Reproduction is the production of offspring that are biologically similar to the parent organism.
An individual organism needs no reproduction to maintain its life but it is necessary for the existence and continuity of life of each species. So an organism spends much energy to create more individuals.
Chromosomes in cell nucleus contain DNA (Deoxyribo Nucleic Acid). It contains information (blueprints) of body designs that inherit from parents to offspring.
DNA is the information source for making proteins. If the information is changed, different proteins will be made. It leads to change in body designs. Therefore, creation of a DNA copy is the basic event in reproduction.
A reproducing cell creates two DNA copies and additional cellular apparatus. Then they separate and the cell divides into two.
In each process of DNA copying, some variations are created. As a result, DNA copies will be similar, but not identical to the original.
If large variations occur, the new DNA copy may not work with the cellular apparatus. So, the new-born cell will die. If variations are mild, cells are survived.
Variation during reproduction is the basis of evolution.
Consistent DNA copying maintains body design. It helps the organism to use a particular niche. So, reproduction is linked to the stability of populations of species.
However, niches can be changed due to many reasons. E.g. change in temperature & water level, meteorite hits.
If a niche is drastically altered, the population not suited to that niche is wiped out. Individuals having variations may be survived. E.g. Consider a bacterial population in temperate waters. If the water temperature is increased by global warming, most of the bacteria die, but the few variants resistant to heat survive. Variation is thus useful for the survival of species over time.