Abstract

A reduction in population size due to habitat fragmentation can alter the relative roles of different evolutionary mechanisms in phenotypic trait differentiation. While deterministic (selection) and stochastic (genetic drift) mechanisms are expected to affect trait evolution, genetic drift may be more important than selection in small populations. We examined relationships between mature adult traits and ecological (abiotic and biotic) variables among 14 populations of brook trout. These naturally fragmented populations have a common ancestor but experienced considerable variability in habitat characteristics and differ by up to two orders of magnitude in population size (49 < Nc <10,032; 3 < Nb < 567). Populations differed markedly in body size, shape, and colouration, with a tendency for more variation among small populations in both phenotypic trait mean and CV when compared to large populations. These differences were more frequently and directly linked to habitat variation or operational sex ratio than to population size, suggesting that selection may overcome genetic drift at small population size. Phenotype-environment associations were also stronger in females than males, suggesting that natural selection due to abiotic conditions may act more strongly on females than males. Our results suggest that natural and sexual-selective pressures on phenotypic traits related to mating systems change during the process of habitat fragmentation, and that these changes are largely contingent upon existing habitat conditions within isolated fragments. Our study provides an improved understanding of the ecological and evolutionary consequences of habitat fragmentation, and lends insight into the ability of some small populations to respond to selection and environmental change.