Mass Selection in Nature: Mk2

20 October 2019

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This article is a modification of Paul Nel's previous article, Mass Selection of Quantitative Genes in Nature, from 20171.

Mass selection is a procedure that is used by plant and animal breeders within the species level to increase both yield and adaptation to the various environments where the organisms are to be grown. Brown, J., Caligari, P.D.S. and Campos, H.A. (2014)2 provide a definition and explanation of the term as it is used by plant breeders. Basically, a number of desirable individuals (phenotypes) is selected and allowed to intercross and produce a new generation. This is repeated for subsequent generations.

It can be maintained that a similar form of mass selection for adaptation to the environment and for producing progeny, which is Nature’s equivalent of yield, takes place in many sexually reproducing species in Nature. How might such a process operate?

A possibility follows. Background information about qualitative and quantitative genetics is available in many textbooks, for example Brown, et al.2 It is well-known that quantitative genes (polygenes) are Mendelian genes. A large number of them determine a trait and the individual genes make small contributions collectively to the phenotype, which is also influenced by the environment.

To illustrate this, consider a hypothetical case where a clone of many plants has been grown in a field and their heights (phenotypes) at maturity have all been measured. They cannot be grouped into classes because the variation is continuous. If plotted graphically, a bell-shaped curve is obtained (Figure 1, below). The curve is called a normal curve and the distribution of heights is a normal distribution. There is a clustering of plants around the population mean. Since the plants are a clone and genetically identical to each other, the spread of heights is due to the many environmental differences to which the individual plants are exposed.

Normal_distbn-NEL_fig1.JPG

Plant height for clone follows a Normal distribution.

Where there is continuous genetic variation, the contribution of the quantitative genes to the phenotype is expressed in the equation P = G + E + GE + σe2 where P is the phenotype, G is the genetic effect, E is the environmental effect, GE is the interaction between the genotype and the environment and σe2  is a random error term. The genetic and environmental contributions are evaluated from scientific field trials undertaken by breeders.

Suppose the plants are a natural population of a randomly cross-pollinating (by wind for example) species, with quantitative genetic variation for height. The contributions of the many genes are small and cannot be classified into groups, the distribution is a normal one, resulting in a normal curve, again with a clustering around the mean (Figure 2, below).

Normal_distbn-NEL_fig2.JPG

Normal curve for the contribution of quantitative loci in plants.

If there is a change in the environment by some factor, say, by a taller species that is migrating in and causing competition for light, there is increasing potential for producing progenies that will be taller, exposed to more light and better adapted to the new environment, by those plants towards the right of the mean in Figure 2 in contrast to those towards the left. The normal curve with its mean will move slightly to the right in the following and subsequent generations, despite random cross-fertilization by the plants in the population, resulting in an increase in mean plant height. Because many plants, showing increasing adaptation to the environment towards the right of the x-axis, are contributing together to increase height each generation, it is a case of mass selection in operation. At the same time, there is increasing mass selection against the less adapted plants towards the left of the x-axis.

The situation is much more complex than indicated so far. Environmental changes can result in a population having to adapt in multiple ways. In this example, plant height is not the only trait that would have to be modified. Some features, to name a few, might be changes to leaf structure and arrangement, flowering time, number of seeds produced, the seed dispersal mechanism and also root properties to enable the population to compete for soil space, nutrients and moisture.

Various genetic factors (see Brown, et al, for example) can affect the properties of the normal curve, as well as the E and GE factors in the above formula. However, it can be assumed that in general, the over-all curve for the population at the species level is more or less normal with a clustering around the population mean and that natural selection will operate in the same way as above.

It is proposed that in many species of sexually reproducing plants, animals and possibly fungi, there is natural mass selection at the same time, of individuals which have the better combinations of a large number of qualitative and quantitative genes for adaptation, and selection against those individuals with inferior combinations. It is a procedure whereby a species can adapt to multiple environmental features and changes in the environment that occur with the passage of time.

Mass selection takes place at the species level, and in many cases also at the subspecies level. What may or may not take place above the species level is not of concern here. For a species to survive in response to changes in the environment, that population of individuals must reproduce. If they cannot do so, they will die, either as a result of environmental factors or aging and death, and the species will become extinct.

Crops, which are populations of individuals, are bred for yield and adaptation to the environments where they will be grown by farmers. It should be possible to obtain some idea of rates of adaptation to various environments under different intensities of selection by examining the results of many scientific field trials undertaken in the past by plant breeders, particularly maize breeders. It should also be possible to design trials along similar lines in natural populations.

To conclude, although some aspects of the above argument might need to be corrected, modified or replaced with a better one by others, the fact remains that mass selection is used with success by breeders. Unless conclusive proof is provided, which I doubt is possible, that a form of mass selection cannot operate in Nature, the hypothesis is a valid one. A form of mass selection is an important aspect of genetics, natural selection, adaptation and ecology.

I have been unable to find any published references to this kind of reasoning so far. If it has been published, it should be made more widely known and credit given to the author(s).

References

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