Daphne J. Fairbairn

This is a book about the strange paths taken by evolution. Sadly, it follows a strange path of its own.

If you've ever seen a photo of a harem of gorillas, you'll have some idea of what this book is about: Male gorillas are twice the size of the females. This sort of "sexual dimorphism" (that is, cases where the two sexes have very different body organization -- in this case, where one is much larger than the other) is common among animals, and this book takes a look at why. It doesn't have a chapter on gorillas -- with so many options to choose from, it can take more extreme examples, and so it chooses as its example among mammals the southern elephant seal. Which is indeed an extreme case: a breeding male weighs at least four times as much as a breeding female. The males are perfectly happy to flatten females, or pups, if they don't get out of his way as he chases off other males; the life of a male elephant seal, during the non-breeding season, is to build up as much weight as he can so he'll better be able to fight during the breeding season, and its life during the breeding season is fighting other males and oppressing every nearby female. The life of the males is a painful one, and the life of the females is basically one of staying as far away from their mates as possible! It sounds horrid, but it makes evolutionary sense.

I've no problem with that; it's a fascinating topic. The problem -- and the reason that I give this book two stars instead of three -- is that the whole book describes this problem of differential evolution of the sexes by the term "sexual selection." But it is not sexual selection; it is natural selection resulting in sexual divergence. Sexual selection is the other method of evolution (besides natural selection), which Darwin recognized although it took a century after his time for other biologists to figure out what he meant. Sexual selection is, for example, female fish which prefer males with a red hue over one with a brown hue, and so "pushing" the males to become more red. Red fish are not more likely to survive than brown fish -- they may even be less likely to survive -- but, because they are attractive to females, they are more likely to breed. With the result that the males become redder and redder -- and the females may, too, because they share most of the same genes. The two sorts of selection often coexist (e.g. it has been suggested that, in humans, males are bigger because of differential natural selection but females have less hair because of sexual selection), but they are entirely distinct. And this book consistently and reliably uses "sexual selection" when it means males and females evolving in different ways because of plain old natural selection.

It doesn't really destroy the descriptions of the subject matter, but it is very grating when one knows better.

Also, while the bulk of this book is about eight examples of species with extreme sexual dimorphism of body size (one species of mammals, one of birds, two of fish, one of spiders, one of octopus, and two species of invertebrates that I will admit to not knowing exactly how they classify), the final section is a sort of classification of sexual dimorphism: Where it is found in the animal kingdom, based on phyla. This would be tremendously interesting -- if one could understand it. The table on page 162, for instance, appears to examine a bunch of classes within 27 phyla in four taxa to see which classes at least sometimes exhibit dimorphism. Wonderful. I'd love such a table. Except... what are these things? "Chordata" I know -- that's mostly vertebrates, although it also includes things like lancelets. To be sure,, Amphibia, Aves (birds), Mammalia, and Reptilia are in there as well, along with a bunch of other chordates. (Which, incidentally, is wrong again; "Reptilia" is not the same class-level structure as Aves and Mammalia, since Aves and Mammalia are both sub-clades of the clade Reptilia.) But Eutardigrada and Heterotardigrada? Pure Greek says that those are "good tardigrades" and "other tardigrades," and I know tardigrades are the so-called "water bears," but what makes them "good" or "other"? And what's a "Polychaeta" or an "Echiurida"? Most of us aren't graduate zoologists. The chart may demonstrate just how widespread sexual dimorphism is, but it doesn't really tell an ordinary user which sorts of critters show dimorphism. This makes the final chapter -- the one that's supposed to wrap it all up -- very difficult to follow.

I suppose this argues for two audiences for this book: The ones who don't care about actual biology at all, who want stories of really strange critters. (And, yes, this book has that.) The first ten chapters are for them. And the ones who are so advanced that those class names mean something, for whom the twelfth chapter is the Big Deal. But for those of us who know enough evolutionary biology to spot all the wrong terminology in the first part, but who don't have the tools to deal with the second, it's very disappointing. And I fear that the errors of the first part of the book will keep the first group of readers from learning all that they could.… (más)