Quotes About Genetics

So, the question is: If greenflies and elm trees don't do it, why do the rest of us go to such lengths to mix our genes up with somebody else's before we make a baby? It does seem an odd way to proceed. Why did sex, that bizarre perversion of straightforward replication, ever arise in the first place? What is the good of sex?*

Perhaps Islam is analogous to a carnivorous gene complex, Buddhism to a herbivorous one.

According to this theory then, senile decay is simply a by-product of the accumulation in the gene pool of late-acting lethal and semi-lethal genes, which have been allowed to slip through the net of natural selection simply because they are late-acting.

A gene is not indivisible, but it is seldom divided. It is either definitely present or definitely absent in the body of any given individual. A gene travels intact from grandparent to grandchild, passing straight through the intermediate generation without being merged with other genes. If genes continually blended with each other, natural selection as we now understand it would be impossible.

The life of any one physical DNA molecule is quite short—perhaps a matter of months, certainly not more than one lifetime. But a DNA molecule could theoretically live on in the form of copies of itself for a hundred million years.

We have now also seen that, in precisely the same sense as it is ever possible to talk of a gene 'for' a behaviour pattern, it is possible to talk of a gene, in one organism, 'for' a behaviour pattern (or other phenotypic characteristic) in another organism.

Genetically speaking, individuals and groups are like clouds in the sky or dust- storms in the desert. They are temporary aggregations or federations. They are not stable through evolutionary time. Populations may last a long while, but they are constantly blending with other populations and so losing their identity.

I define a replicator as anything in the universe of which copies are made.

The ESS has been rigorously defined (Maynard Smith 1974), but it can be crudely encapsulated as a strategy that is successful when competing with copies of itself.

When we talk of a program as 'doing better' or as being 'successful' we are notionally measuring success as capacity to propagate copies of the same program in the next generation: in reality this is likely to mean that a successful program is one which promotes the survival and reproduction of the animal adopting it.

To the extent that active germ-line replicators benefit from the survival of bodies other than those in which they sit, we may expect to see 'altruism', parental care, etc.

Most of the replicators in the world have won their place in it by defeating all available alternative alleles. The weapons with which they won, and the weapons with which their rivals lost, are their respective phenotypic consequences

Just as every gene is the centre of a radiating field of influence on the world, so every phenotypic character is the centre of converging influences from many genes, both within and outside the body of the individual organism.

An extended phenotypic character is the product of the interaction of many genes whose influence impinges from both inside and outside the organism. The interaction is not necessarily harmonious—but then nor are gene interactions within bodies necessarily harmonious

What does complementariness mean for genes? Two genes may be said to be complementary if the survival of each, relative to its alleles, is enhanced when the other is abundant in the population.

It is fundamental to the idea of a replicator that when a mistake or 'mutation' does occur it is passed on to future copies: the mutation brings into existence a new kind of replicator which 'breeds true' until there is a further mutation

The word replicator is purposely defined in a general way, so that it does not even have to refer to DNA.

In principle, we may consider any portion of chromosome as a potential candidate for the title of replicator.

Once the vital ingredient—some kind of genetic molecule—is in place, true Darwinian natural selection can follow, and complex life emerges as the eventual consequence.

The word allele is nowadays customarily used of cistrons but it is clearly easy, and in the spirit of this chapter, to generalize it to any portion of chromosome.

This, then, is our candidate replicator. But a candidate should be regarded as an actual replicator only if it possesses some minimum degree of longevity/fecundity/fidelity (there may be trade-offs among the three).

An arbitrarily defined length of chromosome, or potential replicator, may be said to have an expected half-life, measured in generations.

Phenotypic effects of genes, whether at the level of intracellular biochemistry, gross bodily morphology, or extended phenotype, are potentially devices by which genes lever themselves into the next generation, or barriers to their doing so. Incidental side-effects are not always effective as tools or barriers, and we do not bother to regard them as phenotypic expressions of genes, either at the conventional or the extended phenotype level.

But if we set selection pressures on one side, we can say something about the half-life of a replicator on the basis of its length alone. If the stretch of chromosome we choose to define as our replicator of interest is long, it will tend to have a shorter half-life than a shorter replicator, simply because it is more likely to be broken by crossing-over. A very long portion of chromosome ceases to deserve the title of replicator at all.