Abstract
According to conservative estimates there are at least 10 million eukaryotic species on Earth and perhaps as many as 100 million or even a trillion. Speciation is the process primarily responsible for this great diversity of life. Here, we summarize our current understanding of speciation and highlight some of the important factors and mechanisms involved in the process. We begin by defining the speciation problem as one involving the evolution of barriers to genetic exchange between populations, with the product being differentiated species that maintain their distinction if, when, and where they happen to co-occur. We then categorize the major different types of barriers to genetic exchange and outline several different modes of speciation (largely geographically defined) under which these barriers can evolve. We next elaborate on different mechanisms that can underlie the evolution of reproductive isolation and the formation of new species. We conclude by discussing future avenues of research, including the study of the genomics of speciation and new developments in the field.