Why are cell membranes composed of two phospholipid layers, with the fatty acid tails of each layer directly interacting with each other?

I think it is "The fatty acid tails are hydrophilic, and the negative charges on one set of tails ate attracted to the positive charges on the other set of tails

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".