Isolated table mountains with sheer cliffs in South America should be natural laboratories for evolution. Why aren’t they?
The pantepui region spanning northern Brazil and parts of Venezuela and Guyana contains some of the most isolated ecological environments on earth. The table mountains (tepui) are so remote and difficult to reach, some have been less visited than the moon. Because of their sheer cliffs 1000 meters on all sides, evolutionists expected the habitats to be natural laboratories for evolution, because organisms managing to eke out a living on top of one tepui would be prevented from sharing genes with those on others. Since the sandstone is said to be 1.5 billion years old, there has been ample time for the animals on top to evolve and diverge from one another in isolation. Time estimates for the isolation of the tepui go back to the Cretaceous.
With this “ideal nursery of speciation” tempting scientists to look for tens or possibly hundreds of millions of years of evolution on these natural laboratories, an international team undertook the arduous task of visiting 17 tepui and collecting samples from amphibians and reptiles to compare their genes. They expected differences; after all, “If individual tepui summits were indeed reservoirs of ancient endemism, phylogenetic analyses of these taxa would identify genetically distinct populations on each tepui without close relatives elsewhere.” And outwardly, “Some of the lowest genetic distances are observed for populations that are currently recognized as distinct species and show striking phenotypic differences,” they said. Their paper was published this week in Current Biology.1
Substantial diversity was the expectation based on the amount of time these creatures are believed to have been isolated. But when they made the “analyses of two mitochondrial gene fragments evolving at different rates,” they were very surprised: “populations of a given species on individual summits are often closely related to those on other summits (e.g., Oreophrynella), or to those from the surrounding uplands (e.g., Tepuihyla).” Many of the differences were less than 1%. “Uncorrected pairwise distances in both genes indicate unexpectedly low genetic divergence — as low as zero — among multiple tepui summit species or populations in five of the six groups (Stefania being the only exception), as well as among some summit species or populations and uplands populations described as distinct species.”
With such a dramatic clash between theory and practice, the scientists went into damage-control mode. No one is going to buy the idea that the frogs and snakes decided to move from one tepui to another. That would mean going down one 1000 meter cliff, crossing a completely different ecosystem at lower elevation, then climbing up another 1000 meters. The scientists looked at other options:
If the tepuis are indeed as ancient as often stated, the young age of extant summit fauna can only be explained by active dispersal among summits with subsequent extinction in the intervening uplands, e.g., during ice ages, or by passive dispersal, e.g., by birds or storms. The highly specific ecological niche preferences of some taxa restricted to tepui summits are likely to have limited active dispersal. Most Oreophrynella species, for example, exclusively occupy rocky habitats with extremely impoverished flora, which are absent in the intervening areas. Time estimates for the isolation of individual tepuis range from the Cretaceous to the Quaternary. The youngest estimates, although widely neglected in biological studies, could be compatible with the low genetic diversity and leave vicariance [geographical isolation] as a possible mechanism for speciation.
Regardless of the mechanism, our study shows that, even in small vertebrates restricted to summit-specific habitats, gene flow has been maintained until recently, making single-tepui endemism an exception rather than a rule. Nevertheless, as several of the taxa studied here (e.g., Oreophrynella and Stefania; Supplemental information) represent phylogenetically distinct lineages restricted to the Pantepui region, this area as a whole may still act as a reservoir of high-level endemism.
Jargon aside, these paragraphs should be read with the astonishment evident in the scientists’ prose. They are grasping for alternatives. “If the tepuis are indeed as ancient” sets up the problem: they are looking at isolated habitats the geologists tell them are tens of millions of years old. In evolutionary theory, a dog-like cow became a whale in less time than that. The fauna look young. How to explain this? They considered active or passive dispersal: maybe a storm blew a frog from one tepui across miles to another one, where the frog continued on as if nothing happened. Maybe a bird carried a snake in its talons from one summit to another. Yet the animals live in highly specialized habitats that don’t exist between the summits.
Their best guess was to choose the lowest possible time estimate for isolation of the tepui (Quaternary, the latest two million years), even though these young ages are “widely neglected in biological studies.” The extinction might have occurred in Ice Ages, they say, leaving the summit organisms isolated to evolve very recently. This explanation, though, raises other questions. What raised up these islands in the sky so recently, if the rock is 1.5 billion years old? Why do they show such little erosion? Why are all 17 tepuis in the study showing endemism (isolated populations with unique fauna) to be the exception rather than the rule?
Their answers to the puzzle of the tepuis were mere suggestions. Ending on a positive note, they suggested that even if the tepuis do not qualify as the “ideal nursery of speciation” they hoped for, maybe the entire pantepui region could be looked at that way. They left that job for someone else.
1. Kok, McCulloch et al., “Low genetic diversity in tepui summit vertebrates,” Current Biology Volume 22, Issue 15, R589-R590, 7 August 2012.
This study is reminiscent of a similar study decades ago that compared fauna on isolated buttes in the Grand Canyon. Shiva Temple in particular, with its forest on top, was thought to have been genetically isolated for millions of years. As with the tepui of South America, no evolution was detected. Funny that you don’t hear much in textbooks or TV programs about these falsifications in their rush to celebrate evolution as biology’s answer to everything.
Astute Darwin critics might notice that the observations from the tepui are compatible with a recent creation, a worldwide flood, and a single ice age. Such notions are ruled out of bounds by the current mandarins of science. There are, however, alternative media where those inclined to consider this option can make their case.