Ecological Genetics of Geographic Adaptation in Plant Populations
Douglas W. Schemske,
Department of Plant Biology, Michigan State University, East Lansing, MI 48824
Understanding the ecological and genetic mechanisms of adaptation remains one of the
central goals of evolutionary biology. What is the extent of geographic adaptation? What
traits contribute to adaptive differentiation? What is the genetic basis of adaptive traits?
How many traits and genes are required for major adaptations? Is geographic adaptation
an important first step in the origin of species?
Studies of geographic adaptation in plants have long provided some of our best examples
of adaptive evolution. I will discuss three research projects that illustrate different
approaches to the study of plant adaptation. Experimental studies of hybrids between the
perennial plant Lobelia cardinalis collected from Michigan and its close relative L.
graminea collected from Panama reveal that rosette production is a major factor
contributing to winter survival in Michigan. A genetic mapping study found that a single,
major QTL controlled rosette production, and a field study of experimental evolution
indicated that plants homozygous for the Michigan allele at the rosette QTL had much
higher survival than the heterozygous genotype.
We are in the early stages of investigating the ecological genetics of geographic
adaptation in natural populations of the model plant Arabidopsis thaliana. We find
evidence of striking geographic adaptation from reciprocal transplant experiments
conducted in Sweden and Italy. Laboratory studies are underway to investigate the
genetic basis of freezing tolerance and flowering time, and field experiments of
experimental evolution have identified major QTL for fitness.
Field and greenhouse studies of two closely related, Neotropical gingers (Costus spp.)
indicate that local adaptation to edaphic factors has played an important role in adaptation
and speciation. Costus allenii inhabits dense forest understory along streams while C.
viollosissimus is found along forest edge in full sun. The two species are easily crossed to
produce fully fertile F1 hybrids, yet their strong habitat preferences leads to a microallopatric distribution, thus preventing gene flow. Differences in drought tolerance and in
the timing of seed germination contribute to their spatial isolation. Field experiments with
F2 hybrids are in progress to investigate the ecological genetics of adaptation in this
system.
Taken tougher, these studies provide insight into the ecological and genetic mechanisms
of adaptation in natural plant populations. The findings thus far add to the growing
evidence that adaptation often involves genes of large effect, and that strong geographic
adaptation on a local scale may be sufficient to cause reproductive isolation.
Evolution in the Tropics: Dobzhansky revisited
Douglas W. Schemske
Department of Plant Biology, Michigan State University, East Lansing, MI 48824
A steep latitudinal gradient in species richness exists for most groups of organisms with
many more species observed in tropical regions. Explanations for this pattern focus
largely on historical factors or ecological mechanisms of coexistence and ignore
latitudinal differences in evolutionary processes that may contribute to the gradient. One
exception is Dobzhansky's classic paper "Evolution in the Tropics" (American Scientist.
1950. 38:209-221), in which he proposed that the greater importance of biotic
interactions in the tropics increases the opportunity for the evolution of novel forms. Here
I provide evidence in support of Dobzhansky’s assertion that biotic interactions are
stronger in the tropics and propose a mechanism linking biotic interactions and rates of
speciation. The central premise is that latitudinal differences in the importance of biotic
factors will influence the opportunity for coevolution, and hence rates of divergence. In
the tropics, strong biotic interactions promote coevolution, and as interacting species
coevolve, the optimum phenotype is constantly changing. In temperate regions, the
phenotypic optimum is more static since abiotic factors do not coevolve and biotic factors
are responsible for a much lower proportion of overall selection. Nascent populations
isolated by geographic barriers in tropical regions will diverge in traits related to species
interactions, while those in temperate regions will diverge primarily in response to
differences in abiotic factors. Such latitudinal differences in the role of biotic interactions
should result in faster speciation in tropical environments as species interactions continue
to coevolve. Moreover, in the tropics, stochastic variation in community composition
following geographic isolation, i.e. ‘biotic drift’, may amplify geographic differences in
species interactions, and further catalyze divergent selection and speciation. Where biotic
interactions prevail, each new species becomes a resource for others, and the expanded
number of niche dimensions fuels ever-higher species richness. In this way,
diversification in tropical regions becomes an autocatalytic process.