Natural Selection- Definition, Theory, Types, Examples

Natural selection is the process whereby organisms better adapted to their environment tend to survive, pass on the genes, and produce more offspring.

It is a mechanism of evolution that causes species to change and diverge over time.

Natural Selection
Natural Selection

More than a century ago, in 1958, Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) combinedly credited the theory of evolution by natural selection and published the book “Origin of species”.

In 1859, Darwin separately published the book called “On the origin of species by means of natural selection”. 

After the death of Darwin, Neo-Darwinism theory was developed based on genetic character when the gene was discovered in 1901 by American scientists Margon, Shutton, etc.

Facts that influenced Darwin’s thought or Development of the theory of natural selection

  • In 1831, Darwin was the naturalist on the Voyage when he was just 22 years old. As a naturalist, he observed, collected, and analyzed specimens of plants, animals, rocks, and fossils. He spent more than three years exploring nature on the island and distant continents.
  • Darwin made many observations during the long Voyage, which helped him develop the theory of evolution.
  • Darwin follows the idea of Lamarck and assumes that species can change over time. Fossils he found helped conveniently follow Lamarck’s theory.
  • From Charles Lyell’s book “Principles of Geology”, Darwin concluded that there had been enough time for evolution to cause the great diversity of life because he saw the earth and its life were very old.
  • In late 1830, Darwin attended a meeting of animal breeders and intently read their publications. He was aware that humans could bread animals and plants to have valuable traits by selecting plants or animals which are allowed to reproduce over time to time. They could change an organism’s traits. He called this type of change variation in an organism; artificial selection.
  • Darwin wondered how artificial selection could be carried out in nature, he found the solution in Thomas Robert Malthus’s book, and he knew that the population could grow faster than their resources, which eventually caused a struggle for existence.
  • The term fitness was coined by Darwin and referred to an organism’s relative ability to survive and produce offspring. Variation occurs naturally, So he called this type of selection natural selection.
  • Darwin spent several years thinking about the work of Lamarck, Lyell, and Malthus, what he knew about artificial selection and observation, and the results he had made his Voyage.  Collecting all ideas, and all together, helped formulate his theory.
  • Wallace also traveled to different Places to study nature, and he also developed the same theory of evolution as Darwin. Wallace sent a paper he had written explaining his evolutionary theory to Darwin. Wallace’s idea helped to confirm Darwin’s theory.
  • At the time of controversy about the theory of natural selection, Wallace was in Indonesia, and he could not champion Darwin’s defense by adding his point of view. Thus, the theory is called the Darwinian rather than the Darwin-Wallace theory of evolution.

Darwinian theory of natural selection

  • Darwin-Wallace theory states that “The change in species by the survival of an organismal type exhibiting a natural variation that gives it an adaptive advantage in an environment, thus, leading to a new environmental equilibrium, is evolution by natural selection”. 
  • The natural selection process is a continuous process of trial and error on an enormous scale for all living matter.
  • Natural selection includes the following five elements (factors or causes of natural selection). 
  1. The universal occurrence of variation: 
  • Every group of organisms (animal and plant) may differ in many ways, known as variation. The Darwin and Wallace period did not know the source of variation and assumed it might be one of the innate properties of the organism. Still, now we know that inherited variations are caused by mutation.
  1. An excessive natural rate of multiplication:
  • Without environmental checks, every species has great reproductive potential (reproductive rate tends to increase geometrically). If all the species remained alive and reproduced, it would soon be challenging to survive, obtain food, and crowd all other species from earth.
  1. Struggle for existence:
  • There is an interspecific or intraspecific, or environmental struggle for survival (competition for food, mates, space, and as well as survival in drought or cold).
  1. The consequent elimination of the unfit and the survival of only those that are satisfactory adaption:
  • Some of the variations shown by living things make it better adaption for them to survive; others are handicaps that bring about the elimination of the possessors. The core of the natural selection theory is the idea of the survival of the fittest.
  1. The inheritance of mutations or recombination that make for success in the struggle for existence:
  • The individual who survives will give rise to the next generation, and with this method, the successful variations are transmitted to the succeeding generation.
  • A less fit individual is eliminated before being reproduced.
  • Successive generation becomes better adapted to their environment. If the environmental condition changes, further adaption occurs.
  • The operation of natural selection over time in many generations may produce descendants (different from their ancestors). This way, two or many more species may arise or produce from a single ancestral stock.

Criticisms of Darwinism or natural selection theory

  • Darwin did not mention vestigial organs, which are found in animals.
  • The role of mutation in the origin of new species is not included. Without mutation, new species never evolved by natural selection.
  • Variation, whether genetic or somatic, because only genetic variations are heredity.
  • Darwin’s theory became a failure in Human being due to these reasons; the human population never becomes constant, not only struggling for existence other also continuing life cycles like beggars, and instead of survival, the fitness in man only arrival become most fitted.

Types of natural selection

Population geneticists have categorized three types of natural selection that can occur in nature. These are as follows:

1. Directional selection

  • The variation that can occur when a population shows a particular trend through time is referred to as directional selection. It happens when the environment changes constantly. The directional selection process favors individuals that are better adapted to a new environment or situation or new ecological situation.
  • The directional selection also transforms the gene pool of a species toward the highest level of adaptedness that can be reached in the new environment. 
  • A very simple example of directional selection is the neck of a giraffe. Each giraffe has a different neck length. This variation in the trait gives each giraffe various advantages like a giraffe with a longer neck is easy to reach the food source, so better able to survive and reproduce. Over time, long-neck giraffes will reproduce more offspring and pass on their gene to the next generation. Thus, the future generation of giraffes has longer necks. If directional selection remains, this new population can eventually become a new species.

2. Stabilizing selection

  • In this type of selection, average values for the given trait are favored eliminating extreme value in a population. It is the most common type of selection occurring in the population and is homeostatic that maintains the status quo. This selection reduces the variability in the population. 
  • For example: In a plant, plants that are a more height are exposed to more wind and are at risk of being blown over, whereas a short-height plant fails to get an abundant amount of sunlight to prosper. Thus, the plants are average height between the two get both enough sunlight and protection from wind.

3. Disruptive selection

  • In this selection, two adaptive traits are selected when the population exists in a heterogeneous environment. It favors the extreme if they have better traits or fitness and intermediates are disadvantageous. This selection results in diversification concerning traits. 
  •   For example, an area that has black, grey, and white bunnies contains both white and black rocks. Natural selection will favor both the traits of white and black since they both prove useful for camouflage. The intermediate trait of grey does not prove useful so selective pressure act against the trait.  

The evolutionary biologist also recognized sexual selection and group and kin selection.

  1. Sexual selection 
  • Most species of animals are dimorphic (male and female). Males and females are different in terms of color, specialized song patterns, behaviors, sex organs, capability, etc. 
  • Sexual selection is directly related to differential reproduction, including finding and acquiring a mate, copulation, fertilization, and parental care.
  • Those organisms whose more capable of securing mates and is more fitted to the environment. Sexual selection aims to reproduce in which an individual needs to be able to find and protect a mate and produce viable offspring.
  • In 1938, Huxley recognized two types of sexual selection; Epigamic selection (based on the choice made between male and female) and Intrasexual selection (this selection is based on the interaction between animals of the same sex, generally between males).
  1. Group and kin selection
  • Group selection is a selection of a group of individuals, favoring one group over the other, leading to the evolution of a trait that is group advantageous.
  • The altruistic behavior of an individual involved in kin selection occurs when natural selection favors a trait that benefits related group members. 

Importance of natural selection

  • Natural selection drives the evolution and diversity of life on earth.
  • Favorable traits are transmitted through generations.
  • Natural selection can lead to specification, where one ancestor species gives rise to a new and distinctly different species.

Example of natural selection

  • Galapagos Finches: Galapagos finches have larger and small beaks. During drought time, the finches with long beaks survived better than those with smaller beaks however, in rainy times, more small seeds were produced, and finches with smaller beaks survived better. Thus, the environment supports both types of beaks.
  • The tail of peacock: Peacock females choose their mate according to the male tail. Peacocks, who have the largest and brightest tails mate more often. Tail characteristics are passed from mother breeding to their offspring because of this reason, today, most male peacocks have large and bright tails, it is rare to find a male that does not have large and bright feathers. 

References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S . Chand and Company Ltd.
  2. Keshari, A.K., Ghimire, K.R., Mishra, B.S. and Adhikari, K.K.(2018). A Textbook of Higher Secondary Biology, XI. Vidhyarthi Pustak Bhander, Kamalpokhari/Bhotahiti, Kathmandu, Nepal.
  3. Wallace, A. R. (2007). Darwinism: an exposition of the theory of natural selection with some of its applications. Cosimo, Inc.
  4. https://www.biologyonline.com/dictionary/natural-selection.
  5. https://www.britannica.com/science/natural-selection.
  6. https://sciencing.com/natural-selection-definition-darwins-theory-examples-facts-13719065.html.
  7. https://schooltutoring.com/help/the-3-types-of-natural-selection.
  8. https://examples.yourdictionary.com/examples-of-natural-selection.html.
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  11. Salzano FM. The role of natural selection in human evolution – insights from Latin America. Genet Mol Biol. 2016 Jul-Sep;39(3):302-11. doi: 10.1590/1678-4685-GMB-2016-0020. Epub 2016 Aug 4. PMID: 27561111; PMCID: PMC5004836.

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