How Does Natural Selection Work?

Darwin proposed the concept of survival of the fittest. What is the definition of 'fittest'? Those who survive are the most common response. It is easy to see that this is a circular argument that does not assist us in comprehending what happens after natural selection. The concept of fitness as a question of survival will not suffice. As a result, throughout the formation of what we previously referred to as the modern synthesis, fitness was linked to an individual's capacity to maximize the number of children left in the following generation.

The concept of fitness was converted into reproductive success. Given that fitness is observable regarding reproductive success, we can now return to the fundamental Darwinian principles. The first of the five findings presented was the tendency of living entities to proliferate exponentially. We highlighted that as a result, competition for available resources occurs, resulting in "survival of the fittest." Nature is genuinely prodigal in terms of reproductive capabilities.

The Prodigality of Nature

Nature's prodigality refers to creatures' immense reproductive potential. A carp or salmon may lay over a million eggs. A frog may lay up to 12,000 eggs. If all the eggs survive and breed, the entire planet will be littered with frogs. Darwin calculated the reproduction rates of elephants, believed to be the slowest breeders of all known mammals. If an elephant lives for 100 years and gives birth to six calves throughout its active reproductive period, at least 19 million elephants should be on the planet in 750 years. Several instances might be given to demonstrate how creatures have an enormous capacity for reproduction.

How is it that, despite their vast reproductive capacity, most species' populations are constantly kept at ideal levels? Why are lakes not clogged with fish, fields not covered with frogs, and the planet not overrun with elephants? This is because different environmental limiting variables, both biotic and abiotic, monitor population growth.

Factors that Limit the Reproductive Potential

Organisms are designed to compete for their necessities from their surroundings. As previously said, rivalry might be for food and territory, overcoming severe weather circumstances, evading predators, or combating infectious diseases. This is the "struggle" This "struggle" is what keeps population numbers under control and from expanding on a logarithmic scale.

We already discussed frog reproductive capacity. If the potential is wholly reached, they will quickly pressure the resources they rely on, and all of them will starve. This, however, is not the case. Most population sizes do not expand dramatically but vary from time to time.

On the other hand, other populations expand considerably in certain seasons and fall even more dramatically in others. Cell cultures may be grown in a laboratory with limited resources such as food, space, and so on. Under these conditions, the cell population growth rate, which was initially exponential, slows, and levels of the ensuing sigmoid curve are typical of biological growth in general.

The criteria stated previously limit the number or size of natural populations to the ecosystem's carrying capacity. For example, the quantity of space required by individuals of their own and other species determines the environment's plant-carrying capacity. No new seeds or shoots may grow in that location until the elder plants die. As previously stated, food supplies can restrict animal population size.

The population grows in this area until there is food scarcity. A scarcity of resources would reduce reproduction. Now a situation would emerge where the population size is reduced, and the environment's carrying capacity becomes large enough to accommodate more people. As a result, the cycle continues.

As a word of warning, the term "struggle" should not be taken too literally. Indeed, after Darwin publicized 'The Origin of Species,' there was criticism that Darwin's theory implied conflict, aggressiveness, warring between races and classes, and ethnic contests. However, as G.G. Simpson correctly points out, such a "mature red in tooth and claw" approach to natural selection is regrettable. The battle described by Darwin is a delicate one. Plants that struggle in dry settings, for example, evolve systems that limit water loss from them rather than sucking water from each other.

So, if only a certain proportion of a population can overcome the struggle' and survive to produce the next generation's offspring, the questions that arise are: Is it that chance alone determines which individuals should survive and which should die? If not, what factors determine an individual's survival? These considerations lead us to the next step in the natural selection process, which is population variability.

Variability in Population

The differences in heritable features displayed by individuals of a species are referred to as variability. Darwin's 1868 book, 'The Variation of Animals and Plants Under Domestication,' was an essential addition to studying evolution. Darwin demonstrated that groups of living creatures are not made up of identical individuals. Even monozygotic twins can indeed differ from one another. Various examples may demonstrate the disparities in various features in a group. Certain features, like height or color, constantly change, while others indicate two or a few distinct, diverse types of individuals with no intermediates.

In the human population, for example, there are PTC tasters and non-tasters; people with blood types A or B or AB or O; people with black, brown, or blue eyes; people with sickle cell anemia; and ordinary people, and so on. Sweet peas can have either red or white blooms; Drosophila can have regular or vestigial wings, red or white eyes, and so on.

Mutations of various types induce genetic diversity, and genetic recombination is organisms' principal source of variation. The subtle impacts of the environment also contribute to the variety of characteristics in natural populations. The variances in various features displayed by monozygotic quadruplets of a nine-banded armadillo are a frequently cited example. Because of variances in the uterine environment in which they grow, monozygotic kids may display such differences. It should be evident that four genetically distinct zygotes growing in the same uterus should be much more dissimilar.

Natural Selection

Assuming that population variability is non-genetic and not controlled by genetic material, random occurrences will select which creatures survive again. However, all species' phenotypic features contain a genetic component and are somewhat heritable. If a person possesses any characteristic that improves his chances of survival in a particular environment, his progeny will undoubtedly acquire the trait. This is referred to as differential survival.

Individual survival in an environment to which they are well adapted is referred to as differential survival. Individuals that lack such adaptations to their environment may not live to reproduce. The adaptations are heritable and genetically regulated—differential survival results in differential reproduction of alleles responsible for the following generation's survival. It is common to associate natural selection with unequal reproduction. The result of differential reproduction is that people not only survive but also can pass on their genes to future generations.

Thus, the collection of genes is what indeed remains. As a result, natural selection might result in unequal reproduction among individuals of the same gene pool. We previously said that selection would favor genes that result in phenotypes more suited to a particular environment. In other words, the frequency of such alleles would grow while the frequency of ill-adapted ones would tend to decline. We mean that natural selection is caused by variations in gene frequencies (a brief description of gene frequencies may be found in the box). Differential reproduction has a twofold purpose.


Darwin's theory of natural selection is founded on several observations and deductions. The fact that organisms have a great capacity to reproduce but only a tiny fraction survives leads to the conclusion that there is competition among individuals of a species for their diverse requirements as given by the environment.

This deduction, combined with the fact that heritable trait variations are universal, suggests that in such a competition, genetic variations that are useful to the organism in a given environment become adaptations, and such adaptations improve their reproductive efficiency. The Darwinian idea of natural selection is fundamentally linked to the differential reproduction of species.

Updated on: 05-May-2023


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