a week ago, i reported that I was reading THE ARRIVAL OF THE
FITTEST, by andreas wagner, a fascinating book about evolutionary biology
recommended by my big sister barbara. in
my slow, steady way i have now finished the book, and in this relatively brief
post i am going to try to say just enough to persuade you to read it.
even though the book is intended as a popularization, it is
quite demanding for someone [like me] who is not already fully conversant with
microbiology and evolutionary genetics.
Let me try to state the central idea as simply and clearly as i can.
organisms evolve by changes in their genotypes -- which is
to say, changes of a single gene at a time[there are other changes, but i am
desperately trying toi keep this simple].
these in turn alter what the organism can do. can it metabolize sucrose? or fructose? or ethanol? or acetate? All of this shapes the way that genes are
'expressed,' which means which of them are turned on to produce proteins, which
block the expression of other genes, in what environment, in what sequence, and
so forth. these changes in the genotype
produce changes in the organism's phenotype -- how it is shaped, what it can do
as an organism, what parts it has and what they can do. natural selection, or selection of the fittest,
now determines which of these changes survive and flourish, and which result in
the death of the organism and hence do not get passed on. if a change to the genotype of a simple
bacterium like e. coli enables the individual bacterium with the change to
metabolize fructose, and if that bacterium happens to be in a fructose rich
environment [like a bit of rotting pear], then it will survive and reproduce,
passing on the changed genotype to its descendants. by this process, one gene at a time,
primordial slime evolves into rick santorum [not such a reach, as it happens.]
my sister, who vetted this for obvious mistakes, made the
following comment: "You employ a usage that is essentially universal, among
experts as well as science writers, namely imputing agency to genes. I am not
suggesting you change this. I just thought you should be aware of it. A
"gene," that is to say the DNA in the genome, doesn't DO anything. It
is just a code that specifies the order of amino acids in a protein (or
alternatively, the order of nucleotides in an RNA molecule. Many of the
regulatory molecules are RNA, not protein). Almost all the time, using this
shorthand is perfectly reasonable, but it lies at the heart of the newspaper
language that talks about genes for homosexuality, or aggression, or infidelity
or whatever anyone likes. There are no such things!!! Even the gene for cystic
fibrosis doesn't do anything. It codes for a protein that is essential for a
specific process (I think it is uptake of mucus in the lung or something like
that) and when the protein is made incorrectly the process doesn't work, and
symptoms ensue. But it is WAY too awkward to say things like that all the time,
so everyone talks about the gene for cystic fibrosis."
continuing with my exposition, there is a problem, a very big problem, one that, i confess, i had
never thought about until reading wagner's book. the development of a more fitting phenotypic
change, one that will make the organism better able to survive and reproduce,
will typically involve a series of changes to the genotype. a number of molecules may have to be altered
or swapped or dropped out of the genotype to get from the old phenotype to the
better adapted new one. and in nature,
there is of course no intelligence guiding this process, no purpose, no
pre-identified goal [now i shall develop an eye because with an eye i can avoid
predators more successfully]. it might,
let us say, require six changes to the genotype to produce what turns out to be
a better adapted phenotype. but the
changes to the genotype are utterly random.
some result from copying errors when the dna replicates itself. some result from gamma rays [or something] hitting
a strand of dna and knocking out a gene.
some result from one organism actually inserting some new genetic
material into another organism's dna [this apparently happens quite a lot to
microbes and other tiny critters; not so
much to elephants.]
since all this is unplanned and not guided by any telos, we can compare it, as wagner
brilliantly does, to a random walk through a digital library of all the logically
possible genotypes. we, looking at the
process in retrospect, may say that the organism was 'seeking' the new genotype
that will result in a phenotype better fitted to survive, but of course it was
now comes the kicker.
suppose the genome of an e. coli strain enables it to metabolize
sucrose. if it drops on a piece of
rotting pear, rich in fructose, it might do much better if it were to randomly
go through the six changes needed to enable it to morph into a strain of e.coli
that can metabolize fructose. but if the first of those changes,
happening randomly, makes it unfit anymore to metabolize sucrose, then it will
die out before it can go through enough random changes to reach the better
adapted ability to metabolize fructose.
and this problem, quite obviously, is not peculiar to e.coli and
sucrose, but arises in the case of every single evolutionary change to the
genome of any organism.
well, you might say, given millions of critters and billions
of year, one of them will hit on the right combination by chance, survive,
flourish, and pass it on. alas, not so
simple. wagner does some elementary but
startling calculations that show that the number of genomes is vastly greater
than the number of hydrogen atoms in the universe! There has barely been enough time since the
earth was formed for primordial slime to evolve into slightly less primordial
slime, let alone into rick santorum.
UNLESS one at a
time changes to an organism's genome are not by and large fatal, but permit the
organism to continue to function in the old way. if, as wagner puts it, the organism is robust, if it can survive and function
despite a number of genomic changes, then perhaps it can, by a random 'walk,'
make its way to the new genome that translates into a phenotype better fitted
combining some very elegant computer simulations and
calculations with the vast amount of laboratory experimental work that has been
done on many different species by tens of thousands of research scientists,
wagner has been able to ascertain that there is an astonishing degree of
robustness in the organisms studied. what
is more [this involves more technical detail than i am capable of summarizing
easily], the 'neighborhoods' into which an organism will wander randomly by
genomic change, one gene at a time, are sufficiently diverse genomically that
the probability is not bad of an organism stumbling, as it were, on a promising
well, i am sure you can see that like wile e. coyote, i am
now way over the edge of the cliff with nothing underneath me, so before i fall
down splat on the ground, to be crushed by an ACME safe landing on top of me, i
if you want a charming, fascinating, important read, try THE
ARRIVAL OF THE FITTEST by andreas wagner.