Four Postulates of Evolution
(1) The potential for a species to increase in number
In nature not all organisms are strong enough to survive. Some of them die earlier before they reach the reproductive age, leading to a decrease in their species and their population as a whole. Then there are others who reach the reproductive age but don't reproduce easily because their reproductive age is much more older than other species. For example turtles, they have a long life-span but little signs of reproductive senescence. They reach their reproductive age somewhere between 10 to 59 years of age. Their long life-span compensates the fact that they don't reproduce that often or that easily.
(2) The heritable genetic variation of individuals in a species due to mutation and sexual reproduction;
(3) Competition for limited resources
(4) The proliferation of those organisms that are better able to survive and reproduce in the environment.
Charlesworth, B., & Charlesworth, D. (2009, November 1). Darwin and Genetics. Retrieved April 5, 2016, from http://www.genetics.org/content/183/3/757
Lavergne, S., & Molofsky, J. (2007, March 6). Increased genetic variation and evolutionary potential drive the success of an invasive grass. Retrieved April 5, 2016, from http://www.pnas.org/content/104/10/3883.full.pdf
Michigan, U. O. (2008, November 26). Competition and Resource Scarcity. Retrieved April 5, 2016, from http://www.globalchange.umich.edu/globalchange1/current/lectures/competition/competition.html
Nguyen, D. H. (2016). Ways in Which Sexual Reproduction Provides Greater Genetic Diversity Than Asexual Reproduction. Retrieved April 5, 2016, from http://education.seattlepi.com/ways-sexual-reproduction-provides-greater-genetic-diversity-asexual-reproduction-6284.html