What is a Scientific Evolution Theory?
The scientific evolution theory by natural selection was independently conceived by Alfred Russel Wallace and Charles Darwin in the mid - 19th century, and it was set out in detail in Darwin's book On the Origin of Species. Scientific evolution by natural selection was first demonstrated by the observation that often, more offspring are produced than may possibly survive.
Types of Selection
Stabilizing Selection
Natural selection may be studied by analyzing its effects on the changing gene frequencies, but it may also be explored by examining the effects on observable characteristics - or phenotypes - of the individuals in a population. Phenotypic traits' distribution scales such as weight, height, number of progeny, or longevity typically exhibit a greater number of individuals with intermediate values and fewer toward the extremes—this is called the normal distribution. When individuals with the intermediate phenotype are favored, and extreme phenotypes are selected against, the selection is called stabilizing.
The distribution and range of phenotypes then remain nearly similar from one generation to the other. Stabilizing selection is common. Individuals with moderate phenotypic values have a better chance of reproducing and surviving. For example, newborn infant mortality is highest when they are either very large or very small; babies of moderate size have a higher chance of survival.
The below figure shows the types of natural selection or the natural selection and its types.
Three types of natural selection representing the effects of each on the distribution of phenotypes within the population. The downward arrows, which point to those phenotypes against that selection act, which is a stabilizing selection example.
Stabilizing selection (left column) acts against the phenotypes at both extremes of the distribution, favoring the intermediate phenotype multiplication. Directional selection (on the center column) works against one extreme of phenotypes by shifting the distribution to the opposite extreme. By dividing the distribution at each extreme, diversifying selection (on the right column) works against intermediate phenotypes.
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Often, the stabilizing selection is noticeable after the artificial selection. Chickens that lay larger eggs, cows that produce milk, and corn with a high protein content are chosen by breeders. At the same time, the selection should be reinstated or continued from in time, even after the desired goals have been achieved. If this is fully prevented, natural selection takes over and eventually returns the traits to their original intermediate value.
As a result of stabilising selection, populations often retain a consistent genetic constitution across a range of traits. This attribute of populations is known as genetic homeostasis.
Directional Selection
The phenotypes' distribution in a population at times changes systematically in a specific direction. The biological and physical aspects of the environment are changing continuously, and over long time periods, the changes can be substantial. The climate and configuration of the waters or land differ incessantly. Also, changes occur in the biotic conditions, which means, in the other organisms present, whether prey, predators, parasites, or competitors. Genetic changes take place as a consequence because the genotypic fitnesses can shift so that various sets of alleles are favored. Also, the opportunity for directional selection arises when the organisms colonize new environments, but the conditions are different from their original habitat.
The below figure shows the light gray peppered moth (Biston betularia)
On the soot-covered oak tree's trunk, a light grey peppered moth (Biston betularia) and a darkly pigmented form rest near each other. Against this background, the light gray can be noticed more easily than the darker variant.
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The directional selection process occurs in spurts. The substitution of one genetic constitution for another alters genotypic fitness at other loci, causing changes in allelic frequencies, which in turn stimulates further changes, and so on in a cascade of events.
Directional selection can be possible only if there is a genetic variation with respect to phenotypic traits under the selection. Natural populations have wide stores of genetic variation, which are constantly replenished by variations that produce new variants. The nearly universal success of the artificial selection and rapid response of the natural populations to new environmental challenges is the evidence, which existing variation provides the required materials for directional selection.
Diversifying Selection
Diversifying selection may favour two or more divergent phenotypes in the same environment at the same time. None of the natural environment is homogeneous; rather, the environment of any animal or plant population is a mosaic consisting of either less or more dissimilar sub environments. There is heterogeneity with respect to the food resources, climate, and living space. And, the heterogeneity can be temporal, with the change taking place over time and spatial as well. Species cope with environmental heterogeneity in diverse ways.
A strategy is a genetic monomorphism, which is a generalist genotype selection that is well-adapted to all of the species' sub-environments. The other strategy is genetic polymorphism, which is the selection of a diversified gene pool that yields various genotypes, each adapted to a specific sub-environment.
FAQs on Types of Selections in Scientific Evolution Theory
1. What is the scientific theory of evolution by natural selection?
The theory of evolution by natural selection, independently developed by Charles Darwin and Alfred Russel Wallace, states that organisms with heritable traits better suited to their environment tend to survive and produce more offspring. This process leads to the gradual accumulation of favourable traits in a population over generations. The core idea is that more offspring are produced than can possibly survive, leading to a 'struggle for existence' where advantageous variations prevail.
2. What are the three main types of natural selection that affect a population's traits?
The three primary types of natural selection are distinguished by their effect on the distribution of phenotypes in a population:
- Stabilizing Selection: Favours the intermediate or average phenotype and selects against extreme variations.
- Directional Selection: Favours one of the extreme phenotypes, causing a shift in the population's average trait over time.
- Disruptive (or Diversifying) Selection: Favours individuals at both extremes of the phenotypic range over the intermediate phenotypes.
3. Can you explain stabilizing selection with an example from the NCERT syllabus?
Stabilizing selection is a process that maintains genetic consistency by favouring the average phenotype. A classic example is the birth weight of human infants. Newborns who are either very small or very large have a higher mortality rate compared to babies of an average, moderate weight. This selection pressure ensures that the population continues to favour the intermediate birth weight, thus stabilizing this trait.
4. What is directional selection, and what is a classic example of it in action?
Directional selection occurs when environmental changes favour an extreme phenotype, causing the allele frequency of a population to shift in one direction. The most famous example is industrial melanism in the peppered moth (Biston betularia) in England. Before the industrial revolution, light-coloured moths were camouflaged against lichen-covered trees. However, as pollution darkened the tree trunks with soot, the dark-coloured (melanic) moths had a survival advantage and became more common.
5. What is disruptive selection, and how does it work?
Disruptive selection (also called diversifying selection) simultaneously favours individuals at both extremes of the phenotypic spectrum while selecting against the intermediate forms. This can happen in a heterogeneous environment with multiple distinct niches. For instance, in a habitat with both light-coloured rocks and dark soil, birds with either very light or very dark feathers might be better camouflaged than birds with intermediate grey feathers, leading to the selection of both extreme phenotypes.
6. Why is genetic variation considered the raw material for natural selection?
Genetic variation is essential because natural selection can only act on the differences that already exist within a population. Without a range of heritable traits (different alleles for genes), all individuals would be identical, and there would be no basis for selection. Environmental pressures can only 'select' the fittest individuals if there is a variety of traits to choose from. This variation primarily arises from genetic mutations and recombination.
7. How do directional and disruptive selection differ in their long-term evolutionary impact?
The key difference lies in their outcome on a population's diversity. Directional selection leads to a systematic change, shifting the entire population's average trait towards one extreme, reducing variation around the new norm. In contrast, disruptive selection increases diversity by favouring two or more distinct forms. Over a long period, disruptive selection can split a population into two separate groups, which may eventually evolve into different species (a process known as sympatric speciation).
8. What is the role of sexual selection in evolution?
Sexual selection is a mode of natural selection where members of one biological sex choose mates of the other sex to mate with (intersexual selection) and compete with members of the same sex for access to mates (intrasexual selection). It doesn't primarily concern survival but focuses on traits that enhance mating success. Often, this leads to sexual dimorphism, where males and females of a species look different, such as the elaborate plumage of a male peacock, which is selected for by females.
9. How can human activities like agriculture and medicine act as powerful agents of selection?
Human activities create strong and rapid selective pressures, often leading to directional selection. For example:
- Antibiotic Resistance: The widespread use of antibiotics has selected for bacteria strains that possess genes for resistance, making infections harder to treat.
- Pesticide Resistance: The use of pesticides in agriculture selects for insects that are naturally resistant, leading to a population of 'super pests' over time.
In both cases, humans have drastically altered the environment, favouring the survival of rare variants that would not have otherwise proliferated.
10. Does natural selection always result in a 'perfectly' adapted organism?
No, natural selection does not produce 'perfect' organisms. It is not a goal-oriented process. Evolution is constrained by a species' evolutionary history and can only work with the existing genetic variation. An adaptation is often a compromise between different selective pressures. For instance, a trait that increases mating success (sexual selection) might decrease an organism's chances of survival from predators (natural selection). Therefore, an organism is simply 'fit enough' for its current environment, not perfectly engineered.
