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7.4 Population Genetics

5 min readjanuary 10, 2023

Samantha Himegarner

Samantha Himegarner

Jed Quiaoit

Jed Quiaoit

Samantha Himegarner

Samantha Himegarner

Jed Quiaoit

Jed Quiaoit

Evolution acts upon the genetic variety within a population, and genetic variety can also be driven by random occurrences.

Mutations

, for one, are a source of genetic variation within a population, and this variation is the raw material upon which natural selection can act. can arise from a variety of causes, such as errors in DNA replication, exposure to radiation or certain chemicals, and viral infections. 🧪

The vast majority of are neutral, meaning they have no effect on the organism's fitness. These neutral accumulate over time and can be used to infer the evolutionary relationships between different species.

However, some can have an effect on the fitness of an organism, and these are known as beneficial or deleterious . are those that increase an organism's chances of survival and reproduction, while decrease those chances.

The is a well-known example of how can occur in response to environmental changes. The moths changed the color of their wing from light to dark to better camouflage from predators in the industrial revolution.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-JRxSwGOTLzHs.jpeg?alt=media&token=1e90907c-7bfe-469f-a72b-ae3160cad314

Source: BioNinja

It's worth noting that the effects of are not always straightforward and can depend on the context. A mutation that is beneficial in one environment might be deleterious in another, or a mutation that is beneficial for one aspect of an organism's fitness might be deleterious for another. Overall, are an important driver of evolution and help to create the diversity of life we see around us today! 👶

Genetic Drift and Gene Flow

occurs due to random fluctuations in the frequency of (versions of a gene) in a population. These fluctuations can happen for a variety of reasons, such as chance events like a (e.g. ), which kills a significant proportion of a population and disproportionately affects certain individuals. Because of this, can lead to a loss of genetic variation within a population. 🌋

The effect of is more pronounced in small populations, because the random fluctuations in allele frequency are more likely to be large relative to the overall size of the population. This can result in the loss of certain from a small population, or even the fixation of (when an allele reaches 100% frequency in a population).

Another important mechanism of evolution is , which is the movement of genes from one population to another. can occur when individuals migrate from one population to another, bringing their genes with them. This can introduce new into a population and increase its genetic diversity. can also act to homogenize the genetic makeup of populations that are geographically close to each other, by reducing the frequency of rare in a population.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-gbs3MermdiLk.webp?alt=media&token=7d06dd21-068e-433e-acfe-673bc229e1fe

Source: Storyboard That

Both and can affect the genetic makeup of a population and therefore the evolution of a species, and both are different from natural selection that is driven by the environment. While natural selection can only act on the current phenotypic expression and the environment, the other two mechanisms act more on the genetic makeup itself. Understanding the different ways that evolution can happen is important for understanding how species change over time and how different populations are related to each other.

The Founder Effect and The Bottleneck Effect

The and the are both types of that can occur when a small subset of a population becomes isolated from the rest. 🍾

The occurs when a small group of individuals breaks away from a larger population and colonizes a new area.

This small group is likely to have a different genetic makeup from the larger population, due to chance events such as or genetic differences that existed prior to the colonization. As this small group reproduces and grows, the unique genetic characteristics that are present in the founders will become more prevalent in the new population. This can lead to the overrepresentation of certain and the loss of others, which can affect the allele frequencies of the new population compared to the original population.

The is similar to the in that it also occurs when a population becomes smaller, but it differs in that it is a result of a catastrophic event, such as a , disease outbreak, or human activity, that results in a reduction of population size. The can result in a loss of genetic variation in the population due to chance events, and the population will become less diverse than the original population.

Both of these effects can have a significant impact on the genetic makeup of a population and can lead to changes in allele frequencies, which can in turn influence the evolution of the population. It's also worth noting that these effects can also interact with natural selection: after the population has passed through the bottleneck or , the remaining genetic variation will be more strongly shaped by natural selection because of the loss of some genetic variants.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FBottleneck_effect_Figure_19_02_03.jpg?alt=media&token=22d51380-1ab5-46d5-9769-d6da0d71266a

Image courtesy of WikiMedia Commons.

is grounded in a similar concept, but with a less extreme outcome. It simply describes the phenomenon of how immigration (movement of individuals into a population) and emigration (movement of individuals out of a population) can affect allele frequencies as well. is more common and less disastrous, and helps to explain how and phenotypes can be added and removed from populations. 💀

Speciation

Together, and can lead to the separation of populations and the development of new species, a process known as . When populations are separated and can no longer interbreed, they will evolve independently, and over time, the genetic differences between the populations will become so great that the populations will be unable to produce viable offspring if they do interbreed. At this point, the populations will be considered separate species. Especially in an example with a founder’s effect, the two populations may begin to reproduce and evolve separately, each with their own unique favorable traits.

It's important to note that, this process can be gradual or rapid, and also it may be influenced by other factors such as environmental changes, and , leading to different modes of . ☁️

Key Terms to Review (14)

Beneficial Mutations

: Beneficial mutations are changes in an organism's genetic material that improve its chances of survival and reproduction.

Mutations

: Mutations are changes in the DNA sequence of a cell's genome. They can occur naturally during DNA replication or as a result of environmental factors like radiation and chemicals.

Deleterious Mutations

: Deleterious mutations are harmful changes in an organism's genetic material that decrease its chances of survival and reproduction.

Genetic Drift

: Genetic drift is a mechanism of evolution that causes random changes in allele frequencies within a population over time due to chance events.

Gene Flow

: Gene flow is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.

Peppered Moth Example

: The Peppered Moth Example is a classic case of natural selection where the population of moths in industrial areas of England changed from mostly light-colored to mostly dark-colored due to pollution. This change was because dark-colored moths were better camouflaged against soot-covered trees, making them less likely to be eaten by predators.

Natural Disaster

: A natural disaster is a major adverse event resulting from natural processes of the Earth. These include floods, hurricanes, tornadoes, volcanic eruptions, earthquakes, tsunamis and other geologic processes.

Alleles

: Alleles are different versions or forms of genes that are found at the same place on a chromosome. They can be dominant or recessive, and they determine different characteristics in an organism.

Volcanic Eruptions

: A volcanic eruption is the release of stored energy from below the surface of the earth due to tectonic activity. This energy is released as magma (lava), ash, gas and rock fragments.

Founder Effect

: The founder effect is a type of genetic drift that occurs when a small group of individuals becomes isolated from the larger population, leading to changes in the gene pool.

Speciation

: Speciation is the evolutionary process by which populations evolve into distinct species over time due to factors such as geographic isolation or differing selection pressures.

Bottleneck Effect

: The bottleneck effect is another form of genetic drift where an event drastically reduces the size of a population and consequently its genetic variation.

Adaptation to Different Environments

: This refers to how organisms change over time in response to their environment. These changes enhance their ability to survive and reproduce.

Competition for Resources

: This is a biological concept where organisms vie for the same limited resources in an environment, such as food, water, light, space or mates.

7.4 Population Genetics

5 min readjanuary 10, 2023

Samantha Himegarner

Samantha Himegarner

Jed Quiaoit

Jed Quiaoit

Samantha Himegarner

Samantha Himegarner

Jed Quiaoit

Jed Quiaoit

Evolution acts upon the genetic variety within a population, and genetic variety can also be driven by random occurrences.

Mutations

, for one, are a source of genetic variation within a population, and this variation is the raw material upon which natural selection can act. can arise from a variety of causes, such as errors in DNA replication, exposure to radiation or certain chemicals, and viral infections. 🧪

The vast majority of are neutral, meaning they have no effect on the organism's fitness. These neutral accumulate over time and can be used to infer the evolutionary relationships between different species.

However, some can have an effect on the fitness of an organism, and these are known as beneficial or deleterious . are those that increase an organism's chances of survival and reproduction, while decrease those chances.

The is a well-known example of how can occur in response to environmental changes. The moths changed the color of their wing from light to dark to better camouflage from predators in the industrial revolution.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-JRxSwGOTLzHs.jpeg?alt=media&token=1e90907c-7bfe-469f-a72b-ae3160cad314

Source: BioNinja

It's worth noting that the effects of are not always straightforward and can depend on the context. A mutation that is beneficial in one environment might be deleterious in another, or a mutation that is beneficial for one aspect of an organism's fitness might be deleterious for another. Overall, are an important driver of evolution and help to create the diversity of life we see around us today! 👶

Genetic Drift and Gene Flow

occurs due to random fluctuations in the frequency of (versions of a gene) in a population. These fluctuations can happen for a variety of reasons, such as chance events like a (e.g. ), which kills a significant proportion of a population and disproportionately affects certain individuals. Because of this, can lead to a loss of genetic variation within a population. 🌋

The effect of is more pronounced in small populations, because the random fluctuations in allele frequency are more likely to be large relative to the overall size of the population. This can result in the loss of certain from a small population, or even the fixation of (when an allele reaches 100% frequency in a population).

Another important mechanism of evolution is , which is the movement of genes from one population to another. can occur when individuals migrate from one population to another, bringing their genes with them. This can introduce new into a population and increase its genetic diversity. can also act to homogenize the genetic makeup of populations that are geographically close to each other, by reducing the frequency of rare in a population.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-gbs3MermdiLk.webp?alt=media&token=7d06dd21-068e-433e-acfe-673bc229e1fe

Source: Storyboard That

Both and can affect the genetic makeup of a population and therefore the evolution of a species, and both are different from natural selection that is driven by the environment. While natural selection can only act on the current phenotypic expression and the environment, the other two mechanisms act more on the genetic makeup itself. Understanding the different ways that evolution can happen is important for understanding how species change over time and how different populations are related to each other.

The Founder Effect and The Bottleneck Effect

The and the are both types of that can occur when a small subset of a population becomes isolated from the rest. 🍾

The occurs when a small group of individuals breaks away from a larger population and colonizes a new area.

This small group is likely to have a different genetic makeup from the larger population, due to chance events such as or genetic differences that existed prior to the colonization. As this small group reproduces and grows, the unique genetic characteristics that are present in the founders will become more prevalent in the new population. This can lead to the overrepresentation of certain and the loss of others, which can affect the allele frequencies of the new population compared to the original population.

The is similar to the in that it also occurs when a population becomes smaller, but it differs in that it is a result of a catastrophic event, such as a , disease outbreak, or human activity, that results in a reduction of population size. The can result in a loss of genetic variation in the population due to chance events, and the population will become less diverse than the original population.

Both of these effects can have a significant impact on the genetic makeup of a population and can lead to changes in allele frequencies, which can in turn influence the evolution of the population. It's also worth noting that these effects can also interact with natural selection: after the population has passed through the bottleneck or , the remaining genetic variation will be more strongly shaped by natural selection because of the loss of some genetic variants.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FBottleneck_effect_Figure_19_02_03.jpg?alt=media&token=22d51380-1ab5-46d5-9769-d6da0d71266a

Image courtesy of WikiMedia Commons.

is grounded in a similar concept, but with a less extreme outcome. It simply describes the phenomenon of how immigration (movement of individuals into a population) and emigration (movement of individuals out of a population) can affect allele frequencies as well. is more common and less disastrous, and helps to explain how and phenotypes can be added and removed from populations. 💀

Speciation

Together, and can lead to the separation of populations and the development of new species, a process known as . When populations are separated and can no longer interbreed, they will evolve independently, and over time, the genetic differences between the populations will become so great that the populations will be unable to produce viable offspring if they do interbreed. At this point, the populations will be considered separate species. Especially in an example with a founder’s effect, the two populations may begin to reproduce and evolve separately, each with their own unique favorable traits.

It's important to note that, this process can be gradual or rapid, and also it may be influenced by other factors such as environmental changes, and , leading to different modes of . ☁️

Key Terms to Review (14)

Beneficial Mutations

: Beneficial mutations are changes in an organism's genetic material that improve its chances of survival and reproduction.

Mutations

: Mutations are changes in the DNA sequence of a cell's genome. They can occur naturally during DNA replication or as a result of environmental factors like radiation and chemicals.

Deleterious Mutations

: Deleterious mutations are harmful changes in an organism's genetic material that decrease its chances of survival and reproduction.

Genetic Drift

: Genetic drift is a mechanism of evolution that causes random changes in allele frequencies within a population over time due to chance events.

Gene Flow

: Gene flow is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.

Peppered Moth Example

: The Peppered Moth Example is a classic case of natural selection where the population of moths in industrial areas of England changed from mostly light-colored to mostly dark-colored due to pollution. This change was because dark-colored moths were better camouflaged against soot-covered trees, making them less likely to be eaten by predators.

Natural Disaster

: A natural disaster is a major adverse event resulting from natural processes of the Earth. These include floods, hurricanes, tornadoes, volcanic eruptions, earthquakes, tsunamis and other geologic processes.

Alleles

: Alleles are different versions or forms of genes that are found at the same place on a chromosome. They can be dominant or recessive, and they determine different characteristics in an organism.

Volcanic Eruptions

: A volcanic eruption is the release of stored energy from below the surface of the earth due to tectonic activity. This energy is released as magma (lava), ash, gas and rock fragments.

Founder Effect

: The founder effect is a type of genetic drift that occurs when a small group of individuals becomes isolated from the larger population, leading to changes in the gene pool.

Speciation

: Speciation is the evolutionary process by which populations evolve into distinct species over time due to factors such as geographic isolation or differing selection pressures.

Bottleneck Effect

: The bottleneck effect is another form of genetic drift where an event drastically reduces the size of a population and consequently its genetic variation.

Adaptation to Different Environments

: This refers to how organisms change over time in response to their environment. These changes enhance their ability to survive and reproduce.

Competition for Resources

: This is a biological concept where organisms vie for the same limited resources in an environment, such as food, water, light, space or mates.