Chapter 23: The Evolution of Populations

Merged flashcards from Chapter 23 of Pearson's Campbell Biology, Twelfth Edition.

Natural selection

Force in nature that allows populations of animals to adapt to their environment through genetic components

• Seen with ground finches in the Galapagos Islands evolving to have larger beaks in response to a small seed shortage

  • This led to an increase in average beak depth

Microevolution

The change in allele frequencies in a population over generations, caused by:

• Natural selection (environmental adaptations)

• Genetic drift (chance events)

• Gene flow (population allele transfers)

Genetic variation

The differences in genes or DNA sequences among individuals; this is a prerequisite for evolution by natural selection

• The discovery of this was aided by Gregor Mendel's work on pea plant heritability

• It can be quantified by the percentage of heterozygous loci in a population or the nucleotide sequences of individuals

Gregor Mendel

Scientist that discovered evidence of discrete heritable units (genes) in pea plants

Phenotype

How the trait is expressed in the real world, affected by the inherited genotype and environmental influences

• Though most of these are expressed as two traits, a gradient of traits may appear when determined by two or more genes

  • Seen in horse coat colors or height in humans

Introns

Noncoding DNA segments where most nucleotide differences occur, resulting in no change

Exons

Coding DNA segments where fewer nucleotide differences occur

• Even if DNA is changed here, it rarely results in a change to the amino acid sequence of an encoded protein

Genetic variation sources

Includes:

• Mutation

• Gene duplication

• Other processes

These are accelerated through organisms with short generation times or reproduction through the combination of existing alleles

Mutation

A change in the nucleotide sequence of DNA as a source of genetic variation

• Replication errors, radiation, or chemicals are common sources of this — overall, though, these are rare

• Even a single change can result in a significant phenotypical impact, often becoming at least slightly harmful if not hidden as a recessive trait

• If an organism can produce gametes, these can be passed to offspring

Heterozygote protection

The maintenance of a pool of alleles that could be beneficial with environmental changes through heterozygous individuals

Point mutation

A mutation of a single nucleotide

Neutral variation

A variation with no selective advantage or disadvantage as a result of a point mutation

• Occurs due to genetic redundancy

Gene duplication

The duplication of small segments of DNA, is a key source of genetic variation

• This can result in the accumulation of mutations and new functions, such as mammals' heightened sense of smell

Virus

An infectious agent with rapid mutation rates and short generation times, increasing drug resistance and survival rates

Evolution

The changing of a species' genes over time, requiring genetic variation alongside other factors

Population

A group of individuals of the same species that live in the same area and interbreed

• If geographically isolated, genetic material is rarely exchanged

• Individuals typically only breed with members of their own population

Gene pool

All copies of every allele at every locus in all members of the population

Fixed locus

A locus that cannot change due to all individuals in a population being homozygous for the same allele; otherwise, individuals may be homozygous or heterozygous for two or more alleles

Genotype frequency

How frequent a specific genotype is in a population

• Calculated through the frequency of an individual genotype (like C^RC^R or C^RC^W) over the entire population combined

Allele frequency

How frequent a specific allele is in a measured population

• The total amount of alleles for diploid organisms is the amount of individuals times two

• Count two recessive or dominant alleles for each homozygous individual, and one of each allele for each heterozygote

• The frequency for each allele takes the number of each allele over the total amount of alleles, adding up to 1 when combining dominant (p) and recessive (q) alleles

p

The letter used to represent the allelic frequency of a dominant allele

q

The letter used to represent the allelic frequency of a recessive allele

Hardy-Weinberg Equation

An equation that describes the expected genetic makeup for a population not evolving at a particular locus, resulting in constant frequencies

• If the expected makeup differs from the population, evolution may be occurring — either through mutations, selective mating, natural selection, small population size, or gene flow

• Expressed as p² + 2pq + q² = 1

  • p² and q² represent the expected frequency of homozygous genotypes

  • 2pq represents the expected frequency of the heterozygous genotype

  • This is calculated separately from genotype frequency, only considering the allele frequency in an observed population

Hardy-Weinberg equilibrium

State at which a population is not evolving, given the required conditions of no mutations, random mating, no natural selection, large population size, and no gene flow

Most affected by:

• Natural selection

• Genetic drift

• Gene flow

Natural selection

Process in nature based on differential success in survival and reproduction

• Traits better suited to the environment produce more offspring than others, resulting in different proportions

Adaptive evolution

A process in which traits that enhance survival or reproduction increase in frequency over time as a result of natural selection

Genetic drift

A process in which chance events cause allele frequencies to fluctuate unpredictably from one generation to the next

• Tends to reduce genetic variation through the random loss of alleles

• More significant in small populations; can lead to the fixation of harmful alleles

Founder effect

Occurring when a few individuals become isolated from a larger population, allele frequencies in the smaller population are different from those in the parent population

Bottleneck effect

Occurring when there is a drastic reduction in population size due to a sudden change in the environment, the resulting gene pool may no longer be reflective of the original population’s gene pool

• Seen with the severe reduction of prairie chicken habitats in Illinois leading to low genetic variation and an increase in the frequency of harmful alleles; solved through the introduction of birds from other populations leading to better egg-hatching rates

Gene flow

The movement of alleles among populations, transferred through the movement of fertile individuals or gametes

• Tends to reduce variation among populations over time

• Can affect adaptation to local environments or different threats in varying ways due to these new traits