Convergents and parallels

A match between the nature of organisms and their environment can often be seen as a similarity in form and behavior between organisms living in a similar environment, but belonging to different phyletic lines (i.e. different branches of the evolutionary tree). Such similarities also undermine further the idea that for every environment there is one, and only one, perfect organism. The evidence is particularly persuasive when the phyletic lines are far removed from each other, and when similar roles are played by structures that have quite different evolutionary origins, i.e. when the structures are analogous (similar in superficial form or function) but not homologous (derived from an equivalent structure in a common ancestry). When this is seen to occur, we speak of convergent evolution. Many flowering plants and some ferns, for example, use the support of others to climb high in the canopies of vegetation, and so gain access to more light than if they depended on their own supporting tissues. The ability to climb has evolved in many different families, and quite different organs have become modified into climbing structures (Figure 1.15a): they are analogous structures but not homologous. In other plant species the same organ has been modified into quite different structures with quite different roles: they are therefore homologous, although they may not be analogous (Figure 1.15b).

Other examples can be used to show the parallels in evolutionary pathways within separate groups that have radiated after they were isolated from each other. The classic example of such parallel evolution is the radiation amongst the placental and marsupial mammals. Marsupials arrived on the Australian continent in the Cretaceous period (around 90 million years ago), when the only other mammals present were the curious egg-laying monotremes (now represented only by the spiny anteaters (Tachyglossus aculeatus) and the duckbill platypus (Ornithorynchus anatinus)). An evolutionary process of radiation then occurred that in many

Parthenocissus (Vitaceae), sticky pads analogous and homologous structures

Figure 1.15 A variety of morphological features that allow flowering plants to climb. (a) Structural features that are analogous, i.e. derived from modifications of quite different organs, e.g. leaves, petioles, stems, roots and tendrils.

Figure 1.15 (continued ) (b) Structural features that are homologous, i.e. derived from modifications of a single organ, the leaf, shown by reference to an idealized leaf in the center of the figure. (Courtesy of Alan Bryant.)

(b) Lathyrus

(b) Lathyrus

Figure 1.15 (continued ) (b) Structural features that are homologous, i.e. derived from modifications of a single organ, the leaf, shown by reference to an idealized leaf in the center of the figure. (Courtesy of Alan Bryant.)

ways accurately paralleled what occurred in the placental mammals on other continents (Figure 1.16). The subtlety of the parallels in both the form of the organisms and their lifestyle is so striking that it is hard to escape the view that the environments of placentals and marsupials provided similar opportunities to which the evolutionary processes of the two groups responded in similar ways.

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