Figure 2.

Landscapes of niche evolution in Daphnia (an abstraction of the study in Hooper et al. 2008). A, Presence (filled circles) and absence (open circles) of Daphnia in water bodies across a landscape. B, Daphnid “niche”: laboratory characterization of the growth rate of a clone of Daphnia, as a function of two abiotic variables. C, Position of water bodies from the actual landscape in this abstract environmental space. To a reasonable approximation, the distribution matches the niche requirements of the species, but with exceptions; the text notes several plausible explanations for these. D, Arrows show two possible colonization episodes in the real landscapes. The models discussed in the text suggest that adaptive colonization into sites with r ≪ 0 is very unlikely, compared to colonization into sites with only slightly negative r. Colonization with adaptation and an evolved shift in the niche boundary of the clade is more likely just outside the niche boundaries than far outside, simply because extinction should be rapid in the latter. But a series of small evolutionary steps through the environmental space (dotted lines) might ultimately permit adaptation to the extreme environment.