Most sources of variations in sexually reproducing populations are described next.

• The genetic recombination produced through the biological process (recall from previous studies that crossover occurs during meiosis) ends up in gametes with variations.

•Mutation is the ultimate source of variation. Without variation, there could be no evolution. Mutations (changes in DNA (Deoxyribonucleic Acid)) are an important source of variations. Mutations also happen during gametes formation through meiosis.

• During fertilization, one of the legion sperms combines with one egg. The possibility involved during this combination also acts because of the source of variations.

• Gene flow i.e. movement of genes from one population to a different is additionally a vital source of variations.


Discontinuous and Continuous Variations:

      The inheritable variations are of two types i.e. discontinuous and continuous variations. Discontinuous variations show distinct phenotypes. The phenotypes of such variations can not be measured. The individuals of a population either have distinct phenotypes, which might be easily distinguished from one another. Blood groups are a decent example of such variations. in an exceedingly human population, a private has one in every one of the four distinct phenotypes (blood groups) and can't have in between. Discontinuous variations are controlled by the alleles of one gene pair. The environment has little effect on this sort of variation. In continuous variations, the phenotypes show an entire range of measurements from one extreme to the opposite. Height, weight, feet size, intelligence, etc. are examples of continuous variations. In every human population, individuals have a variety of heights (from very small to tall). No populace can show only two or three distinct heights. Continuous variations are controlled by many genes and are often laid low with environmental factors.


1. Variations result in Evolution:

      Organic evolution (biological evolution) is that the change within the characteristics of a population or species of organisms over the course of generations. The evolutionary changes are always inheritable. The changes in a person aren't considered evolution, because evolution refers to populations and not individuals. Organic evolution includes two major processes:

• Alteration in genetic characteristics (traits) of a sort of organism over time; and

• Creation of recent sorts of organisms from one type.

        The study of evolution determines the ancestry and relationships among different styles of organisms. The anti-evolution ideas support that everyone living things had been created in their current form only some thousand years ago. it's referred to as the “theory of special creation”. But the scientific add eighteenth century led to the thought that living things might change additionally.

     Charles Robert Darwin (1809–1882) proposed the mechanism of organic evolution in 1838. It had been called “The Theory of Natural Selection”. Darwin presents this theory after his 5-year voyage on the His Majesty’s Ship Beagle. Darwin also published his own book “On the Origin of Species employing Natural Selection” in 1859. Darwin’s theory of evolution wasn't widely accepted due to a lack of sufficient evidence. The modern evolutionary theory began within the late 1920s and early 1930s. Some scientists proved that the idea of selection and Mendelian genetics are identical ideas even as Darwin had proposed.


Mechanism of Evolution - natural action

       Every populace contains several variations for the characteristics of its members. In other words, there are morphological and physiological variations altogether in populations. survival of the fittest is that the process by which the higher genetic variations become more common in successive generations of a population. The central concept of the natural process is that the evolutionary fitness of an organism. Fitness means an organism’s ability to survive and reproduce new generations. Creatures produce more offspring than can survive and these offspring vary in fitness. These conditions produce a struggle for survival Creatures among the organisms of the population. The organisms with favorable variations are ready to reproduce and pass these variations to their next generations.

      On the opposite hand, the speed of the transmission of unfavorable to next generations is low. We can say that the favorable variations are “selected for” their transmission to the next generations, while the unfavorable variations are “selected against” to their transmission to the next generations. In the example mentioned next, we will see a mouse population with variations in skin color. Cat preys for light and medium colored mouses. In first-generation, cat preys light-colored mouse upon.

      The only medium and dark-colored mouse can make their next generations. In next-generation, the population again contains light, medium, and dark-colored mice. Cat preys upon the sunshine and medium-colored mouse. Now only the dark-colored mouse makes a new generation. If this happens in many generations, we are going to see only the dark-colored (favorable variation) mouse within the population. As a result of selection, the allele that offers more fitness of characteristics (favorable variations) than other alleles becomes more common within the population. So, the individuals with favorable variations become a significant part of the population while the individuals with harmful or unfavorable variations become rarer. In England, the moths had 2 (two) varieties, i.e. dark and white-colored moths. The moths were accustomed to rest on the sunshine-colored tree trunks (on which white lichens had grown). within the 19th century when industries were established in England, the lichens on tree trunks died (due to polluted air), and also the naked tree trunks turned dark. Now the white moth variation became harmful because a white moth resting on a dark trunk was easily visible to the predatory birds. The natural action selected dark moths to breed. During this way, dark-colored moths became more common and finally, the white moths disappeared from the population. During this case, the dark color variation in moths could also be considered an adaptation to the environment.


Artificial Selection:

        The term “artificial selection” was revealed by the Persian scientist Abu Rayhan Biruni within the 11th (eleventh) century. The naturalist also used this term in his work on selection. He noted that several domesticated animals and plants had special properties that were developed by:

• Intentional reproducing among individuals with desirable characteristics; and

• Discouraging the breeding of people with less desirable characteristics.

       Artificial selection (or selective breeding) means intentional breeding between individuals, sure enough, traits, or a combination of traits. Selective development has revolutionized agricultural and livestock production throughout the globe. Plants or animals having desirable characteristics are chosen for breeding. In this way, many new generations with desirable characteristics are produced. In artificial choice, the bred animals are called breeds, while bred plants are referred to as categories or cultivars. Many breeds of sheep, goat, cow, hen, etc. are produced by artificial selection to extend the assembly of wool, meat, milk, eggs, etc. Similarly many plant varieties (cultivars) are produced for better quantity and quality of cereals, fruits, and vegetables.