Can Evolution Create Homologous Structures by Different Paths?

Günter Thebien (Friedrich Schuller U, Jena, Germany) is baffled about how two plants arrived at similar structures by different evolutionary pathways. In the April 22 issue of Nature,¹ he asks,

Structures that occur in closely related organisms and that look the same are usually considered to be homologous – their similarity is taken to arise from their common ancestry. Common sense suggests that the more complex such structures are, the less likely they are to have evolved independently and the more valuable they should be for studying systematics. But what if ‘obviously’ identical organs have arisen through two mutually exclusive developmental routes?

He points to a discovery by Glover et al. (Gene 331, 1-7; 2004) of just such a what-if situation. Two species in the nightshade family (of which tomatoes are a member) that have almost identical looking “pepperpots” or anther cones in their flowers. Yet mutation experiments on the genes that develop the structures show that neither could be related to the other by common ancestry, because they develop under different pathways. “So the most plausible conclusion,” he claims, “is that pepperpots originated twice independently in the lineages that led to tomato and bittersweet.” If so, this means trouble for systematists:

Molecular systematic analysis confirms that tomato and bittersweet are closely related, and the traditional view would be that their pepperpot cones are obviously homologous. But genetic tinkering and mutant analysis show that they probably are not – that they are convergent, having taken different routes to the same end. Life’s potential to invent complex structures more than once may worry systematists, who depend on reliable characters to reconstruct relationships between organisms.  ut it will please anyone who admires nature’s innovative power.

¹Günter Thebien, “Developmental genetics: Bittersweet evolution,” Nature 428, 813 (22 April 2004); doi:10.1038/428813b.

Homology is one of those words that embeds Darwinian assumptions into the terminology. The Darwin Party’s word games go like this:

• • Homologous structures are similarities that Darwinians believe are related by common ancestry.
  • Analogous structures are similarities that Darwinians believe are not related by common ancestry. In some unspecified way, they arrived at the same pattern by “convergent evolution.”
    Thus, by waving either hand, the Darwin show can go on. But when both hands are waving, they might collide. Thebein’s hand-waving term “convergent evolution” has just collided with the hand-waving Darwinian concept of homology. Now what? Nature has thrown the Darwinians a curve; a complex structure that “common sense” says could not have evolved twice independently. This is where the Darwinians go to Plan C:

• • Homologous-convergent structures prove Nature is tricky.
    Since, to a Darwinian, Nature is a personified goddess tinkering with her creations, she has free will and even a sense of humor, in addition to “innovative power.” By employing fast-talking equivocation with the science security guards, the Darwinians avoid having their science badges disqualified. They can remain and enjoy the melodrama, chuckling at the dirty trick “Nature” played on the systematists. They never catch on that the joke’s on them.