As the above paragraph suggests, each of my study systems has their own system-specific peculiarities [e.g., cultural differences seem to play a prominent role in driving population divergence in killer whales (Riesch et al. 2012), but not in stick insects or livebearing fishes]. However, they also have a lot in common. For example, selection from predation is integral to both, the livebearing-fish and stick-insect systems I study, while foraging specialization plays a prominent role in population divergence of both, stick insects and killer whales. Thus, the three systems simply constitute different examples of how ecologically-based divergent selection drives population divergence and ultimately (ecological) speciation.
|A rain shower moving through the chaparral near Santa Barbara, California. Timema are often found in this biome of dense thickets and thorny bushes.
This concept of a speciation continuum has gained traction again in recent years (e.g., Hendry et al. 2009). Consequently, studies across closely related taxa at different phases of speciation are beginning to illuminate the processes and genetic changes underlying the formation of new species (Seehausen et al. 2014). It is well-known, of course, that speciation involves genetic differentiation, and that, in the absence of gene flow, genome-wide differentiation can readily build up by selection and drift. If speciation is to happen in the face of gene flow, however, the picture gets more complex. According to the genic model of speciation (Wu 2001), speciation is initiated by a few genetic regions that become resistent to gene flow before others. This results in a localized pattern of genetic differentiation, which becomes more genome-wide as speciation progresses.
In a recent study just published in the April issue of Nature Ecology and Evolution (http://www.nature.com/articles/s41559-017-0082), we took a closer look at the transitions between phases of genomic differentiation during speciation of Timema stick insects. Like other studies on the speciation continuum (including my other study systems), we were faced with a key problem: speciation is often slow enough that we cannot simply follow a single lineage through time to see in real-time how the process unfolds. The solution then is to take as many different snapshots of the process from different pairs of natural populations as possible, and to then start to reconstruct a bigger picture of what might be happening across different moments in time. This is exactly what we did using data from >100 populations of 11 species of Timema stick insects. Our work suggests that speciation can be initiated by few genetic changes associated with natural selection on few loci, but the overall process is multi-faceted and involves mate choice and genome-wide differentiation.
|A male Timema cristinae on one of its host plants (genus Ceanothus).
Photo: Moritz Muschick
|Evaporating hexane samples in Santa Barbara, California, as part of the perfuming experiment on Timema mate choice.
|Preparing another CHC-hexane sample for analysis with the gas chromatograph in the Gries lab at Simon Fraser University in 2014. Photo: Sean McCann
|A female Timema bartmani, cryptic against the needles of white fir. Photo: Moritz Muschick.