Darwin's Logic: Ingredients of Natural Selection
Fact 1: All species have great potential fertility - population size would increase exponentially if all individuals born reproduced successfully.
Fact 2: Populations display stability.
Fact 3: Natural resources are limited. Resources remain constant in a stable environment.
Fact 4: Populations have variability.
Fact 5: Variation is heritable.
From these, we get the four postulates of Darwin's Theory of Evolution through Natural Selection.
Zimmer, Carl and Emlen, Douglas J. Evolution: Making Sense of Life. Greenwood Village, CO: Roberts and Company, 2013. Print.
1) Individuals within populations have genetic variation.
2) The variations are heritable.
3) Differential survival and reproduction must exist. More offspring are produced than survive.
4) Individuals exhibiting the more favorable, inherited variation have an advantage to survive and reproduce successfully- these are naturally selected.
This leads to evolution because it changes the population's frequency of alleles.
This video does a good job of explaining Natural Selection in a very basic way. A lot of the examples can be compared to the examples we discussed in class. Instead of the turtles in the video, we discussed the finches on the islands.
Link on Darwin's theory
Here is a website explaining these requirements with an awesome cartoon picture included.
Here is a lab simulation that helps showcase the relationship between natural and sexual selection.
It is possible for natural selection to select for a trait that is not necessarily in the organism's best interest as a population, this leads to what is known as a "Tragedy of the Commons."
"Tragedy of the Commons" → depletion of shared resources by individuals who are acting in their own self interest, despite their knowledge that depleting the common resource is detrimental to everyone in the long term.
An extreme tragedy of the commons can result in the extinction of a population. This concept is called evolutionary suicide. Natural selection can select for an adaptation of a trait that may lead to the endangerment and/or extinction of a population; this is an example of the "Tragedy of the Commons" as detailed above.
This "Tragedy of the Commons" though may be averted if selection does manage to switch to a phenotype that is non lethal, either through chance mutation or enhanced survival and frequent mating of the desirable non lethal phenotype.
Unit of selection: populations
How Population Size Affects Selection:
In a small population, genetic drift has a greater impact than selection. Why?
If your sample size is small then you have a non-representative sample of the entire population. By having a small sample size the probability of having allele frequencies that deviate strongly from the frequencies of the entire population are high. Therefore, the greater your sample/population size, the closer your allelic frequencies match that of the entire population and the greater impact selection has. This increases the allelic variety within a population, which is useful if a need to evolve arises.
**It is also important to note that just because a population experiences selection it does not necessarily mean it's evolving in response to it. Whether a population is evolving in response to selection depends on the phenotypic variation that the selection causes and the ability to transmit these phenotypic characteristics to offspring.
Here is an article by E.J Eisen that studies population size and selection in mice!
Measuring Natural Selection
Natural selection is not something that is only observable over large time periods. As long as Darwin's postulates are not violated a field exercise can study the strength of selection of a specific species for traits such as survivorship. 3 main ways to do this would be to-
1. Sampling from a population within a generation
2. Comparing live versus dead individuals in a population
3. Non-random mating
**It should also be noted that episodes of natural selection can be associated with seasons or events like droughts or floods so it's not always measurable.
More info is provided in the link at the bottom of the page
Natural Selection in Bacteria