Introduction
Welcome to AP Biology Topic 7.4: Population Genetics! So far in Unit 7, we’ve reviewed the factors that drive evolution, natural and artificial selection, and genotypic and phenotypic variations. In 7.4, we’ll build on what we previously learned and specifically discuss the different phenomenons that affect the makeup of population genetics.
Overview
The genetics that make up a population seems like a complex concept. To truly grasp this, we need to start with understanding the basics: what a population is, what makes up a gene pool, and microevolution.
A population is a group of individuals of the same species that live in the same area and interbreed to produce fertile offspring. For example, the global human population of 8-10 billion people only includes humans, because we’re only able to produce viable offspring with humans. The population of the United States has roughly 350 million people, because this only includes all of the above that lives in the United States.
A gene pool is the genetic makeup of an entire population, consisting of all copies of every type of allele. The gene pool for all humans would consist of all alleles for our genes, such as every allele for every eye color, hair color, etc.
If there’s only one allele present for a particular locus–the physical location of a gene on a chromosome–, then it is fixed. For humans, fixed alleles typically code for crucial biological processes that, if varied, would be fatal, such as the alleles that code for the cytochrome and ribosomal proteins.
Imagine a jar of 100 marbles, with half being red and the other blue. Think of these marbles as individuals with a specific allele that codes for their color. Now imagine that this population lives in a dark environment. In 10 years, it’s likely that a significantly larger portion of the marbles will be red compared to blue, because the trait of being red helps them camouflage. Because the portion of the allele coding for the red marbles significantly increased while that of the allele coding for blue marbles decreased, we have evidence of evolution. This is because changes in gene pools and allele frequencies provide evidence of evolution in populations, which is why the gene pool is so important in studying evolution.
Not all evolution is drastic. Do you ever wonder why we have to get a new flu vaccine every year? It’s because the disease evolves and can become resistant to the previous year's vaccine. Think of it: the vaccine kills most of the virus, but some can survive because they have genes that make them resistant to the vaccine. These resistant viruses can reproduce and make an entire population that's resistant to the vaccine, so scientists are tasked with making a whole new vaccine. This phenomenon is microevolution, or the small-scale genetic changes that occur within a population.
While natural selection is a major mechanism of evolution, evolution itself is driven by random occurrences, such as mutations, genetic drift, the bottleneck effect, the founder effect, and migration, that can affect the genetic makeup of a population.
Mutations
As a review, mutations are changes in the genetic material of a cell that can alter its phenotype, such as the mutation for down syndrome that codes for a trisomy on chromosome twenty-one.
Mutations are the primary source of genetic variation and can result in the formation of new alleles. A harmful mutation could be fatal or harmful to the quality of one's life, such as cystic fibrosis or albinism. Some mutations have no effect, such as silent mutations or ones that don’t alter protein synthesis. Some can even be beneficial, such as lactose tolerance and increased bone density.
Because natural selection can act on varied phenotypes, mutations can lead to evolution and changes in allele frequency. For example, if a mutation led to humans being stronger and more resilient to illness, it would increase the frequency of the allele and ultimately lead to human evolution.
Genetic Drift
The gene pool of a population can shift due to natural selection, or sometimes, purely by chance. Genetic drift describes chance events that cause a change in allele frequency from one generation to the next.
Genetic drift has a more substantial effect on smaller populations because random events have a greater proportional impact on a smaller population than that of a larger population. Genetic drift can increase or decrease genetic variations, can cause harmful alleles to become fixed, and doesn’t produce adaptations.
Genetic drift can allow a small population to diverge from other populations of the same species and evolve into a separate species. There’s two forms of this: the bottleneck effect and the founder effect.
The Bottleneck Effect
Imagine if I took a big scoop from the jar and took 20 blue marbles and 40 red marbles. There would be only 30 blue marbles and 10 red marbles left, causing a shift in the gene pool proportions. This phenomenon is known as the bottleneck effect, which is when a large population is drastically reduced by a non-selective disaster, such as floods, famine, fires, hurricanes, or hunting, for at least one generation.
Oftentimes, this can lead to some alleles being overrepresented, underrepresented, or absent. With the marbles, this caused the blue alleles or marbles to be overrepresented, leaving the red alleles underrepresented.
The Northern Elephant Seal is a real-life example of the bottleneck effect. Overhunting in the 1980s drastically declined the population, reducing it to nearly 20 individuals. Although the population has since increased, the gene pool remains with limited genetic variation.
The Founder Effect
What if, let's say, four marbles, three red and one blue, got bored of being in the jar and instead, rolled over to a drawer. These marbles will reproduce and have a different gene pool than the original population. This is an example of the founder effect, which occurs when a small group of individuals becomes isolated from a larger population and establishes a new, smaller population with a distinct gene pool, resulting in a loss of genetic diversity. The frequency of genes and traits will shift in response to the genes present in this new founder population.
A real-life example of a potential founder effect is the arrival of the Mayflower in the United States in 1620, which carried only 102 passengers, of whom 53 survived. If this colony were left alone without immigration, its gene pool would have been significantly limited, and genetic characteristics would have been magnified in future generations.
Migration
When we move, we meet an entire community of new people and organisms that have different genetics than us. Migration of species can result in the exchange of genes, in a process known as gene flow. Additionally, when individuals move into a new area and interbreed, they introduce new alleles and gene combinations that may not have previously existed in that local gene pool. In this way, migration serves to increase genetic diversity.
Gene flow refers to the transfer of alleles into or out of a population through the movement of fertile individuals or gametes. Gene flow between two populations prevents them from diverging into separate species. A real-life example of gene flow is bees carrying pollen from one flower to another, thereby transferring genetic material and alleles between populations.
Summary
We evolve through mutations, but not all mutations lead to evolution. Some of these mutations can create phenotypes that are harmful, neutral, or beneficial. Changes in a population's gene pool are due to mutations, genetic drift, whether it’s the bottleneck or founder effect, and migration. Changes in allele frequencies provide evidence for the occurrence of evolution in a population.