Genetic variation A species refers to a population of organisms potentially capable of interbreeding to produce fertile offspring. Variation can be shown within a species. Variation is the difference between individuals within a species. The variation can be caused by genetic factors, environmental factors, or a combination of both. Environmental factors that influence variation include diet, climate and pollution. Sexually reproducing organisms exhibit genetic variations. Genetic variation is the result of meiosis. Meiosis allows for the independent assortment of chromosomes which contributes to variation. Independent assortment occurs in metaphase I, when bivalents align on the equator. Each bivalent is made up of two homologous chromosomes, one maternal and the other paternal. The orientation of bivalents across the spindle is completely random, so maternal and paternal chromosomes can mix in the final gametes. Each homologous pair aligns on the mandrel completely independently of any other pair. This contributes to genetic variation. Another factor that contributes to genetic variation is "crossing over". Crossover occurs in prophase I, when bivalents are formed. When the two chromatids join together to form a synapse, parts of one chromosome can be exchanged with the corresponding parts of the other chromosome. The places where chromosomes cross are called chiasmata. There are usually many chiasmata in a bivalent and it is the chiasmata that hold the bivalent together. Ultimately, crossbreeding means that maternal and paternal alleles can be mixed. The result… middle of the paper… includes hybrid sterility and hybrid sendability. 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 invibility occurs when development is abnormal due to incompatible fertilization and as a result the zygote dies and is aborted at an early stage. As a result, due to any of the above factors, part of the population may become isolated, preventing reproduction with the rest of the population. Both populations experience different environmental conditions that have different selection pressures. This changes the genetic frequency in the next generation. Eventually genetic differences accumulate so that individuals from separate populations can no longer interbreed, forming a new species.