While there are many factors that can affect allele proportions in a population, selection is thought to be among the most powerful. Selection effectively shapes the heritable physiology, morphology and behaviors of populations through differential reproduction among individuals. Natural selection affects traits related to survival and producing offspring, while sexual selection specifically affects traits related to obtaining mates. Often traits which increase an individual’s access to mates, actually decreases their survival. One of the models in this section deals with this evolutionary trade-off.