How and when should subspecies be defined? Analyses of geographical
populations of the mangrove tree, Avicennia marina
The designation of subspecies has long been controversial in
systematics. In addition to phenotypic divergence, subspecies
designation may need to incorporate population genetic analyses. In this
study, we perform such a survey on three subspecies of the mangrove tree
Avicennia marina, distributed along the Indo-West Pacific coasts.
Samples from 16 populations (577 individuals) were collected and 94
nuclear genes were sequenced. We identify four genetic features that
support the subspecies designation in this genus. First, genetic
divergence that delineates the three subspecies is evident, with
discordance found mainly in zones of secondary contact. Moreover, levels
of genetic diversity within local populations differ among subspecies.
Second, the three subspecies have separate demographic histories
inferred by computational modeling. Third, gene flow is detected between
subspecies indicating little or no reproductive isolation. Fourth, the
delineation of the subspecies varies from locus to locus across the
genome, thus hinting continual but uneven exchanges of genes. All these
features indicate that the three taxa have proceeded far beyond
structured populations. Since they have not satisfied the criteria for
full-species designation, the subspecies designation is warranted. We
believe these considerations can be generalized to other taxa.