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posted 09. June 2004 12:11
Nature 428, 703 - 704 (15 April 2004)
Evolutionary biology: Lost and found
Neil H. Shubin and Randall D. Dahn
Summary: Can we ever hope to pin down the genetic changes that underlie the big steps in evolution? Possibly so, if a study of the variation in the pelvic fins of sticklebacks is anything to go by.
Darwin's lament that "Our ignorance of the laws of variation is profound" has described one of the persistent problems in evolutionary biology for the past 145 years. How does genetic variation - the raw material of evolution - arise within populations, and how does it evolve to make species anatomically and behaviourally distinct? Ultimately, answers to these questions will be derived from understanding the actual genetic changes that control the evolution of anatomy or behaviour. These insights could explain a host of puzzles, from how rapid evolutionary change can occur in populations, to why morphological evolution often follows certain well-defined trends. On page 717 of this issue, Shapiro et al. describe the genetic basis for one such trend: limb reduction in threespine stickleback fish.
Surprisingly, some of the most significant novelties in the history of life are associated not with the evolution of new structures but with the loss or reduction of primitive ones. In vertebrates, for example, the invasion of new ecological niches and the origin of new locomotor adaptations involve either the complete loss or partial reduction of appendages3, 4. The complete loss of appendages has been involved in the evolution of new aquatic lifestyles in whales and burrowing niches in snakes, amphisbaenians (worm lizards) and caecilians (rubber eels). The reduction or loss of fingers and toes is associated with the evolution of jumping, flying and running in some amphibians, reptiles and mammals. In sticklebacks (Fig. 1), a large spine in the pelvic fin serves to limit predation in marine populations. But in some freshwater stickleback populations this pelvic spine is reduced or lost entirely - a trend that might be associated with lower concentrations of available calcium ions in freshwater lakes, or with the presence of large invertebrates that feed on sticklebacks by grasping the pelvic spines.