Genetic variationSpecies refers to a population of organisms potentially capable of interbreeding to produce fertile offspring. Variations 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 that affect variation include nutrition, climate and pollution. Organisms that reproduce sexually exhibit genetic variation. Genetic variation is the result of meiosis. Meiosis allows for the independent assortment of chromosomes that contributes to variation. Independent assortment occurs at metaphase I, when the bivalents align on the equator. Each bivalent is made up of two homologous chromosomes, including one maternal and one paternal. The orientation of bivalents across the spindle is completely random, so maternal and paternal chromosomes may be mixed in the final gametes. Each homologous pair aligns on the spindle completely independently of any other pair. This contributes to genetic variation. Another factor contributing to genetic variation is “crossover.” Crossover occurs at prophase I, when bivalents first form. When the two chromatids come together to form a synapse, parts of one chromosome can be exchanged with the corresponding one. parts of the other chromosome. The points where chromosomes cross are called chiasmata. There are usually many chiasms in a bivalent and it is the chiasms that actually hold the bivalent together. Ultimately, crossing over means that maternal and paternal alleles can be mixed. Result...... middle of paper ...... include hybrid sterility and hybrid inviability. A hybrid may be healthy but sterile, unable to produce viable gametes because the chromosomes inherited from its parents do not pair and cross over properly during meiosis. Hybrid inviability occurs when development is abnormal due to incompatible fertilization and as a result the zygote will die and be aborted at an early stage. Therefore, due to any of the above factors, part of the population may become isolated, preventing reproduction with the rest of the population. The two populations experience different environmental conditions which result in different selective pressures. This changes the frequency of genes in the next generation. Eventually, genetic differences accumulate, so that individuals from distinct populations can no longer interbreed, forming a new species..