Rapid evolution of pre-zygotic reproductive barriers in allopatric populations

Adaptive divergence leading to speciation is the major evolutionary process generating diversity in life forms. The most commonly observed form of speciation is allopatric speciation which requires that gene flow be prevented between populations by physical or temporal barriers, as they adapt to their respective environments. Eventually, these adaptive responses lead to accumulation of mutations in different lines. The increased genetic distance between the lines is known to lead to the populations becoming reproductively isolated. A widely accepted theory is that speciation simply occurs as a by-product of adaptive response of the populations. Several examples of allopatric speciation from ecology and laboratory exist. However, we know little about the nature (pre- or post-zygotic) of barriers that arise first in this process. Understanding the first barriers that arise between populations is key to understanding how the process of speciation initiates. In recent years, fungi have been used as model organisms to answer questions related to the evolution of reproductive isolation. Here, we show rapid evolution of pre-zygotic barriers between allopatric yeast populations. We further demonstrate that these pre-zygotic barriers arise due to altered mating kinetics of the evolved population. Moreover, our non-adaptive evolution experiments with yeast under limited selection pressure also show rapid emergence of reproductive isolation. Overall, our results show that evolution of pre-zygotic reproductive barriers can occur as a result of natural selection or drift. IMPORTANCE A population diversifies into two or more species-such a process is known as speciation. In sexually reproducing microorganisms, which barriers arise first-pre-mating or post-mating? In this work, we quantify the relative strengths of these barriers and demonstrate that pre-mating barriers arise first in allopatrically evolving populations of yeast, Saccharomyces cerevisiae. These defects arise because of the altered kinetics of mating of the participating groups. Thus, our work provides an understanding of how adaptive changes can lead to diversification among microbial populations.