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Speciation is the process by which populations evolve reproductive isolation and become distinct species. It happens when populations build up genetic differences over time.

Genetic changes can affect traits like body features, behavior, or the timing of reproduction. Over time, these differences can build barriers that reduce interbreeding.

For example, changes in a major pigment gene can shift flower color—and that can change which animals visit the flowers.

If different pollinators prefer different colors, then plants with different flower colors get fewer chances to cross, so gene flow drops.

Some Petunia species attract bees, others attract hummingbirds, and others attract hawkmoths—often linked to color, scent, and nectar traits.

Another genetic barrier, especially in plants, is polyploidy, where an organism has extra sets of chromosomes.

For example, the interbreeding—or hybridization—of different species of Tragopogon plants led to the formation of new Tragopogon species. Because these hybrids have more than two sets of homologous chromosomes, they can’t reproduce with either parent species, even though they are fertile.

In some animals, interactions between the host genome and its symbiotic microbes can contribute to reproductive isolation.

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

Experiments suggest these hybrids die due to interactions between the wasp’s genome and certain residing bacterial communities in the reproductive tissues, so the microbes help keep the species separate.

While the role of genetics in speciation is an active field of research, speciation can start with a change in one key gene, a whole-genome change like polyploidy, or the interaction of multiple genomes involving microbes.