Genetic Variation

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Genetic Variation

Species refers to a population of organisms which are potentially

capable of interbreeding to produce fertile offspring. Variation can

be shown within a species. Variation is the difference between

individuals within a species. Variation can be caused by genetic

factors, environmental factors or a combination of both. Environmental

factors which affect variation include nutrition, climate and

pollution. Organisms that reproduce sexually display genetic

variation. Genetic variation is a result of meiosis. Meiosis allows

the independent assortment of chromosomes which contributes to

variation.. Independent assortment happens at metaphase I, when the

bivalents line up on the equator. Each bivalent is made up of two

homologous chromosomes, one of which is maternal and one which is

paternal. The orientation of the bivalents across the spindle is

completely random so the maternal and paternal chromosomes can be

mixed up in the final gametes. Each homologous pair aligns on the

spindle completely independently to any other pair. This contributes

to genetic variation.

Another contributing factor to genetic variation is ‘crossing over.’

Crossing over occurs at prophase I, when the bivalents first form.

When the two chromatids come together to form a synapsis, parts of one

chromosome can be exchanged with the corresponding parts of the other

chromosome. The points at which the chromosomes cross over are called

chiasmata. There are usually many chiasmata in a bivalent and it is

the chiasmata which actually hold the bivalent together. Ultimately,

crossing over means that maternal and paternal alleles can be mixed


... middle of paper ...

... include hybrid sterility and hybrid

inviability. A hybrid may be healthy but sterile, it is unable to

produce viable gametes because the chromosomes inherited from its

parents do not pair and cross over correctly during meiosis. Hybrid

inviability occurs when development is abnormal due to incompatible

fertilization and consequently the zygote will die and be aborted at

an early stage.

Consequently due to any of the above factors part of the population

may become isolated, preventing breeding with the rest of the

population. Both populations experience different environmental

conditions which have different selective pressures. This changes the

gene frequency in the next generation. Eventually genetic differences

accumulate so individuals from separate populations can no longer

interbreed, forming a new species.
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