Mutation-The changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes.Genetic variation comes about through new combinations of alleles at different loci caused by recombination in sexual reproduction and by new mutations creating new sources of variation.
Each individual carries a unique combination of alleles. Some of these are better than others in terms of survival in the natural environment. This is termed fitness.
As there is not an endless supply of resources in the environment individuals must compete with one another to obtain them (Darwin's struggle for existence). Those individuals with lower fitness will fail to obtain the resources they need and will fail to pass on their genes to the next generation. This is natural selection - only the fittest individuals in each generation will survive and reproduce.
A couple of examples: chicks in the nest must beg for food otherwise their parents don't feed them. A chick with reduced fitness won't beg (enough) won't be fed and will die. Also among birds - migratory birds that arrive early on breeding grounds generally get the best sites and their eggs hatch when the food supply is peaking. Birds that arrive late (have lower fitness in terms of arrival time) have a much higher chance of failing to breed and don't pass on their genes. Both of these are examples of natural selection at work.
In terms of evolution, this means that at every generation only the fittest individuals survive and so over long periods populations become highly adapted to their environments - this is evolution.
Different environments place different selective pressures on organisms. For example different attributes are required to survive in Siberian than in tropical Sumatra - so tigers in these two environments have become differently adapted - size, pelage etc. Given enough time without any gene flow between the two subspecies then they would diverge completely. At all stages from the origin of the species until it eventually splits into two or more new species evolution (change) is occurring - fuelled by genetic variation.
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