Episode 41 recAPs the random side of evolution.
Episode 41 recAPs the random side of evolution. Melanie begins with a vocab review - allele, gene, heterozygous (1:00). This episode is about population genetics because populations are the level of ecological organization where evolution occurs (1:40). Mutations are the primary source of genetic variation, providing new phenotypes on which natural selection can act (2:40). It's important to distinguish between gene flow and genetic drift (3:24). Two types of genetic drift are the bottleneck effect (4:10) and the founder effect (4:50).
The Question of the Day asks (6:30) “What do the p and q represent in Hardy-Weinberg equilibrium?
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap Population Genetics
Lets Zoom out:
Unit 7 - Natural Selection
Topic - 7.4
Big idea - Evolution
Evolution is random right? There needs to be some meat and potatoes for natural selection to act upon, but how we get to be what we are has a good amount of random to it. Look around - who is a part of your population?
Lets Zoom in:
Just in case we are a little rusty on vocab -
Alleles are alternate forms of a gene, a piece of DNA that codes for a trait. Alleles are found at the same place, or locus, on a chromosome. As a eukaryotic organism with homologous chromosomes - you inherit two alleles for each gene. There might be more than two alleles out there in the gene pool, but you can still only get two. One from Mom and one from Dad. We represent these as capital and lowercase letters in genetics problems (sometimes superscripts - but that's another episode). So if you inherited a capital A from mom and a lowercase a from dad - your genotype is heterozygous and you would express the dominant capital A for Awesome. Congrats.
This episode is about population genetics because populations are the level of ecological organization where evolution occurs. First you have your organism, then population, and beyond that - community and ecosystem. It's a hierarchy. By definition, a population is a group of organisms of the same species living in the same area. Close quarters drive competition, causing differential survival and allele frequency changes over time. Lets recap some of those random things that can influence the population gene pool.
First up, and the unsung hero of evolution - mutations! Mutations in and of themselves aren’t negative - they are just genetic changes. The change may have an effect on the nucleic acid sequence, resulting protein, and phenotype. Recall that the mRNA is translated by codons, three base pair sequences long. First - the mutation in question has to be within the coding region of a gene to have effect. Second, the location of the change within a codon matters. If the mutation changes the amino acid sequence of the polypeptide and alters the structure and function of the protein, then it affects the phenotype of the organism.
There are many reasons for mutations - including errors in DNA replication, environmental or chemical interactions, or more commonly, random. Mutations are the primary source of genetic variation, providing new phenotypes on which natural selection can act. Genetic changes that enhance survival and reproduction can be selected for by environmental conditions, leading to adaptations. Check out episode 40 for our pesticide example of natural selection.
On deck, genetic drift. Organisms move around all the time - some on purpose and with the seasons as more of a migration pattern. This is known as gene flow and introduces new alleles into other populations through immigration or emigration. But if an existing population is very small, or lacks genetic diversity - geographic movement or catastrophic event can isolate alleles from the rest of the group. Gene flow and genetic drift are distinct. Genetic drift is a nonselective process that occurs in small populations with two main categories.
The first type of genetic drift is the appropriately named bottlenecks effect. When you look at the structure of a bottle, it narrows at the neck. If you were to tip it upside down - only a small amount of material would come out at one time. The bottleneck effect occurs when there is a dramatic and sudden decrease in population size. This can be caused by environmental factors such as fires, droughts, disease, floods or human factors like the illegal poaching of animals. The remaining surviving population has a random sampling of genes. These remaining alleles increase in frequency, as the population reproduces. The American Bison is an example of the bottleneck effect. There were once millions of bison in North America that were brought to near extinction in the1880s from hunting. The current bison are descendants from less than 100 ancestral survivors.
The second type of genetic drift is the founder effect. With this process, a very small group of individuals migrates away or becomes isolated from the original population and settles in a new geographic area. Unlike with the bottleneck effect, the original population remains intact. And typically, the genes are not randomly sampled from the main population. So the founder effect is when a population is descended from a small number of colonizing ancestors. This is essentially how the United States was populated. Even calling it the 13 colonies. We often don’t think of ourselves as part of evolution’s story - but we are right there in the thick of it. Allele frequencies changing over time.
Both the founder and bottleneck effect result in reduced genetic diversity. And reduction of genetic variation within a given population can increase the differences between populations of the same species. Get enough genetic drift and you are looking at speciation, or the emergence of a new species. Galapagos Islands and Darwin’s finches anyone?
We would be remiss if we didn’t give a shout out to Mr. Paul Anderson of Bozeman Science. He is an amazing resource for anyone interested in biology. Make sure to check out his TedEd collaboration titled the “5 fingers of Evolution.” If you don’t meet any of these conditions - then your population is at Hardy-Weinburg equilibrium. A topic for another time.
To recap….
Evolution is characterized by a change in the genetic makeup of a population over time. Much of the genetic change is random - through mutation, migration and genetic drift. Your own genes may be the result of a founder effect.
Coming up next on the Apsolute RecAP Biology Edition: Evidence of Evolution
Today’s Question of the day is about equilibrium.
Question “What do the p and q represent in Hardy-Weinburg equilibrium?”