Reproductive success of gene combinations is measured using fitness, which is the probability of an individual contributing all or part of its genotype to its following generations..
A low fitness value means that there is little likelihood of an individual surviving long enough to reproduce, whereas a high fitness value means that there is a good chance for an individual to pass its genes onto its offspring.
Every aspect of an organism's phenotype is, in a way, an adaptation; therefore, the individual that results from all these genes, and gene products, is an interlocking mesh of overlapping structural, behavioral and metabolic adaptations.
These adaptations enable the organism to acquire energy, escape danger, exchange gases, replenish its stores of energy and materials, and so on. As one generation passes on its genes to the next generation, the codes for these successful adaptations are passed on too.