Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L.

The central-marginal hypothesis (CMH) posits that range margins exhibit
less genetic diversity and greater inter-population genetic
differentiation compared to range cores. CMH predictions are based on
long-held ‘abundant-centre’ assumptions of a decline in ecological
conditions and abundances towards range margins. Although much empirical
research has confirmed CMH, exceptions remain almost as common. We contend
that mangroves provide a model system to test CMH that alleviates common
confounding factors and may help clarify this lack of consensus. Here, we
document changes in black mangrove (Avicennia germinans) population
genetics with 12 nuclear microsatellite loci along three replicate
coastlines in the United States (only 2 of 3 conform to underlying
‘abundant-centre’ assumptions). We then test an implicit prediction of CMH
(reduced genetic diversity may constrain adaptation at range margins) by
measuring functional traits of leaves associated with cold tolerance, the
climatic factor that controls these mangrove distributional limits. CMH
predictions were confirmed only along the coastlines that conform to
‘abundant-centre’ assumptions and, in contrast to theory, range margin A.
germinans exhibited functional traits consistent with greater cold
tolerance compared to range cores. These findings support previous
accounts that CMH may not be a general rule across species and that
reduced neutral genetic diversity at range margins may not be a constraint
to physiological adaptation to context-specific environmental factors.