Johnson, Marc

Assistant ProfessorBiology
Marc Johnson

Contact Information

(905) 569-4484
(905) 828-3792
DV 3040


To understand the dynamic interplay between the ecology and evolution of multitrophic communities, our research examines two sides of the same coin: how evolution and genetic variation shape community processes and patterns, and in turn, how ecological interactions among species drive evolution within populations.

From an ecological perspective, we are trying to understand whether genetic variation and evolution within plant populations have cascading effects on multitrophic communities. For example, we have found that genetic variation in plant populations can affect the coexistence among competing plant species, the abundance and population dynamics of herbivores, and the diversity and composition of large arthropod communities that rely on plants for food and shelter. Surprisingly, these community-level effects of genetic variation are often stronger than those factors often cited as being most important in community ecology (e.g., environmental variation, competition, density dependence, mutualisms, etc.). We have also found that natural selection on plant traits is predicted to lead to evolutionary changes within plant populations that cause the composition of arthropod communities to change over time.

From an evolutionary perspective, we are investigating two related problems. First, the wide diversity of chemical and morphological defenses found in plants are believed to be adaptations against herbivores, but despite numerous studies that document natural selection by herbivores on plants, and macroevolutionary comparative evidence consistent with a history of coevolution between plants and herbivores, there is little convincing evidence that selection by herbivores actually results in an evolutionary response of defensive traits within plant populations. In collaboration with colleagues from several institutions we are conducting a long-term selection experiment in the field, to test whether selection by a community of over 60 herbivores species leads to predictable changes in the frequency of clonal genotypes within populations, where genotypes differ in chemical and morphological plant traits related to resistance against herbivores. This work integrates ecological, molecular, and phytochemical techniques.

The second problem we are addressing examines whether there is an association between plant sexual reproduction and the evolution of defenses against plant parasites (herbivores and pathogens). Existing theory on the evolution of sex predicts that sexual reproduction can spread and be maintained within populations as a mechanism to reduce linkage disequilibrium, purge deleterious mutations and enable populations to adapt to selection by parasites. The evening primroses (Onagraceae) have experienced multiple independent transitions (>20!!!) from sexual reproduction to a functionally asexual reproductive system called permanent translocation heterozygosity (it sounds scary but it really quite fantastic!). These repeated transitions from sexual to asexual reproduction allow for a powerful test of how recombination and segregation (sex) affect the molecular and phenotypic evolution of plant defenses, plus the ecological consequences of these defenses for insect herbivores and pathogens.