By Archit Kothari
The definition of what differentiates species has always pitted biologist against each other. This is because a definition that might apply to an asexually reproducing bacteria, can’t apply to a sexually reproducing bird. Charles Darwin, one of the first to question the concept of species even said in On the Origin of Species, “I was much struck how entirely vague and arbitrary is the distinction between species and varieties.” Fortunately, the advent of molecular biology and population genetics has allowed us to study our’s and our ancestors genes to figure out how humans are fundamentally different from other primates.
DNA sequence polymorphisms can be used to understand the subtle differences in the past Homo genuses, other primates, and us. Scientists in the Department of Ecology and Evolutionary Biology and UC Irvine focused on the major histocompatibility complex (MHC), a group of genes that play an important role in defending vertebrates from parasites and pathogens. This specific group of genes is a great one to study because it has been present in the gene pool for over 30 million years, and the polymorphisms (base pair changes in the gene) have been either passed down or wiped out between millions of generation of humans, apes, and primates. Within the MHC or as it is referred to in humans as human leukocyte antigen (HLA) is a gene, DRB1, which we can study across species. When the scientists at UC Irvine sequenced this gene in humans and chimpanzees, they reached to the conclusion that two human alleles (variations in the genotype) were actually farther from each other than the two chimpanzee alleles. In this case, genetic distance refers to the number of base pairs that differ between two genes.
This raises other questions about polymorphisms such as: when did the polymorphism occur in relation to the speciation of the organism? whether the polymorphism provided selective advantages, and how many organisms had this polymorphism when it occurred. In a statistical analysis factoring in generation time, affected population, rate of mutation, and the selective value of the polymorphisms. It was concluded that the average duration of a species, or the amount of time till it diverges into different species is 4 million years, and places approximately 100,000 individuals as the ancestral population that lead to divergence that gave us the modern human. This leads us to believe that at any given time the population of individuals that gave us the modern human was at least 100,000 individuals.
However, it is a fact of life that drastic events known as population bottlenecks occur. These events challenge the population, and the few individuals who survive, carry the polymorphism with them. These population bottlenecks marks a turning point for species’, and those who survive are pioneers for future of their kind. A series of population bottlenecks is what gave us modern humans, and these events in the future will be what change our species on an epic level.