Answer:
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation during sexual reproduction. Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate. or Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote. Recombination or crossing over occurs during prophase I .
Without variation in traits among the population, there would be no variation in the relative fitness of individuals within the population (i.e. (higher level) no differential reproductive success between individuals). Without varying reproductive success between individuals in population based on traits, there will be no change in frequency of traits in the population over time. To put it simply, there is nothing for natural selection to act upon.
I found some articles on the internet that may be helpful (took me three minutes of searching). Hope these help?
http://www.huffingtonpost.com/ravishly/neuroscience-proves-what-_b_6494820.html
http://sitn.hms.harvard.edu/flash/2016/gender-lines-science-transgender-identity/
Answer:
The human body uses 21 amino acids to make the proteins it needs to function.
