Alleles are alternative characteristics at a particular loci (place) on the chromosome. Mendel's pea experiments illustrate alleles. You might have white flowers or yellow flowers. One might be dominate and the other recessive. This gives rise to hybrids.
A mutation might occur that makes a red flower. Then there is a third allele. An individual can only have a pair of alleles. Populations have the total allele alternatives for a given loci. Each gene loci might have a number of different alleles (short or tall, big leaf or little leaf etc)
There may be tens or hundreds of different alleles for the same loci. As the total percentage of each allele changes or by the addition of new alleles through mutation or other mechanisms, evolution in the population occurs. As natural selection operates on these alternatives, the optimum mixture for survival in the enviroment of the population begins to emerge.
You also need to understand the difference between phenotype and genotype.
And that is why the understanding of allele frequency change is crucial to understanding evolution. It is evolution.
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