In the absence of large-scale environmental change, populations frequently retain a stable genetic constitution for numerous features in more or less homogenous settings. Natural selection preserves genetic equilibrium in the absence of environmental change. This is referred to as normalizing selection. Several of an individual's phenotypic characteristics can be placed on a linear scale. The distribution curve of the attributes generally takes the shape of a bell, with more people at intermediate levels and fewer people at the extremes.
Natural selection that effectively preserves genetic equilibrium especially in the uniform environments, is passed down from generation to generation. If there is a significant selection pressure against the phenotypes at the extremities of the standard curve, the population may exhibit reduced variability even if the mean remains constant. Natural selection tends to normalize or stabilize populations with intermediate trait values, and individuals with intermediate trait values have a greater chance of surviving.
For example, newborns weighing significantly less or much more than average have a high death risk. Infants of moderate weight, on the other hand, have fewer survival issues. To understand the notion of normalizing selection, we will look at two examples, one from nature and one from Dobzhansky and Spassky's research.
An American scientist, H.C. Bumpus (1899), presented an intriguing insight that reasonably explains normalizing selection. After a heavy snow and sleet storm with high winds at Woods Hole, Bumpus gathered 136 wounded house sparrows. Sixty-four of these birds perished, resulting in two groups of sparrows: those killed by the storm and those that survived. Bumpus measured various randomly selected features like wing length, wing span, tarsus length, and so on and discovered that those killed by the storm had measures that fell at the extremities of the bell-shaped curve. In other words, birds with mean or near-mean measures were the ones who survived.
Normalizing selection often removes people whose features deviate significantly from the mean values during a disaster or a stressful event. According to Bumpus' observations, the birds that were easily blown down by the winds either had wings that were too long for their body weight and thus presented a larger surface area, or they had wings that were too short for their body size and thus could not fly against strong winds. Before the disaster, the range of individual measures was more comprehensive than the range of measurements that survived the disaster. Every disaster can reduce population diversity, selecting inferior genes.
Boris Spassky and Theodosius Dobzhansky established the operation of normalizing selection on a behavioral feature in two populations of Drosophila pseudobscura. Artificial selection was used on both populations. One group was chosen for positive phototactic conduct, while the other was for negative behavior. Flies were placed in a container and allowed to fly toward light or darkness.
Photopositive flies, or those that went towards the light, were caught and bred, as were photonegative flies (those that moved away from light). The breeding trials were repeated for numerous generations, with artificial selection maintained for each generation. After multiple generations of artificial selection, two populations emerged, one highly photopositive and the other becoming photonegative.
More crucially, after the artificial selection was removed, natural selection rewarded individuals with neutral light behavior, and both populations recovered to an intermediate phototactic score. The above two examples demonstrate how selection often favors phenotypes in the middle of the distribution, lowers variability around the mean, but does not affect the mean value.
In explaining normalizing selection, it was stated that in uniform conditions, selection restricts population variability and generates genetic homeostasis. Diversifying selection refers to the sort of selection process that takes place in diverse contexts. The inverse of normalizing selection is diversifying selection. Assume that a population inhabiting a specific environment contains two or more genotype groups (AA, Aa, and aa) and encounters sub-environments or habitats. Among the two or more genotypes, a rare genotype (aa) that is well suited to its environment will be encouraged by the selection, and its frequency will grow as long as the habitat is not entirely occupied.
However, once the habitat is saturated, there is no further growth in the frequency of that genotype. The extra population is quite likely to spread to another sub-environment. Diversifying selection may generate a population of diverse genotypes if the genotype is not suited to the new habitat. Genetic polymorphism refers to the presence of two or more genotypes for a particular characteristic in a population. Distinct genotypes occupy distinct sub-environments; such occupancy is as thorough and efficient as feasible.
Through their research on bentgrass growing on heavy metal-polluted soils, A.D. Bradshaw and D. Jonell showed that populations might become genetically diverse while remaining physically adjacent. Heavy metal pollutants such as lead and copper are prevalent in mine refuse heaps, and the pollution is poisonous to most plants, even bentgrasses growing in uncontaminated soils. On spoil heaps, however, abundant growth of bentgrasses could be detected. Essentially, such plants contain genes that give tolerance to high levels of lead and copper.
A few meters from the uncontaminated soils, one could see resistant bentgrass plants surrounded by non-resistant kinds. Diversified selection is efficient. Although cross-pollination between resistant and non-resistant types is possible, the genetic distinction is preserved due to non-resistant seedlings' incapacity to develop in contaminated soil. In contrast, they outgrow resistant kinds in uncontaminated soils. Given that some of the mines are less than 400 years old, it is clear that diversifying selection has resulted in resistant forms in a relatively short period.
Selection operates on genotypes based on the contexts in which they exist. If the environment is uniform and homogenous, the type of selection that occurs is the normalizing or stabilizing type, which weeds out phenotypes with extreme measurements, providing the selection pressure on such characteristics is quite strong. Under these conditions, population variability is reduced, but the mean value of the characteristic stays constant.
However, if the environment changes, the mean value of the feature will vary since selection acts unequally on the two extremes of the normal distribution. The mean value is moved to a new location. Insect resistance to pesticides is one example of this selection, also known as directed or progressive selection.
Finally, diversifying or disruptive selection occurs when selection acts on a population spread throughout a varied environment. Each genotype occupies a heterogeneous sub-environment, causing the standard distribution curve to become bimodal. Each genotype has a different mean value for the characteristics in question.
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