Environmental variation and biotic interactions limit adaptation at ecological margins: lessons from rainforest Drosophila and European butterflies

Models of local adaptation to spatially varying selection predict that
maximum rates of evolution are determined by the interaction between
increased adaptive potential owing to increased genetic variation, and the
cost genetic variation brings by reducing population fitness. We discuss
existing and new results from our laboratory assays and field transplants
of rainforest Drosophila and UK butterflies along environmental gradients,
which try to test these predictions in natural populations. Our data
suggest that: (i) local adaptation along ecological gradients is not
consistently observed in time and space, especially where biotic and
abiotic interactions affect both gradient steepness and genetic variation
in fitness; (ii) genetic variation in fitness observed in the laboratory
is only sometimes visible to selection in the field, suggesting that
demographic costs can remain high without increasing adaptive potential;
and (iii) antagonistic interactions between species reduce local
productivity, especially at ecological margins. Such antagonistic
interactions steepen gradients and may increase the cost of adaptation by
increasing its dimensionality. However, where biotic interactions do
evolve, rapid range expansion can follow. Future research should test how
the environmental sensitivity of genotypes determines their ecological
exposure, and its effects on genetic variation in fitness, to predict the
probability of evolutionary rescue at ecological margins. This article is
part of the theme issue ‘Species’ ranges in the face of changing
environments (Part II)’.