Candidate species
The genetic resolution of candidate species has often relied on
identifying lineages using either gene genealogies (e.g., mtDNA or nDNA
gene trees) or multi-locus population trees (for allozymes or genomic
data). However, as such tree-only approaches detect genetic structure
rather than candidate species per se(Sukumaran & Knowles, 2017;
Unmack et al., 2022), lineages delineated in this manner need not
directly equate to biological or evolutionary species but may instead
reflect major phylogeographic breaks within a species or a composite of
two or more species plus admixed individuals.
While there is no simple formula for deciding whether two genetically
distinctive allopatric populations are conspecific or represent
different species, we have recently advocated a six-step approach to
assist in this task (Unmack et al., 2022). These steps are: identify
lineages, hybrids, and introgressed populations using a combination of
ordination of individuals (step 1) plus phylogenetic methods (step 2),
followed by pairwise assessments of lineage diagnosability (step 3),
comparative geographic distribution (step 4), and sampling intensity
(step 5), and concluding with a review of any other biological
information that might indicate that lineages are not conspecific (step
6). Unfortunately, observations relevant to this final step are largely
unavailable in the literature, since many ecological studies ofHypseleotris in eastern Australia have not attempted to reliably
distinguish H. klunzingeri from a suite of congeneric and often
co-occurring taxa (e.g., Meredith, Matveev & Mayes, 2003; Lintermans,
2007), now known to comprise a complex of sexual species and ‘unisexual’
(hybridogenetic) lineages (Unmack et al., 2019; Thacker et al., 2022b).
We hope that a recent taxonomic revision by Thacker et al. (2022a) for
this hemiclonal species complex, which includes five sexual species and
multiple unisexual combinations, will help establish a more robust
taxonomic framework for identifying individuals to their correct sexual
group and hence facilitate the documentation of comparative biological
information for all sexual forms of Hypseleotris , including those
referrable to the H. klunzingeri complex.
Table 4 summarizes the outcomes of applying steps 3–5 to the primary
taxa identified for H. klunzingeri using steps 1 and 2. As shown,
there is strong evidence that KN, KE+, and KS are all valid candidate
species, being unequivocally or effectively diagnosable from each other
at hundreds of unlinked genes and displaying distributional patterns
that are inconsistent with being phylogeographic lineages within a
single species (Table 4, Fig. S5). Given their comparatively low number
of diagnostic differences, the decision as to whether the allopatric
taxa KS and KW+ are conspecific or represent distinct evolutionary
species remains the only taxonomic question not fully resolved by our
stand-alone genetic datasets. However, as the number of molecular
characters that diagnose KS from KW+ greatly exceeds the nine
partially-diagnostic morphological characters that delineate other
co-occurring species of Hypseleotris (Thacker et al., 2022a), we
have concluded that KW+ ‘probably’ represents a fourth candidate
species. A full resolution of its taxonomic status will require
additional targeted assessments of any morphological and other
biological differences between KS and KW+, and must include exemplars of
pure KS, pure KW, and KWm. The scenario of sister
Cooper versus MDB candidate taxa is also evident in another co-occurring
freshwater fish (Australian smelt, Retropinna spp.; Unmack et
al., 2022).