Christopher Lord

When does evolution proceed faster, when the population is large and can draw from a large body of mutations, or when it is very small and those changes actually make a difference in the genome in a timely manner? (ie: propgate to all animals)

I tend to lean towards the small-population-is-swifter side, but I realized I dont know what the current thought regarding this is, and several people in the past here have echoed the larger-is-faster position.

cricket

Small. Mutations don't go far if they're just drops in the bucket.

Christopher Lord

Is there any sort of paper on the topic though? I agree with you, but I cant recall ever reading anything authoritive on the topic.

tronvillain

Well, you really have to specify what you mean by "evolution proceeds faster."

Christopher Lord

By 'proceeds faster' I mean: salient changes of the genome occur more frequently over time.

Quetzal

CL: From what I've read, most of the current thinking on your question boils down to: "it depends".

Apparently, average rate of evolutionary change (if you mean speciation or phyletic evolution) is primarily ecologically dependent (see E.O. Wilson's "Diversity of Life" for a good discussion). Also Mayr maintains that insular or mosaic population distribution patterns (leading, like cricket mentioned, to relatively isolated "small" populations), tend to cause relatively "rapid" and frequent speciation. OTOH, in large, uniformly distributed "continental" distribution patterns there will be little speciation. This pattern is still undergoing study, and is one of the most interesting theoretical research areas of modern evo biology (IMHO). In general, the more gene flow between populations, the less speciation occurs.

There are some observations, however, for which the above doesn't really explain the data. It appears that there may be differential rates of speciation between lineages. IOW, there are some lineages that speciate veeeerrry slowly if at all, whereas others speciate very quickly. There is NO evidence that this differential rate is environmentally dependent. One example Mayr uses is Symplocarpus foetidus (skunk cabbage), which has populations found in both Asia and the northeastern US. Not only are these populations nearly identical morphologically, but they apparently can interbreed! This means we have two populations that have been separated by, what, ~8 my? that haven't speciated. On the other extreme, you have the cichlids of Lake Victoria, which differentiated into 400+ different species in less that 12,000 years. This is the problem Gould/Eldredge were trying to solve (I haven't picked up my copy of "Structure of Evolution" yet, but I imagine this is one of Gould's key points). Naturally, we have ample evidence that agamospecies (distinct bundles of closely-related asexual lineages) of bacteria, for instance, speciate REALLY rapidly.

I know I didn't really answer your question. I guess the problem is there really ISN'T an answer yet. I'd say the overall concensus so far is that evolutionary rates are highly variable.

Aren't you glad you asked?

Christopher Lord

thanks for the answer none the less. 'we almost know' is acceptable for me, I use it all the time

I think my confusion comes from sexual versus asexual. sex speeds up smaller populations while stabilizing large ones, but asexual critters perhaps go faster linearally with population change, since they relies on raw mutations moreso. Does that sound plausable? I'm really tired right now, hehe.

Oolon Colluphid

Ref Morpho's post, I think it comes down, as he says, to the availability of ecological niches, as with the cichlids and Galapagos and Hawaiian birds.

Mutations are, of course, only beneficial or detrimental wrt the environment the phenotype is in. If a large number of niches are potentially available, that makes the probability of a random mutation being beneficial in one of the slightly different niches higher.

IOW, in Dawkins's terminology, a neighbouring position in 'animal space' will have somewhere different ecologically where it can still fit in. If the population isn't forced to abide by the rules of a single niche, variations from the norm may be more likely to find a home in the next door bit of ecological space, if it is available -- at a slightly higher altitude, in stronger winds etc (plants), or in colder waters or weather (animals), and so on.

Erm, I think...?

TTFN, Oolon

Quetzal

CL: I'll have to think about the sexual vs asexual question a bit. I'm not sure that's necessarily valid, but I need to dig up some old references.

Oolon: Niche availability is certainly a crucial issue in biodiversity. Besides "Rappaport's rule" on increasing biodiversity (because of energy input) as you approach the equator, and altitude stratification, you have ecological release (founder effect). The poster child is Pinaroloxias inornata, Darwin's 14th finch from Cocos Island. Because of near-complete lack of competitition, this bird - within a single freely-interbreeding species - has adapted to nearly every single conceivable bird-niche on the island: from shore to hilltop, from snail eater to seed, fruit, and even nectar eater. Even more interesting is that there's no behavioral or morphological barriers yet - in other words all these birds are still the same species. Apparently the young birds pick another bird to copy, either for a few weeks or its entire life. The even emulate other species (there's a warbler and a sandpiper species there). Bill size/shape is about intermediate between finch and warbler. Each individual bird picks its own specialization!

Why is this so cool? Because it is speciation frozen at the moment of speciation. The island is too small and too isolated for the actual budding out of distinct, non-interbreeding species. I'd like to see cretinists wave this one away. (Probably, "They're still finches.")

Of course, niche availability doesn't appear to be the ONLY criteria for speciation, hence the problem. Otherwise, why didn't the stupid clams - one of the few large scale marine survivors of the Permian extinction event - rapidly speciate? All they did was massively reproduce the same flippin species (apparently) all over the world, leaving meters-thick beds of crushed clam shells. Obviously there's other factors involved here. Not to pretend I know what they are, of course.

Oolon Colluphid

Some sort of genetic constraints seem to be part of it, by which I mean that the appropriate mutations -- appropriate initial changes in phenotype -- just don’t seem to turn up, or if they have, they’ve not, at that crucial early stage, been advantageous. Maybe the ‘new gene’ has some other, disadvantageous, pleiotropic effect, or something along those lines.

It is odd. The example I have in mind is the one Dawkins quotes, the lensless but otherwise excellent pinhole camera eye of the Nautilus. They’re the only shelled cephalopod, so an early offshoot of the line that led to squid, octopuses and cuttlefish, all of which have excellent -- lensed -- eyes. As Dawkins says, the nautilus could so easily and immediately benefit from a lens. Why hasn’t it? (The flip side of it is, why didn’t god give it a lens? )

Oolon