MECHANISM OF EVOLUTION
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Evolution is a change in a population, alleles and genotype from
generation to generation.
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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.
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Speciation it is the process of forming one or more new species from
an existing species through evolutionary means.
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Allopatric speciation refers to populations of a species that are
geographically separated.
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Sympatric speciation is the formation of two or more descendant species
from a single ancestral species occupying the same geographical location.
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Peripatric speciation is when new species are formed in isolated, small
peripheral population that are prevented from exchanging genes with the main
population.
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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.
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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.
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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.
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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.
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Recombination the non-parental arrangement of alleles in a progeny is
called recombination. It results from independent assortment and crossing over.
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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.
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Founder effect changes in gene frequencies that usually accompany starting
a new population from a small number of individuals.
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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.
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Selection the process by which those organisms which appear
physically, physiologically and behaviourally better adapted to the environment
survive and reproduce.
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There
are three types of selection processes occurring in a population, they are
stabilising, directional and disruptive.
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Stabilising or normalising selection this occurs when environment does not change and it
causes no pressure on well-adapted species.
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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.
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Disruptive or diverging selection this occurs when environmental change may produce
selection pressure that favours two extreme characteristics.
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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.