Peter & Rosemary Grant—Natural Selection, Speciation, and Darwin’s Finches

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.