杏吧原创

The Variety of Life by Colin Tudge, Oxford University Press, 拢35, ISBN
0198503113

SHAKE a tree in a rainforest and most of the organisms that fall into your
collecting nets will be new to science. Yet it is a common lament of academic
biologists (of a certain age) that within their classrooms basic knowledge of
the diversity of organisms isn鈥檛 what it used to be.

Are school curricula to blame? Or is it the rise of molecular genetics and
the concentration on the gene rather than the organism? Whichever, in The
Variety of Life, Colin Tudge attempts to redress this imbalance.

Tudge would be the first to admit that this is no definitive treatise. How
could it be? At the last count, about 1.7 million species were known. A list of
all their names would, he notes, fill seven fat books. Proper descriptions of
each would fill a library. And these known species almost certainly represent
only a small fraction of what is out there. Conservative
estimates鈥攅xtrapolating from the rainforest experience鈥攕uggest that
there may be about eight million species, but others put the figures much
higher. Taking bacteria alone, some estimates suggest that the current count of
40 000 types is only one ten-thousandth of the real number.

The first task is to organise this vast diversity. Unusually, before
introducing us to life鈥檚 splendour, Tudge takes us on a historical and
philosophical tour of the way biologists classify life. To cut a long story
short, modern taxonomy relies on the understanding that every species is derived
from another species. None is generated de novo鈥攏one was dropped onto the
stage from the gods. Every organism can therefore be represented as a twig on a
very large, multi-branched, very twiggy tree.

This tree description of a group is otherwise known as its phylogeny. An
alternative representation of the same set of relationships is a 鈥淩ussian doll鈥�
system of classification, in which humans sit in the primate box, which sits in
the mammal box, which sits in the vertebrate box . . . and so on and
outwards.

The problem of relating species鈥� phylogenetic positions to their names
generates much hot air. Many a taxonomist will castigate you for calling a bird
鈥渁 bird鈥� and a reptile 鈥渁 reptile鈥�. A bird is a reptile, you will be told.
Reptiles evolved into birds, so the class that includes all the birds must be a
subdivision of reptiles. If a reptile is a reptile, a bird must be a reptile as
well.

Oh, please! A prime function of any naming system must be to ensure easy
dialogue, but the efforts of many taxonomists seem to hinder rather than help.
The many changes made to the official name of the bluebell, to take one example,
are a recipe for confusion. On this Tudge is refreshingly pragmatic, arguing
that we shouldn鈥檛 get too worked up about the naming business. It鈥檚 the
phylogeny that鈥檚 the important thing.

The going gets a bit trickier when it comes to working out where any given
species fits on the tree of life. Tudge is an able guide through a minefield of
appalling terminology, but there are weaknesses. One is that he never touches on
the problem of 鈥渉orizontal鈥� transfer of genes between 鈥渟pecies鈥�, which is
commonplace in bacteria, for example. This inconvenient reality seriously messes
up the notion that any given individual can be placed neatly into a unique
nested box. If you reconstruct the phylogeny from one gene, it can show a
completely different pattern to that reconstructed from another gene in the same
organism, which makes the concept of a 鈥渟pecies鈥� begin to look a bit vaguer than
intuition demands.

Tudge also fails to introduce us to the many statistical methods for
connecting gene sequences to phylogenetic trees. Indeed, many in the field would
regard as dated his concentration on 鈥減arsimony鈥� methods鈥攊n essence,
constructing the tree that contains the fewest differences between the species
its nodes represent.

From the tree of life, Tudge moves to its diversity. This, the bulk of the
book, is unashamedly biased. Bacteria get a chapter, as do my personal
favourites, the unicellular eukaryotes. Fungi get one and plants get three, of
which one is given to the largest group within the flowering plants, the
Compositae (daisies, artichokes and so on). No fewer than 18 chapters are
devoted to animals鈥攁 share that is out of all proportion to their
abundance: the book might better be called 鈥淭he diversity of fairly big things a
bit like us, with notes on the rest鈥�.

But this is typical of the wonderful eccentricity that imbues the book, which
was clearly not written with money or a particular market in mind. Regrettably,
academics won鈥檛 have much use for it, as it lacks appropriate referencing. And
parts are out of date: five years ago the small unicellular parasites known as
microsporidia were classed as early eukaryotes, but all recent phylogenies point
to them being weird fungi. This omission in the one group whose phylogeny I have
been watching closely saps my confidence in the rest of Tudge鈥檚 trees.

Nor is it as easy a read as popular science should be. Granted, any attempt
to explain in a sexy and accessible style what a 鈥渟ynapomorphy鈥� might be is
doomed from the start. But then, if you wanted to write a popular science
best-seller you wouldn鈥檛 write about the philosophy of taxonomy. It鈥檚 as dry as
old bones.

To complain about flaws and lack of focus is to miss the point. There is an
inscription on the original score of Beethoven鈥檚 great mass, the Missa
Solemnis, which translates as 鈥渋t comes from the heart, may it go to the
heart鈥�. This could be the inscription on this book. This is very much a great
mass to nature. If Tudge communicates the wonder and richness of life, even in
some small part, then he has gone one step to achieving his ends.

What are these ends? The bottom line is conservation. The present rate of
species loss has only been matched during periods of mass extinction. So what,
you might ask. Didn鈥檛 nature bounce back every time? Mass mortality of species,
Tudge insists, is like that of humans a moral issue. And on all moral issues the
fundamental desire should come from the heart, rather than the mind or the
pocket. If Tudge can make you feel as he does about nature, then he has
succeeded.