Crimea State Medical University, Simferopol

1.

Crimea State Medical University,
Simferopol
Biology Project
Teacher – Anna Zhukova
Made By - Mohammad Imran Sheikh
Batch- LA 1 – 194 A

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In the introduction to The Cost of Natural Selection Haldane
writes that it is difficult for breeders to simultaneously
select all the desired qualities, partly because the required
genes may not be found together in the stock; but, he
writes, especially in slowly breeding animals such as cattle,
one cannot cull even half the females, even though only
one in a hundred of them combines the various qualities
desired.

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That is, the problem for the cattle breeder is that
keeping only the specimens with the desired
qualities will lower the reproductive capability too
much to keep a useful breeding stock.

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Haldane states that this same problem arises with
respect to natural selection. Characters that are
positively correlated at one time may be
negatively correlated at a later time, so
simultaneous optimization of more than one
character is a problem also in nature. And, as
Haldane writes
in this paper I shall try to make quantitative the
fairly obvious statement that natural selection
cannot occur with great intensity for a number of
characters at once unless they happen to be
controlled by the same genes.

6.

Selection Intensity
Haldane proceeds to define the intensity of
selection regarding "juvenile survival" (that is, survival to
reproductive age) as
I=ln(So/S)
where So is the proportion of those with the optimal
genotype (or genotypes) that survive to reproduce,
and S is the proportion of the entire population that
similarly so survive.

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The proportion for the entire population
that die without reproducing is thus 1-S,
and this would have been 1- So if all
genotypes had survived as well as the
optimal. Hence So-S is the proportion of
"genetic" deaths due to selection. As
Haldane mentions, if So approx S, then I
approx So-S.

8.

Origin of the term "Haldane's
dilemma"
Apparently the first use of the term "Haldane's dilemma" was by
Palaeontologist Leigh Van Valen in his 1963 paper "Haldane's
Dilemma,
Evolutionary Rates, and Heterosis".
Van Valen writes :
Haldane (1957 [= The Cost of Natural
Selection]) drew
attention to the fact that in the process of the evolutionary
substitution of one allele for another, at any intensity of
selection and
no matter how slight the importance of the locus, a substantial
number
of individuals would usually be lost because they did not already
possess the new allele.

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Kimura (1960, 1961) has referred to this loss as the
substitutional (or evolutional) load, but because it
necessarily involves either a completely new mutation or
(more usually) previous change in the environment or
the genome, I like to think of it as a dilemma for the
population: for most organisms, rapid turnover in a few
genes precludes rapid turnover in the others. A corollary
of this is that, if an environmental change occurs that
necessitates the rather rapid replacement of several
genes if a population is to survive, the population
becomes extinct.

10.

The Cost
Haldane writes,
I shall investigate the following case mathematically. A
population is in equilibrium under selection and mutation
One or more genes are rare because their appearance by
mutation is balanced by natural selection.

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A sudden change occurs in the environment, for
example, pollution by smoke, a change of climate, the
introduction of a new food source, predator, or
pathogen, and above all migration to a new habitat. It
will be shown later that the general conclusions are not
affected if the change is slow. The species is less adapted
to the new environment, and its reproductive capacity is
lowered. It is gradually improved as a result of natural
selection. But meanwhile, a number of deaths, or their
equivalents in lowered fertility, have occurred.

12.

The selective death that must occur for a gene to be
substituted was called the cost of selection by the
biologist J.B.S. Haldane.
The higher the intensity of natural selection, the higher
the amount of selective death (or infertility) there must
be. A population cannot tolerate an indefinitely large
amount of selective death: if selection is too strong it
will drive the population extinct.
The cost of selection places an upper limit on the rate of
evolution:
Suppose there are two alleles in a population, A and A'
with fitnesses 1 and (1-s) and frequencies p and q (=1-p)
respectively.

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In any generation, of the q A' -bearers, s will die
without reproducing and (1-s) will survive likeA bearers. A proportion, sq, of the population dies
without reproducing because of selection at this
locus.
We can now define a ratio of the proportion of
individuals in the population that survive to the
proportion that die: sq die, and p+q(1-s) = 1-sq
survive. The ratio in one generation is sq/(1-sq).
The ratio is the same every generation until A' is
eliminated. As selection operates each
generation, more and more selective death
accumulates.

14.

We can now define the total cost of natural selection as:
C = S [sq /(1 - sq)]
(The summation is over all the generations it takes to fix the A
gene.)
If a population is to maintain itself, the individuals that survive
have to produce sufficient extra offspring to make up for those
that die before reproduction. Because of this there will be an
upper limit to the possible cost of natural selection. If the ratio
was 0.999/0.001, each survivor would have to leave 1000
surviving offspring, which would be much more difficult.
The upper limit suggested by Haldane for a diploid population was
one gene substitution per 300 generations. Haldane’s cost of
selection was used to argue that the rates of molecular evolution
are too fast to be explained by natural selection.

15.

Haldane was born
in Oxford to John
Scott Haldane, a physiologist,
scientist, a philosopher and
a Liberal, and Louisa Kathleen
Trotter, a Conservative. His
younger sister, Naomi
Mitchison, became a writer,
and his uncle was Viscount
Haldane and his aunt the
author Elizabeth Haldane.
J.B.S. Haldane
1892-1964