Sticklers for sympatry.

W hat is the origin of species? Nearly a century and a half after the question was first posed 1 , evolutionary biology has yet to come up with a satisfactory answer. This is largely because of the timescales involved: an entire speciation event is unlikely to occur within the lifetime of a research scientist, let alone the lifetime of a research grant. Nevertheless , it is still possible to deduce a substantial amount by piecing together snapshots of different systems at different stages of the speciation process. The latest results from the study of two recently formed stickleback species 2,3 provide valuable insights into the early stages of speciation, and generate interesting contrasts with our understanding of what maintains boundaries between older, long-established species. The work also highlights a striking difference between the results of measuring fitness in the laboratory and in a natural environment. One evolutionary process assumed to generate species diversity is 'ecological' speciation. If alternative ecological niches require different adaptations for their exploitation, populations will diverge as they adapt to those niches. Divergence will isolate them from each other, and might eventually lead to complete reproductive isolation and hence speciation 4. This all seems perfectly reasonable. However , surprisingly few empirical studies have demonstrated a driving role for ecological selection pressures in the early stages of speciation – particularly in comparison with the volume of work that exists documenting hybrid dysfunction, where changes in the respective genomes have eventually rendered them intrinsi-cally incompatible (for example, Refs 5,6). For these reasons, the detailed picture emerging from studies of the ubiquitous threespine stickleback (Gasterosteus aculeatus complex) in North American lakes is particularly exciting. Approximately 12 500 years ago in southwestern British Columbia, Canada, populations of the marine threespine stickleback (Gasterosteus aculeatus) were stranded in freshwater lakes because, following deglaciation, the land uplifted rapidly. Several of these lakes now contain a pair of coexisting trophic forms: a 'limnetic' form, which feeds mainly on plankton in open water, and a 'benthic' form, which forages almost entirely on the lake bottom. Corresponding to these different feeding strategies are morphological differences: the plankton-feeding limnetic is slim-bodied with numerous, long gill-rakers, whereas the benthic form is larger, with a deeper body, wider mouth and fewer gill-rakers. It is unclear exactly when one species became two. The two forms might have arisen through sympatric speciation following a single invasion by the marine species. Alternatively, …