Chapter 21: Microevolution

What is Microevolution?

A change in gene frequency within a population, observable over short periods, responding to shifting environmental circumstances, or small-scale genetic changes in populations

What is the scale of genetic changes described by microevolution?

Small-scale genetic changes within populations

Over what period of time can microevolution be observed?

Short periods, such as between one generation and the next

What are examples of microevolution?

Antibiotic resistance and pesticide resistance

How does microevolution relate to the environment?

It responds to shifting environmental circumstances and can fine-tune populations within their environments

What is Phenotypic Variation?

Variation in the appearance or function among individuals in a population

Describe phenotypic variation in European Garden Snails.

Their shells vary considerably in appearance, including background color, number, and color of stripes (Polymorphic traits)

What is a Quantitative Trait?

A measurable phenotype that depends on the cumulative actions of many genes and the environment, varying among individuals over a range

How is Quantitative Variation often visually represented?

A bar graph of the data often approximates a bell-shaped curve

What does the width of the curve indicate in a graph of quantitative variation?

It is proportional to the variability among individuals; a broad curve indicates a lot of variation, while a narrow curve indicates little variation

What is Qualitative Variation?

Variation existing as discrete phenotypic variations (e.g., blue or white snow geese)

What can cause phenotypic variation within populations?

Genetic differences, differences in environmental factors, or an interaction between genetics and the environment

Can organisms with different genotypes exhibit the same phenotype?

Yes

Can organisms with the same genotype exhibit different phenotypes?

Yes

What are the two potential sources that generate genetic variation?

Production of new alleles and rearrangement of existing alleles into new combinations

How do most new alleles arise?

From small-scale mutations in DNA

How do rearrangements of existing alleles result in new combinations?

Larger scale changes in chromosome structure or number, or several forms of genetic recombination

What are some forms of genetic recombination?

Crossing over, independent assortment, and random fertilizations

Genetic recombination process/effect:

Shuffles existing alleles into new combinations, which can produce an extraordinary number of novel genotypes

Can genetic variations in protein-coding regions be silent?

Yes, if they do not change the amino acid sequences of the proteins the genes encode for

What techniques can biologists use to detect genetic variation?

Gel electrophoresis and DNA sequencing technologies

Gel electrophoresis process/technique:

Identifies biochemical polymorphisms by separating different forms of a protein that differ significantly in shape, mass, or net electrical charge

DNA sequencing technology process/technique:

Allows direct survey of genetic variation in coding and non-coding DNA sequences

What are the three premises from which the theoretical edifice of population genetics stems?

DNA can replicate, DNA mutates and recombines, and the information in DNA interacts with the environment to produce traits

What is the Gene Pool?

Sum of all alleles or gene copies at all gene loci in all individuals in a population

What are Genotype Frequencies?

Percentage of individuals in a population possessing a certain genotype

What is an Allele?

One of two or more versions of DNA sequence at a given genomic location

What are Allele Frequencies?

The relative abundance of the different alleles in a population

What should the sum of the allele frequencies and genotype frequencies add up to?

1

For a gene locus with two alleles (p and q) in a diploid organism, how many genotype frequencies are there?

Three

For a gene locus with two alleles (p and q) in a diploid organism, how many allele frequencies are there?

Two (p and q)

Calculating genotype frequencies process:

For a gene locus with two alleles (p and q) in randomly mating organisms, genotype frequencies are p^2, 2pq, and q^2

Calculating allele frequencies process (example with 1,000 individuals, 1,400 CR alleles, 600 CW alleles):

Divide the total number of each allele by the total number of alleles (e.g., frequency of CR = 1400/2000 = 0.7, frequency of CW = 600/2000 = 0.3)

What are Null Models used for in population genetics?

As theoretical reference points against which observations can be evaluated

What is the Hardy-Weinberg Principle?

A mathematical model that describes how genotype frequencies are established in sexually reproducing organisms

What is Hardy-Weinberg Equilibrium?

A state where a population's allele and genotype frequencies remain constant unless there is some type of evolutionary force acting

Under what conditions is genetic equilibrium (Hardy-Weinberg equilibrium) possible?

No mutations, population is closed to migration, population is infinite in size, all genotypes survive and reproduce equally well, individuals mate randomly

Do natural populations typically meet all the conditions for Hardy-Weinberg equilibrium?

No, natural populations never meet all five requirements

What is the role of the Hardy-Weinberg principle as a null model?

Under its conditions, microevolution will not occur

What is a Mutation?

A spontaneous and heritable change in DNA

What is the immediate effect of new mutations on allele frequencies in populations?

Little or no immediate effect

What is the long-term effect of mutations over evolutionary time?

They create new genetic variations and are a major source of heritable variation

For animals, what cell line must mutations occur in to be heritable?

The germ line (cell lineage that produces gametes)

What is Genetic Drift?

Random changes in allele frequencies caused by chance events

What is the effect of genetic drift on genetic variation?

Reduces genetic variation, especially in small populations, and can eliminate rare alleles

What is a Population Bottleneck?

A dramatic reduction in population size caused by factors such as disease, starvation, or drought

Population bottleneck process/effect:

A drastic reduction in population size shakes through only a few survivors, leading to a loss of genetic variability and potential loss of rare alleles in the surviving population

Give an example of a population bottleneck.

Northern elephant seals descended from a small number of survivors after hunting, now exhibiting no variation in certain proteins

What are the conservation implications of endangered species experiencing population bottlenecks?

Loss of genetic variability means the population will be less resistant to diseases and less able to cope with environmental change, even if numbers rebound

What is Gene Flow?

Change in allele frequencies as individuals join a population and reproduce

What is the effect of gene flow on genetic variation?

May introduce genetic variation into a population

What is Nonrandom Mating?

Choice of mates based on their phenotypes and genotypes

What is the effect of nonrandom mating on genotype frequencies?

Increases the frequency of homozygous genotypes and decreases the frequency of heterozygotes

What is Inbreeding?

A form of nonrandom mating in which genetically related individuals mate, with self-fertilization being an extreme example

What is the effect of inbreeding on genotype frequencies?

Increases the frequency of homozygous genotypes and decreases the frequency of heterozygotes

What is the effect of inbreeding on phenotypes?

Recessive phenotypes are often expressed

What is Natural Selection?

Differential survivorship or reproduction of individuals with different genotypes

What is the effect of natural selection on genetic variation?

One allele can replace another or allelic variation can be preserved

What is the effect of natural selection on average fitness?

Positive effect on fitness through the evolution of adaptations

Does natural selection exert a strong effect on traits appearing during an individual's post-reproductive life?

No, it exerts little effect

What are the three modes of natural selection?

Directional selection, Stabilizing selection, and Disruptive selection

How do biologists measure phenotypic variation changes over time due to natural selection?

By recording changes in the mean and variability of characters over time

What is Stabilizing Selection?

Natural selection that favors intermediate phenotypes

Give an example of stabilizing selection in humans.

Birthweight, where newborns of intermediate size experience lower mortality than very small or very large newborns

Stabilizing selection process/example (Gall-making flies):

Tiny wasps parasitize fly larvae in small galls (favoring large galls), while birds open large galls to feed on larvae (favoring small galls). The opposing pressures result in stabilizing selection for intermediate gall size

What is Disruptive Selection?

Natural selection that favors both extreme phenotypes

What is Directional Selection?

Natural selection that favors phenotypes at one extreme

Why are rare recessive alleles protected from the action of natural selection in diploid organisms?

Because they are masked by dominant alleles in heterozygous individuals

What is a Balanced Polymorphism?

A polymorphism in which two or more phenotypes are maintained in fairly stable proportions over many generations

How does natural selection preserve balanced polymorphisms?

Through mechanisms such as heterozygote advantage, frequency-dependent selection, and selection by varying environments

What is Heterozygote Advantage?

A mechanism that preserves balanced polymorphism where the heterozygote has higher fitness than either homozygote

Give an example of heterozygote advantage.

Individuals heterozygous for the sickle-cell allele (sickle-cell trait) are resistant to malaria

How does Selection by Varying Environments maintain genetic variability?

Different phenotypes are favored in different local habitats, maintaining variability in the population as a whole by preserving different alleles in different places

Give an example of selection by varying environments maintaining variability.

European garden snails with different shell patterns are favored in different habitats (woods vs. hedges/fields) because of camouflage

What is Frequency-Dependent Selection?

Genetic variability is maintained because rare phenotypes have a reproductive advantage

Frequency-dependent selection process/example (Elderflower orchids):

Bumblebees visit elderflower orchids; the rarer the flower color (yellow or purple), the higher the reproductive success of plants with that color

What is the Neutral Variation Hypothesis?

Some genetic variations at loci coding for enzymes or proteins are selectively neutral when different forms of the proteins function equally well

Why might different alleles be selectively neutral according to the Neutral Variation Hypothesis?

Even if they code for slightly different amino acid sequences, the different forms of the proteins may function equally well

What is an Adaptive Trait?

Any product of natural selection that increases the relative fitness of an organism in its environment

What is Adaptation?

The accumulation of adaptive traits over time

When studying adaptive traits, how might biologists proceed if traits do not vary much within a population?

They may compare variations in closely related species living in different environments

What is Artificial Selection?

Using variations to produce lines of organisms that exhibit desired traits, such as increased wheel running behavior in mice

What is an organism's genotype?

The genetic makeup of an organism or its complete set of genes

Why is the study of DNA polymorphisms in humans especially important?

In understanding the genetic origins of certain diseases

Do null models predict what will happen under experimental treatment?

No, they predict what would happen if the experimental treatment had no effect

What makes up a population?

Individuals, each with its own genotype

What is the definition of an allele given in the source?

One of two or more versions of DNA sequence at a given genomic location

If allele frequencies p and q add up to 1, what does this signify?

That all alleles at that locus in the population are accounted for

If genotype frequencies p^2, 2pq, and q^2 add up to 1, what does this signify?

That all individuals in the population are accounted for in terms of these genotypes

In snapdragons, what phenotype is produced by the CRCW genotype?

Pink flowers (intermediate phenotype due to incomplete dominance)

If a population deviates from the conditions required for Hardy-Weinberg equilibrium, what can occur?

Microevolution

What is one reason why natural populations rarely meet the conditions for Hardy-Weinberg equilibrium?

Factors like mutation, gene flow, genetic drift, nonrandom mating, and natural selection are typically present

Effect of Mutation on Genetic Variation (Summary):

Introduces new genetic variation into the population but does not change allele frequencies quickly

Effect of Gene Flow on Genetic Variation (Summary):

May introduce genetic variation from another population

Effect of Genetic Drift on Genetic Variation (Summary):

Reduces genetic variation, especially in small populations; can eliminate rare alleles

Effect of Natural Selection on Genetic Variation (Summary):

One allele can replace another or allelic variation can be preserved

Effect of Nonrandom Mating on Genetic Variation (Summary):

Does not directly affect allele frequencies, but usually prevents genetic equilibrium

Effect of Mutation on Average Fitness (Summary):

Unpredictable effect; most mutations in protein-coding genes lower fitness

Effect of Gene Flow on Average Fitness (Summary):

Unpredictable effect; may introduce beneficial or harmful alleles

Effect of Genetic Drift on Average Fitness (Summary):

Often harmful because of lost genetic diversity