Selection in MicroevolutionNatural Selection in microevolution has 3 main types. (FYI, Microevolution is change over time within populations.) Here's a great overview visual I found:
And here is a closer look at each.
Directional selection can cause (eventually) loss of alleles, because more advantageous alleles are being selected for. In these graphs, you can think of the heterozygotes (Aa) as being in the middle of the curve, and both types of homozygotes (AA / aa) on either end. So if you are losing an allele, you're essentially losing the homozygotes on one side, AND losing the heterozygotes. The allele that gets selected for can eventually become the only option. This is known as "fixing" that allele. So your distribution shifts in one particular direction, like so:
Stabilizing/ Balancing Selection
Instead of heterozygosity decreasing as it does in directional selection, with stabilizing selection it increases. The curve becomes taller as there are fewer homozygotes (aa/ AA) on the ends, and more heterozygotes (Aa) in the middle. This maintains diversity because both alleles are still in the population.
Heterozygote advantage is a term used in this kind of situation. Selection is acting to increase the number of heterozygotes, so for some reason this genotype has advantages to survival and reproduction. An example would be in Africa with sickle cell anemia. If you are homozygous for the disease, your red blood cells can't function and it is very detrimental. But if you don't have any of the disease you're more susceptible to malaria. Heterozygotes win in this case because they aren't as affected by malaria since they have some sickle cells, but they can also function well with the good red blood cells they have from the one dominant allele.
Diversifying/ Disruptive Selection
This type of selection also maintains the diversity of multiple alleles, but this time, heterozygotes are selected against. There could be a barrier to gene flow that is causing two populations of a species to diverge in this manner. An example is peppered moths that diversified into white and black depending on the environment they lived in. You can read about and even play a simulation here: http://www.biologycorner.com/worksheets/pepperedmoth.html
Speciation is a common consequence of this type of selection.
Negative Frequency-Dependent Selection
This is a matter of "the more of you there are, the crappier job you do". Too many cooks in the kitchen. As a population size increases, their fitness will decrease. In other words, population size and fitness have an inverse relationship.
|Scarlet King Snake (mimic) and Eastern Coral Snake (poisonous) Source|
Positive Frequency-Dependent Selection
As one goes up the other does as well. I couldn't find a graph of this but it's the same as a positive correlational relationship. (As one increases, so does the other.) If the x- axis is one species, the y-axis would be another.
A good example of Positive Frequency-Dependent Selection is mullerian mimicry. These are the"truth telling" species in which they are both venomous/ toxic. In the example below, the 3 frogs on the top row are all from the same species. But they have had very different coloration phenotypes selected for. They have each converged on a pattern that is strikingly similar to other frog species that live in their same area and are also toxic. Each of these sympatric species are pictured on the second row. (Click "source" link below for the species' names and more info.)
|Mullerian Frogs - Source|
Selection in Molecular Evolution
Neutral Evolution is just the absence of Natural Selection at the molecular level. It is the "null hypothesis" which states no selection is happening (similar to the Hard-Weinberg Equillibrium which is a null hypothesis for no selection at the population genetics level).
Increase frequency of adaptive traits. In other words, this is a term to differentiate active selection from neutral evolution. This violates the Neutral Evolution "null hypothesis".
Negative / Purifying / Background Selection
(This term or category has given me some confusion. Many sources have listed them all as being synonymous, but I'm not so sure. So this is my best understanding to this point and I am waiting for confirmation from an Evolution expert. Just FYI.)
Negative Selection is getting rid of deleterious alleles- actively selecting against them and weeding them out. I believe it's a more general term like Positive Selection is. But it can also be used to refer to Purifying/ Background
Purifying Selection and Background Selection are synonymous. It is specific to molecular evolution. It is the process in which there is a gene locus that is highly conserved- it's been around a long time and will stick around a long time. Basically I think of it as something that works really well and is crucial to the organism. If it gets tampered with it will break and not be able to even get passed on to another generation. So any of these little tweaks don't stick, and you end up with a highly specific and conserved bit of DNA. It "purifies" the gene by selecting against extreme values of the character (see this Wikipedia page). It is a common type of stabilizing/ balancing selection.
Mechanisms of MacroevolutionFYI, in case you are wondering about Macroevolution, I decided to let you know about that as well. Selection acts on populations and genes, but with accumulation of enough of these changes, you can get Macroevolutionary changes: evolution that is above the level of a species. This is the stuff that people normally think of when they think "evolution".
This is when a lineage splits into two different species, or a new species is otherwise defined. When enough changes accumulate or certain barriers are in place, a group can become speciated from another, so they no longer are considered the same species. Pretty self-explanatory.
Adaptive Radiation is when one lineage diversifies quickly into many species. This happens as a result of a number of open niches to be filled. The niches could have been recently opened up by others going extinct (like the Dinosaurs), changes in the environment, moving to a new area and colonizing it (such as islands), or if a lineage evolves a new trait (such as flight) that gives them a great advantage or takes them into previously empty niches.
I'll be very unimaginative and point out the class example of Galapagos Finches. It's always a great example. :)
|Galapagos Finches, example of Adaptive Radiation due to colonization Source|
Coevolution is when two species drive the evolution of one another. A perfect example is host/ parasite. They are constantly having to try to "outwit" each other evolutionarily. Other great examples are a flowering plant and their pollinator, mimic species, predator and prey.
When more than one species converges onto the same characteristics even though the characteristic didn't evolve from a common ancestor, this is convergent evolution. The body forms of sharks and dolphins are one example. It's often a product of similar adaptations to similar environments, as in these examples of mammals on different continents.
Hope you learned something about evolution and selection! Please leave a comment! Thanks. :)