Reproductive advantage is a mechanism of evolution that works by altering the heritable traits of a population.
In the theory of evolution, natural selection is the means by which the phenotype (the visible and expressed genetic characteristics) of a species can change over time. In every species there are traits that nature deems superior to others; size, intelligence, natural weaponry, athleticism, etc. It is part of the reproductive instinct to choose a mate with superior traits.
These superior traits (traits that provide some type of advantage) are passed along to offspring. These superior traits are believed to confer a reproductive advantage in that more reproductive success is attained by having positive traits therefore more of these genes are present in the following generation.