Question

what is the basic cause of variation

Answer 1

Variation between individuals and different species is due to a slow rate of mutation which provides the changes that can be beneficial, silent or detrimental to survival. No organism perfectly copies it's DNA, but mistakes are made at a limited enough rate that there will always be variation in a population if sufficient size. A good average rate might be one mistake in a cell's DNA per 100,000 bases copied.

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Answer 2

There are the following causes of variations:--
¡)At the time of DNA Replication.
¡¡)At the time of gamete formation due to crossing over and reshuffling of chromosomes during meiosis.
¡¡¡)At the time of fertilisation due to chance union of male and female gamete.