MrDarwin

What kind of evolution are we talking about here--micro or macro? Phyletic change or speciation?

Either way, I'm not sure that population size makes much difference. I think the most rapid evolution occurs when a species:

(1) survives a mass extinction and is faced with a paucity of predators, a lack of competition from other species, and a large number of empty ecological niches; or

(2) moves into a new habitat and/or exploits a new or previously unexploited niche (as when insects or dinosaurs acquired wings--rare, one-time events that were not apparently directly related to a mass extinction).

In some cases #2 may relate directly to #1, as when the ancestors of whales moved into the oceans, where several niches had been vacated by the extinction of large marine reptiles.

MrDarwin

What is an "appropriate mutation"?

There are genetic constraints, and as you note, there are external constraints: selective pressure. As we've noted here before, a particular mutation may be disadvantageous in one situation, but highly advantageous in another. And when a species moves into a new habitat--one example I use is the European dandelion moving into the entirely artificial habitat of the North American lawn--the external selective constraints change, allowing the populations in these new habitats to go in an entirely different evolutionary direction than the populations that stayed behind in the ancestral habitat.

Oolon Colluphid

It was a waffly was of saying the initial slight modifications that could be picked up and turned into something drastically different, many generations down the line. (Actually, that was the waffly way of saying it ) I was thinking specifically of the early hypothetical proto-lens in a nautilus eye, the mutations causing, say, a layer of jelly or transparent cells ono the retina, for instance. If such mutations occurred, they weren't picked up on by NS for some reason, so they were either somehow detrimental in their early stages, or else the genetics of it never happened. I find the former more plausible, but can't really justify saying so

Oolon

Peez

There are two processes that are known to drive evolution: natural selection and genetic drift.

Genetic drift is a change in the frequency of genes in a population due to "sampling error" (random chance). This process tends to be more important in small populations (especially less than about 10 individuals), but not in large populations (very weak if there are more than about 250 individuals). So, evolution by genetic drift tends to be faster in smaller populations, but does not produce adaptations.

Natural selection is responsible for the evolution of adaptations. We know that natural selection can cause evolutionary rates far higher than those observed in the fossil record, and almost all traits examined have "genetic variance" (differences in genes in the population that would allow natural selection to act), so a lack of genetic variance does not seem to typically constrain evolutionary rates, so the higher number of mutations observed in a larger population seems unlikely to lead to a higher rate of evolution by natural selection. Conversely, since natural selection may act on an entire population, there is no reason to expect a smaller population to evolve by natural selection any faster than a large one.

Things can get more complicated when both genetic drift and natural selection are acting, as genetic drift can sometimes result in changes that allow evolution by natural selection to move in a new direction. Also, the total population size is less important than the "effective" population size, a mathematical quantity that reflects the population-genetic characteristics of the population.

By the way, evolutionary rate (that is, the rate at which a character or suite of characters change over time) should be expressed proportionally, to avoid scaling effects (how do you compare the rate of evolution of body size in a blue whale and a shrew?). The most widely-used units are Darwins: 1 Darwin is equivalent to an increase by a factor of e (~2.718) in 1 million years (a population of organisms that evolves from 1 cm long to 2.718 cm long in 1 million years has an evolutionary rate of 1 Darwin).

Peez