if you examine the gene pool of a population that is evolving due to genetic drift, what do you predict will occur with the allele frequencies?

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The Variety of Genes in the Gene Pool Can Be Quantified within a Population

Genes exist in multiple forms called alleles, which vary in quantity between different groups of organisms.

The Variety of Genes in the Gene Pool Can Be Quantified within a Population

Most populations have some degree of variation in their gene pools. By measuring the amount of genetic variation in a population, scientists can begin to make predictions about how genetic variation changes over time. These predictions can then help them gain important insights into the processes that allow organisms to adapt to their environment or to develop into new species over generations, also known as the process of evolution.

Genetic variation is usually expressed as a relative frequency, which means a proportion of the total population under study. In other words, a relative frequency value represents the percentage of a given phenotype, genotype, or allele within a population.

Relative phenotype frequency is the number of individuals in a population that have a specific observable trait or phenotype. To compare different phenotype frequencies, the relative phenotype frequency for each phenotype can be calculated by counting the number of times a particular phenotype appears in a population and dividing it by the total number of individuals in the population.Relative genotype frequency and relative allele frequency are the most important measures of genetic variation. Relative genotype frequency is the percentage of individuals in a population that have a specific genotype. The relative genotype frequencies show the distribution of genetic variation in a population. Relative allele frequency is the percentage of all copies of a certain gene in a population that carry a specific allele. This is an accurate measurement of the amount of genetic variation in a population.

Examining allele frequencies

A gene that can occur in two forms is said to have two alleles. Body color in fruit flies is an example of a gene with two alleles: a dominant allele for brown body color, and a recessive allele for black body color. The brown body color allele can be represented as "B" and the black body color allele as "b." The allele frequencies for a gene with two alleles are usually represented by the letters p and q, where the relative frequency of the B allele is p and the relative frequency of the b allele is q.

Symbolically, these relative allele frequencies can be represented as:

relative frequency (B) = p and relative frequency (b) = q

Remember the Punnett square?

Figure 1: A Punnett square showing how p and q alleles combine.

Figure Detail

If B and b are the only two alleles of a gene, then possible genotypes can be predicted by arranging the alleles in a Punnett square, in their p and q representation (Figure 1). This exercise can help to visualize the computation of relative allele frequencies and their corresponding relative genotype frequencies in a population.

The possible combinations can be represented mathematically as:

[p × p] + [2 × p × q] + [q × q]

or p2 + 2pq + q2

How can relative frequencies be used to study populations?

The mathematical expression p2 + 2pq + q2 can be used as a platform for understanding both allele frequencies and genotype frequencies in real populations. For instance, if a population does not change over time, then scientists can make certain predictions about its relative allele frequencies, and about its relative genotype frequencies. In other words, if they have information about its relative genotype frequencies, they may also make predictions about its relative allele frequencies.

When a population is in equilibrium, the BB homozygotes (individuals that carry the same two dominant B alleles) will have a relative genotype frequency of p2: freq (BB) = p2. Similarly, bb homozygotes (individuals that carry the same two recessive b alleles) will have a relative genotype frequency of q2: freq (bb) = q2. Finally, the Bb heterozygotes (individuals that carry both the dominant B allele and the recessive b allele) will have a relative genotype frequency of 2pq: freq (Bb) = 2pq.

In a stable population, the sum of all these relative genotype frequencies remains equal to 1 over successive generations. This is a mathematical way of expressing that the sum of all relative genotype frequencies always equals 1 because if one relative genotype frequency increases, another will decrease in tandem, and alleles become redistributed rather than increasing in proportion to the population. Therefore, this relationship can be expressed mathematically as follows:

This equation is known as the Hardy-Weinberg equation, and it defines a population in which relative allele frequencies do not change over successive generations. Such a population is said to be in equilibrium. This state of equilibrium represented by the Hardy-Weinberg equation is an ideal model against which to compare observed changes in relative allele and genotype frequencies in natural populations.

How is the Hardy-Weinberg equation used?

The Hardy-Weinberg equation describes a population at equilibrium. This can only occur in the absence of disturbing factors and when mating between individuals is completely random. When mating is random in a large population, both the relative genotype and allele frequencies will remain constant.

Source : www.nature.com

Genetic drift (article)

Evolution due to chance events. The bottleneck effect and founder effect.

Population genetics

Genetic drift

Evolution due to chance events. The bottleneck effect and founder effect.

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Key points

Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error).

Genetic drift occurs in all populations of non-infinite size, but its effects are strongest in small populations.

Genetic drift may result in the loss of some alleles (including beneficial ones) and the fixation, or rise to

100\% 100% 100, percent

frequency, of other alleles.

Genetic drift can have major effects when a population is sharply reduced in size by a natural disaster (bottleneck effect) or when a small group splits off from the main population to found a colony (founder effect).

Introduction

Natural selection is an important mechanism of evolution. But is it the only mechanism? Nope! In fact, sometimes evolution just happens by chance.

In population genetics, evolution is defined as a change in the frequency of alleles (versions of a gene) in a population over time. So, evolution is any shift in allele frequencies in a population over generations – whether that shift is due to natural selection or some other evolutionary mechanism, and whether that shift makes the population better-suited for its environment or not.

In this article, we’ll examine genetic drift, an evolutionary mechanism that produces random (rather than selection-driven) changes in allele frequencies in a population over time.

What is genetic drift?

Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to "sampling error" in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations.

Genetic drift example

Let's make the idea of drift more concrete by looking at an example. As shown in the diagram below, we have a very small rabbit population that's made up of

8 8 8

brown individuals (genotype BB or Bb) and

2 2 2

white individuals (genotype bb). Initially, the frequencies of the B and b alleles are equal.

Genetic drift at work in a small population of rabbits. By the third generation, the b allele has been lost from the population purely by chance.

Image credit: "Population genetics: Figure 2," by OpenStax College, Biology CC BY 3.0.

What if, purely by chance, only the

5 5 5

circled individuals in the rabbit population reproduce? (Maybe the other rabbits died for reasons unrelated to their coat color, e.g., they happened to get caught in a hunter’s snares.) In the surviving group, the frequency of the B allele is

0.7 0.7 0, point, 7

, and the frequency of the b allele is

0.3 0.3 0, point, 3 .

In our example, the allele frequencies of the five lucky rabbits are perfectly represented in the second generation, as shown at right. Because the

5 5 5

-rabbit "sample" in the previous generation had different allele frequencies than the population as a whole, frequencies of B and b in the population have shifted to

0.7 0.7 0, point, 7 and 0.3 0.3 0, point, 3

, respectively. [Do allele frequencies always match the sample?]

From this second generation, what if only two of the BB offspring survive and reproduce to yield the third generation? In this series of events, by the third generation, the b allele is completely lost from the population.

Population size matters

Larger populations are unlikely to change this quickly as a result of genetic drift. For instance, if we followed a population of

1000 1000 1000 rabbits (instead of 10 10 10

), it's much less likely that the b allele would be lost (and that the B allele would reach

100\% 100% 100, percent

frequency, or fixation) after such a short period of time. If only half of the

1000 1000 1000

-rabbit population survived to reproduce, as in the first generation of the example above, the surviving rabbits (

500 500 500

of them) would tend to be a much more accurate representation of the allele frequencies of the original population – simply because the sample would be so much larger. [Why would this be the case?]

This is a lot like flipping a coin a small vs. a large number of times. If you flip a coin just a few times, you might easily get a heads-tails ratio that's different from

50 50 50 - 50 50 50

. If you flip a coin a few hundred times, on the other hand, you had better get something quite close to

50 50 50 - 50 50 50

(or else you might suspect you have a doctored coin)!

Source : www.khanacademy.org

CH.14 Quiz Flashcards

Study with Quizlet and memorize flashcards terms like The collection of _____ in a population constitutes that population's gene pool., The proportion of alleles present in a gene pool is called the _____ and is an important tool in measuring evolutionary changes., In a population of 75 mice, what would be the size of their gene pool for a particular gene? and more.

CH.14 Quiz

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The collection of _____ in a population constitutes that population's gene pool.

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alleles

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The proportion of alleles present in a gene pool is called the _____ and is an important tool in measuring evolutionary changes.

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allele frequency

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Terms in this set (17)

The collection of _____ in a population constitutes that population's gene pool.

alleles

The proportion of alleles present in a gene pool is called the _____ and is an important tool in measuring evolutionary changes.

allele frequency

In a population of 75 mice, what would be the size of their gene pool for a particular gene?

150

In a population of 100 mice, if the recessive allele frequency is 0.25, how many copies of the recessive alleles are found in the population?

Which of these does NOT influence allele frequencies in a population?

all of these DO influence allele frequencies in a population

Inbreeding can lead to a decrease in genetic diversity and _____.

inbreedng depression

Which of these is NOT an example of adaptive evolution?

all of these are non-adaptive forms of evolution

Darwin's finches came from a mainland species where some individuals were able to move to the islands and then spread from one island to another. Is the genetic diversity of the new population of finches on the Galápagos increased or decreased in relation to the population on the mainland, and what is this an example of?

It decreased their genetic diversity because this represents an example of genetic drift.

You examine a population and note that its allele frequencies for a particular gene are p = 0.55 and q = 0.45. If you came back and examined the population after several generations and found that the values for p and q had not changed, which statement would be FALSE for this gene?

The population is evolving.

If you examine a population for a particular gene where B represents the dominant allele and b represents the recessive allele, you find that the allele frequencies for this gene are p = 0.3 and q = 0.7. If this population is not evolving, what would be the frequency of the heterozygous genotype?

0.42

Which scenario would represent a situation that reduces the effect of genetic drift?