What makes an evolutionary branch dead?
Apart from the obvious - when a species is out competed by a competitor - two things:
The achievement of an almost ideal state for any given niche. Examples of these would be crocodiles, sharks and cephalopods. Evolution is driven by the natural selection of one body type over the other based on the ability of the new shape or type to outcompete the incumbent type. At some point though an ideal is reached - the shark isn't called 'the perfect killing machine' for nothing and it's hard to improve on perfection. At that point natural selection tends to stall, or to be more precise the incumbent tends to outcompete any possible challenger. What you end up with is a body type with minor evolutionary changes, more teeth, increase or reduction in size etc. etc. but the basic form stays the same.
The second reason is that, in my opinion, hybridisation is more important to speciation (a major evolutionary change) than most people would think. Dawkins argues that evolution is a continuing steady process; Gould argued that the fossil record doesn't reflect that and that a state of punctuated equilibrium better fits the evidence.
I think that they're both right but that they've missed an important evolutionary trick.
Minor evolutionary change, called microevolution, occurs continuously, sharks over time get bigger or smaller, evolve sharper teeth or even less teeth dependent on the requirements of the environment. This constant change can, over a very long period, lead to major changes, or macroevolution, that's where the cumulative changes are so great that the animal you end up with looks nothing like its antecedents - it is in fact a new species.
Gould noticed however that the fossil record indicates that at several points in the geologic record speciation seems to be accelerated - the Cambrian explosion being one major example, he thought that the mechanism was an increase in the rate of natural selection with micro evolution speeding up leading to macro evolution and speciation. I believe the viability of hybridisation between species is the driving factor during those punctuated events in the normal equilibrium. A species unable to hybridise under those circumstances is, to all intents and purposes, a dead branch.
[This message has been edited by Uncas (07-03-2011 07:59 AM).]