Figure 1. Types of selection: (a) directional, (b) stabilizing, (c) stabilizing and directional, (d) disruptive, and (e) “squashed stabilizing selection”. Top panels show the fitness function: fitness as a function of phenotype. Bottom panels show the effect on a population’s phenotypic distribution, before (solid line) and after (dotted line) selection. See thesis for details.
Figure 2. Absolute frequency histograms of linear selection gradients, β, and quadratic selection gradients, γ, observed for the modeled population. White bar areas represent non-significant selection, black bar areas represent significant selection. The left column summarizes model realizations without competition, showing that stabilizing selection is detected much more frequently than disruptive selection in that scenario. The right column summarizes model realizations with negative frequency-dependent intraspecific competition, showing that disruptive selection is more commonly detected in that scenario. Overall, significant selection is rarely detected, as in studies in natural populations.
Figure 3. A complexly heterogeneous landscape. In the steep gradient on the right side of the landscape (blue), refugia are generated by the spatially continuous patchiness (green). These refugia can provide evolutionary stepping-stones that promote adaptation to the harsh conditions of the gradient, which might otherwise be too extreme to allow evolutionary branching to occur. This “refugium effect” appears to promote branching in complexly heterogeneous landscapes.
Figure 4. An illustration of the hypothesized mechanism of reproductive isolation between populations of heterostylous plants, as a result of the combination of precise pollen transfer and reciprocal herkogamy. Differential organ positioning between the populations is the result of adaptation to different local pollinators, and results in decreased pollen flow between the differently adapted flowers.
Figure 5. A flowchart depicting the chain of causation that propagates heterogeneity upward from its initial ecological causes (squashed stabilizing selection and environmental heterogeneity) into heterogeneity among individuals, populations, and species.
Until the fall, I’m more or less homeless; my wife and I plan to road-trip around the U.S., visiting friends and going to the Evolution 2013 conference in Snowbird, Utah. Maybe I’ll see you there!
Canyonlands National Park in Utah, not so very far from Snowbird. If you attend Evolution 2013, take some extra days (or weeks!), rent a car, and see the Southwest!
(Photo credit: Ben Haller)