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If you can't be arsed skip to the last paragraph (nothing earth
shattering - just nice to see it presented in Nature, without too many
Predicting the future
JOHN F. Y. BROOKFIELD
Institute of Genetics, University of Nottingham, Nottingham NG7 2UH, UK.
Nature 411, 999 (2001) © Macmillan Publishers Ltd.
The idea of fitness is central to evolutionary biology. The British
philosopher Herbert Spencer characterized Darwin's theory as "the
survival of the fittest" — the survival of the individuals that best fit
their environment. In the 1930s, J. B. S. Haldane, Sewall Wright and
Ronald Fisher quantified 'fitness' to express the strength of natural
selection. Thus, if a mutant genotype suffers a 10% selective
disadvantage relative to the wild type, it has a fitness of 90%.
This concept of fitness has prompted sterile debate along the lines
that, as natural selection states that the fittest survive, and as 'the
fittest' is defined as those that survive, the whole concept of natural
selection is tautological. This misses the point: the main thing is that
it is extremely useful to have a quantifiable, measurable description of
Fitnesses of genotypes tend, empirically, to be roughly constant.
Calculations based on this assumption give good predictions of the time
course of the spread of advantageous alleles in populations. Fisher
showed that when the fitness of each genotype is constant with time, the
mean fitness of a population increases. Fitness describes the present
success of a genotype, not the probability of its survival in the long
term, which might depend on its capacity to adapt to new environments.
One fundamental misunderstanding is that any differences in survival or
reproduction between individuals reflect differences in fitness. This is
not what fitness means. Fitness represents an expected outcome, and what
actually happens in small populations differs from expectation because
each generation's genotypes represent a sample, with an attendant
sampling error, of the gametes produced by the previous generation; this
is the basis of the phenomenon known as 'genetic drift'. The fitness of
a genotype is related only probabilistically to real events; weakly
advantageous mutations are usually lost by chance. Weakly deleterious
mutations are much less likely to spread than advantageous alleles, but
may arise much more often. Indeed, it is probable that most evolution of
amino-acid sequences has occurred by fixation of weakly deleterious
Fitness is hard to measure, particularly in wild populations, because it
summarizes expected survival and reproduction. In particular, measuring
'lifetime reproductive success' by painstakingly tracking cohorts of
individuals throughout their lives gives data that are difficult to
interpret. Each individual in a sexual wild population has a genotype
that, as an entity, is unique. An individual's genotype will have a
fitness, which will thus be the individual's fitness. But random events
cause the lives of individuals to differ, even those with identical
fitnesses, and variation in the lifetime reproductive success of
individuals does not represent variation in fitness between their
genotypes. The only way to measure differences in fitness is to measure
differences in mean survivorship and in mean reproductive rate between
classes of individuals (groups of individuals that differ biologically).
If one is interested in evolution, the only interesting classes are
those that differ in their genotypes. A genotypic class, for example,
might include all individuals that share the same genotype at a single
Now, if one is looking at the mean survival and reproduction of
genetically defined classes of individuals, there is no point in looking
at lifetime reproductive success. It is not worth measuring the
fertility of young adults, for example, and then monitoring the
fertility of the same individuals year after year as they grow older.
You can find out all you need to know by looking at the fertility of
different age classes in the same year — the fact that the individuals
are different has no effect on the expected mean fitness.
Many view natural selection as an environmental force that acts on the
phenotypes of populations, by analogy with artificial selection in
animal or plant breeding. Although differences in genotypic fitness are
caused by differences in phenotype, the widespread occurrence of
pleiotropy — whereby a single genetic change has multiple phenotypic
effects — means that it is very difficult to identify the true ways in
which fitness differences arise. The most obvious phenotypic differences
may not be the most important.
Does Fisher's theorem predict whether organisms become better adapted to
their environment with time? In principle, yes, but there are important
caveats. First, environments change. It is futile to try to explain
human behaviour in adaptive terms, as the environments in which the
genes responsible evolved were so different. For example, it may be that
bad drivers crash cars more often, and so there is natural selection
against individuals who are poor at driving. But this has no causal
connection with the fact that humans can drive cars. The genes for this
skill were not created by selection against individuals who crashed. Of
course, nobody would suggest that they were, yet there are consistent,
misguided attempts to explain other aspects of contemporary human
behaviour in terms of the consequences, in effects on fitness, of those
behaviours in modern environments.
Second, although genes that improve survival tend to increase fitness,
so do genes that increase sexual attractiveness — such as those that
create the peacock's tail. A population of peacocks that did not evolve
a spectacular tail might have been more successful in terms of
population density or the probability of survival. Equally, in a
population in which random genetic changes reduced male fitness to make
all males slightly less attractive to females, the females would settle
for second-best, and the species would get along fine. There is no
necessary agreement between mean fitness and ecological variables.
I have used the term 'fitness' to describe differences between genotypes
within species. What about the 'fitnesses' of different entities? Some
species spread at the expense of others, and some ideas (memes) become
better known by imitation. Can the spread of religion, for example, be
explained in terms of 'memes' with high 'fitness', as some believe?
Logically, 'fitness' could be used for these other entities, in which
case its use would remain as circular and non-explanatory as it is for
genotypes. But here, fitness is not constant over time, so there is no
pragmatic justification for it as a predictive mathematical tool.
References 1. Smith, J. M. Evolutionary Genetics 2nd edn (Oxford Univ.
Press, Oxford, 1998).
Chris Taylor (firstname.lastname@example.org)
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