When Size Doesn't Matter: Microevolution, a Rambling

 

Microevolution is one of my favorite things to talk about in regards to evolution because, at least in my humble opinion, it contains the most raw, compelling data that you can observe with your own eyes. Most of the time you don't even have to draw any conclusions (an exaggeration of course, always question), you can just *see* that A became B! "Why", that's another story, but the fact remains that A became B and now they can do C instead of D. 

Microevolution can be defined as the change in allele frequency in a population over generations. The only requirements are that:

1) Microevolution cannot occur in individuals. There needs to be a change within the entire breeding population. 

2) The change needs to be genetic and able to be passed on from parent to offspring

3) There needs to be at least one generation between 

Some of the easiest and quickest ways to observe microevolution are in pathogens. Penicillin was invented in 1941, and in only 1942 a strain of Penicillin-resistant Staphylococcus aureus was identified. Antibiotic resistance occurs when the strain is not eradicated and allowed to either mutate or continue to populate using the individuals in the population that happened to possess the gene for resistance. This quickly causes a case of microevolution, the original strain has died but the new, resistant strain is left to repopulate, no longer the same entity as the one that died off. The new strain possess new genetic frequencies, might express different phenotypic traits, and would not or could not procreate with the previous generation if given the opportunity. 

http://modmedmicro.nsms.ox.ac.uk/learn-more-about-antibiotic-resistance/

A quick story: 

In the 1800s the trees in Brooklyn were being invaded by moth larvae. In order to preserve them some well-meaning but stupid citizens imported not-yet-seen House Sparrows to eat the larvae. The sparrows did their due-diligence and, with no natural predators, thrived. Over the years the House Sparrows have spread across the United States and can now be found in every nook and cranny; a nuisance, one might say.

The point:

House Sparrows were introduced less than 200 years ago. Though no one who remembers their introduction is still alive, there is well documented evidence of what they looked like initially. Today, however, House Sparrows come in a variety of sizes. When looking in warmer climates you'll find smaller birds. Alternatively, as the temperature averages colder, so do the birds average larger. They are all from the same base species, but this can be considered an example of microevolution as the bird adapts to it's current climate. This builds the barrier for speciation, as smaller birds tend to mate with smaller birds and vice versa, as well as the literal geographical/climatological barrier dividing them. Size is a trait that is passed down from parent to offspring: if you come from a line of very tall people and they only ever "mate" with other tall people, the offspring will continue to be tall. Therefore from this example we can see that:

- a heritable trait is being passed down

- natural selection is taking place 

- there is, at the very least, geographical speciation taking place

- House Sparrows in one area are averaging a different size than House Sparrows in another area and there appears to be a pattern related to climate, though it is still speculation. 

University of California Museum of Paleontology's Understanding Evolution (http://evolution.berkeley.edu).

Generally, microevolution begins from some sort of mutation. It's not "normal" for S. aureus to be resistant to Penicillin, but it also was not deadly. Because of this, the code was set in place for the generation to become resistant, allowing for the process of natural selection to take place. Polar bears didn't look outside one day, think to themselves, "Gosh, it's a bit chilly out!" and then *decide* to be Polar bears, with their perfect adaptations to maintain homeostasis. Sometime in the line there was a bear or bear-like creature whose genes happened to mutate, creating solar capturing tubes for fur. That bear or bear-like creature then went on to survive better and procreate, passing on its weird mutation that helped it stay warmer in sub-freezing temperatures. There always has to be a baseline for the evolution to work off of. Because of this, it's pretty easily seen and accepted in society. Yes, it would be weird for things to magically appear out of thin air, but when put in the perspective of "this thing was already here, there's just more of them now" it's a bit easier to digest. 

Links:

https://evolution.berkeley.edu/evolibrary/article/0_0_0/microexamples_01

https://www.nature.com/scitable/knowledge/library/speciation-the-origin-of-new-species-26230527/

https://examples.yourdictionary.com/examples-of-speciation.html

http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Passer_domesticus.htm

https://www.allaboutbirds.org/guide/House_Sparrow/overview

https://courses.lumenlearning.com/wmopen-nmbiology1/chapter/mutations-and-evolution/

https://www.cdc.gov/drugresistance/about.html