Darwin's Postulates
The Factors that Affect the Process of Evolution
A species' potential to increase in number
The potential a species has to increase in number comes primarily form one thing: an individual organism's fitness level and the ability to survive in the environment they live in. Thus a species' ability to continue existing and increase in population as a whole is solely dependent on the thriving of the individual organisms that make it up. This shapes evolution and the development of species in many ways; mainly because only the fittest genes are allowed to be passed down by the forces of evolution and thus the organism's offspring are more likely to survive and reproduce in greater number than others (continuing this cycle).
Genetic variation of individuals
Genetic variation in a species, due to mutation and sexual reproduction, are some of the main factors that push evolution. Through the inherent variation of all genes, individuals are more, or less, fit for the environment that they live in. Thus through natural selection and the other forces of evolution, the environment "decides" which are the fitter genes that get to proliferate and continue to be passed down through reproduction.
Competition for limited resource
The fact that there are only a limited amount of resources available to an organism in an environment means that organisms will have to compete and fight for these. Food, shelter, water, and other assets are what's at stake. By only allowing the fittest organisms to take these for grabs, letting them thrive and letting other less-fit organisms plunge, the environment allows only the fit-enough organisms to thrive and reproduce, passing these superior genes on to their offspring. This leads to fitter, more evolved, organisms.
The proliferation of fit organisms
Through the forces of evolution, only the fittest organisms and species are allowed to thrive, thus increasing their chance of reproducing and passing-down their genes in larger numbers than those that are less fit and less likely to reproduce and survive. This allows for the best genes to be passed down more constantly and in greater numbers, leading to organisms that are the best fit to survive and continue the process in the current environment.
Sources
Evolution. (2015). Funk & Wagnalls New World Encyclopedia, 1p. 1. from http://web.b.ebscohost.com/ehost/detail/detail?vid=4&sid=6a3d5718-34c0-4360-a7a3-2374025a90d5%40sessionmgr114&hid=107&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#AN=EV073800&db=funk
Woman Frantically Begs Bear To Not Eat Her Kayak; Bear Eats It Anyway. 2016. Web. 2 Aug. 2012. from http://i0.wp.com/cdn.bgr.com/2015/10/bear.jpg?w=625