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An Inordinate Fondness for Confounding Darwinians


Write FAIL by another Darwinian prediction: there's no relationship between the length of a branch on Darwin's "tree of life" and how many leaves it has. Evolutionists find this result of a massive study surprising and disconcerting.

The question is this: Shouldn't groups of organisms that have been evolving the longest have the most species? If neo-Darwinism could make any law-like predictions, this should be it: the inexorable pressure to evolve or perish should lead to the most species in the oldest groups:

the most fundamental expectation in macroevolutionary studies is simply that species richness in extant clades should be correlated with clade age: all things being equal, older clades will have had more time for diversity to accumulate than younger clades.
So say Rabosky, Slater and Alfaro, who have just published the most exhaustive study to date of species richness as a function of time. They examined species counts for 1,397 clades, representing 1.2 million species "for taxa as diverse as ferns, fungi, and flies" (emphasis added throughout). Here's what they expected, as reported in their paper in PLoS Biology:
The most general explanatory variable of all is clade age: clades vary in age, and this age variation should lead to differences in clade diversity, particularly if all clades have identical net rates of species diversification through time. If clade diversity is generally increasing through time, there is a strong theoretical expectation that species richness should be associated with their age (Figure S1). Even if individual clades are characterized by a "balanced" random walk in diversity, such that speciation and extinction rates are exactly equal, we may still observe a positive relationship between age and richness through time if clade diversity is conditioned on survival to the present day (Figure S1). Stochastic models of clade diversification through time consistently suggest that species richness and clade age should be correlated. These expectations differ from patterns observed for extinct clades, presumably because living clades have survived to the present to be observed. The expectation that age and diversity should be correlated does not minimize the importance of evolutionary "key innovations" and other factors as determinants of clade richness. In fact, to the extent that such factors influence net diversification rates, their effects should further accentuate differences in richness attributable to age variation alone.
Well, guess what. They aren't correlated. "Clade Age and Species Richness Are Decoupled Across the Eukaryotic Tree of Life," says he paper's title. "At the largest phylogenetic scales, contemporary patterns of species richness are inconsistent with unbounded diversity increase through time," the researchers found. "These results imply that a fundamentally different interpretative paradigm may be needed in the study of phylogenetic diversity patterns in many groups of organisms." Much to their consternation, they couldn't wiggle out of this result (readers can check the open-access paper for how many ways they tried).

The three biologists certainly are aware of complicating factors that might rule out a neat, clean graph. They know that "Some groups, like beetles and flowering plants, contain nearly incomprehensible species diversity, but the overwhelming majority of groups contain far fewer species." Only one species of tuatara, for example, remains after 200 million years on the planet. Sometimes extinction rate exceeds speciation rate; sometimes the ecological niche puts constraints on the ability to diversify. Or, species counts might be artifacts of our taxonomic system or the habits of collectors. Still, even when correcting for these factors, Rabosky et al. expected some remnant of a law-like trend between clade age and species diversity. Not only was no correlation found at the large scale, it was not found at finer scales either. When they authors examined beetles in more detail, for instance, age and diversity showed an even lower correlation than for the bigger picture.

This failure of expectations left them scrambling. It's important to understand the causes for this decoupling, they point out, because most phylogenetic models rely on the implicit assumption that clades should diversify over time at some kind of predictable evolutionary rate. "If age and richness truly are decoupled, then species richness in clades should not be modeled as the outcome of a simple time-constant diversification process, as is done in the overwhelming majority of evolutionary and biogeographic studies." Note that point: the "overwhelming majority of ... evolutionary studies" is based on an assumption that is demonstrably wrong!

Commentary by Harmon

When faced with contrary data this strong, evolutionists have to be immensely creative in coming up with ways to dodge the implications. Luke J. Harmon, for instance, commenting on this paper in the same issue of PLoS Biology, tries humor. He tinkers with an irrelevant joke by J. B. S. Haldane who, noting the 400,000-some-odd species of Coleoptera, quipped that "God has an inordinate fondness for beetles." Harmon titled his paper, therefore, "An Inordinate Fondness for Eukaryotic Diversity."

The point of his commentary is that this is not really a problem; sure, the study showed that it is "difficult or impossible to predict how many species will be found in a particular clade knowing how long a clade has been diversifying from a common ancestor" -- but one thing evolutionists can take heart about, he assures us: we're slowly becoming ever wiser and more knowledgeable about Darwin's world:

This pattern suggests complex dynamics of speciation and extinction in the history of eukaryotes. Rabosky et al.'s paper represents the latest development in our efforts to understand the Earth's biodiversity at the broadest scales.
Where is the understanding exactly? Evolutionists predicted a trend, and found none. Does labeling the situation "complex " help? Does a drunken sailor's staggering suddenly make sense simply by speaking of it as reflecting a "complex dynamic"?

Harmon praises Rabosky et al. for "the most ambitious study to date" saying, "This provides a remarkably complete view of what we currently know about the species diversity of clades across a huge section of the tree of life." He imagines an escape hatch in the future, saying that their "analysis is not the final chapter" because "the tree of life is still under construction, and the total number of species in some clades is best viewed as an educated guess." Maybe somebody else will find a pattern some day. With more genomes, or with improved species counts, who knows?

"Still, the results in Rabosky et al. are intriguing and will certainly inspire further study, which I expect will be focused on testing more sophisticated mathematical models, beyond the constant-rate birth-death models prevalent today, that might be able to explain patterns in the data." Yes, falsifying evidence is indeed "intriguing." After that, Harmon wanders off into a distracting diversion about another evolutionist's quip, this one by Huxley, who joked about "Santa Rosalia as the patroness of evolutionary studies." Pay no attention; there's no falsification here. Look at this nice shrine!

News Coverage

How did the science news media spin this result? Michael Alfaro, senior author of the paper, works at UCLA, where a press release written by Stuart Wolpert gave the official interpretation for public consumption (for instance, on PhysOrg). "Why evolution has produced 'winners' -- including mammals and many species of birds and fish -- and 'losers' is a major question in evolutionary biology," we're told.

Scientists have often posited that because some animal and plant lineages are much older than others, they have had more time to produce new species (the dearth of crocodiles notwithstanding). This idea -- that time is an important predictor of species number -- underlies many theoretical models used by biologists. However, it fails to explain species numbers across all multi-cellular life on the planet, a team of life scientists reports Aug. 28 in the online journal PLoS Biology, a publication of the Public Library of Science.

"We found no evidence of that," said Michael Alfaro, a UCLA associate professor of ecology and evolutionary biology and senior author of the new study. "When we look across the tree of life, the age of the group tells us almost nothing about how many species we would expect to find. In most groups, it tells us nothing."

Another idea, that some groups are innately better or worse at producing species, similarly fails to explain differences in species number among all of the major living lineages of plants and animals, the life scientists found.

So far, this is a forthright statement of the findings. Wolpert gives significant space to Alfaro's favorite rescue strategy, that of "adaptive zone carrying capacity" -- the notion that speciation will proceed up to the point where an adaptive zone is filled to its carrying capacity, then will stop. "Most of the groups that we studied have hit their limits," Alfaro said. "Ecological limits can explain the data we see." This is, of course, not an explanation but a post-hoc rationalization.

So despite the despairing tone of the paper, Alfaro finds a little light in the darkness: "The ultimate goal in our field is to have a reconstruction of the entire evolutionary history of all species on the planet," he says. "Here we provide a piece of the puzzle. Our study sheds light on the causal factors of biodiversity across the tree of life."

But in the paper, the three authors jointly considered and rejected adaptive zone carrying capacity as a suitable explanation for the data. The idea of adaptive zones is not new; George Gaylord Simpson coined the phrase in 1953. Adaptive zone carrying capacity was one of several "diversity-dependent processes" the authors investigated that might result in the decoupling of time and diversity they found. The explanation would be that "ecological opportunity influences the tempo and mode of species diversification through time."

A fallacy in this explanation, though, is its assumption that carrying capacity is static: "We may not understand the ecological mechanisms underlying 'carrying capacity dynamics, but we must still wrestle with substantial neontological and paleontological evidence for their existence." The dynamics exist, they mean. Organisms have uncanny abilities to break out of the box and enter new niches, or to rebound after mass extinctions; the explanation, therefore, fails when considered in the long term. It certainly does not explain why one species of tuatara survives in the same adaptive zone as hundreds of species of beetles.

The authors would not have left time-richness decoupling as an unsolved problem if any number of explanations they considered were of any help: "we are not presently aware of any non-biological mechanism that can account for this lack of relationship," they conclude. Maybe in the future someone will find a law-like pattern; for now, it's a failed prediction of Darwin's tree of life that may require a "fundamentally different interpretive paradigm," as yet unknown.

Intelligent design theory holds no fixed view on common descent per se, with some in the ID camp being personally skeptical of the idea and other more accepting. Either way, from an ID perspective, there seems no reason to expect species richness to correlate with time. The data fit well with ID predictions, therefore, but represent a strong disconfirmation of neo-Darwinian predictions. Once again, nature seems to have an inordinate fondness for confounding Darwinians.

Image credit: Will Montague/Flickr.