<i>Science</i> Paper Admits Evolution Mechanism of "Adaptive Radiation" Lacks Empirical Evidence - Evolution News & Views

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Science Paper Admits Evolution Mechanism of "Adaptive Radiation" Lacks Empirical Evidence

A recent article in Science titled "Adaptive Radiation: Contrasting Theory with Data" admits that the evidence documenting the precise workings of a key macroevolutionary mechanism -- "adaptive radiation" -- is missing. The article concedes that "how exactly radiation occurs, and how it differs among taxa and in different settings, as well as why some lineages radiate and others do not, are still unclear."

When studying evolutionary biology in college, I learned that new types of organisms have commonly appeared abruptly in the history of life. Since Darwinian evolution is supposed to proceed by "numerous, successive, slight modifications," this data made little sense in the light of evolution. Our professors always reassured us that rapid evolution or abrupt appearance of major macroevolutionary innovations could be explained by a mechanism they called "adaptive radiation." For example, my intro-level college biology textbook, Campbell's Biology, gives the short explanation that "[t]he evolution of many diversely adapted species from a common ancestor is called adaptive radiation." (Biology, 4th ed., p. 444) Very few additional details are provided about this mechanism, which is supposed to explain how many species rapidly diversify and fill new niches quickly. This always left me wondering: is "adaptive radiation" simply a magic wand that can be waved over the history of life to explain how diverse macroevolutionary innovations can arise quickly in the history of life, or is there actually hard evidence for this theory? This recent article in Science has confirmed my suspicions that it's the former.

As noted, the article is titled, "Adaptive Radiation: Contrasting Theory with Data" (the title provides another example where scientists use the word "theory" in a sense that doesn't mean a well-tested explanation) and it admits that our empirical examples of adaptive radiations are all within low-level taxa groups: "Darwin's finches on the Galápagos islands, Anolis lizards on Caribbean islands (Fig. 1), Hawaiian silverswords, and cichlids of the East African Great Lakes." According to the article, some of these examples (like Darwin's finches, or the silverswords, which only have about 5 species) are not impressive because they have "relatively low species richness." Given the similarity of these species (in each case, we're still dealing with finches, lizards, silverswords, or aquarium fish), these do not inspire confidence in the creative power of adaptive radiation to produce large-scale changes in organisms.

The mechanism of adaptive radiation is said to account for some of the most important events in the history of life. Robert Carroll observes in Trends in Ecology and Evolution that "within less then [sic] 10 million years, almost all of the advanced phyla appeared, including echinoderms, chordates, annelids, brachiopods, molluscs and a host of arthropods" and concludes that "extreme speed of anatomical change and adaptive radiation during this brief time period requires explanations that go beyond those proposed for the evolution of species within the modern biota." Indeed, this recent article in Science admits molecular and fossil evidence for abrupt appearance, stating, "An increasingly common theme in molecular studies of speciation is the finding of a burst of species diversification early in a clade's history (37, 38). This pattern is also seen in the fossil records for some groups." The paper subtly acknowledges evidence for rapid appearance of species, and then openly admits that we don't have sufficient empirical to understand this allegedly crucial evolution mechanism of "adaptive radiation" to explain abrupt origins of species:

"More empirical studies are needed specifically aimed at assessing the predictions discussed above. At present, we are left with a haphazard set of studies that happen to be relevant."

"More generally, evolutionary biology is an inductive science in which we establish generalities by the accumulation of case studies. The number of adaptive radiations that have been extensively studied from the many different perspectives relevant to our discussions is surprisingly small. More detailed studies, integrating across a variety of approaches and disciplines, is needed to build a reservoir of case studies from which generalizations can be drawn."

(Sergey Gavrilets and Jonathan B. Losos, "Adaptive Radiation: Contrasting Theory with Data," Science, Vol. 323:732-737 (February 6, 2009).)

Despite the candid admissions about the lack of data showing how adaptive radiations proceed, this magic wand of "adaptive radiation" is waved over the history of life to account for the huge portions of macroevolutionary innovation. This does not inspire confidence in modern evolutionary biology.

The article further admits that a problem when studying radiations is that you're dealing with one-time events that are unpredictable: "extrapolation from processes operating today to what happened early in the history of a radiation is problematic; in the past, different processes may have operated or the outcome of these processes may have been different." In other words, when we're dealing with one-time events that can't be repeated or tested in the laboratory, we can't predict the outcome:

"In the 150 years since publication of the Origin, adaptive radiations have continued to astonish and inspire scientists and the public alike. But how exactly radiation occurs, and how it differs among taxa and in different settings, as well as why some lineages radiate and others do not, are still unclear. Most likely this is because there is no single answer: Lineages vary in manifold ways, various evolutionary factors act simultaneously, similar evolutionary outcomes can be achieved via alternative paths, and the contingencies of place and time play a large role in guiding the evolutionary process."

(Sergey Gavrilets and Jonathan B. Losos, "Adaptive Radiation: Contrasting Theory with Data," Science, Vol. 323:732-737 (February 6, 2009).)
The lack of predictability inherent in this crucial evolutionary mechanism doesn't mean that adaptive radiations are unscientific as explanations. Historical sciences often deal with unique historical events that could not have been predicted and certainly cannot be repeated. But the admissions of this paper show that evolutionary biology does not always rely on predictable processes.

This article should be admired for its candidness about the lack of data for adaptive radiation. Somehow I suspect that when biologists answer its calls for more studies, they won't find evidence that natural selection--even operating upon species expanding into new, open habitats and niches--can produce large-scale evolutionary change at the rapid rates required by the fossil and molecular data.