Neo-Darwinism / R.J. Berry.

  • Berry, R. J. (Robert James)
Date:
1982
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Neo-Darwinism / R.J. Berry. Source: Wellcome Collection.

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    WHY BE VARIABLE? 31 better called the 'environmental load' since it depends on selection exerted against one or both homozygotes, and the strength of this selection is directly related to environmental pressures. In the classical example of the sickle cell haemoglobin polymorphism, the advantage of the heterozygote disappeared in negro slaves taken to North America where there was no malaria. The frequency of the sickle cell allele is now declining there entirely through (directed) selection against the anaemic homozygotes. In formal terms, selection pressures may be density or frequency dependent or independent. For example, a low density of a prey species attracts neither predators nor intra-specific pressures, and a struggle for survival only intensifies when numbers increase. It is also possible to distinguish between 'hard' and 'soft' selection. Under hard selection, an individual has to meet certain requirements to survive. For example, a marine organism is unable to cope with the osmotic problems of living in fresh-water; most plants are poisoned by the soil concentrations of heavy metals on mine spoil heaps; unselected mosquito populations cannot tolerate a high concentration of insecticide; and so on. In contrast 'soft' selection begins to operate only when the normal carrying capacity of an environment is filled. When we examine the situations where genetically caused deaths occur, we are faced with the enigma that a genetically invariable population has no 'load', but may be faced with extinction if the environment deteriorates without new variants appearing. This is true of the classical case of the spread of the melanic form of the peppered moth (section 4.1), which Haldane used as his example of the cost of selection. The species only survived in polluted areas because of the occurrence of melanic mutants. In situations where an environment changes and reduces the chance of survival of a species living in it, a 'load' is placed upon the species which has nothing to do with genetics. A gene replacement process offers an escape from this external load. If the replacement is rapid enough, the gradual decrease in environmentally-caused deaths will enable the population to survive while another population without an alternative allele may succumb to the changed environment. Although different genetical causes of death all contribute to 'genetic load' and can all be described by the same algebra, they differ considerably in their contribution to the species. Mutational load - the original Mullerian load - is bad; the elimination of mutants is a maintenance job to keep the population in working condition. Substitutional load is definitely favourable to the popula¬ tion, although it may coincide with a time of increased mortality if it occurs in response to an environmental crisis. Segregational load is a paradox. It can result in clearly identifiable genetical deaths, yet its overall effect on the species is to increase its tolerance to environmental variation. Perhaps it is offensive to our sense of fairness: it would be more democratic to have everyone sickly than to have a few die outright. Much of the difficulty of the load concept is due to the fact that it wrongly combines the mean and variance of a population's viability through a false assumption that increase in the number of segregating genes must be associated with decreased mean fitness. It would be an advantage to keep the fitness and