Contrasting genetic and morphological differentiation among geographical lineages of a stenotopic miniature rasborine, Boraras maculatus, in Peninsular Malaysia

Abstract

The variability in the stenotopic miniature rasborine Boraras maculatus (Cypriniformes: Danionidae: Rasborinae) across acidic‐water habitats of Peninsular Malaysia (PM) was investigated using two molecular markers (the mitochondrial cytochrome c oxidase subunit I [COI] gene and the nuclear rhodopsin gene), as well as morphological evidence. Molecular phylogenetic analyses revealed differentiation among populations of B. maculatus in PM with the distinction of four allopatric lineages. Each of them was recognized as a putative species by automatic species delimitation methods. These lineages diverged from each other between 7.4 and 1.9 million years ago. A principal component analysis (PCA) was conducted to examine the multivariate variation in 11 morphometric measurements among three of these lineages. PCA results showed a significant overlap in morphological characteristics among these lineages. Additionally, a photograph‐based machine learning approach failed to fully differentiate these lineages, suggesting limited morphological differentiation. B. maculatus represents a case of morphological stasis in a stenotopic miniature species. Strong habitat preference, coupled with long‐term habitat fragmentation, may explain why each lineage of B. maculatus has a restricted distribution and did not disperse to other regions within and outside of PM, despite ample possibilities when the Sunda shelf was emerged and drained by large paleodrainages for most of the past 7 million years. The conservation status of B. maculatus and its peat swamp habitats are discussed, and it is concluded that peat swamps comprise several evolutionary units. Each of these units is considered a conservation unit and deserves appropriate protection.

Genetic and phenotypic diversification in a widespread fish, the Sailfin Molly (Poecilia latipinna)

“…Yet they appear to exhibit little to no diversification at the species level [ 2 ], which could be due to a host of factors, including a lack of geographic isolation amongst populations, intense gene flow, demography, or short time since population establishment [ 3 , 4 ]. There are examples of genetic differentiation in the absence of phenotypic differentiation, for example in Bororas maculatus from Malaysia [ 5 ] and in Microrasbora rubescens from Myanmar [ 6 ]. This is in stark contrast to the rapid differentiation and even speciation found in some taxa, such as cichlid fishes of the East African rift valley [ 7 ], Laupala crickets in Hawaii [ 8 ], and toads [ 9 ].…”