Evolution is the key process to know from where species have been derived from. A question was always there of where the rhinoceros species have diverged since the time of Darwin. One such evolutionary history of rhinoceros family is unravelled now. At present there are only five species of Rhinos which are white, black, Sumatran, greater one horned, Javan. Three known extinct rhinos are Siberian, Merckes, woolly. Only five species remain, out of which scientists have sequenced five (3 extinct and 2 extant) species and compared to existing data of living species. Divergence between extant African and Eurasian lineages was distinguished which resolves key debates on phylogeny of extant.
According to shanlin et al’s paper in Cell journal the horn hypothesis, geographical hypothesis, mitochondrial genome hypothesis proves that the Sumatran rhinoceros is sister to the clade comprising the four other extant species. Genetic variation is the raw material of evolution. Without genetic variation, a population cannot evolve in response to changing environmental variables and may face an increased risk of extinction. A 100-Kb genomic region alignments simulated gene tree distribution along with D-statistics that analyzed ancient gene flow events having similar levels of gene flow and low levels of genetic diversity between the Sumatran rhino and other Rhinoceros species. Genome wide heterozygosity (GWH) for all eight rhinoceros species is compared with other animal species, including ruminants and, more broadly, mammals. Results show that present-day rhinoceros genomes exhibit significantly lower GWH compared to the historical Javan and extinct genomes and mainly due to transversions. Main reasons include limited genetic diversity and moderate inbreeding levels, these are intrinsic features of rhinoceros life history, where low population densities and limited dispersal result in increased genetic drift as well as occasional mating between relatives. Eight species striked a decrease in population size in last 2 million years. Low population size over extended periods of time allows purging of deleterious alleles keeping low genome wide levels of genetic diversity. The high number of Loss of function mutations were seen in extinct species.
Rhinocerotidae diverged from tapir’s age like 55-60 mya in Eurasia or North America. Later, radiated into 100 species across Africa, Eurasia, North, and Central America. Most rhinocerotids went extinct prior Pleistocene with survival of 9 species. Upon using fossil data to calibrate phylogeny lineage divergence it resulted in a 65 mya estimate for the common ancestor of horses, tapirs, and rhinoceroses and a 36 mya estimate for the common ancestor of the extinct rhinoceros subfamily Elasmotheriinae and the extant subfamily Rhinocerotinae.The three major clades within the Rhinocerotinae subfamily diverged 16 mya, at the end of the early Miocene and around the time of the Miocene climatic optimum (17–14 mya). Diversification occurred after the formation of the terrestrial connection between the Afro-Arabian and Eurasian landmasses 20 mya. Novel frameshift mutations in IFT43 contribute to rhinoceroses’ poor eyesight, also is involved in the formation and maintenance of cilia, which are important for the development and function of the light-sensitive tissue at the back of the eye. Relatively low genomic heterozygosity kept rhinos in the same state for many years. Present genomes indicates low levels of genetic diversity and high inbreeding which is affecting population size decline. Genomics can surely complement actions by closely looking into ongoing changes in genetic variation, inbreeding, and mutational load.