Evolution in 1-2-3

1) Descent with modification
2) Adaptation and natural selection
3) Speciation


Population genetics

Genes and mutations, time and mathematics

The process of evolution can be shown using mathematics where the governing principles are translated into formulae. The output of the model then show how these principles work in practice.

In physics and chemistry the governing principles are derived from known natural laws. Thanks to Keppler and Newton we can calculate the future position of the planets. In evolution, however, the governing principle is random mutation followed by natural selection, none of which we can predict. But knowing the numbers involved -- the size of the genome, the size of certain genes, the size of a population, its generation time, etc. -- we can apply the governing principle of random mutation and natural selection in simulations to verify the past. Or vice versa, if the outcome of the mathematical model is different from what we observe in nature,

  1. the mathematics must be wrong, or
  2. the parameters must be wrong, or
  3. the governing principles must be wrong, or
  4. any combination of the above.

A very simple example of a population genetics simulation is Richard Dawkins' METHINKS IT IS A WEASEL, a target sentence which will be reached by first starting with giberish and then exchanging the letters randomly one by one. Every new sentence is a generation and every time a "child" sentence is closer to the target than its "mother" sentence, it will be chosen as parent for the next generation. In other words, random mutation coupled with non-random natural selection.

Once maximal fitness is achieved, all new mutations will be less fit than the mother sentence and thus discarded. Only perfect sentences are kept. Only when the target sentence is changed, i.e. new fitness requirements are introduced, will the process start over and through trial and error reach the new target. In other words, once evolution has reached its target it will cease unless something in the environment changes -- and the environment changes continuously.

However, detrimental mutations outnumber beneficial mutations by a thousand to one, i.e. for every new letter, word or sentence that take you closer to the manual for a space shuttle there are a thousand new letters, words or sentences that corrupt the manual. Before you get from a regular wheelbarrow to one with two wheels and headlights the wheelbarrow goes extinct because the manual in effect has been destroyed by all the typos.

In nature, however, the accumulated errors in the manual are countered by high reproduction rates. A new generation of ants can consist of several hundred thousand new members in an ever increasing population. A new generation of bacteria can consist of millions or even billions of new members. Thus, one in a thousand beneficial mutations suffices to take a species on the journey to larger populations, greater variation and higher probability of fitness.

For humans, the thousand to one relationship is not as detrimental as one might think. Some population genetics models show that in order to avoid an accumulated reduction in fitness each woman has to produce forty kids in order for two to survive and thus maintain a constant population size. Other models show that every person on average is born with three harmful mutations. This means that 75% of the population must die prematurely in each generation in order to eliminate the accumulated effect on reduction in fitness.

None of these models are what we currently observe in nature so, obviously, nature must have another mechanism to get rid of all the bad mutations in humans faster than they are produced. The simple solution is sex which shuffles genes around and eliminates detrimental mutations in large numbers at a time. Thanks to sex, some individuals inherit many bad mutations at once, die prematurely due to natural selection and thus rid the gene pool of several accumulated bad traits simultaneously. Similarly, some individuals inherit many good mutations at once and thus boost fitness.

Thanks to medical, moral and social progress, humanity is now able to counter most of the harsh effects of natural selection. Some scientists therefore estimate that the fitness of the human race degenerates at a rate of about 1 to 2% per generation. As no population can sustain a continual loss of viability of 1.5% the human race will thus probably be extinct between 10 000 to 100 000 years from now.

This expectation is surely exaggerated, however. Influential thinkers like  John Hardwig and Peter Singer promote that old people have a duty to kill themselves when they become a burden and that the life of a newborn is of less value than the life of a pig, a dog, or a chimpanzee. Furthermore, Exit International and dr. Philip Nitschke offer those of us between childhood and old age pills, tools and advice on how to commit suicide. In other words, moral and social progress is moving in the opposite direction of what some scientists think.

Proponents of present day utilitarianism may find this booklet from 1922 inspiring. The authors were no doubt a hundred years ahead of their time in their scientific, moral and ethical thinking even though their ideas were implemented on a large scale just ten years after publication.