Jeff Houlahan writes: >I think it's a reasonable question to ask 'How important is >competition in driving evolutionary change?'. Fitness is a measure of how >well an organism is adapted to its environment. An environment is more than >just competing species. I can imagine many systems where the way an >organisms looks, acts, respires, metabolizes etc. has been shaped relatively >little by competitors but relatively a lot by predators, mutualists, >pathogens and abiotic environmental conditions. It may not be an either-or >question but it certainly is a 'relative importance' question.
It may be a surprising answer, but it shouldn't be with just a little thought, but intense ecological competition brings evolutionary change to a halt. It's only during periods when competitive pressures are released (generally following environmental catastrophes) that evolution is allowed to explore a variety of new biotypes and evolution "advances." Olivia Judson, who will be one of the keynote speakers at this year's Evolution conference, wrote a particularly nice essay on this subject 18 months ago in the NY Times entitled, "Evolution 101: No Vacancy, No Evolution." I've put it on-line at: http://67.41.4.238/evo101-no-vacancy-no-evol.html As Judson writes in the piece, this is one of those repeated patterns for which we now have a great deal of evidence. Just a little before Judson wrote her piece, Johnson et al. also published a particularly nice study in Science regarding the recent evolution of cats, and it too recapitulates the basic story. In their case, they relate the rising and falling of the global sea level to the episodic migrations of cats onto new continental territories. With each new unsettling of the competitive landscape, new bursts of evolutionary novelty were introduced into the cats, resulting in episodes of speciation. In essence, the changing environment is acting as a species diversity pump. I've put a copy of their paper up at: http://67.41.4.238/cats.pdf Just a little earlier than either of these papers though, Richard Bambauch and Andy Knoll performed what I believe to be some truly inspired work, based on the late Jack Sepkowski's data. They "binned" species from the fossil record based on some very simple minded categories (physiologically buffered from the environment or not, predacaeous or not, motile or not). From these binnings, they derive three extremely interesting charts. The two great extinction events on earth clearly reset the nature of life with each major extinction. Following the catastrophes of the Permo-Triassic and Cretaceous-Tertiary boundaries, life became more mobile, more buffered from environmental vagaries, and more predacaeous as a step-wise function. Their paper (and its figures) is on-line at: http://www.pnas.org/cgi/reprint/99/10/6854.pdf These two great extinction events did not act merely as diversity pumps, but as a complexity pump. Life became more complex and more "intelligent" following each catastrophe (predators are intrinsically more intelligent than their prey; as one wag once said, "how much intelligence does it take to sneak upon a blade of grass?"). Why should this complexity pumping action exist? The answer is relatively simple. Although life on the planetary surface came close to being extinguished twice, particularly in the first instance, when the time came to repopulate the arena that is the earth again, it didn't start from scratch. It built on the complexity that previously existed. But for a time, in the absence of competition, it was allowed to explore a great variety of biotypes before competition set in again once more slowed evolution to a halt. Wirt Atmar
