Darwins 4 Postulates
Why do species evolve?
Offsprings are of the same parents are not identical, even if some seem to be identical we now know they are different because we can compare DNA. Sexually reproducing organisms get genes from 2 different parents and this creates variation. So organisms of the same species may still have different colours, shapes, etc.
Variations are hereditable.
Some differences in traits are passed on from parents to their offsprings. Parents with long tails may produce an offspring that also carries that trait. Traits may change because of mutations but they are also passed down to later generations.
Individuals vary in survival and reproductive success.
Not all organisms are equally likely of surviving and reproducing.
Survival and reproduction are NOT random.
Some variations between organisms make them more or less likely to survive and reproduce. This is not random because some traits are responsible for an organisms survival or reproductive success. Organisms that are not well enough suited to their environment will go extinct because they won't be able to adapt.
These two flowers belong to the same species and have variations. One flower has a mutation that makes it look different. If the flower with the crooked petals survives and gets pollinated then it might produce offsprings that express the same trait. There is also a possibility that it won't reproduce as much or not at all. However, their reproductive success is not random.
Bees or other insects are more likely to be attracted to the "nicer" flower, therefore those are more likely to reproduce and become more abundant. The mutation for the ugly looking flower has more trouble passing to another generation and that is why they are not as numerous.
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M. (n.d.). Phenotypic Variation Explained. Retrieved April 04, 2016, from http://nov79.com/mr/exph.html
Phillip, M. (1997). “Evolutionary Genetics”. Population and Evolutionary Genetics. North Dakota State University. Accessed on 2016, April 4. Retrieved from https://www.ndsu.edu/pubweb/~mcclean/plsc431/popgen/popgen4.htm