Understanding how evolutionary processes structure genetic and phenotypic variation across spatial and temporal environments has been a central goal of evolutionary biology since the modern synthesis. Clinal variation reflect the balance of natural selection and gene flow across an environmental gradient, and can be used as a predictive tool for evaluating the agents of selection that drive evolution in natural populations. Much of our work targets widespread species where similar ecological gradients are found in disparate parts of the species range, as these systems provide a replicated series of natural experiments with abundant power to parse apart the specific agents and targets of selection, thus allowing mechanistic insights into historical and contemporary microevolutionary processes at play. Such replicated clines across similar environmental gradients also speak to the degree of predictability in evolution.
Figure (left): Example of a cline in plant size along an altitudinal gradient. In this example, plants were sampled from populations A, B and C.
Research in the Kooyers lab focuses on understanding the evolution of clines in a classic discrete polymorphism found in white clover called cyanogenesis, the ability to produce hydrogen cyanide following tissue damage. Cyanogenesis clines, where there are higher frequencies of cyanogenic plants relative to acyanogenic in warmers areas, have evolved in several areas around the world where white clover was introduced as a forage crop. These clines are one of the best examples of recent, rapid parallel evolution and were thought to evolve as a defense against greater herbivore pressure in warmer climates. However, many other selective pressures including freeze tolerance and drought have been implicated in the formation and maintenance of these clines making this an ideal system to investigate how multifarious selection pressures across space and time can result in seemingly simple recurrently forming clines. Current work in the lab (some of which is in collaboration with Ken Olsen at Washington University in St. Louis and Marc Johnson at University of Toronto) involves establishing a long-term experimental evolution study to parse the ecological factors involved in the cyanogenesis polymorphism, using herbarium specimens to determine how the temporal scale of cyanogenesis cline evolution following invasion, and using common garden/isotope studies to identity the physiological tradeoffs to producing cyanogenesis precursors.
Figure (above): Cyanogenesis cline across the central United States.