30.4: Genetics of Speciation

Speciation is the evolutionary process resulting in the formation of new, distinct species—groups of reproductively isolated populations.

The genetics of speciation involves the different traits or isolating mechanisms preventing gene exchange, leading to reproductive isolation. Reproductive isolation can be due to reproductive barriers that have effects either before or after the formation of a zygote. Pre-zygotic mechanisms prevent fertilization from occurring, and post-zygotic mechanisms reduce the viability, or reproductive capacity, of the hybrid offspring.

For example, pre-zygotic mechanisms act early in the life cycle of an organism, imposing the strongest impediment to gene flow, and preventing unfavorable mating combinations. Some mating combinations produce hybrid individuals. Natural selection can work against the production of hybrids with low fitness, thereby increasing reproductive isolation between two species.

Post-zygotic reproductive barriers can be due to the intrinsic inviability of hybrids. Genetic complications resulting from aberrant ploidy levels, different chromosomal arrangements, or gene incompatibilities where the alleles do not function properly contribute to different genetic makeup and alternative developmental pathways in hybrids. These genetic alterations affect both plants and animals, leading to post-zygotic isolation and speciation.

Epistasis, or non-allelic gene interactions, is a distinctive feature contributing to speciation. The effect of a gene variant is dependent on the genetic background in which it appears. For example, an allele giving rise to a normal phenotype in members of the same species may function poorly in the genetic environment of hybrids. This hybrid weakness can also lead to reproductive isolation and speciation.

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.