Leaf after leaf. A series of opportunities. I have been thinking about evolutionary opportunities. Charles Darwin saw a source of evolutionary opportunity in bodies in which parts were “many times repeated.” He wrote in the first edition of Origin of Species that:
“We have formerly seen that parts many times repeated are eminently liable to vary in number and structure; consequently it is quite probable that natural selection, during a long-continued course of modification, should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes.” (p. 418)
In the spiral stairs of leaves around the artichoke, we see an example of Darwin’s “parts many times repeated.” These leaves of the artichoke, however, are all alike—little evolutionary opportunity has been taken on the artichoke’s leafy head. In contrast, flowers demonstrate well the sense of Darwin’s idea. Orchid flowers often have one petal, the lip, that is highly modified relative to the other petals. We might even think about the transition in structure from petals to stamens, as in the photograph below of a nasturtium, as an example of modification among ‘parts many times repeated’ in flowers.
These evolutionary opportunities presented when parts are ‘many times repeated’ in the body of plants interest me. How does evolution go from simple repetition of parts, as we see among the leaves of an artichoke, to the variation and specialization we find along the short length of a flower?