Evolution Revision Notes For NEET

MECHANISM OF EVOLUTION

        Evolution is a change in a population, alleles and genotype from generation to generation.

        Species it is a group of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups that maintain connectedness over geographic distances.

        Speciation it is the process of forming one or more new species from an existing species through evolutionary means.

        Allopatric speciation refers to populations of a species that are geographically separated.

        Sympatric speciation is the formation of two or more descendant species from a single ancestral species occupying the same geographical location.

        Peripatric speciation is when new species are formed in isolated, small peripheral population that are prevented from exchanging genes with the main population.

        Parapatric speciation is the partial separation of zones of two diverging populations afforded by geography individuals may come in contact but reduced fitness of hybrid prevents their interbreeding.

        A population constitutes all individuals of the same species occurring in the same area at a particular time. Random mating exists in a naturally occurring population. Evolution occurs through the various agents in a population like mutation, recombination, genetic drift, gene flow, inbreeding and natural selection.

        Mutation it is sudden and heritable change in an organism. Joshua Lederberg and Esther Lederberg (1952) conducted experiments to demonstrate the genetic basis of drug resistant mutation in E. coli by using the replica plating technique. It helps in the accumulation of variations which results in large variations and formation of new species.

        Gene migration (Gene flow) the movement of individuals from one place to another is called migration. It can be a powerful agent of change because the members of two different populations may exchange genetic material.

        Recombination the non-parental arrangement of alleles in a progeny is called recombination. It results from independent assortment and crossing over.

        Genetic drift (Random drifts) in small population, frequencies of particular allele may change drastically by chance alone. Such change in allele frequencies occurs randomly as if the frequencies were drifting and are thus known as genetic drift. There are two special cases of genetic drift.

        Founder effect changes in gene frequencies that usually accompany starting a new population from a small number of individuals.

        Bottle neck effect an event in which a populations size is greatly reduced. When this happens, genetic drift may have a substantial effect on the population.

        Selection the process by which those organisms which appear physically, physiologically and behaviourally better adapted to the environment survive and reproduce.

        There are three types of selection processes occurring in a population, they are stabilising, directional and disruptive.

        Stabilising or normalising selection this occurs when environment does not change and it causes no pressure on well-adapted species.

        Directional selection this occurs when environment changes in a particular way and it causes a selective pressure for the species to change in response to environment.

        Disruptive or diverging selection this occurs when environmental change may produce selection pressure that favours two extreme characteristics.

        Hardy-Weinberg equilibrium the equation (p²+2pq+q²=1) used by population geneticists to determine probable genotype frequencies of a population for specific traits.

        Polymorphism it is the existence of two or more forms or phenotypes of the same species within the same population. 

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