How do different varieties become species? Darwin credited selection. What are the details of this process? Speciation is a booming topic in the science literature, with 25,000 titles last year. Need to define a species to begin. Uses Mayr's biological species concept, which focuses on the importance of reproductive isolating factors.
Darwin on speciation: recommends Stauffer's compilation of Darwin's notes as much more thorough and specific than the Origin. Darwin explained speciation as a consequence of selection, divergence, and extinction. Mayr thought Darwin considered geographic isolation to be unimportant; the big book does infer that "some degree of geographical isolation would be indispensible". Darwin had much more sophisticated views of speciation than Mayr ascribed to him.
What are reproductive barriers in nature? Case study of monkeyflowers (Mimulus) that show very different morphologies and different pollinators (bees vs. hummingbirds). Ecogeographic barriers, premating isolation, postmating prefertilization barriers, hybrid unfitness. In Mimulus, species separated by elevation with a very narrow band of sympatry. Reciprocal translocation showed that the two species are highly adapted to their native ranges.
Premating isolation imposed by bees/hummingbirds that very rarely feed on different flowers.
Postmating isolation: artificially pollinated with cross-species gametes. One species can produce hybrids, but the other does so only at low frequency.
Hybrid fitness: F1 hybrids are not as viable.
Cited Coyne's work on how these factors work sequentially. Each step in isolation has different degree of contribution, but ecogeographic isolation is the most important component.
What traits contribute to reproductive isolation? They made hybrid F1s, crossed them to produce F2 hybrids that vary widely in morphology. Transplanted them to Yosemite, where they kept records of what pollinators visited which hybrids. Key factors were nectar production (hummingbirds favor lots of nectar); bees shunned hybrids rich in carotenoids — reds were invisible to bees.
Now looking at QTLs (quantitative trait loci) that affect reproductive isolation, and those genes that affect carotenoid production seem to be important. Isolated strains that only carried trans-specific carotenoid genes. This single gene has dramatic differences in visitations by bees vs. hummingbirds.
A single gene substitution seems to be responsible for the reproductive isolation. It takes a long period of time for post-zygotic barriers to evolve, and the most important barriers are in the habitat. Comparative studies of other species were cited to show that ecogeo barriers are the main agents of speciation.