{"id":9576,"date":"2026-06-01T21:33:48","date_gmt":"2026-06-01T21:33:48","guid":{"rendered":"https:\/\/kapdec.com\/help\/?p=9576"},"modified":"2026-06-01T21:33:48","modified_gmt":"2026-06-01T21:33:48","slug":"natural-selection","status":"publish","type":"post","link":"https:\/\/kapdec.com\/help\/natural-selection\/","title":{"rendered":"Natural Selection"},"content":{"rendered":"

Unit :  Natural selection<\/strong><\/h2>\n

Chapter: Introduction to natural selection<\/strong><\/h3>\n

Reference:<\/em><\/strong> Darwin theory of natural selection or Darwinism, Feature of Darwin’s theory of natural selection, Criticism of natural selection theory, Evidence in the favour of theory of natural selection, Effect of industrial melanism on peppered moth (Biston betularia), Pesticide (DDT) resistance in mosquitoes, Types of Natural selection<\/em><\/p>\n

Learning objectives<\/strong><\/p>\n

    \n
  • To understand Darwin theory of evolution<\/li>\n
  • To describe the evidences for the theory<\/li>\n<\/ul>\n

    Darwin theory of natural selection or Darwinism-<\/strong><\/p>\n

    Charles Darwin, observed variations in the organisms of South America and Islands of Atlantic Ocean and Pacific Ocean. He also studied 13 species of finches (birds) of genus Geospiza, on Galapagos Islands (named after Giant Tortoises).  He noticed that insect eating birds, like woodpeckers and Warblers were absent on Galapagos Islands. But, some of the finches, who were originally seed-eating had developed insect-eating habits.<\/p>\n

    In 1859 Charles Darwin published his ‘Theory of natural selection’ in the book ‘Origin of species’. The theory of natural selection is based on 5 important observations-<\/p>\n

    1. All species have great reproductive or fertility potential.<\/p>\n

    2. Most populations are stable in size.<\/p>\n

    3. Natural resources are limited.<\/p>\n

    4. Individuals of a population have variations.<\/p>\n

    5. Most of the variations are heritable<\/p>\n

    Feature of Darwin’s theory of natural selection-<\/strong><\/p>\n

      \n
    • Over production<\/strong>– Every organism has a high reproductive potential, or over produces the number of off-springs than survive.<\/li>\n
    • Struggle for existence<\/strong> – As the space, food or other resources are limited, there starts a struggle for existence amongst the organisms. The struggle can be intra-specific (amongst the members of the same species) or inter-specific (amongst the member of the different species). The intraspecific struggle is more severe due to similar food habits in the organisms.<\/li>\n
    • Variations<\/strong> – There can be micro (graded) and macro variations in the organisms. Charles Darwin considered micro variations significant for the evolutionary process. These variations, according to him, are small and directional. Such variations can however, be useful, harmful, or neutral for the survival of the organism. Darwin also recognized macro variations, in Ancon sheep, and called them sports.<\/li>\n
    • Natural selection \/ Survival of the fittest<\/strong> – According to Darwin the organisms which have maximum useful variations are selected by nature (Natural selection). A.R. Wallace called it ‘Survival of the fittest’. Such individuals, according to Darwin, chances of reproduction than the animals having less useful variations. This ‘differential reproduction’ increases the number of the organisms having useful variations and thus changes allelic frequency in the population.<\/li>\n
    • Origin of species<\/strong> – The individual selected by nature in that environment, later accumulate more and more variations and ultimately form a new species, which is better adapted in the changed conditions.<\/li>\n
    • Inheritance of useful variations<\/strong>– Organism after getting fitted into the surrounding transmit their useful variation to next generation and no-useful variation are eliminated. Darwin however, is unable to differentiate between continuous and non-continuous variations.<\/li>\n<\/ul>\n

      Criticism of natural selection theory-<\/strong><\/p>\n

        \n
      • Failed to explain presence of vestigial organ<\/li>\n
      • Failed to explain inheritance of small variations<\/li>\n
      • No explanation of connecting links<\/li>\n
      • Failed to explain inheritance of overspecialized characters<\/li>\n
      • There is no distinction between germplasm and somatoplasm<\/li>\n
      • The theory is failed to explain the evolution of terrestrial animal from the aquatic animals.<\/li>\n
      • The theory is failed to explain the discontinuous variations.<\/li>\n<\/ul>\n

        Evidence in the favour of theory of natural selection-<\/strong><\/p>\n

        Effect of industrial melanism on peppered moth (Biston betularia) <\/strong><\/p>\n

          \n
        • These moths during 1850, were mostly grey (Biston betularia typica) and during day time used to rest on tree-trunks which had lichens grown on them. They resembled with the background and could not be detected by their predators, mainly birds.<\/li>\n
        • During industrial revolution in England, the coal was burnt and the soot got deposited on the tree-trunk. Moreover, sulphur-di-oxide killed lichens.  In such polluted areas all the moths of grey colour now could be spotted by birds.<\/li>\n
        • A few black individuals which had mutant gene were selected by nature. Such individuals got more chances of reproduction and the grey were reduced in number.<\/li>\n
        • After about 100 years (by 1950), more than 90% of the individuals had become black, ‘Melanic form’ (Biston betularia carbonaria).<\/li>\n
        • This evolution from grey to black (Melanic) forms was not due to mutation but due to selection of pre-existing mutant allele by nature (Natural selection).  Now, when electricity is being used in industries, the number of grey moths is again increasing. This also indicates that industrial pollution has not eliminated the genes responsible for light colour of the moth.<\/li>\n<\/ul>\n

          Pesticide (DDT) resistance in mosquitoes <\/strong><\/p>\n

            \n
          • When DDT was sprayed to kill the mosquito, there were very few mosquitoes which were already resistant to DDT. Most of the mosquitoes, who were sensitive to DDT, died, but the resistant ones survived.<\/li>\n
          • Due to differential reproduction, the resistant mosquito got more chances of reproduction and multiplied. After a few generations most of the available mosquitoes were resistant. The sensitive were very few.<\/li>\n<\/ul>\n

            Types of Natural selection<\/strong><\/p>\n

            1. Stabilizing selection<\/strong> – In this type of selection the frequency of average sized individuals increases further but the frequency of smaller and larger sized individuals is reduced and finally becomes negligible.<\/p>\n

            2. Directional selection<\/strong> – In such selection the longest or the shortest individuals are selected by nature and increase in frequency in the fourth-coming generations. In directional selection it is always the ‘extreme’ (end one) which is favoured. Evolution of DDT resistant mosquitoes, evolution of giraffe, industrial melanism in peppered moth are the examples of directional selection.<\/p>\n

            3. Disruptive selection<\/strong> – In this case the individuals of both the extremes (ends), the smallest and the largest ones, are favoured, and thus increase in the frequency or percentage. Such type of selection is rare in nature, but occurs when the environment is changing frequently.<\/p>\n

                        \"\"<\/p>\n

            Solved examples<\/strong><\/p>\n

            Example 1<\/strong>. According to natural selection, light colored tree trunks would favor—-<\/p>\n

            a) light colored moths  b) dark colored moths   c) red colored moths  d) red and light colored moths<\/p>\n

            Solution 1:<\/strong> a. According to natural selection, light colored tree trunks would favor light coloured moths.<\/p>\n

            Example 2<\/strong>. Over the years, white mice became extinct and darker mice survived. This is<\/p>\n

            a) stabilizing selection  b) directional selection  c) disruptive selection  d) due to exercise<\/p>\n

            Solution 2:<\/strong> b. Over the years, white mice became extinct and darker mice survived. This is directional selection.<\/p>\n

            Summary<\/strong><\/p>\n

              \n
            • Natural selection is the evolutionary process that Charles Darwin initially described, and it is the image that most people think of when they think of evolution.<\/li>\n
            • The study of the peppered moth (Biston betularia) in England during the Industrial Revolution in the 1800s is a famous example of natural selection.<\/li>\n
            • Natural selection is a key idea in evolutionary theory. It is frequently referred to as the survival of the fittest, and Charles Darwin and Alfred Russel Wallace proposed it.<\/li>\n
            • Natural selection explains why living creatures evolve to have the architecture, functions, and behaviors over time.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

              Unit :  Natural selection Chapter: Introduction to natural selection Reference: Darwin theory of natural selection or Darwinism, Feature of Darwin’s theory of natural selection, Criticism of natural selection theory, Evidence in the favour of theory of natural selection, Effect of industrial melanism on peppered moth (Biston betularia), Pesticide (DDT) resistance in mosquitoes, Types of Natural […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[629],"tags":[],"class_list":["post-9576","post","type-post","status-publish","format-standard","hentry","category-ap-biology"],"_links":{"self":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9576","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/comments?post=9576"}],"version-history":[{"count":0,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9576\/revisions"}],"wp:attachment":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/media?parent=9576"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/categories?post=9576"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/tags?post=9576"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}