The Origin of Species and a couple of others are good reading, and show the enormity and thoroughness of Darwin's mind and method. He covered the waterfront, from orchids to earthworms. But less well known and hardly read is the way he covered the waterfront's huge population of barnacles. A fairly recent book, Darwin and the Barnacle (Stott, 2003) is about Darwin's fascination, if not obsession, with barnacles, but if you actually try to read Darwin's four barnacle volumes from scratch you'll find them as dry as an Iraqi sandstorm. Most of Darwin's books are dense compendiums of facts, but the barnacle books are more like telephone books than narratives.
At the same time, they tell an incredibly fascinating story, and Darwin was well aware of it (and his letters show that they were far from boring or dry material to him). In fact, while he deals with many issues including species relationships and anatomy, the most fascinating part of the barnacle story is their bizarre, and highly variable reproduction. It's as near to a natural X-rated story as you could image.
Most barnacles are hermaphrodites, carrying both male and female sex organs. But they can't self-fertilize and must mate with nearby barnacles. For this, males in some species have the longest penises in relation to body size that are found in nature (or they may have multiple penises!). Some barnacle species are only partly hermaphrodites, having degenerated organs of one sex or the other. Yet others, discovered by Darwin to his utter amazement, have males as parasites living within the body of the female, which in some cases only have retained a partly hermaphrodite body! In some, these 'husbands' are so rudimentary that they're little more than sex organs.
Darwin noted that in the countless species of barnacles, every trait in every species was variable. He used the data on variation to determine barnacle phylogeny: their relative relationships and order of evolution. By looking at contemporary species with various stages of sexual reproduction strategy, he inferred that some stages, such as live-in husbands, and degenerate husbands, represented stages along the way to being non-hermaphroditic.
The complexity and variation among species must also be found within species, in order for the traits observed today to have evolved over time -- the trait must have varied within species so that today's form would gradually become the standard for that species.
Darwin was trying to solve the species (or 'transmutation') problem of his day. But his very sexy story also reveals the problem of reconstructing adaptations. Darwin assumed that different stages he saw in different species were traits along the way to some final trait. But that's not necessarily so and it nearly assumes predestination, a total no-no in evolution. Just because a species of barnacle has host-husbands does not imply that it, like some other existing barnacle species, is on the way to having only rudimentary husbands or to losing hermaphroditic structure. Or, at least, such arguments about how predictable evolution is are highly debatable based on what we know today.
Darwin is great reading, as a way to see a great mind at work. His utterly dull but sexy barnacle books show some of the problems we face in reconstructing selective stories from the past, because when so many traits vary at the same time, over so much time, it's hard to know why each stage on the way was favored by selection. This is especially true because we can only tell so much from fossils.
Maybe everything in life, even everything totally boring, eventually boils down to sex. But whether HBO can make an exciting film about barnacle bedroom behavior is unclear.
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