Darwin's Postulates
Four main factors underlying evolution
Variation
Genetic variation is a result of genetic mutations, random mating or fertilization, and recombinations of chromatids of homologous chromosomes in meiosis, and is crucial to natural selection because the frequency of alleles increases or decreases depending on which traits prove useful for survival: the more variety, the more chances a population has at obtaining a favorable allele to keep in the upcoming generations.
Inheritance
Selective Pressures
An excellent and detailed example of a selective pressure is food: when there is only one piece of bread for 10 people, then getting the bread can be very competitive -- only those with the strength or wit get the bread, while the others are at a loss in comparison.
Fitness and Survival of the Fittest
Fitness may be contingent on physical characteristics, such as an organism's ability to camouflage with its environment to hide from predators, or another's long legs to run away more hastily; however, they may also be intellectual advantages, as we may see in humans today. Some satirists give "Darwin Awards" ironically to people who act irrationally, with no apprehensions to protect themselves from death or accidents. For instance, someone who crosses the street without looking may deserve a "Darwin Award", implying that natural selection will ensure their genes are not around long enough to be passed on to the next generation. It's a very cynical way of looking at the world, but in actuality, that is precisely what Darwin postulated. And, in the natural world, animals do not grow sheltered; rather, they grow facing the perils and obstacles and are therefore forced to compete amongst each other to survive.
If the predators like the green bugs
The orange bugs have an advantage...
Similarly, giraffes get taller each generation
Bibliography
Biology Department. (2008). "Chapter 3. Darwinian natural selection." Florida International University. Retrieved 7 April 2016. bioserv.fiu.edu/ortegaj/CHAPTER3_DARWIN_SPR.pdf
Bitesize. (n.d.). "Natural selection." BBC. Retrieved 6 April 2016. www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/understanding_environment/natural_selectionrev3.shtml
Math & Science. (2011, Aug 25). 12.2.2 darwin's Principles of Natural Selection [Video file]. Retrieved 7 April 2016. https://www.youtube.com/watch?v=jQf1owiEje8
Pujari, S. (n.d.). "4 main theories of evolution." Your Article Library. Retrieved 5 April 2016. www.yourarticlelibrary.com/biology/4-main-theories-of-evolution-explained-with-diagram-and-tables-biology/27220/