Population Evolution

Defining Population Evolution

Genetic variation in a population is determined by mutations, natural selection, genetic drift, genetic hitchhiking, and gene flow.

Learning Objectives

Describe how the forces of genetic drift, genetic hitchhiking, gene flow, and mutation can lead to differences in population variation

Key Takeaways

Key Points

  • The theory of evolution gives us a unifying theory to explain the similarities and differences within life’s organisms and processes.
  • Populations (or gene pools ) evolve as gene frequencies change; individual organisms cannot evolve.
  • Variation in populations is determined by the genes present in the population’s gene pool, which may be directly altered by mutation.
  • Natural selection is the gradual process that increases the frequency of advantageous inherited traits (allowing it to survive and reproduce) and decreases the frequency of detrimental inherited traits within a population.
  • A population’s genetic makeup can also be affected by random chance events like genetic drift, or when genes are inherited together in genetic hitchhiking.

Key Terms

  • gene flow: the transfer of alleles or genes from one population to another
  • genetic hitchhiking: a phenomenon in which a gene increases in a population because it lies near genes on the same chromosome that are advantageous to an organism
  • genetic drift: an overall shift of allele distribution in an isolated population, due to random fluctuations in the frequencies of individual alleles of the genes
  • fitness: an individual’s ability to propagate its genes
  • natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce
  • mutation: any heritable change of the base-pair sequence of genetic material

The Evolution of Populations

According to evolutionary theory, every organism from humans to beetles to plants to bacteria share a common ancestor. Millions of years of evolutionary pressure caused some organisms to died while others survived, leaving earth with the diverse life forms we have today. Within this diversity is unity; for example, all organisms are composed of cells and use DNA. The theory of evolution gives us a unifying theory to explain the similarities and differences within life’s organisms and processes.

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Evolution on earth: Evolution has resulted in living things that may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution.

Genetic Variation in Populations

A population is a group of individuals that can all interbreed, often distinguished as a species. Because these individuals can share genes and pass on combinations of genes to the next generation, the collection of these genes is called a gene pool. The process of evolution occurs only in populations and not in individuals. A single individual cannot evolve alone; evolution is the process of changing the gene frequencies within a gene pool. Five forces can cause genetic variation and evolution in a population: mutations, natural selection, genetic drift, genetic hitchhiking, and gene flow.

Mutations

Why do some organisms survive while others die? These surviving organisms generally possess traits or characteristics that bestow benefits that help them survive (e.g., better camouflage, faster swimming, or more efficient digestion). Each of these characteristics is the result of a mutation, or a change in the genetic code. Mutations occur spontaneously, but not all mutations are heritable; they are passed down to offspring only if the mutations occur in the gametes. These heritable mutations are responsible for the rise of new traits in a population.

Natural Selection

Just as mutations cause new traits in a population, natural selection acts on the frequency of those traits. Because there are more organisms than resources, all organisms are in a constant struggle for existence. In natural selection, those individuals with superior traits will be able to produce more offspring. The more offspring an organism can produce, the higher its fitness. As novel traits and behaviors arise from mutation, natural selection perpetuates the traits that confer a benefit.

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Mutation and natural selection: As mutations create variation, natural selection affects the frequency of that trait in a population. Mutations that confer a benefit (such as running faster or digesting food more efficiently) can help that organism survive and reproduce, carrying the mutation to the next generation.

Genetic Drift

When selective forces are absent or relatively weak, gene frequencies tend to “drift” due to random events. This drift halts when the variation of the gene becomes “fixed” by either disappearing from the population or replacing the other variations completely. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations.

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Genetic drift and gene fixation: In this simulation, there is fixation in the blue gene variation within five generations. Images these dots are beetles and some of them are destroyed by a wildfire. As the surviving population changes over time, some traits (red) may be completely eliminated from the population, leaving only the beetles with other traits (blue).

Genetic Hitchhiking

When recombination occurs during sexual reproduction, genes are usually shuffled so that each parent gives its offspring a random assortment of its genetic variation. However, genes that are close together on the same chromosome are often assorted together. Therefore, the frequency of a gene may increase in a population through genetic hitchhiking if its proximal genes confer a benefit.

Gene Flow

Gene flow is the exchange of genes between populations or between species.If the gene pools between two populations are different, the exchange of genes can introduce variation that is advantageous or disadvantageous to one of the populations. If advantageous, this gene variation may replace all the other variations until the entire population exhibits that trait.

Population Genetics

Population genetics is the study of the distributions and changes of allele frequency in a population.

Learning Objectives

Define a population gene pool and explain how the size of the gene pool can affect the evolutionary success of a population

Key Takeaways

Key Points

  • A gene pool is the sum of all the alleles (variants of a gene) in a population.
  • Allele frequencies range from 0 (present in no individuals) to 1 (present in all individuals); all allele frequencies for a given gene add up to 100 percent in a population.
  • The smaller a population, the more susceptible it is to mechanisms like natural selection and genetic drift, as the effects of such mechanisms are magnified when the gene pool is small.
  • The founder effect occurs when part of an original population establishes a new population with a separate gene pool, leading to less genetic variation in the new population.

Key Terms

  • allele: one of a number of alternative forms of the same gene occupying a given position on a chromosome
  • gene pool: the complete set of unique alleles that would be found by inspecting the genetic material of every living member of a species or population
  • founder effect: a decrease in genetic variation that occurs when an entire population descends from a small number of founders

Population Genetics

A gene for a particular characteristic may have several variations called alleles. These variations code for different traits associated with that characteristic. For example, in the ABO blood type system in humans, three alleles (IA, IB, or i) determine the particular blood-type protein on the surface of red blood cells. A human with a type IA allele will display A-type proteins (antigens) on the surface of their red blood cells. Individuals with the phenotype of type A blood have the genotype IAIA or IAi, type B have IBIB or IBi, type AB have IAIB, and type O have ii.

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ABO blood type in humans: In humans, each blood type corresponds to a combination of two alleles, which represent a the type of antigens displayed on the outside of a red blood cell. Human blood types are A, B, AB, and O.

A diploid organism can only carry two alleles for a particular gene. In human blood type, the combinations are composed of two alleles such as IAIA or IAIB. Although each organism can only carry two alleles, more than those two alleles may be present in the larger population. For example, in a population of fifty people where all the blood types are represented, there may be more IA alleles than i alleles. Population genetics is the study of how selective forces change a population through changes in allele and genotypic frequencies.

Allele Frequency

The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Frequencies range from 0, present in no individuals, to 1, present in all individuals. The gene pool is the sum of all the alleles at all genes in a population.

Using the ABO blood type system as an example, the frequency of one of the alleles, for example IA, is the number of copies of that allele divided by all the copies of the ABO gene in the population, i.e. all the alleles. Allele frequencies can be expressed as a decimal or as a percent and always add up to 1, or 100 percent, of the total population. For example, in a sample population of humans, the frequency of the IA allele might be 0.26, which would mean that 26% of the chromosomes in that population carry the IA allele. If we also know that the frequency of the IB allele in this population is 0.14, then the frequency of the i allele is 0.6, which we obtain by subtracting all the known allele frequencies from 1 (thus: 1 – 0.26 – 0.14 = 0.6). A change in any of these allele frequencies over time would constitute evolution in the population.

Population Size and Evolution

When allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies, the phenomenon is called genetic drift. The smaller a population, the more susceptible it is to mechanisms such as genetic drift as alleles are more likely to become fixed at 0 (absent) or 1 (universally present). Random events that alter allele frequencies will have a much larger effect when the gene pool is small. Genetic drift and natural selection usually occur simultaneously in populations, but the cause of the frequency change is often impossible to determine.

Natural selection also affects allele frequency. If an allele confers a phenotype that enables an individual to better survive or have more offspring, the frequency of that allele will increase. Because many of those offspring will also carry the beneficial allele and, therefore, the phenotype, they will have more offspring of their own that also carry the allele. Over time, the allele will spread throughout the population and may become fixed: every individual in the population carries the allele. If an allele is dominant but detrimental, it may be swiftly eliminated from the gene pool when the individual with the allele does not reproduce. However, a detrimental recessive allele can linger for generations in a population, hidden by the dominant allele in heterozygotes. In such cases, the only individuals to be eliminated from the population are those unlucky enough to inherit two copies of such an allele.

The Founder Effect

The founder effect occurs when part of a population becomes isolated and establishes a separate gene pool with its own allele frequencies. When a small number of individuals become the basis of a new population, this new population can be very different genetically from the original population if the founders are not representative of the original. Therefore, many different populations, with very different and uniform gene pools, can all originate from the same, larger population. Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population.

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The Founder Effect: Here are three possible outcomes of the founder effect, each with gene pools separate from the original populations.

Hardy-Weinberg Principle of Equilibrium

The Hardy-Weinberg principle can be used to estimate the frequency of alleles and genotypes in a population.

Learning Objectives

Use the Hardy Weinberg equation to calculate allelic and genotypic frequencies in a population

Key Takeaways

Key Points

  • The Hardy-Weinberg principle assumes that in a given population, the population is large and is not experiencing mutation, migration, natural selection, or sexual selection.
  • The frequency of alleles in a population can be represented by p + q = 1, with p equal to the frequency of the dominant allele and q equal to the frequency of the recessive allele.
  • The frequency of genotypes in a population can be represented by p2+2pq+q2= 1, with p2 equal to the frequency of the homozygous dominant genotype, 2pq equal to the frequency of the heterozygous genotype, and q2 equal to the frequency of the recessive genotype.
  • The frequency of alleles can be estimated by calculating the frequency of the recessive genotype, then calculating the square root of that frequency in order to determine the frequency of the recessive allele.

Key Terms

  • genotype: the combination of alleles, situated on corresponding chromosomes, that determines a specific trait of an individual, such as “Aa” or “aa”
  • phenotype: the appearance of an organism based on a multifactorial combination of genetic traits and environmental factors, especially used in pedigrees

Hardy-Weinberg Principle of Equilibrium

The Hardy-Weinberg principle states that a population’s allele and genotype frequencies will remain constant in the absence of evolutionary mechanisms. Ultimately, the Hardy-Weinberg principle models a population without evolution under the following conditions:

  1. no mutations
  2. no immigration/emigration
  3. no natural selection
  4. no sexual selection
  5. a large population

Although no real-world population can satisfy all of these conditions, the principle still offers a useful model for population analysis.

Hardy-Weinberg Equations and Analysis

According to the Hardy-Weinberg principle, the variable p often represents the frequency of a particular allele, usually a dominant one. For example, assume that p represents the frequency of the dominant allele, Y, for yellow pea pods. The variable q represents the frequency of the recessive allele, y, for green pea pods. If p and q are the only two possible alleles for this characteristic, then the sum of the frequencies must add up to 1, or 100 percent. We can also write this as p + q = 1.If the frequency of the Y allele in the population is 0.6, then we know that the frequency of the y allele is 0.4.

From the Hardy-Weinberg principle and the known allele frequencies, we can also infer the frequencies of the genotypes. Since each individual carries two alleles per gene (Y or y), we can predict the frequencies of these genotypes with a chi square. If two alleles are drawn at random from the gene pool, we can determine the probability of each genotype.

In the example, our three genotype possibilities are: pp (YY), producing yellow peas; pq (Yy), also yellow; or qq (yy), producing green peas. The frequency of homozygous pp individuals is p2; the frequency of hereozygous pq individuals is 2pq; and the frequency of homozygous qq individuals is q2. If p and q are the only two possible alleles for a given trait in the population, these genotypes frequencies will sum to one: p2 + 2pq + q2 = 1.

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Hardy-Weinberg proportions for two alleles: The horizontal axis shows the two allele frequencies p and q and the vertical axis shows the expected genotype frequencies.Each line shows one of the three possible genotypes.

In our example, the possible genotypes are homozygous dominant (YY), heterozygous (Yy), and homozygous recessive (yy). If we can only observe the phenotypes in the population, then we know only the recessive phenotype (yy). For example, in a garden of 100 pea plants, 86 might have yellow peas and 16 have green peas. We do not know how many are homozygous dominant (Yy) or heterozygous (Yy), but we do know that 16 of them are homozygous recessive (yy).

Therefore, by knowing the recessive phenotype and, thereby, the frequency of that genotype (16 out of 100 individuals or 0.16), we can calculate the number of other genotypes. If q2 represents the frequency of homozygous recessive plants, then q2 = 0.16. Therefore, q = 0.4.Because p + q = 1, then 1 – 0.4 = p, and we know that p = 0.6. The frequency of homozygous dominant plants (p2) is (0.6)2 = 0.36. Out of 100 individuals, there are 36 homozygous dominant (YY) plants. The frequency of heterozygous plants (2pq) is 2(0.6)(0.4) = 0.48. Therefore, 48 out of 100 plants are heterozygous yellow (Yy).

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The Hardy-Weinberg Principle: When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined.If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

Applications of Hardy-Weinberg

The genetic variation of natural populations is constantly changing from genetic drift, mutation, migration, and natural and sexual selection. The Hardy-Weinberg principle gives scientists a mathematical baseline of a non-evolving population to which they can compare evolving populations. If scientists record allele frequencies over time and then calculate the expected frequencies based on Hardy-Weinberg values, the scientists can hypothesize the mechanisms driving the population’s evolution.