Mass Selection of Quantitative Genes in Nature

14 June 2017
Posted by Paul Nel.

The mechanism by which natural selection operates on qualitative genes is well-known. However, it is worth considering the action of the quantitative genes that control the continuous genetic variation of many traits such as weight, height, fat content of meat, and ear length, internode length and seed weight in maize.

An important aspect of selection which appears to have been overlooked is some of the research undertaken on crop plants over a period of many years. Brown, J., Caligari, P. and Campos, H. (2014) provide an explanation of qualitative and quantitative genetics. Whitehouse, H. L. K. (1969) describes the early experiments that led to an understanding of quantitative genetics. 

Quantitative genes are Mendelian genes. A large number of them determine a trait and the individual loci 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).

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 were a clone and genetically identical to each other, the spread of heights is due to the many environmental differences to which the individual plants were 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 plant height is expressed in the equation P = G + E + GE + σe2  where P is the phenotype, G is the genetic effect, E is the effect of the environment, GE is the interaction between the genotype and the environment and σe2  is a random error term.

Suppose the plants are a natural population of a randomly cross-pollinating species with continuous genetic variation for height. Since the contributions of the many loci (genes) are small and cannot be classified into groups, the distribution is a normal one, resulting in a normal curve which is adapted to the environment, again with a clustering around the mean (Figure 2).

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 combinations of loci towards the right of the X-axis in contrast to those towards the left. It can perhaps be visualised more easily by considering the potentials of the plants to the right of the mean versus those to the left.

The Normal curve, with its mean, can be expected to 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 (depending on the circumstances, the spread of heights could increase or decrease, and the variance, which is a measure of the degree of clustering around the mean, could change, to give a broader or narrower bell-shape between the two extremes of height).

Since the E and GE factors also contribute to the phenotypes, the normal curve of the phenotypes will differ to some extent from that of the genotypes, but the weight of the phenotypes contributing to increased height in subsequent generations would still be towards the right of the X axis. It is a case of mass selection, explained more fully below.  At least one kind of objective experiment can be designed to test this idea.

However, 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. Those which are controlled by quantitative genes will have their own normal curves, which will be influenced by interactions between them and also some qualitative genes.

Crops, which are populations of individuals, are bred for yield and adaptation to the environments where they will be grown by farmers. Mass selection is one of the techniques used by breeders. Seed from a number, which can be up to 200 or more, of desirable phenotypes is harvested, bulked and planted out for the next generation. The procedure can be repeated for later generations. It is a matter of good economics that works for them. If it didn’t, they would not use it. It will be difficult to disprove that mass selection operates in a similar way in many species, besides higher plants, in nature.

Selection of individual qualitative genes and mass selection of quantitative genes are two fundamental features/laws of natural selection. They complement each other. Mass selection takes place in populations of individuals, at least up to the species level. For the 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.

It follows that mass selection in Nature is not entirely a chance process. Although chance mutations take place at a low frequency in individual quantitative genes, mass selection operates on the large numbers of quantitative genes that are already in the population and on new recombinants that arise every generation.

There is wide scope for immediate mass selection of the best combinations of those genes, for adaptation. It is akin to an organism fighting for survival. Adaptation to all the features in a changed environment via chance mutations of individual qualitative genes would be a very lengthy process, during which time the environment might have undergone several changes. Most chance mutations are deleterious, even fewer would be advantageous, and then the best combinations of the latter would have to be brought together in individuals of the population.

To conclude, although some of the mathematics above may need to be corrected or modified by persons who design and analyse the results of mass selection trials, the fact remains that, since mass selection has been used with success by humans, there is no reason to suppose that natural selection does not operate in a similar way on quantitative genes. 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 widely known and credit given to the author(s).

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