This variance in the appearance of living things is called phenotypic variance. There are a number of reasons for the existence of such phenotypic variance. In other words there are a number of underlying variances that can explain phenotypic variances.
Primary source of phenotypic variance
The two most obvious source of phenotypic variance are
- genetic variance
- environmental variance
Genetic variance is caused by individuals having different genetic predisposition in the population. Environmental variance is caused by the fact that different individuals experience different environments (both macro and micro - environmental differences).
Monozygotic twins look alike because, among them, they have no genetic variance. Even if raised in the same home, they still experience slightly different environments. If raised separately (which sometimes happen) then twins will differ a little more as they experience very different environments.
Other source of phenotypic variance
The other sources of phenotypic variances include
- epistatic variance
- developmental noise variance
- all covariances between all of these causal factors.
Are there any articles or research about this?
Well yes! Your question is underlying an entire field of knowledge which is called quantitative genetics.
For more information, I recommend having a look at the post Why is a heritability coefficient not an index of how “genetic” something is?