Variation can come about due to genes and the enviroment they both play roles for example you may have genes which suggest that you will be tall but if you dont have a good diet with the correct vitamins and minerals for growth your potential height will not be reached.Height is an example of a continuous variable where as Eye colour or blood type are far more clear cut so are discontinuous.
Genes are heritable variation , the enviroment in non inhertiable the combination of the two is epigenetics.Variation from sexual reproduction is brought about by random fertilisation,random assortment in metaphase one and crossing over in prophase one of meiosis.The is also Random fertilisation and random fusion of gamates and mutations which may occur.
Competition as we learnt in unit 4 can be interspecific or intra specific.Interspecific competion is competition between organisms of differing species , intra specfic competition is competition between organisms of the same species.Competiton comes into play when organisms compete for the limited resources availble to them in their habitat such as food and water or in plants sunlight and mineral ions.
When Charles Darwin came up with nautral selcetion he said that more offspirng than can survive are produced , there is selection pressure and those with the more helpful alleles will survive and reproduce to form the next generation of organisms. Things which could cause a struggle for survival are coat colour in wild rabbits. For example a rabbit with a brown coat will be more likely to survive than a white rabbit which would be far more visible to predetors like foxes so slowly this allele will die out as none of these white rabbits will survive and reproduce.This in turn causes change in the genepool.
Lets say that a group of birds only breed in cool climates and the climate warmed and the group of birds split up to find cooler places.
In this case their food source may change and lead to a different beak shape or different length claws to get food. Over a prolonged period of time this will cause changes so great that the two groups of bird is forced back together by further enviromental change would no longer be able to breed and have fertile young. If this was the case they are now two different species of bird. The has arisen from geographical isolation.
Fertile young may be born but be sterile because they have an odd number of chromosomes ( this is the case of a mule). Some embryos may not even implant .The Gentailia of the birds or the courtship rituals may have changed so much that they may choose not to breed with one another.Gametes may also not survive in the body of another species. This type of isolation is reproductive isolation.
Selection pressures and population genetics
In a group of a type of organism there will be a large range of phenotypes.This called a genepool.There will be different proportions of each allele within a group of organisms this is known as gene frequency.If the gene pool remains stable this means that the enviroment has also stayed stable.
Factors which change the genepool are variation, genetic drift and natural selection. Genetic drift is more likely to occur when you have small groups which have been isolated this is because there are a smaller number of organism with that allele even though the proportions of allele are equal to that of a larger group.This could lead to loss of an allele in a small group quite quickly.An example of a small group could be a founder colony( a small isolated group) which may have a very different gene frquency to that of the rest of the population else where and become very different to that of the main group so the become a new species.