Speciation—the evolutionary formation of new species—is associated with genetic changes in one or more populations. 

Genetic changes may alter an organism’s molecular composition, behavior, and physical structure, creating genetic barriers resulting in the separation of species.

For example, in species of the flowering plant genus Petunia, a single gene codes for flower color. Alteration of that gene can impose such a genetic barrier.

The flower color can determine which pollinator visits the flower, effectively causing the reproductive isolation of populations with different flower colors.

Solitary bees pollinate species with purple flowers, hummingbirds pollinate species with bright red flowers, and hawk moths pollinate those with white flowers. Eventually, different Petunia species evolved. 

Another genetic barrier is the alteration of the total chromosome content of an organism. 

For example, the interbreeding—or hybridization—of different species of Tragopogon plants led to the formation of new Tragopogon species. Because the hybrid offspring have more than two sets of homologous chromosomes, they are incapable of reproducing with either parent species, despite being fertile. 

Even the specific combination of a host organism’s genome and the genomes of all the symbiotic microbes associated with it may impose genetic barriers and ultimately lead to speciation.

For example, in crosses between certain Nasonia wasp species, up to 90% of offspring perish during larval development.

Experiments suggest that this hybrid lethality results from interactions between the wasp’s genome and its residing bacterial communities, illustrating how gene-microbe interactions can maintain species separation by preventing reproduction.

While the role of genetics in speciation is an active field of research, genetic changes spanning single genes, genome composition, and the interaction of multiple genomes can contribute to reproductive isolation and speciation.