Evolutionary Mechanisms and Population Genetics

• Evolution occurs when allele frequencies in a population change over time.
• Natural selection that causes adaptive evolution can be shown.
• The importance of genetic variation can be shown.
• The use of the Hardy-Weinberg Equation can be illustrated.
• An understanding of the effects of genetic drift and migration on a population can be gained

The Smallest Unit of Evolution

• Natural selection acts on individuals, but only populations evolve.
• Organisms do not evolve during their lifetimes.
• During a drought on Daphne Major Island, large-beaked medium ground finches capable of cracking large seeds were more likely to survive.
• The finch population then evolved by natural selection.

Early Criticism of Natural Selection

• A long time span would be required for natural selection to occur, meaning that it was untestable experimentally
• It only described selection among pre-existing variations and provided no source of variation

Origin of Species

• Mendel likely read Origin of Species, but Darwin did not know of Mendel's work studying inheritance.
• A connection was not made between the two until the 20th century.
• New alleles arise by mutation.
• Sexual reproduction creates new combinations of those alleles.

The Evolution of Populations

• Microevolution consists of adaptations that evolve within a population confined to one gene pool.
• Evolution can be defined as changes in the genetic makeup of a population over time.
• Experiments to test natural selection are possible given shorter time spans.
• Kettlewell's experiment involved the peppered moth.
• Soapberry bugs were studied by Carroll & Boyd.
• Resistance to Antibiotics has been studied
• Rosemary and Peter Grant have done experiments.
• Literally, hundreds of others also show that it occurs reliably

Trees and Moths

• Before industry, most moths were the light variety.
• After, most moths were the dark variety

Beaks and Bugs

• Beak length in mm, based on distribution: on native species, southern Florida
• Beak length in mm, based on distribution: on introduced species, central Florida

Resistance

• Selection works on individuals, but populations evolve.

Genetic Variation

• Genetic variation ultimately results from mutation, which creates new alleles.

Gene Pool

• A gene pool consists of all the alleles for all the loci in all individuals in an interbreeding population.

The handy Weinberg Equation

• This equation is designed to identify how we measure the changing genetic makeup of a population
• p² + 2pq + q² = 1
  • p = frequency of one allele (dominant)
  • q = frequency of the other allele (recessive)
  • p² = frequency of homozygous dominant

Genotype frequencies

• With two flower color genes (CR and CW), the alleles are partially dominant
• The Hardy-Weinberg equation allows for allelic frequencies to be any number
• p + q = 1
• p = frequency of a red allele
• q = frequency of a white allele
• The genotype frequencies and allelic frequencies do not change from one generation to the next if a population is at a constant

Population Change

• Factors affecting a population's gene pool resulting in evolutionary change:
  • Mutation – creates new alleles
  • Non-random mating
  • Gene flow - migration of individuals in or out of the population
  • Genetic drift – chance events
  • Natural selection – non-random reproductive success

Gene Flow

• Gene flow is the transfer of alleles into or out of a population by migration.

Genetic Drift

• Random events likely affect small populations.
• Random events that affect small populations can change subsequent population allele ratios

Bottleneck and Founder Effects

• When population size crashes, it can dramatically affect the gene pool.
• The same thing can happen if a small group splits from the population and colonizes a new location.
• A frequency-dependent selection selects for equal numbers of “right-mouthed” and “left-mouthed" scale-eating fi

Not Evolving

• A population in Hardy-Weinberg equilibrium remains constant from one generation to the next and is not evolving.
• The Hardy-Weinberg equation measures the change due to various factors such as natural selection.

Inherited Traits

• Natural selection can alter the frequency distribution of inherited traits in three basic ways, depending on the phenotypes that are beneficial

Natural Selection

• Can lead to adaptation
• Some traits are adaptive only in terms of sexual selection.
• Genetic variation is preserved in populations in various ways.
  • Neutral Variation
  • Diploidy
  • Heterozygote Advantage (balancing Selection)
  • Frequency Dependent Selection (Balancing Selection)

Diploidy

• Diploidy maintains genetic variation in the form of hidden recessive alleles.
• Heterozygotes can carry recessive alleles that are hidden from the effects of selection.
• Alleles that lower an individual's fitness can be maintained in a population if they are recessive.

Stabilizing Phenotypic Forms

• Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population. Balancing selection includes:
  • Heterozygote advantage
  • Frequency-dependent selection

Natural Selection

• Frequency-dependent selection occurs when the fitness of a phenotype declines if it becomes too common in the population.
• Selection favors phenotypes that are less common in a population.

Heterozygotes

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes.
• Natural selection tends to maintain two or more alleles at said locus
• The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance.

Genetic Disorder

• Sickle-cell disease is a genetic disorder that strikes individuals with two copies of the sickle-cell allele.
• This allele causes mutations in hemoglobin but also confers malaria resistance.
• This allele affects the structure and function of hemoglobin, reducing the oxygen-carrying capacity of red blood cells. Though sickle-cell disease is lethal, the frequency of the allele can be as high as 15-20% in some regions

Point Mutation

• A point mutation in the sickle-cell allele changes one amino acid, leading to a fiber chain that may contribute negatively to proper protein folding

###Normal and Sickle Cell Hemoglobin

• The difference between normal and sickle cell hemoglobin is one base substitution leading to one amino acid difference in the protein.
• Val 6 is a mutation within the molecule which may cause bloods cells to stick together

Malaria Protection

• Having one copy of the sickle cell allele is protective against malaria.
• Being homozygous for sickle cell lowers one's fitness
• Being heterozygous for sickle cell increases fitness in areas where malaria is common

Mutation

• New alleles arise by mutation: a change in the nucleotide sequence of DNA
• Mutations can result from replication errors or exposure to types of radiation or chemicals
• Even a point mutation: change in a single nucleotide, has a can have a significant impact on phenotype

Natural Selection

• Natural Selection:
  • Can act only on existing variations.
  • Is limited by historical constraints.
  • Adaptations are often compromises.
  • Chance, natural selection, and the environment interact.

Explore industrial melanism, the Hardy-Weinberg principle, and the concept of gene pool. Understand the sources of genetic variation and the relationship between natural selection and evolution. Learn how new alleles arise and are combined through sexual reproduction.