Hybridization is mating between two different species. Hybridization was originally thought to be both uncommon and evolutionarily unimportant, typically leading to evolutionary dead ends, like mules (hybrids between horses and donkeys) that cannot produce offspring. In more recent years, however, thousands of examples of hybridization have been uncovered in plants, animals, and fungi. Further, research has suggested that the genetic mix produced when two divergent genomes unite via hybridization might trigger important events, including speciation, adaptive radiation (the rapid appearance from a common ancestor of many morphologically divergent species, adapted to different environments), and could also facilitate species invasions and range expansions. Pinpointing the role of hybridization in these events is tricky, however, because usually the putative hybridization happened in the distant past, and we cannot directly observe the critical early generations post-hybridization.

To sidestep this problem, we examined the role of hybridization in evolution experimentally, by creating hybrids from two parental species and then comparing evolution in the hybrids to parental controls.  We chose a system of two species of annual Texas sunflowers (Helianthus annuus and H. debilis) that have formed a natural hybrid in the wild (H. annuus ssp. texanus). By creating our own hybrids and letting them evolve in the field, we "replayed the evolutionary tape" to understand what might have happened in the early generations of this system (Fig. 1). We found that hybridization increased the rate of adaptation over seven generations, with fitness (seed production) of the hybrid lines exceeding that of the controls by 14-51% by the end of the experiment (Fig. 2). More traits evolved significantly in hybrids relative to controls, and hybrid evolution was faster for most traits. Further studies mapped the genetic loci controlling particular sunflower traits and traced changes in allele frequencies across the generations of the experimental plants. These findings show a causal pathway from hybridization to rapid adaptation and suggest an explanation for the frequently noted association between hybridization and adaptive radiation, range expansion, and invasion.

We further examined ideas about hybridization at larger, non-experimental scales. Across a phylogeny (a tree-shaped diagram depicting relatedness) of nearly 1800 plant genera, we asked what traits were associated with higher hybridization rates. We found that woodier, more perennial genera hybridized more than genera dominated by herbaceous and annual plants. Further examination also determined that groups with higher rates of hybridization also had higher diversification rates (produced more new species, relative to extinctions, over a given period of time), consistent with the idea that hybridization can increase rates of evolutionary innovation and can have a net positive effect on diversity.

This research has advanced our understanding of hybridization's sometimes creative role in evolutionary change. Beyond the scientific advances, this project also provided training opportunities for dozens of undergraduate and graduate students, as well as postdoctoral fellows, and included over 10 trainees from groups that are under-represented in STEM fields.

Last Modified: 02/15/2020
Modified by: Kenneth D Whitney