Water chemistry variation promotes adaptive radiation in three-spined
stickleback (Gasterosteus aculeatus)
Abstract
The context and cause of adaptive radiations has been widely described
and explored but why rapid evolutionary diversification does not occur
in related evolutionary lineages has yet to be understood. One possible
answer to this is simply that evolutionary diversification is provoked
by environmental diversity, and that some lineages do not encounter the
necessary environmental diversity. Three-spined stickleback on the
Scottish island of North Uist show enormous diversification, which seems
to be associated with the diversity of aquatic habitats. Stickleback on
the neighbouring island of South Uist have not been reported to show the
same level of evolutionary diversity, despite levels of environmental
variation that we might expect to be similar to North Uist. In this
study, we compared patterns of morphological and environmental diversity
on North and South Uist. Ancestral anadromous stickleback from both
islands exhibited similar morphology including size and bony ‘armour’.
Resident stickleback showed significant variation in armour traits in
relation to pH of water. However, North Uist stickleback exhibited
greater diversity of morphological traits than South Uist and this was
associated with greater diversity in pH of the waters of lochs on North
Uist. Highly acidic and highly alkaline freshwater habitats are missing,
or uncommon, on South Uist. Thus, pH appears to act as a causal factor
driving the evolutionary diversification of stickleback in local
adaptation in North and South Uist. This is consistent with
diversification being more associated with ecological constraint than
ecological opportunity.