Ecological constraint, rather than opportunity, promotes adaptive radiation in three‐spined stickleback (Gasterosteus aculeatus) on North Uist

• Published in: Ecology and evolution Vol. 13; no. 1; pp. e9716 - n/a
• Main Authors: Begum, Mahmuda, Nolan, Victoria, MacColl, Andrew D. C.
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01-01-2023; John Wiley and Sons Inc
• Subjects: Adaptive radiation; Aquatic habitats; Armor; Brackish water; Competition; Diversification; Ecology; Evolution; Evolutionary Ecology; Freshwater environments; Islands; Morphology; morphometric study; North Uist; pH effects; Salinity; South Uist; stickleback; United Kingdom > UK; Scotland; North Uist; morphometric study; South Uist; stickleback
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.9716

The context and cause of adaptive radiations have been widely described and explored but why rapid evolutionary diversification does not occur in related evolutionary lineages has yet to be understood. The standard answer is that evolutionary diversification is provoked by ecological opportunity and that some lineages do not encounter the opportunity. Three‐spined sticklebacks on the Scottish island of North Uist show enormous diversification, which seems to be associated with the diversity of aquatic habitats. Sticklebacks on the neighboring 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 sticklebacks from both islands exhibited similar morphology including size and bony “armor.” Resident sticklebacks showed significant variation in armor traits in relation to pH of water. However, North Uist sticklebacks 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.
Evolutionary diversification is often thought to arise from “ecological opportunity:” A reduction in competition for resources following the invasion of a new habitat. Here, we show that diversification may be provoked by ecological constraint: Organisms invading habitats, in this case, acid freshwater, may be free from competition but have poor resource availability.