Natural selection is the process that allows certain traits to become more prevalent in populations. Sexual selection is one form of selection based on mating choices. In evolution, failure to mate is the same as dying.
The natural selection process takes place when more offspring are produced than can survive to sexual maturity due to competition and other abiotic factors. Genetic variation is present and some members of the population are better able to survive due to phenotype variations. These individuals make it to breeding age and reproduce passing on these traits.
Sexual selection is one form of natural selection where individuals of a population are selected for specific traits which are then passed on to future generations.
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Freeman, S. & Herron, J. (2007). Evolutionary Analysis 4th Ed. Sexual Selection, pp 401-441
"natural selection" why does it take so long? [Web]. (2011). Retrieved from http://www.youtube.com/watch?v=0lluuX7lsFk