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Adapting to Environmental Change: The Role of Directional Selection
The mechanism of selection related to environmental change is directional or progressive selection. However, in a changed context, the prior mean may no longer be the best measure of the feature. Under these conditions, selection would unequally cull individuals from the standard curve's two extremes, shifting the characteristic's mean to a new preferred extreme. In that it does not act evenly at both ends of the distribution curve, directional selection is a modified variant of stabilizing selection.
Directional Selection in Changing Environment
The potential of insect populations to evolve pesticide resistance is an instructive example of directional selection in action. Environment of many species have changed due to changes caused by man. Organisms respond quickly to change in their environment by utilizing directional selection. Since the first report on DDT resistance in houseflies in 1947, there have been increasing reports of insects developing resistance to various pesticides throughout the last four decades. In all circumstances, a low dosage of a new pesticide is adequate to control the pests at first. The concentration is gradually raised as the insects gain resistance to greater concentrations. Finally, the pesticide becomes completely ineffective, or its usage becomes economically unfeasible.
Directional selection removes insects more or less resistant to the pesticide with each generation. With each generation, more and more insects with stronger pesticide resistance are chosen. Pesticide resistance in insects demonstrates the success of directional selection since pesticides are often artificial compounds that the insects have never been exposed to in natural surroundings. Another fascinating example of directional selection in a changing context is industrial melanism, which occurs when a species of melanic moth escapes predation due to environmental changes.
Example of Moth in Industrial Melanism
Natural selection continually seeks to eliminate alleles less suited to their surroundings. Selection, on the other hand, indirectly seeks to favor those alleles or people that are well suited to a specific environment. Promoting and establishing an allele and the resulting phenotype in natural populations might take hundreds of years. However, there are cases where selection pressures have caused changes in the character of individuals through time.
Moths with speckled wings Biston betularia comes in two varieties: melanic and nonmelanic. Melanic forms are black, whereas non-melanic forms are mottled grey. Until the early nineteenth century, insect collectors considered the melanic species rare and valuable, while the light non-melanic forms were plentiful in natural populations. The scarcity of black forms was mainly owing to increased avian predation. The birds were drawn to the dark figures lying on light-colored, lichen-encrusted branches. Light-colored nonmelanoma mix in with their surroundings and are hence invisible to predators.
The distribution of melanic forms, particularly in industrial hubs, changed dramatically with the advent of the industrial revolution. An investigation of museum collections of insects over the last 100 years revealed that melanic forms grew in abundance while nonmelanoma was increasingly rare. What may be the cause of such an occurrence? Sooty smoke from factories darkened the bark of trees in industrial areas, preventing the establishment of lichens. As a result, the black-colored forms resting on such trees were undetectable to predators.
In contrast, the light-colored ones became increasingly vulnerable to predator attack due to their contrast with the backdrop. In industrial regions, birds preyed on non-melanics in huge numbers, but in other places, melanics were preferred. Because the number of melanics grew due to industrial activities, this case is frequently called industrial melanism. Industrial melanism is an example in which selection promoted a specific trait in moths, namely protective coloring.
H.B.D. Kettlewell and E.B. Ford, who examined this phenomenon in England, explained that the population of Bistort betularia has a single dominant gene for melanism. In fewer than fifty generations, the frequency of the gene grew from less than 1% in pre-industrial times to more than 90% with the advent of the industrial revolution.
The occurrence of industrial melanism was later identified in over 100 species of moths, including Biston betularia. Moreover, severe pollution control measures undertaken in certain industrial cities have restored the original ecosystem: tree trunks free of soot deposits and lichens growing on them. Natural selection has preferred mottled-grey individuals over melanic forms in such locations—another idea of selection, generally known as the directed selection, was explained by industrial melanism. One sort of natural selection that occurs when organisms must adapt to a changing environment is directional selection.
Example of African Cichlids
African cichlids are regarded to be among the most diversified fish, evolving at a breakneck pace. These fish originated in the same environment but have distinct morphologies, particularly in the mouth and jaw. Albertson et al. 2003 put this theory to the test by crossing two African cichlid species with significantly diverse mouth morphology.
The hybrid between Labeotropheus fuelleborni (which has a subterminal mouth for biting algae off rocks) and Metriaclima zebra (which has a terminal mouth for suction eating) enabled for the mapping of QTLs impacting feeding morphology. The QTL sign test provided conclusive evidence that directional selection was happening in the oral jaw apparatus. This was not the case for the suspensorium or the cranium, implying genetic drift or stabilising selection.
Example of Sockeye Salmon
Sockeye salmon are among the many anadromous fish species. Individuals return to the rivers where they were born to breed. These migrations occur at the same time every year, however Quinn et al. 2007 reveal that sockeye salmon found in the seas of Alaska's Bristol Bay have recently undergone directional selection on migratory timing. In this study, two sockeye salmon populations (Egegik and Ugashik) were observed.
Data from the Alaska Department of Fish and Game were separated into five groups of seven years and shown for average arrival to the fishery from 1969 to 2003. After analysing the data, it was discovered that the average migration date in both groups was earlier and that directional selection was taking place. The Egegik population was subjected to more intense selection and relocated four days. Water temperature is supposed to induce earlier migratory dates, however there was no statistically significant association in this study.
According to the article, fisheries may be a factor driving this selection since fishing happens more often during the later stages of migration (particularly in the Egegik area), preventing those fish from reproducing.
Directional selection is a modified variant of stabilizing selection, which seeks to eliminate alleles less suited to their surroundings and favor those alleles or people that are well suited to a specific environment. Examples include industrial melanism and pesticide resistance in insects. Industrial melanism is an example of natural selection that promotes protective coloring in moths.
It is caused by sooty smoke from factories darkening the bark of trees, preventing the establishment of lichens, and natural selection preferring mottled-grey individuals over melanic forms. Natural selection favours or does not eliminate the aberrant allele of the chain gene in certain human populations, suggesting that it is not beneficial. Sickle cell anaemia is an adaptation caused by a faulty allele or component of the genotype, resulting in minimal population diversity.
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