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Recent data help explain how this famous group of Galapagos birds evolved, although gaps in our understanding remain
A powerful metaphor for evolutionary diversification is a tree. A typical spreading tree has a single trunk, two or more major branches, several minor branches and many twigs. Its foundations-its roots and the remnants of the original seed-remain hidden from sight. The metaphor applies to the full set of living things on Earth-"the tree of life"-as well as to small groups of species, whether they are marsupial mammals or Hawaiian Drosophila. We can think of the typical crown of a tree as resembling the shape of an umbrella, with twig-bearing branches radiating in all directions (Figure 2, top). The ends of the twigs represent organisms adapted to separate microenvironments; the tree overall represents an adaptive radiation.
Darwin's finches are a prime example of an adaptive radiation. Fourteen or perhaps 15 species, all derived from a common ancestor, occupy individual ecological niches to which they are adapted, principally because of the size and shape of their beaks in relation to the food they eat. Yet on close inspection, we now know that their evolutionary relationships depart radically from the orthodox view of an adaptive radiation with one trunk, a few branches and many terminal twigs. The most striking difference from the idealized model is that near the base of the tree, where the main trunk-representing the ancestral species-splits, only one of the resulting trunks leads to several branches and many twigs (Figure 2, bottom). The other trunk gives rise to little more than thin twigs, although they have persisted in growing to the crown. The next division is also asymmetrical. The third division is more orthodox, producing two branches that radiate approximately equally, one yielding the ground finches and the other yielding tree finches. Nevertheless, the tree as a whole is lopsided.
Even if a model fails to fit the data perfectly, it is useful to describe evolutionary branching with a metaphor in mind, because the confrontation between data and metaphor encourages the posing of sharp questions. Being forced to fit data to an idealized concept may lead to new insights and revised idealizations. For example, as we discuss in more detail later, thinking about the loss...