Habitat alteration might have triggered formation of a hybrid zone between two species of musk turtle.
“Hybrid swarms can only survive in hybridized habitats.” Edgard Anderson wrote this in his 1948 Evolution paper entitled “Hybridization of the Habitat”. Based on experiments in the plant genus Tradescantia, he argued that human-modified habitats were conducive for hybridization. A recent study in the journal Molecular Ecology provides an example of this scenario in animals.
The distribution of two musk turtle (Sternotherus) species in southeastern USA is a but peculiar. The flattened musk turtle (S. depressus) is endemic to the Black Warrior River system in Alabama. The range of this species is completely encompassed by its sister species, the stripe-necked musk turtle (S. peltifer). The distribution map of these two species is reminiscent of a Russian doll.
The Black Warrior River flows across the Fall Line, an ecological transition from rocky underground to sandy soils. Early studies on the musk turtles uncovered a morphological hybrid zone at this transition. More recent work showed introgression of mitochondrial DNA from stripe-necked into flattened musk turtle. But the exact extent of gene flow between these species remains elusive. Therefore, Peter Scott (University of Alabama) and his colleagues undertook an extensive genetic study of this hybrid zone.
Altered River System
The analyses revealed that all individuals from the hybrid zone were admixed, while the remainder of the samples turned out to be pure. Gene flow was unidirectional from stripe-necked into flattened musk turtle, confirming the mitochondrial pattern. Moreover, hybridization seems to be a recent phenomenon. Most individuals were second-generation hybrids or backcrosses to flattened musk turtle.
Recent hybridization suggests that human intervention triggered the formation of the hybrid zone. In the 1940s-1960s, installation of big dams restructured the river system in Alabama. This change probably led to the breakdown of natural ecological barriers between the two turtle species, resulting in hybridization. A nice example of “hybridization of the habitat”.
Scott, P.A., Glenn, T.C. & Rissler, L.J. (2019) Formation of a recent hybrid zone offers insight into the geographic puzzle and maintenance of species boundaries in musk turtles. Molecular Ecology, 28:761-771.