About the author:
Brinkley Thornton wrote this blog for Dr. Krueger-Hadfield’s Fall 2022 Ecological Genetics course. Brinkley is currently a graduate student in the Department of Biology at the University of Alabama at Birmingham seeking her MS degree in Biology. She works in the Krueger-Hadfield Evolutionary Ecology Lab. Her research focuses on uncovering the life cycle and underlying reproductive mode in an invasive green alga in Hawaii.
Since grade school, we’ve all learned acronyms to help us understand taxonomic terminology. Personally, I was a fan of “Dear Katy Perry Came Over For Grape Soda”. DKPCOFGS. Domain. Kingdom. Phylum. Order. Family. Genus. Species. In that order. However, our acronyms have been leaving out one important level…the subspecies. Subspecies usually diverge from their species in habitat, distribution, and morphology. These differences are driven by genetic differences that allow them to pursue their own evolutionary trajectories. Therefore, to preserve both functional and phylogenetic diversity as well as evolutionary potential in natural populations, subspecies must be viewed as taxonomic units of conservation! In many cases, it is these lineages that are of conservation concern, even if the species they evolved from is not. Enter the rough-footed mud turtle Kinosternon hirtipes (Figure 1).
This water-loving reptile is widespread in relation to others of its genus, ranging from Texas to south-central Mexico. Within the species are 6 subspecies: K. h. hirtipes, K. h. murrayi, K. h. chapalaense, K. h. magdalense, K. h. tarascense, and the extinct K. h. megacephalum. In terms of conservation, K. hirtipes is ranked as globally secure. However, the continued threats of habitat loss and degradation, climate change, small population sizes, and the recent extinction of K. h. megacephalum warn us that security is not the case for the subspecies. Relationships among the populations were unclear because their descriptions have nearly exclusively relied on geographical distribution and morphological characteristics. Additionally, the relationships between subspecies had not been previously analyzed on the molecular level. In order to establish conservation units, the ecological and genetic relationships between these lineages needed resolution.
Weaver et al. (2022) addressed this by sampling across the entire range of K. hirtipes to assess the taxonomic status of the different lineages using population genetics. Analyses included constructing a new phylogeny to assess lineage relationships, assessing population genetic structure, and summarizing genetic diversity. Luckily, the genetics seemed to agree with the currently described subspecies (Iverson, 1981). Phylogenetics inferred two main clades with strong genetic differentiation dividing the K. hirtipes subspecies. Particularly this division seemed driven by the Trans-Mexican Volcanic Belt (TMVB) (Figure 2).
The first clade included populations of K. h. murrayi north of the TMVB (hereafter Northern K. h. murrayi). The second clade consisted of populations within the TMVB representing another lineage of K. h. murrayi (TMVB K. h. murrayi), K. h. tarascense, K. h. chapalaense, and K. h. magdalense. The genetic structure of these populations suggest very little gene flow. However, the K. h. tarascense and TMVB K. h. murrayi seemed to be closely related in terms of both phylogeny and structure. The most genetically diverse populations were those representing TMVB K. h. murrayi. The least amount of diversity was seen in Northern K. h. murrayi, along with higher levels of inbreeding.
Following these analyses, Weaver et al. (2022) strengthened the implications of their genetic findings through an ecological assessment. They developed ecological niche models for three of the subspecies lineages to assess ecological differentiation and investigate the future suitability of each of their defined habitats. They found the ecological differences mirrored the genetic differences. There is strong niche differentiation between Northern K. h. murrayi, TMVB K. h. murrayi, and K. h. chapalaense. These ecological differences appeared to be driven by vicariance from the TMVB as predictions for future habitat suitability based on niche models were grim for the northern lineages. While the TMVB populations are projected to see an increase in suitable habitat, Northern K. h. murrayi populations are expected to lose most of their current suitable habitat by 2070.
The main takeaway from this study is that conclusions and generalizations made about a species certainly do not encompass all its subspecies.
Firstly, these lineages accumulated genetic differences that allowed different evolutionary trajectories. In the case of K. hirtipes subspecies, these genetic differences corresponded with environmental differences and the important phylogenetic barrier of the TMVB, likely blocking gene flow from occurring between the separated populations. Second, while K. hirtipes as a homogenous species may be secure in the eyes of conservation, this study shows that its subspecies are not as secure. Of particular concern are the Northern K. h. murrayi populations, as low levels of genetic diversity threaten this lineage’s adaptive potential. Matters are only made worse for this lineage as both climate change and human-caused reduction in water supply are projected to further dwindle their available habitat. This study by Weaver et al. (2022) shows us is that to properly define conservation units, using both genetics and ecology is key. When considering organisms at the subspecies level, you will find surprising biological diversity that is critical to both ecosystem function and resilience.
Weaver, McGaugh, S. E., Kono, T. J. Y., Macip-Rios, R., & Gluesenkamp, A. G. (2022). Assessing genomic and ecological differentiation among subspecies of the rough-footed mud turtle, Kinosternon hirtipes. J. Hered., 113, 538–551.
Iverson, J. B. (1981). Biosystematics of the Kinosternon hirtipes species group (Testudines: Kinosternidae). Tulane Stud. Zool. Bot. 23, 1-74.