©2003  by Gerard Wakefield
(This article may be copied for educational purposes only.)

"Microevolution vs. Macroevolution Once Again"

In previous columns  [#1  #2  #3], I showed how changes within a single species help that species deal with environmental changes, but that these changes do not lead to a completely new species. This is intraspecific microvolution versus transpecific macroevolution. Evidence that microevolution occurs but macroevolution does not is cropping up everywhere.

Jonathan Losos, professor of biology at Washington University and director of the Tyson Research Center, has studied tree anoles (Caribbean lizards) and witnessed unquestionable microevolution, as anoles' feet and leg size changed when they were introduced to new kinds of tree branches. Since, according to Losos, "[e]volution most often occurs over timescales humans cannot experience," he assumed that the microevolution he witnessed was "a tiny window" into an eons-long macroevolutionary process (Losos 2001: 67). However, the only evidence available to him was the microevolutionary adaptations he saw among the anoles. The "tiny window" was not enough to conclude that these small changes lead to macroevolution, in which new species of anoles are produced over millennia.

He referred to an experiment begun in the 1970s by Thomas and Amy Schoener of UCAL Davis, who introduced brown anoles (Anolis sagrei) onto 20 anole-free islands in the Bahamas and studied them to see if climbing on unfamiliar trees would cause a change in the dimensions of their limbs. When Losos examined the introduced anole populations in 1991, he found that, because each island had different kinds of vegetation (some islands' trees had broad trunks, while other islands only had shrubbery), the A. sagrei populations experienced changes in their physiography that matched the new vegetation. Populations that had been placed on islands with trees that had broad trunks and branches developed legs that were several millimeters longer than members of the same species that had been placed on islands with plants that had narrower surfaces (Ibid. 67-68).

This change comes from "phenotypic plasticity" - the genetically pre-programmed ability of any living organism to undergo microevolutionary changes in response to environmental changes. Losos stated that phenotypic plasticity "could even result in individuals that are genetically identical. Studies on bone growth, conducted on mammals and birds, indicate that the bones of individuals experiencing different stresses and strains during growth develop differently" (Ibid. 68).

He then set up an experiment by raising baby A. sagrei in captivity. He raised one group on wooden dowels 0.7 cm wide, the other on flat boards 8 cm wide. He found that the anoles that grew up on the broad surfaces developed longer limbs than those that grew up on the narrow dowels. He concluded: "These findings suggest that the differences observed among the introduced Bahamian populations might result from phenotypic plasticity" (Ibid. 68). He then made the leap of faith that these minor changes must be his "tiny window" into eons-long macroevolution - with the always necessary "time and chance" - despite the lack of hard evidence for this conclusion:

"[E]ventually, mutations will occur, by chance, in these populations, making the individuals even better adapted to this new habitat. These changes, being genetically based, would lead to evolutionary change. Given enough time, mutation and natural selection could produce species substantially different, and better adapted, than the ancestral form. In this way, phenotypic plasticity could be an important means by which major evolutionary changes are initiated" (Ibid. 68).

Ironically, Losos himself stated that fossil anoles preserved in amber supposedly 20 million years old "are virtually indistinguishable from the tree-canopy habitat specialists living today…" (Ibid. 66-67). This shows that microevolutionary changes, such as the ones he saw with his own eyes, do not become macroevolutionary. Otherwise, the allegedly 20-million-year-old anoles would be substantially different from today's anoles, and not "virtually indistinguishable" from them.


Losos, J. B. 2001. “Evolution: A Lizard’s Tale.” Scientific American 284, no. 3

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