How can new species be formed if animals live together and can be crossed? A team from the Smithsonian Tropical Research Institute in Panama and the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, studied Caribbean reef fish to find out. Their discovery that natural selection combines the evolution of genes for vision and color pattern was published in Nature Ecology and Evolution.
For a new species to evolve, two things are essential: a characteristic such as a color, unique to a species, and a mating preference for that color. For example, individuals of a blue fish species prefer blue pairs and individuals of a red species prefer red pairs. If the two species intersect, the sexual recombination process is expected to destroy the coupling between color and pair preferences, resulting in red individuals with a preference for blue pairs and vice versa. This is one of the reasons why it has long been thought that new species can only evolve in absolute isolation, without crossbreeding.
However, the dynamics of this process depends on the exact number and location of the genes underlying the species characteristics and preferences of the pair, the force of natural selection acting on these genes, and the amount of interbreeding between species. In a recent study, Oscar Puebla, GEOMAR professor and research associate at the Smithsonian Tropical Research Institute, and his colleagues discovered that natural selection can match the evolution of genes for color pattern and pair preferences when species are still crossing. “To address this question, our first challenge was to identify a group of animals in which species still cross and in which the mechanisms underlying reproductive isolation are well understood,” said Puebla.
Hamlets, a group of closely related fish species found on reefs throughout the Caribbean, are genetically very similar. The main difference between the species is the color pattern, and it is the pair’s preference for the different color patterns that keep the species separate.
Two yellow-bellied hamlets (Hypoplectrus aberrans) spawning in Dominica. (Photo: Carlos and Allison Estapé carlosestape.photoshelter.com)
A second difficulty is identifying the genes underlying species differences and pair preferences. The authors of this study sequenced the entire genome of hamlets and then asked how the genome differed in each of the 110 individuals from three species found together on Caribbean reefs.
“This powerful dataset allowed us to identify four narrow regions of the genome that are consistently highly differentiated among species in a context of almost no genetic difference in the rest of the genome,” said co-author Kosmas Hench, a doctoral student at GEOMAR. These four intervals include genes involved with vision and color pattern.
The data also show that the vision genes and color pattern remain coupled despite the fact that they are located on three different chromosomes and that the species are still crossing. Such coupling was previously reported when gene sets are very close together on one chromosome, in which case they are protected from sexual recombination, but not when they are on different chromosomes. By capturing the earliest stages of speciation in hamlets, the team shows how selection can contribute to the creation of new species.
“Many closely related coral reef fish differ little more than in color and pattern,” said Owen McMillan, co-author and academic dean of STRI. I very much hope that the discoveries we’ve made in hamlets will apply to other life forms and, ultimately, can explain the remarkable diversity of fish in coral reefs around the world. (Source: STRI/DICYT)