I have wireless access in the lecture hall today, so I’m going to try liveblogging these talks. This may get choppy! What it will lack in editing will be compensated for by more timely and regular updates. I hope. At least I’ll be able to dump something to the site every 40-60 minutes.
He summarizes the idea that there is a wealth of genetic diversity in populations to allow for effective selection. Lack of mutations should not limit a straightforward selection response. This raises a paradox, however: organisms have phylogenetic niche conservatism. Many species are evolutionarily unadventurous. He works on clades of herbivorous insect species that are sticking to the same plant groups since the Miocene.
May be many niches in nature that are unfilled: example: fish-catching bats have only one species. Where are the nocturnal aerial fish-feeders in other environments? Species don’t just liberally fill every possibility.
Futuyma introduces Gould/Eldredge’s concept of stasis. We need to acknowledge the existence of constraints that are limiting factors on evolutionary possibilities.
Genetic constraints:
In some cases, a “character” doesn’t exist — there aren’t genes or developmental pathways that specify it. For example, thoracic bristle number in flies may not be defined by simple genetic programs. Haldane said humans will not evolve into angels because we lack the required genetic diversity in wings or moral character.
Little or no genetic variance in a character or combination of characters. Looked at Ophraella beetles, asking whether genetic predispositions might limit which species of plants they can feed on. Screened for genetic variation; in about half the cases they found no evidence of genetic variation that would allow for expansion into distantly related plant species. Discussed Bradshaw’s work on genostasis in evolution, which found little genetic variance in heavy metal tolerance in grasses, dessication resistance in rainforest flies, locomotor and life history traits in Hyla. Adaptation observed in some fly species may have been facilitated by hybridization, which introduces the needed variation.
Species evolve along lines of genetic least resistance, where variation is present in the population. Other directions may not be easily followed.
Successful genetic change may require correlated change in multiple other traits, so genetic diversity may hinder evolutionary change by making the optimal combinations rare in the population. Demanding simultaneous changes in larval and adult characters, for instance, might limit rates of change.
Major issue: how much evolutionary novelty is due to new mutations vs. recombination of standing variation in a population?
What accounts for stasis? Most adaptive novelties are associated with shifts to new niches. Because of recombination, new constellations of characters are likely to be ephemeral and not appear in the fossil record — we don’t see them because of issues in population structure. Adaptive gene combinations will be diluted by interbreeding with individuals that lack the combination, so novelties are unlikely to spread very far (unless it’s also associated with reproductive isolation).
During the glacial periods, most species did not adapt to new environments — they used habitat tracking to follow favorable environments. Recombination with more abundant ancestral genotypes leads to collaps of population structures that might favor new forms. Subpopulations lose their character when merged with larger populations, so reproductive isolation is important.
Interesting prediction: ought to be more stasis in times of environmental fluctuation, and more expansion of novelties in subpopulations in times of environmental stability.. Adaptation to rapid environmental change may fail, especially if multiple character changes are required, and extinction is not unllkely. Climate change may simply doom many species. Adaptation to other invasive species is also going to be slow. And many adaptations may be unlikely and evolve only rarely.
Once upon a time, biologists like the idea of convergence — that similar populations might arise in similar environments (I’m thinking of Simon Conway Morris here), but communities are dependent on contingency in evolutionary history, and a deterministic, equilibrial view of ecological “communities” can no longer be supported.
We are seeing a major shift in the discipline to the importance of constraint and evolvability, and the origin of variation. History is important. and there’s increasing integration of disciplines to cover micro- and macro-evolution.
