Section 7.2 (pages 158-60) showed that a ‘fast’ life history of producing numerous small offspring is typical of high mortality environments, whilst a ‘slow’ strategy of few large offspring is typical of low-mortality environments. However, all the evidence we looked at was correlational. There is experimental evidence too. A long tradition of work on Trinidadian guppies (Poecilia reticulata) by David Reznick and colleagues has shown that in the high-predation downstream sections of rivers, the fish have a relatively fast life-history strategy of rapid maturation and many eggs, compared to the predator-free upper reaches. More than that, if you put high-predation guppies into a low-predation environment, their life history changes in the direction predicted by the theory in as little as a few generations (and this seems to be by selection, not by plasticity). Most recently, Gordon et al. (2009) transplanted some guppies from one stream into another. The average life-history of their descendants was measurably different within fewer than 30 guppy generations (10 years). Moreover, the descendants of the transplanted fish now survived better in the new habitat than a newly-introduced fish did. In other words, in less than 30 generations, adaptation to the new habitat by natural selection had occurred. This is some of the strongest evidence of the power of selection in action, and moreover it is strong confirmation of hypotheses that have been developed using correlational evidence and theoretical modelling.

Gordon, S.P. et al. (2009). Adaptive changes in life history and survival following a new guppy introduction. American Naturalist 174: 34-45 [DOI: 10.1086/599300]