Focus Magazine

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PZ Myers change we see in the sequence of ‘letters’, called the constraints of natural selection with our
Associate professor of biology at the nucleotides, in DNA, and it is unavoidable. The technology and social support systems. That does
University of Minnesota Morris most common mechanism is simply errors in not imply an end of evolution, though: selection is
copying. No machine can be immune to error, and a conservative force that culls novelty, or reinforces
There are two things that must happen for human that goes for the biological machinery of the cell, the expansion of a few advantageous forms. What
evolution to come to an end: one, everyone must which is responsible for copying, one by one, each does that mean? That less selection allows greater
be genetically identical, and two, the copying of of the three billion letters in the human genome amounts of genetic diversity in a population.
DNA from one generation to the next must become each time cells divide. And cells have to divide Not that we’ve actually eliminated selection, of
absolutely ﬂ awless. The former is simply not the or we couldn’t produce populations of sperm course. All we have changed are the parameters,
case, while the latter is physically impossible. and egg cells. often in ways we take for granted. Perhaps we don’t
These conditions reﬂ ect the two engines that Our cells are actually very good at copying, only have to outrun sabre-toothed tigers anymore, but
drive the appearance of anything new during the making one error in every 10 billion letters, which our offspring still have to overcome the hurdle
process of evolution. The ﬁ rst is the reshufﬂ ing of really is remarkably good. Imagine transcribing of coping with the opposite sex in the big school
alleles (all the different possible forms of a gene). a million pages of text from one pile of papers to dance, of acquiring skills that will get them a good
We human beings do this by using sex. Every another. How many mistakes would you make? job, of ﬁ nding a mutually fertile mate sometime
child is the product of the random assortment However, that still means a new mutation roughly before they die. Selection can be a subtle force, and
of alleles from two individuals, producing a once every three cell divisions, and since it takes you cannot escape it.
unique combination of alleles every time. This is a several hundred cell divisions between birth and Oh, and there is one other way we could see
signiﬁ cant source of new variants and forms: the ejaculation to produce a sperm cell, that means human evolution end: if we became extinct.
human race contains billions of people, and no two each sperm contains hundreds of new mutations. Otherwise, evolution is a natural and
(with the exception of identical twins) have exactly Most of these are neutral in effect, since we can unavoidable property of living, replicating
the same arrangement of alleles. As long as the survive the many copying errors we accumulate, systems, and it is absurd to talk of any living
pool of parents vary, we will continue to produce but it does mean that every single one of us is a population as not evolving.
children with unique attributes. mutant in the purest sense of the word.
The second engine that keeps things fresh In spite of this, one common argument for the
PZ Myers’s blog, Pharyngula, can be found at
in the human gene pool is mutation. This is the end of human evolution is that we’ve loosened
http://scienceblogs.com/pharyngula
1918-1932 1937-1947 1950s- 1968 1976
Population genetics is Genetics is incorporated After molecular biology techniques According to neutral theory, Richard Dawkins popularises the
developed to study the into Darwinian natural reveal the chemical sequences of most mutations neither selﬁ sh gene theory. Individuals
change in the frequency of selection to produce the proteins (1955) and DNA (1977), harm nor help, so evolution evolve not for the good of the
alleles, gene variants, in a modern evolutionary scientists can compare different is largely dictated by species, but to pass their genes to
population’s gene pool. synthesis. species genetically. chance or genetic drift. the next generation.
www.bbcfocusmagazine.com February 2009 45
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