chekmate

Thinking about evolution. They say that who has the benificial mutation passes it on to the next generation, right? We have arms, right? The arm didn't sprout all at once, right? So, here's my question. How could an animal that had an extra nub, or a partial arm or a leg, be more likely to survive then it's competition?

NialScorva

Because in a world where everything has at most half an arm, it's just as good as anything else

Doubting Didymus

Extra super simple quick answer #1:

We get arms from fishy-arms such as on a lungfish. The lungfish-like things got them from fins, which can be pretty much any size at all and still be useful. That this transition occurred becomes very obvious when you look at fin-bones in modern bony fish, and compare them to a human arm, as the structures are 'homologous'.

Ta daa!

edited for spelling

[ August 11, 2002: Message edited by: Doubting Didymus ]</p>

pz

First step: fin folds. Simple structures that stabilize the fish in movement.

Second step: Fin folds made rigid by incorporation of bony rays.

Third step: Investment by muscle allows controlled movement for steering.

Fourth step: Regionalization/specialization of pectoral/pelvic regions for greater efficiency.

Fifth step: Increasing refinement of pectoral and pelvic motility.

Sixth step: Certain lineages of fishes specialize in 'bottom walking', using the limbs to propel themselves along surfaces.

Seventh step: preadapted bony fins are useful for terrestrial locomotion in ancestral tetrapods.

Eighth step: Further refinement and specialization of limbs for terrestrial life.

All of the intermediate steps were clearly adaptive, and the advantages of subsequent stepwise change are apparent. Furthermore, a large part of this series is reasonably well-represented in the fossil record -- I recommend reading Jenny Clack's _Gaining Ground_ or Zimmer's _At the Water's Edge_ to get the details.

Doubting Didymus

Well yes, if you want to go into a little depth, PZ, you COULD put it that way, but my response was DEFINITELY... shorter.

Camaban

I think the saying "In the world of the blind, the one (or half) eyed man is King"

has relevance here.

braces_for_impact

That's an old question. It has a black and white mentality to it. It assumes the appendage, eye, what have you is useless until it becomes whole, which is not the case. Each step along the way to that whole arm, eye, etc. had it's own usefullness which, when passed on to offspring, gave them an advantage over those that did not have the half appendage, eye, etc.

Caverdude

Okay, let me turn the question around a little. What good are vestigal legs on a whale? They exist on some species (I don't know which right now) - these useless little legs...why are they there? Also, some snakes have them...again why? Maybe because they eveolved from species that originally had these appendages, huh? You can look further and find many examples of evolutionary leftovers, like our nub of a tailbone. Think about it.

monkenstick

one thing I'd like to add

large changes do not neccessarily have to happen gradually - developmental genes and their expression can cause large phenotypic changes even with only very small changes in cis-regulatory elements or in the genes themselves.

lpetrich

According a beanbag model of genetics, that would certainly be a puzzle. However, development-control genes are parts of networks of interacting genes, meaning that a drastic change in one gene can disrupt the network.

Thus, snakes and whales would have vestigial hip and leg bones because a mutation that completely knocks out their growth may have some bad side effect elsewhere, like causing a whale's front fins to be deformed or absent.

Evolutionary developmental ("evo-devo") biology is nowadays a very hot field, and some longstanding riddles have been solved, with continued research likely to solve many others.

[ August 11, 2002: Message edited by: lpetrich ]</p>