How do we distinguish bacterial species? Cohan shows us some nice diagrams of phenotypic and molecular clusters, and they show groups separated by gaps — therefore, species. Unfortunately the species defined thereby are big and contain considerable diversity within them. Darwin defined species as divergent forms between which one finds morphological gaps. Mayr: cohesive set of organisms whose divergence is constrained by genetic exchange. Speciation requires a breakdown of that exchange.

Mallet has developed a version of Darwin’s species definition that includes molecular characters. Under Mayr, speciation is tough, under Mallet, speciation is easy. The two models differ in the frequency of cladogenesis.

How do bacterial species maintain cohesion? Periodic selection purges divergent populations. Diversity within ecotypes is maintained by selective purges, but ecotypes that found new populations in new environments will not face the same selective effects.

Why doesn’t the free(er) exchange of genetic material between bacterial species lead to a convergence or fusion of species? One reason is the rarity of genetic exchange. If two ecotypes have a suite of niche-specifying genes, low frequency of interchange will not be sufficient to prevent divergence.This does not prevent free exchange of niche-transcending genes, genes that are useful in different environments.

Lots of details from Cohan’s work followed, and I confess to being a bit lost in places. He’s looking at different soil bacteria that are found in different ecotypes—for instance, having different characteristic fatty acid content depending on whether they are found on a north-facing or south-facing slope. He argues that speciation is easy because genetic exchange doesn’t prevent speciation. They’re working on finding and confirming ecotypes with whole genome sequencing.

There is cohesion with local populations in one niche, but there is also niche-specifying divergence that is in defiance of cohesion. In animals and plants, niche-transcending genes are only shared between close relatives; in bacteria, they can be shared by the most distant relatives. This sharing doesn’t interfere with divergence.

How do we explain the diversity of species in the world? The core process is speciation, a splitting of a lineage into two divergent lines that at the end, cannot interbreed. What do we know about speciation in Darwin’s finches?

They evolved from a common ancestor in 2-3 million years into 14 different species, filling different ecological niches in the Galapagos, largely free of human interference. Showed us photos of four different species with very different beaks.

Developed predictions of population density from things like available biomass, and worked out relationship of expected density to beak size. It seems to have worked, with good correlations between where the environment provides the best opportunities and the kinds of species that are actually present.

Different birds in different environments have different characters, presumably generated by adaptive processes. They frequently observe matches between species present and available food supplies. This is a historical interpretation: what is needed is direct observation of morphological changes in response to changes in the environment.

How much genetic variation is extant in a population? They assessed this in bird populations on Daphne Major, measuring heritability of beak size (value = 0.74, about the same as heritability of height in human populations).

How much genetic difference is present between species? Two genes show consistent graded pattern that correlate with beak shape: BMP4 and calmodulin. Inserting finch BMP4 genes in chickens produces chickens with larger beaks. Most of the variation is thought to be not in structure of genes, but in their regulation.

Describe size-dependent mortality in birds during drought — large birds survived better. Used r=h²s to predict what the average beak size in subsequent generation, tested it, and found a very good fit.

Later rainy year led to a second evolutionary shift, back to favoring smaller birds. They now have a body of data describing almost 30 years of evolutionary responses to 5 drought years. Mean trait values are changing over these years. The birds are not the same morphologically now as they were at the start of their study. Have seen an identical drought condition in 2005 to drought in 1983, but in this recent drought, saw a different reaction. Now, there is a substantial population of magnirostris on the island, so the response to drought is decline in beak size: they are seeing character displacement to increase differences between two species.

Rosemary Grant took the lectern to talk about courtship. Finches can recognize conspecifics by both morphology and song. THere are individual differences, but also larger species differences. Song is learned early by young birds, mainly from the father, and once learned, it is retained for life. Song is learned in a Lorenzian fashion by imprinting, forming a pre-mating species barrier. Sometimes, males will take over a nest of another species and fail to toss out all the chicks, so you sometimes (1%) get individuals that learn a foster-father’s song, of a different species…so you get hybrids later in life.

Hybrids were not seen to survive any of the drought years. Hybrids had intermediate sized beaks that did not thrive when only large, tough seeds were present, but could do well in wet years with abundant small seeds. In those cases, hybrids survived as well as parental types, so their death is not a result of genetic incompatibility.

These hybrids trickle cross-species genes into the foster parents’ species. Will this lead to fusion of the two species? Maybe not, because drought reinforces differences.

Also, some hybrids with magnirostris seen — they don’t breed back into the population. They can’t compete with the purebred magnirostris, and the purebreds also beat up the hybrids.

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.

It was a fine evening here in Chicago, with all these superstars of evolutionary biology in attendance. It was also an information-dense evening — I tried to keep up on my little laptop, but I know I missed a lot. Fortunately, I’m not alone: Rob Mitchum and Jeremy Manier were also covering the event, and have a play-by-play available. I’ll just dump what I’ve got here tonight. I do have wi-fi passwords so I can get things up a little more promptly tomorrow and Saturday.

Richard Lewontin opened up with a few deprecatory comments about the religiosity of our surroundings (the talks were given in a chapel) and our purpose, the reverence given to Saint Darwin. He was there to talk about the importance and danger of metaphors, and addressed two of them. The New Testament metaphor of genes make organisms, and the Old Testament metaphor that organisms adapt to the environment.

It’s not true that genes make organisms. Organisms are consequence of interactions between inside and outside, genes and molecules, and the phenotype is not predictable from the genotype. He discussed the classic example of norms of reaction in Achillea, showing growth of clones in different environments. Cloned plants taken from cuttings — so they’re genetically identical — and grown in different environments show different patterns of growth, and, for instance, don’t show a simple relationship between morphology and the elevation at which they’re grown. Another example is bristle number in Drosophila which show similar unpredictable pattern of response to temperature. Another thing to think about: look at the fingerprints on your left and right index fingers. They’re not identical, but they have the same genes and formed in the same environment at the same time. Living organisms are the outcome of developmental and physiological processes influenced four factors: genes, non-genic molecules in the embryo, environment, and random variation.
Biologists have known this for years but have fallen prey to the metaphor of genetic determinism. (He also mentioned another bad metaphor as an aside: the cell as a machine.)

The other bad bad metaphor is the idea that organisms adapt to ecological niches. Organisms do not fit into preexisting niches. You can’t look at “niches” in the environment…there are an infinity of them. The organism determines the niche. A better idea is the concept of niche construction, in which niches change as organisms evolve. Organisms take whats available and integrate it with their biology, and the life activities of organisms determine what is relevant. When did living in water become the niche of the ancestral seal?

Organisms seek out appropriate environments, the idea of microclimate. Put mesic (adapted to environments with a moderate amount of moisture) and xeric (dry or desert) flies in evironments with different zones of humidity, one surprising result is that the xeric flies move most quickly and determinedly to moist areas, more so than mesic flies. It’s not that dry-adapted flies can handle dryness better…it’s that they’re better at finding damp microenvironments.

Lewontin gave several other examples of organisms that respond in sophisticated ways to confound simple interpretations of adaptation: that we all produce shells of altered microenvironments around us by our metabolic activity; that trees can count the number of days of a certain temperature to trigger flowering; that Daphnia measure the rate of environmental change to determine whether to reproduce sexually or asexually; that organisms modulate the statistical properties of their environment by storage.

He suggested that we need to set aside the bad metaphor of adaptation for a less bad metaphor of construction. Unfortunately, this creates a difficult situation for scientists interested in selection, because, for instance, frequency dependent selection means that the addition of new genotypes to the gene pool (which happens constantly) causes fitness to change in unpredictable ways. It’s a game of rock-paper-scissors with a lot more than just three possibilities. He closed by saying that addressing this kind of problem should be the goal of the next generation of evolutionary biologists.