Haldane's dilemma

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Haldane's Dilemma refers to a limit on the speed of beneficial evolution, first calculated by J. B. S. Haldane in 1957, and clarified further by later commentators. It remains a source of unresolved controversy in the evolutionary genetics literature, particularly concerning the fundamental causes of the speed limit, and the suitability of the special evolutionary processes invoked to solve it. Today, Haldane's Dilemma is raised mostly in anti-evolutionary circles, where it is claimed as evidence against large-scale evolution, and as an example of evolutionist negligence.

1 The Substitution Cost
2 Origin of the term "Haldane's Dilemma"
3 Walter ReMine and Haldane's Dilemma
4 See also
5 References
6 External links

[edit] The Substitution Cost

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, writes Haldane (p. 511),

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.

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

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 (loc. cit.)

[i]n 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.

In faster breeding species there is less of a problem. Haldane mentions (loc. cit.) the peppered moth, Biston betularia, whose color is determined by two allele genes C and c. The CC and Cc moths are dark, while the cc moths are light. Against the originally pale lichens the darker moths were easier for birds to pick out, but in areas, where pollution has darkened the lichens, the cc moths had become rare. Haldane mentions that in a single day the frequency of cc moths might be halved.

But even here there is a potential problem, if "ten other independently inherited characters had been subject to selection of the same intensity as that for colour, only $(1 / 2) 10$ , or one in 1024, of the original genotype would have survived." The species would most likely have become extinct; but it might well survive ten other selective periods of comparable selectivity, if they happened in different centuries.

[edit] Selection Intensity

Haldane proceeds to define (op. cit. p. 512) the intensity of selection regarding "juvenile survival" (that is, survival to reproductive age) as $I =$ ln $(s 0 / S)$ , where $s 0$ is the quotient of those with the optimal genotype (or genotype) that survive to reproduce, and $S$ is the quotient for the entire population. The quotient of deaths $1 - S$ for the entire population would have been $1 - s 0$ , if all genotypes had survived as well as the optimal, hence $s 0 - S$ is the quotient of deaths due to selection. As Haldane mentions, if $s 0$ ≈ $S$ , then $I$ ≈ $s 0 - S$ - since ln(1) = 0.

[edit] The Cost

At p. 514 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. 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. If selection at the $i t h$ selected locus is responsible for $d i$ of these deaths in any generation the reproductive capacity of the species will be Π( 1 - $d i$ ) of that of the optimal genotype, or exp( -Σ $d i$ ) nearly, if every $d i$ is small. Thus the intensity of selection approximates to Σ $d i$ .

Comparing to the above, we have that $d i = s 0 i - S$ , if we say that $s 0 i$ is the quotient of deaths for the $i t h$ selected locus and $S$ is again the quotient of deaths for the entire population.

The problem statement is therefore that the genes (actually alleles) in question are not particular beneficial under the previous circumstances; but a change in environment favors these genes by natural selection. The individuals without the genes are therefore disfavored, and the favorable genes spread in the population by the death (or lowered fertility) of the individuals without the genes. Note that Haldane's model as stated here allows for more than one gene to move towards fixation at a time; but each such will add to the cost of substitution.

The total cost of substitution of the $i t h$ gene is the sum $D i$ of all values of $d i$ over all generations of selection; that is, until fixation of the gene. Haldane states (loc. cit.) that he will show that $D i$ depends mainly on $p 0$ , the small frequency of the gene in question, as selection begins - that is, at the time that the environmental change occurs (or begins to occur).

[edit] A Mathematical Model of the Cost for Diploids

Let A and a be two alleles with frequencies $p n$ and $q n$ in the $n th$ generation. Their relative fitness is given by (cf. op. cit. p. 516)

Genotype AA Aa aa

Frequency $p_n^2$ $2 p n q n$ $q_n^2$

Fitness 1 $1 - λ K$ $1 - K$

where 0 ≤ $K$ ≤ 1, and 0 ≤ λ ≤ 1.

If λ = 0, then Aa has the same fitness as AA, e.g. if Aa is phenotypically equivalent with AA (A dominant), and if λ = 1, then Aa has the same fitness as aa, e.g. if Aa is phenotypically equivalent with aa (A recessive). In general λ indicates how close in fitness Aa is to aa.

The fraction of selective deaths in the $n th$ generation then is

$d_n = 2\lambda Kp_nq_n + Kq_n^2 = Kq_n[2\lambda + (1 - 2\lambda)q_n]$

and the total number of deaths is the population size multiplied by

$D = K \sum_0^\infin q_n \; [2\lambda + (1 - 2\lambda)q_n]$

[edit] The Magic Number 300

Haldane (op. cit. p. 517) approximates the above equation by taking the continuum limit of the above equation. This is done by multiplying and dividing it by dq so that it is in integral form

d q n = - K p n q n [λ + (1 - 2λ) q n]

substituting q=1-p, the cost (given by the total number of deaths, 'D', required to make a substitution) is given by

$D = \int_0^{q_n} \frac{[2\lambda + (1 - 2\lambda)q]}{(1 - q)[\lambda + (1 - 2\lambda)q]}dq = \frac{1}{1 - \lambda} \int_0^{q_n} \left[\frac{1}{1 - q} + \frac{[\lambda(1 - 2\lambda)}{(\lambda + (1 - 2\lambda)q}\right]dq$

Assuming λ < 1, this gives

$D = \frac{1}{1 - \lambda} \left[-\mbox{ln } p_0 + \lambda \mbox{ ln }\left(\frac{1 - \lambda - (1 - 2\lambda) p_0}{\lambda}\right)\right]$ ≈ $\frac{1}{1 - \lambda} \left[-\mbox{ln } p_0 + \lambda \mbox{ ln }\left(\frac{1 - \lambda}{\lambda}\right)\right]$

where the last approximation assumes $p 0$ to be small.

If λ = 1, then we have

$D = \int_0^{q_n} \frac{2 - q}{(1 - q)^2} = \int_0^{q_n} \left[\frac{1}{1 - q} + \frac{1}{(1 - q)^2}\right]dq = p_0^{-1} - \mbox{ ln } p_0 + O(\lambda K)$

In his discussion Haldane writes (op. cit. p. 520) that the substitution cost, if it is paid by juvenile deaths, "usually involves a number of deaths equal to about 10 or 20 times the number in a generation" - the minimum being the population size (= "the number in a generation") and rarely being 100 times that number. Haldane assumes 30 to be the mean value.

Assuming substitution of genes to take place slowly, one gene at a time over n generations, the fitness of the species will fall below the optimum (achieved when the substitution is complete) by a factor of about 30/n, so long as this is small - small enough to prevent extinction. Haldane doubts that hight intensities - such as in the case of the peppered moth - have occurred frequently and estimates that a value of n = 300 is a probable number of generations. This gives a selection intensity of $I = 30 / 300 = 0.1$ .

Haldane then continues (op. cit. p. 521):

The number of loci in a vertebrate species has been estimated at about 40,000. 'Good' species, even when closely related, may differ at several thousand loci, even if the differences at most of them are very slight. But it takes as many deaths, or their equivalents, to replace a gene by one producing a barely distinguishable phenotype as by one producing a very different one. If two species differ at 1000 loci, and the mean rate of gene substitution, as has been suggested, is one per 300 generations, it will take 300,000 generations to generate an interspecific difference. It may take a good deal more, for if an allele a1 is replaced by a10, the population may pass through stages where the commonest genotype is a1a1, a2a2, a3a3, and so on, successively, the various alleles in turn giving maximal fitness in the existing existing environment and the residual environment.

So the number 300 of generations is a conservative estimate for a slowly evolving species and not at the brink of extinction. For a difference of at least 1,000 genes, then 300,000 generations are needed - maybe more, if some gene runs through more than one optimization.

[edit] Origin of the term "Haldane's Dilemma"

Apparently the first use of the term "Haldane's Dilemma" was by paleontologist Leigh Van Valen in his 1963 paper "Haldane's Dilemma, Evolutionary Rates, and Heterosis".

At p. 185 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. 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

That is, since a high number of deaths are required to fix one gene rapidly, and dead organisms do not reproduce, fixation of more than one gene simultaneously would conflict. Note that Haldanes's model assumes independency of genes at different loci; if the selection intensity is 0.1 for each gene moving towards fixation, and there are N such genes, then the reproductive capacity of the species will be lowered to 0.9^N times the original capacity. Therefore, if it is necessary for the population to fix more than one gene, it may not have reproductive capacity to counter the deaths.

[edit] Walter ReMine and Haldane's Dilemma

Walter ReMine identifies three layers to Haldane’s Dilemma. First, Haldane's calculations (if correct) would establish a limit of 1,667 beneficial substitutions over the past ten million years in a lineage leading to humans, (and according to evolutionary geneticists these substitutions are typically one nucleotide, not an entire block of new DNA). The origin of all the uniquely human adaptations would have to be explained within that limit. Are 1,667 beneficial mutations sufficient to explain the origins of upright posture, tripling of brain size, hand dexterity, vocal speech, language, arrangement of hair, and the appreciation of music, to name a few? (That is to be compared with the power of beneficial mutations observed today, such as those that alter the Galapagos finch beaks.) Is 1,667 enough? This is Haldane’s Dilemma, ReMine says, stated in a manner anyone can understand. (The problem exists also for other organisms, especially those with low-reproduction rates and long generation times, such as whales, elephants, apes, bears, cows, and so forth.)

Second, though everything necessary for that statement was available in 1957, evolutionists did not communicate it to the general public, and still have not, according to ReMine. He has called it “the trade secret of evolutionary genetics.” This layer of Haldane’s Dilemma is about the history of the issue, and how it was mis-communicated, both inside and outside the field, for decades.

Third, ReMine claims the problem’s central concept – known as “the cost of substitution” – was garbled in the technical literature, and because of the confusion, many false “solutions” were promoted. He identifies many needless confusion factors, such as: genetic death, genetic load, extinction, environmental change, soft selection, fluctuations in population size, neutral substitutions, and various combinations of these.

ReMine says the central concept is simple: Every evolutionary scenario requires a certain level of reproduction rate in order to be plausible. A required reproduction rate is called a “cost.” If the species actual reproduction rate is not that high, then the species “cannot pay the cost” and therefore the scenario is implausible.

There are many types of cost. The cost of substitution is the extra reproduction rate required specifically for making substitutions under a given scenario. If the species cannot pay the cost of substitution, then that scenario is implausible. ReMine claims his cost concept reduces exactly to Haldane’s equations (under the same assumptions Haldane used) and reaches the same conclusions.

ReMine objects to the many confusion factors still entrenched in the literature, and has published a paper to clarify the issues. Leading evolutionary geneticists, including James F. Crow and Warren Ewens, peer-reviewed the paper and acknowledge it is correct. The paper is Cost Theory and the Cost of Substitution — a clarification (TJ 19(1), 2005, pp. 113-125).

According to ReMine, evolutionary geneticists contradict each other and do not possess an agreed solution to Haldane's Dilemma. He claims Haldane’s Dilemma was never solved (and notes that evolutionary geneticist, G. C. Williams, concluded the same in his book, Natural Selection: Domains, Levels, and Challenges, p 143-144)

For more details on ReMine's position, see:

Haldane's Dilemma - the trade secret of evolutionary genetics by Walter ReMine.
Also see A Straightforward Introduction to Haldane's Dilemma at CreationWiki.

For criticism of ReMine's position, see:

Haldane's Dilemma by Robert Williams. For rebuttal of Robert Williams see here
talk.origins article CB121. For rebuttal of talk.origins article CB121 see here

[edit] See also

[edit] References

Haldane, J.B.S., "The Cost of Natural Selection", J. Genet. 55:511-524, 1957.
Van Valen, Leigh, "Haldane’s Dilemma, evolutionary rates, and heterosis", Amer. Nat. 47:185-190, 1963.
ReMine, W.J., The Biotic Message, St. Paul Science, Saint Paul, MN, pp. 208-236; 499-507, 1993.
Nunney, Leonard, "The cost of natural selection revisited", Ann. Zool. Fennici. 40:185-194, 2003.

[edit] External links

Haldane's Dilemma by Walter ReMine.
Haldane's Dilemma by Robert Williams.
Rebuttal to Williams' arguments.
talk.origins index to creationist claims, claim CB121.

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