wanneria walcottana from the Kinzers Formation of Lancaster County, Pennsylvania. Right, the Cretaceous ammonite Discoscaphites conradi from the Fox Hills Formation of South Dakota. Scale bars equal 1 cm. Both specimens are from the collections of the Paleontological Research Institution, Ithaca, New York. Image by Jonathan R. Hendricks.

Equally staggering is that each of these tremendously species-rich groups are now entirely extinct. This matches a more universal pattern also common to human existence: sometimes you’re up and sometimes you're down; sometimes you’re the pigeon, other times the statue; every dog has its day, and some days are longer than others.  There are two key sides of the coin when it comes to understanding the evolutionary histories of major clades of organisms: why, and how much, are they speciating?; and why, and how fast, are they going extinct? It is this delicate balance and complex interplay that determines a clade’s fate, be it long-term burgeoning or prolonged withering. This is why research on both fossil and extant organisms will continue to produce fundamental insights into this key area of macroevolution.