Chapters
In this article, we will discuss how selection, the founder effect, and genetic drift, including the bottleneck effect, may affect allele frequencies in populations.
Changes in Allele Frequencies
Due to evolution, allele frequencies change over time. Natural selection can be the cause of this change. Selection pressures boost the chance that some individuals within specific alleles survive and reach reproductive age. Thus, they are able to pass on their alleles to their offspring. Apart from natural selection, allele frequencies also change because of chance. A process called genetic drift can cause this change.
The founder and bottle net effect are the other processes that cause changes in allele frequencies because of the chance events.
Natural Selection
Directional selection occurs with the rise of the new allele in a population. It can also take place because of environmental change. Due to directional change, there is a gradual change in allele frequencies over many generations. Within a population, there is a phenotypic variation. The selection pressure is in the environment. For instance, the presence of a predator is a selection pressure.
In a population, few individuals can have a phenotype that helps them to survive in the presence of selection pressure. Some specific alleles produce this phenotype. Individuals having the beneficial phenotype are fitter and have a greater chance to reproduce and pass on these beneficial alleles to their offspring. On the other hand, the chance of survival of the individuals who do not have these advantageous alleles or phenotypes is quite low, so they are less likely to pass their alleles to their offspring. In this way, over several generations, the frequency of beneficial alleles increases as compared to the frequency of other alleles.
Genetic Drift
In a smaller population, chance may affect the type of alleles that get passed on to the next generation. Meiosis leads to haploid gametes which imply that a fertilization event only passes on half of the alleles of an individual. The half that is passed on is a consequence of random fertilization and the other half of the alleles may get lost to the next generation.
With time and over generations, few alleles can get lost or passed on purely due to chance. This is referred to as genetic drift which is more likely to affect the frequencies of an allele in a small population.
Genetic Drift Example
Suppose there is a small population of five plants that are grown near a playground having a rubber floor. Three of the five plants have blue flowers and two of them have pink flowers. Due to chance, the majority of the seeds from the pink-flowered plants grew on the rubber floor of the playground. On the other hand, blue-flowered plants grew on fertile soil which allowed them to germinate and grow.
Remember that the pink flower seeds do not fall on the impermeable surface just because of the chance. If this occurs due to the change over many generations, then the allele of the pink flowers may get lost from this population.
The Founder Effect
The founder effect takes place when a small number of individuals from a large parent population begin a new population. The change can give rise to a founder effect. For instance, a storm event that happens due to chance may be held responsible for separating a small group of individuals from the original population.
Because of the chance, the new population is composed of only a few individuals, hence only some of the total alleles from the parent population will be there. In other words, the smaller population does not contain all the gene pools. Genetic drift can affect the smaller population more as compared to the main population.
Example of the Founder Effect
The majority of the Caribbean islands inhabit anole lizards. These lizards can move from one island to another through vegetation or floating debris. Due to a chance event such as a large ocean wave or storm, a small number of lizards may get separated from the main population on a larger island and taken away to a smaller island.
The small number of lizards arriving at a new island may only have a small selection of alleles between them. On the other hand, several more alleles are present in the population of the lizard on the original island. For instance, the lizards on the main island can exhibit scale colours ranging from white to yellow and the small number of lizards that arrived on the other island may have white scales. It implies that the entire population that will grow on the smaller island will have white scales as compared to the larger island which has a mixture of yellow and white scaled lizards.
If the yellow allele was recessive and occurred as a single copy in the small population of lizards that arrived on the smaller island due to chance, then there is a greater chance that this allele will get lost as a result of the genetic drift.
The Bottleneck Effect
The bottleneck effect resembles the founder effect and takes place when previously a larger population suffers a tremendous fall in numbers. The number of individuals in a population can be greatly reduced because of the major environmental event.
This event in turn decreases the genetic diversity in the population because alleles are lost. As a result, the surviving individuals of a population may breed and reproduce with close relatives.
Bottleneck Effect Example
A genetic bottleneck effect can be observed in cheetahs today. Approximately, 10,000 years ago, there was a large and genetically diverse population of cheetahs. The dramatic change in the climate killed the majority of the population at the end of the ice age. Consequently, the surviving cheetahs got isolated in smaller populations and lots of inbreeding took place. It implies that the cheetah population today lacks genetic variation.
These poses are a serious threat to conservation because the genetic variation within species increases the likelihood of the species responding to an event of any environmental change. We already know that the selection pressure on the organisms is exerted by the environment.
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