Life Evolving by Christian de Duve

Author:Christian de Duve [Duve, Christian de]
Language: eng
Format: epub
Published: 2011-02-16T05:00:00+00:00

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life evolving

One easily calculates in the same manner12 that a 99.9 percent probability of winning a bet is reached by throwing a die 38 times or by giving a roulette wheel about 250 spins. Even a seven-digit lottery number has a 99.9 percent chance of coming out if some 69 million drawings are done. The number of times the cards must be dealt at bridge to guarantee a given distribution with the same probability is likewise finite and theoretically computable, even though it is beyond the capacities of my hand calculator and, also, beyond the boundaries of any realistic physical possibility.

In conclusion, chance does not exclude inevitability. The probability of an event, however improbable, can become close to certainty if the event is given a sufficient number of opportunities to take place. This is of little help to those who play games of chance, because the games are always rigged so as not to give chance enough opportunities. But it may be relevant to the evolutionary game.

mutations are rarely the limiting factor in evolution

The number of mutations a given genome can undergo is large, but it is finite and, in many cases, not excessively large relative to the number of individuals at risk—often millions, if not billions or more—and to the time—up to millions of years—evolution has available for its experiments. Contrary to what is often assumed, evolution rarely has to wait very long for some favorable mutation to occur. More often than not, mutations are present in a population all the time or come up regularly, waiting, so to speak, for the environment to provide them with an opportunity to prove useful.

This assertion is easily demonstrated in the case of simple point mutations, that is, those in which one base is replaced by another. Considering only replication errors, which are known to occur with a frequency of about one wrongly inserted base in one billion, it is readily calculated13

that the probability of finding a given point mutation in a clone of cells produced by successive division from a single cell becomes 99.9 percent after about 34 generations, that is, less than one day for bacteria, and about one month for animal cells.14 To take a more concrete example, consider red-blood-cell renewal in the bone marrow of an adult human individual. The probability of a given point mutation taking place in the course of this process reaches 99.9 percent after only about two hours.

It is fortunate that most of those mutations are harmless and that, in the arrow of evolution

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addition, evolution has provided elaborate repair mechanisms to offset their effects. There are, however, rare cases in which an accident of this sort may lead to some anomaly, such as cancer.

It may be argued that such calculations are of limited value because point mutations are likely to be of little importance for evolution, which probably depends mostly on more extensive genetic rearrangements whose probability is much more difficult to evaluate. It is therefore significant that all we know of evolution in action tends to confirm the richness of the mutational field.

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