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Section 24_1: Genes in Populations

Page history last edited by Ashish Murthy 11 years, 1 month ago

A. Learning Objectives:

• Explain the Hardy–Weinberg principle.

• Describe the characteristics of a population that is in Hardy–Weinberg equilibrium.

• Outline the differences between allele frequency and genotype frequency and how each contributes to the Hardy-Weinberg equation.

• Compare and contrast various micro-evolutionary forces.

B. Section 24.1 Summary:

This section, 24.1, is called Genes in Populations. It deals with population genetics, which is the study of genes and genotypes within a population. Population Genetics helps us understand why genetic variation within a population occurs, how it changes between generations of the same population, and how genetic variation can lead to varying phenotypes.

During this section, we'll be discussing Gene Pools, which are all of the distinct alleles for genes within a given population. and how variation within a gene pool affect an entire population. This section also delves into exactly what is a population is, looking at where populations are located and how size affects the amount of genetic variation within a population. A population is defined as a group of individuals of the same species who live within the same environment and can breed. Many populations can be subdivided into smaller groups, leading to varying genotypes and phenotypes between different groups within a single population.

Section 24.1 also covers the topics of polymorphism, monomorphism, and Single-nucleotide-polymorphism, examining the differences between the 3 and giving examples of each. The section material is supplemented by real life applications of the topics covered. Polymorphism is when there are 2 or more alleles occurring at a frequency over 1% for a single gene in the gene pool of a population. Monomorphism is when there is 1 allele occurring at a frequency over 99% for a single gene in the gene pool of a population. A single-nucleotide-polymorphism (SNP) is when the changing of a single nucleotide causes polymorphism within a population. An example of this is sickle-cell anemia, which is caused by a SNP in the β-globin gene. The section goes into great detail about how these morphisms create genetic diversity and thus contribute to evolution, and why it is a good thing.

Another interesting concept from this section is that of "Micro-evolution". Micro-evolution is the changes in a gene pool between different generations of the same population. It can be caused by two types of Micro-evolutionary forces: new genetic variation, and evolutionary mechanisms. New Genetic Variation includes mutations, horizontal gene transfer, exon shuffling, and gene duplication. These forces rarely have large effects on the Gene pool. Evolutionary mechanisms include Genetic Drift, Non-random Mating, Migration, and Natural Selection. These forces often have large effects on a population, and can create widespread genetic changes between different generations of a population. This is going to be an important topic in the virtual lecture.

The most important idea from this section is the Hardy-Weinberg Principle. It relates allele frequency and genotype frequency in a single equation. Allele frequency is (the number of copies of 1 allele for a given gene in 1 population)/(the total number of copies for any allele(s) for a given gene in 1 population). Genotype frequency is the (number of individuals with a particular genotype within 1 population)/(the total number of individuals within 1 population). The Hardy-Weinberg Principle uses allele frequency to determine genotype frequency. Though in theory this principle is correct, it is not 100% accurate due to processes like natural selection and random mutations. This will be the main focus of the virtual lecture.

C. Useful Materials:

	This video is quite remarkable. It is a lecture by a professor from U.C. Berkeley. It deals with the Hardy-Weinberg Principle, and the concept of the Hardy-Weinberg Equilibrium. It does an excellent job of explaining how the principle can to develop, and how it is applied. Skip to 18:00 to start the discussion of Hardy-Weinberg. It also explains when this Hardy-Weinberg Principle fails. Due to processes like Natural Selection and Genetic Drift, the professor explains, this equation won't always work. He uses real life examples, like a crime-court-case, to show how this stuff is actually important. It was pretty cool to see him explain it. Check it out if you want to see more of the history and applications of this principle.
	I really like this video because it's not some 50 minute lecture that doles over the same exact concept a million times, an bores everyone to death. This video is short an sweet, explaining the concepts fast and thoroughly without wasting time. This Mini-lecture gives multiple examples of Micro-evolution through Genetic Drift. He uses examples of beads to introduce the concept, and then moves on to using animal and human populations to drive it home. Genetic Drift was one of the hardest topics for me to grasp. But for whatever reason, seeing this lecture made me understand it a lot more than the textbook did. It helped to see visuals of the "Bottleneck Effect" and the "Founder Effect" in different scenarios. Mr. Lima may speak a bit fast, but he did a good job in this video. Be sure to view it, it's really short, and if it helped me out then there's a possibility that it can help you out as well.
Polymorphism and Microevolution	Typically I lean towards visual materials, like videos and graphs as a source of learning, but this paper helped me just as much as a video would. It explained the differences between polymorphism and monomorphism. It talks about how often genes are monomorphic and polymorphic, and differentiates between polymorphism in animals and plants. It also includes information about restoration (which is not a focus point of this section), so if this section really interests you, this is a good article to read up on too gain extra knowledge. It also discusses some micro-evolutionary forces. Much like the video above, it touches on genetic drift, but also explains genetic flow and talks about how migration rate and effective population size affect genetic variation and breeding. The article did a good job teaching me about how these forces can lead to changes in allele frequency. This is a very important concept to grasp, as allele frequency is essential to the Hardy-Weinberg Principle, which is central to section 24.1. Overall, I believe that this article can help you understand this section better and expand your knowledge beyond the scope of this section.
Hardy-Weinberg	In case the uber-long lecture video wasn't enough for you, here's more! This article is yet another explanation of the Hardy-Weinberg Principle. This concept is so essential to this section, so it is imperative that you understand this topic. The article talks a lot about the difference between expected outcomes of breeding between certain individuals and a population as a whole. It gives examples of using the principle to determine expected genotype frequencies. It also does an excellent job of explaining the Hardy-Weinberg Equilibrium. The allele frequencies of the equilibrium in the article's example. equilibrium are .8 and .2, and the genotype frequencies are 64%, 32%, and 4%. Once reached, they will repeat infinitely (assuming evolutionary forces don't disturb the equilibrium). Something I like about this article is that it goes beyond just stating when this principle won't work (like when natural selection and genetic drift occur). It actually explains how natural selection affects the equation and shows you how to calculate genotype frequencies based on a selection coefficient. Check it out if you want to expand your knowledge on this principle.

C. Primary Literature:

Population Genetics of an Ecosystem-Defining Reef Coral Pocillopora damicornis in the Tropical Eastern Pacific

This article is all about the reef coral Pocillopora damicornis in the Tropical Eastern Pacific. This reef coral is found in multiple places around the world, but has different functions relative to each respective ecosystem it's located in. There was not a lot of information about the population genetics of this reef coral prior to this study. The researchers knew there was limited gene flow, but wanted to know more. In order to do this, the researchers looked at 9 different populations of reef coral in 3 distinct regions of Panama.

The findings were significant. They confirmed that there was limited genetic flow, but also pointed to other information. The researchers learned that the species had the potential for differential adaptation, meaning that even though there was limited gene flow, the species could adapt to different environments quite easily. This corroborated the finding that there was high genetic diversity among different populations. Since the Tropical Eastern Pacific is so isolated from other ecosystems, it makes sense that it's environment would differ significantly from other places; thus, only species that could adapt quickly (like Pocillopora damicornis) would be able to thrive in this environment.

The article discusses other aspects of population genetics that we discuss in Section 24.1, like allele frequencies. It also talks about aspects we don't touch on, like clonality. It explains how the different findings about the population genetics allow it to be a minor player in some ecosystems, and a major reef builder in others. The article talks about the tendency of this coral to inbreed, reducing gene dispersion. It even compared the population genetics of this coral in the Tropical Eastern Pacific to those in the Indo-West Pacific. All of these findings were used to determine how to manage the ecosystem of this reef coral. In conclusion, this article took an in-depth look at the topics we cover in Section 24.1 and more.

E. Virtual Lectures:

Part 1: