Testing the Core‐Periphery Hypothesis: a standardised multi‐phylum
assessment of genetic diversity of marine coastal species
Abstract
The Core‐Periphery Hypothesis (CPH) predicts that genetic diversity is
greatest at the centre and lowest at the edges of a species’
distribution because genetic diversity is a function of a species’
abundance, which is also expected to be greatest at the centre and
lowest at the edges of the distribution. Variants of the CPH include the
‘ramped north’ (greatest variation in the north), the ‘ramped south’
(greatest in the south), and the ‘abundant edge’ (greatest at the
distributional edges). Here, we present the first standardised
multi‐phylum analysis of the CPH using 9 indices of genetic diversity
for New Zealand’s marine biota, covering 52 species. Based on 80 studies
across 8 phyla, spatial variation in the genetic indices was tested
against four models (Normal, Ramped North, Ramped South, Abundant Edge).
Only 22.7% of all individual taxon‐specific tests were statistically
significant: Ramped North (10.5%), Ramped South (7.4%), Abundant Edge
(2.6%) and Normal (2.2%). Nonetheless, amongst the Chordata (Ramped
North and Ramped South), Arthropoda (Ramped South) and Mollusca (Ramped
North) a reasonably consistent pattern of genetic variation was observed
within each phylum. Spatially‐explicit genetic diversity of the
remaining taxa fitted different models but without any obvious pattern
across the phyla. Generalised binomial testing of observed p‐values for
each genetic index across all studies revealed that 10 of 29 tests were
significant (5 ramped north, 2 normal, 2 ramped south, 1 abundant edge).
Overall, our meta‐analysis revealed no real support for the CPH and only
limited support for a Ramped model (either Ramped North or Ramped South)
of spatially‐explicit genetic diversity. For New Zealand coastal marine
taxa we conclude that consistently strong patterns of genetic variation
across multiple taxa do not exist and the CPH requires extensive testing
from multiple other regions before we can say that such patterns exist,
let alone explain them.