What allows a population in a heterogeneous landscape to become locally adapted? In general, adaptation to a rare habitat type is difficult because divergent selection is counteracted by the homogenising effects of gene flow. One well-established condition under which adaptation to a rare habitat type may occur is if the rare habitat has higher quality, so that a greater number of offspring can be produced there, to compensate for the habitat’s relative rarity in the landscape. In our new Ecology Letters paper led by Nadiah, we focus on an alternative way in which a habitat may be considered to have higher quality: by increasing the quality, rather than the quantity, of offspring produced. We show, using simulation models, that such “carryover effects” can indeed permit adaptation to a rare habitat type, counteracting gene flow from a more common but lower-quality habitat type.

We also propose an empirical example of carryover effects: the Blue Tit (Cyanistes caeruleus) on the island of Corsica. Blue Tits utilise two habitat types: deciduous and evergreen. There is evidence that the deciduous habitat is higher quality. On the mainland, the deciduous habitat type is more common, and thus Blue Tits who settle in the rarer evergreen patches are maladapted, resulting in source–sink dynamics. On Corsica, however, evergreen habitat dominates, but the source–sink pattern is not simply reversed. Instead, trait divergence into two ecotypes has been documented. We propose that this is because the deciduous habitat, though rare on Corsica, still has higher quality, and that offspring raised there gain advantages that carry over to their fitness later in life. These carryover effects then explain the observed divergence of the Blue Tit on Corsica into two ecotypes.

Tantalisingly, the carryover effects we study provide a potentially novel ecological mechanism whereby two subpopulations can become isolated. Nadiah gives a more comprehensive summary of the paper on her blog.

Kristensen, N. P., Johansson, J., Chisholm, R. A., Smith, H. G., Kokko, H. (2018) Carryover effects from natal habitat type upon competitive ability lead to trait divergence or source-sink dynamics, Ecology Letters (in press)

The divergence of Blue Tits (Cyanistes caeruleus) on Corsica into two ecotypes may have been driven by carryover effects. Image credit: © Francis C. Franklin / CC-BY-SA-3.0