with an example, explain how variation leads to evolution.

Variation can be defined as the changes in the genetic material or in the alleles of the genes of a population. Variations which are beneficial for the population and help in the better survival of the population are naturally selected. Accumulation of many such variations result in development of new species. Therefore, variation is the cause for evolution.

For example: Let us suppose that a population of bird with long beaks inhabit a new area. In that area, they have food which can be consumed properly with short beaks. Due to certain variations, there are some birds with short beaks in that population. So, in the new area they will survive better and reproduce more, hence there number will increase and slowly they will evolve into a new species.

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 The variation can be heritable. One such heritable variation occurs as mutation. Mutation introduces genetic changes that can be passed on to the next generation through reproduction. This results in the evolution of the species with alternative traits.

Another source of variation is genetic recombination, which shuffles the genes into new combinations which can result in organisms exhibiting different traits. This process also leads towvolution.

OR in more simple language

Evolution is the cumulative changes that occur in a population over time. It is the result of change in the genetic make up of the organism due to mutations. The favourable mutations are accepted by nature and provide the organism an adaptive advantage.

Example: Say you have a population of birds, and naturally some have longer beaks than average and some have shorter beaks. This is variation, and the bell curve has it's peak centered where the average beak length is.

Now say those birds are running out of food, but the birds with beaks long enough to reach in holes and pull bugs out can find plenty of food. Those birds survive more and pass their longer beaks down to their offspring, so in the next generation the bell curve is shifted more towards longer beaks.
Do this enough times and what becomes the normal beak length is several times longer than even the longest beaks were in the beginning.

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