Genetically identical mice develop individuality

One of the more interesting findings in behavior genetics is the fact that monozygotic twins (essentially genetic clones of one another) that are raised together (essentially in the same general environment) do not develop identical personalities. Clearly, this means that there is some sort of non-shared environmental factors that condition individuality. It is very difficult to pin these factors down in humans because we can't simply throw them into controlled experimental conditions and watch what happens (and rightfully so).

The journal Science has just published a fascinating study by Julia Freund and her colleagues that explores this issue with mice. What they did was take a group of 40 genetically identical female mice (well, about as identical as you can get; they were super inbred, but may vary by a handful of SNPs and/or VNTRs) and let them roam around an enriched environment for a few weeks. They were able to track their movements (in order to examine how "explorative" each individual was) and their hippocampal development (this part of the brain is thought to regulate behavioral plasticity). I'll let the authors pick it up from here (Freund et al. 2013: 758):

This study shows that adult neurogenesis, as an instantiation of brain plasticity, is linked to individual differences in experience among genetically identical individuals who live in a nominally identical environment. About one-fifth of the experiential effects on adult neurogenesis was captured by a measure of roaming through an enriched environment. This finding supports the idea that the key function of adult neurogenesis is to shape hippocampal connectivity according to individual needs and thereby to improve adaptability over the life course and to provide evolutionary advantage...[t]his is in line with the observation that behavioral treaits can be strongly influenced by external stimuli that vary between individuals or populations of individuals...

They continue (Fruend et al., 2013: 759):

Three months of living in a complex environment led to a massive magnification of individual differences in explorative behavior among genetically identical individuals over time, and these differences were related to adult hippocampal neurogenesis. The rich environment lost its "sameness" over time and gave way to the emergence of a personalized "life space" and a "mouse individuality," similar to what has been observed in humans for personality traits.

There has been a lot of discussion regarding cloning. Now, if you wanted to produce two absolutely identical people, think about all the factors you'd have to keep constant, and we're not just talking about minute changes in the intrauterine environment (temperature, etc.), but also all the experiences an individual will have over their lifetimes. Genetically cloning an individual may not be so difficult, but creating two identical human beings may be nigh impossible.

References:

Freund, J, Brandmaier, AM, Lewejohann, L, Kirste, I, Kritzler, M, Krüger, A, Sachser, N, Lindenberger, U, Kempermann, G (2013). Emergence of individuality in genetically identical mice. Science 340, 756-759.