ANTB14: Evolutionary Anthropology Lecture 3: Microevolution I

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Today's Topics

• Selection cont’d

• Adaptation

• Misuse of ideas of natural selection

• Microevolutionary forces:

  • Natural Selection (NS)

  • Mutation

  • Genetic drift

  • Gene flow

• Hardy-Weinberg Equilibrium (HWE)

• Week #4 assigned readings:

  • Andrews, C. A. (2010) Natural Selection, Genetic Drift, and Gene Flow Do Not Act in Isolation in Natural Populations

  • Andrews, C. (2010) The Hardy-Weinberg Principle

  • Barreiro, L.B. The evolutionary tale of lactase persistence in humans

Adaptation

• An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear it.

• Not all features are adaptive.

Factors besides Adaptation

• A trait may be a necessary consequence of physics or chemistry.

• A character state may be a consequence of phylogenetic history.

• May be correlated with another feature that confers an adaptive advantage.

• The trait may have evolved by other mechanisms.

Determining if a Feature is Adaptive

• Design: How complex is it?

• Experiments: Is performance enhanced?

• Comparative method: Is it correlated with a specific selective pressure?

The Comparative Method

• Comparing sets of species to pose or test hypotheses.

• Takes advantage of "natural evolutionary experiments" provided by convergent evolution.

• Asks: Is the feature consistently correlated with a specific function or selective pressure in multiple lineages?

Use and Misuse of Natural Selection

• Social Darwinism:

  • Application of Darwin’s ideas (e.g., differential reproductive success and descent with modification) to social traits in humans.

  • Advocated for social improvement via the removal of ‘lesser’ classes (Herbert Spencer).

• Eugenics movement:

  • Assertion that some individuals are superior to others based on biology.

  • Raises questions: Superior in what way? Decided by whom?

Scales of Evolution

• Evolution is characterized on two scales:

  • Microevolution

  • Macroevolution

Microevolution

• Evolution happening on a small scale within a single population.

• Affects changes in allele frequencies.

• Typically, evolution that we can observe in real time is microevolution.

Molecular Definition of Evolution

• A change in allele frequency from one generation to the next.

Microevolutionary Processes

• Mutation: Random change in genetic material; introduces new variation.

• Genetic Drift: Random changes; adaptation.

• Natural Selection: Non-random; adaptation.

• Gene Flow: Migration/Hybridization.

Natural Selection

• An explanation (process) for how evolution (pattern) occurs.

• An evolutionary process that occurs when certain phenotypes confer an advantage or disadvantage in fitness.

• Conditions for natural selection to occur:

  1. There must be variation in the population.

  2. A trait must be heritable for selection to act on it.

  3. Differential survival due to competition for resources.

• Natural selection acts on variants in the populations.

• Much like how humans select desirable variants in artificial selection.

Mutation

• Mutation is the only way to produce new allele variants.

• Recombination during meiosis is also important for creating new variation in terms of combinations of alleles, but not new alleles.

• Point mutations occur at a rare but predictable rate.

• Rate of mutation in nuclear genome in humans: 10^{-8} per base pair, per generation.

• Mutations can be:

  • Deleterious (bad)

  • Neutral

  • Beneficial (rare)

Types of Mutations

• Point mutations:

  • Substitution: substitution of a single base.

  • Frameshift mutation: insertion or deletion of a base or bases.

• Chromosomal mutations:

  • Chromosomal deletion: a section of a chromosome is deleted.

  • Chromosomal duplication: a section of a chromosome is duplicated.

  • Chromosomal inversion: the order of genetic loci on a chromosome are flipped.

  • Chromosomal insertion: a portion of a different chromosome is inserted into another.

• A mutation may spread throughout the population with the help of another force of evolution.

  • Beneficial mutations selected for by natural selection.

  • Deleterious mutations selected against.

Lactase Persistence

• Feature common to all mammals: Mammary glands (modified sweat glands that produce milk).

• Lactose: A sugar in milk that all mammals can digest at birth.

• Lactase: An enzyme that facilitates the digestion of milk.

• Typically, the production of enzyme is turned off after weaning = lactose intolerance.

• Lactase persistence (or lactose tolerance):

  • Maintaining benefits of dairy products into adulthood.

  • Regulatory gene turned on by mutation so lactase continues to be produced.

  • Big benefit in pastoralist societies, as dairy is high in protein, etc.

• Lactase persistence (or lactose tolerance) emerged at least six separate independent point mutations in multiple pastoralist populations.

  • First emerged in northern Europe during the Neolithic period (~10ka).

  • Also found in Central Balkans and central Europe among dairying agricultural groups.

  • Independent mutations and selection in Africa and Middle East, all related to domesticating livestock and consumption of dairy.

  • Separate distributions of each mutation/allele.

  • Independent origins of the same trait/phenotype = convergent evolution.

Genetic Drift

• Changes in allele frequency produced by random sampling.

• Chance events, sampling error.

• Fitness is not a factor in genetic drift.

• Effects are greater when population size is smaller.

• Not directional, unpredictable, not adaptive.

• Can reduce genetic diversity.

• Two special types of genetic drift:

  • Bottleneck effect

  • Founder effect

Bottleneck Effect

• Population passes through a ‘bottleneck’, wherein only a small portion survive.

• The new population has different allele frequencies (reduced variation) than the original population due to random chance/sampling error.

• Example: Cheetahs underwent bottleneck, resulting in very low genetic diversity.

• Link between genetic drift and conservation?

• Genetic variation is important for population health.

• Example: Homo sapiens genetic variation is much lower than our closest relatives.

Founder Effect

• A small subset of the original population leaves or is separated, becoming the founding members of a new population.

• Again, the new population has different allele frequencies due to random chance/sampling error.

• Example: ABO blood groups in Homo sapiens.

ABO Blood Groups

• Blood functions:

  • Immune system

  • Antibodies

  • Antigens

  • Nutrient transportation

  • Hemoglobin

• Several blood group systems:

  • ABO blood group (A, B, O types)

  • Rh blood group (Rh D-positive, Rh D-negative)

• Karl Landsteiner (1868-1943) discovered transfusion and blood agglutination and won a Nobel Prize.

• How is your blood type determined?

  • ABO blood group gene is on Chromosome 9.

  • 3 allele system: A B O

  • Blood types (phenotypes): A, B, AB, O

• Antigens: Present on cell, trigger immune response, determine blood type.

• Antibodies: Present in plasma, seek to destroy specific antigens.

• Three allele system = 6 possible genotypes.

• A and B alleles are codominant; O allele is recessive (H antigen).

• Six Genotypes and Four Phenotypes:

  • AA = A (Antigen A, Anti-B antibodies)

  • AO = A (Antigen A, Anti-B antibodies)

  • AB = AB (Antigens A & B, No antibodies)

  • BB = B (Antigen B, Anti-A antibodies)

  • BO = B (Antigen B, Anti-A antibodies)

  • OO = O (No antigens, Anti-A & B antibodies)

• Universal recipients: AB

• Universal donor: OO

• Potential reasons for distributions?

  • Natural selection?

  • Gene flow?

  • Genetic drift?

• Natural selection?

  • Differential susceptibility to certain diseases and environmental stressors (e.g., Smallpox and black death/plague).

  • Balance between evolutionary forces.

• Potential reasons for distributions?

  • Gradual frequency change (clines): Suggests gene flow or natural selection.

  • Abrupt changes: Genetic drift or founder effect.

• Founder effect example: Serial founder effects in Homo sapiens.

• Human genetic variation:

  • Greatest variability within Africa, decreasing with distance from Africa.

  • Serial founder effects.

  • More genetic variation exists in Africa than everywhere else combined.

  • Does not support conceptions of human races.

Common Misconceptions about Genetic Drift

• Changes in allele frequency produced by random sampling.

• Like natural selection, genetic drift results in populations that are better adapted to their environments.

• Role of population size.

Gene Flow

• Exchange of genes between populations.

• Unidirectional migrations.

• Exchanges between two or more populations.

Summary of Evolutionary Forces

Population Genetics

• The study of the frequency of alleles, genotypes, and phenotypes in populations.

• Population: a group of interbreeding individuals.

• The branch of biology that provides the mathematical theory to study microevolution.

• Hardy-Weinberg Equilibrium

Hardy-Weinberg Equilibrium (HWE)

• G.H. Hardy and W. Weinberg (1908).

• A mathematical model expressing the predicted distribution of alleles in populations when no evolution is occurring.

• Measure populations at different points in time to determine if allele frequencies have changed.

• Compare expected and observed results.

• Conditions for HWE:

  • The population in infinitely large.

  • There is no mutation.

  • There is no gene flow.

  • There is no selection.

  • Mating is random.

• A population in equilibrium is a population that isn’t evolving.

• Equations:

  • p + q = 1 (where p = dominant allele, q = recessive allele)

  • p^2 + 2pq + q^2 = 1 (where p^2 = homozygous dominant, 2pq = heterozygous, q^2 = homozygous recessive)

Example

• If A (p) = 0.5 and a (q) = 0.5, then:

  • p + q = 1 which means 0.5 + 0.5 = 1

  • p^2 + 2pq + q^2 = 1 which means (0.5)^2 + 2(0.5 * 0.5) + (0.5)^2 = 1

  • 0.25 + 2(0.25) + 0.25 = 1 which means 0.25 + 0.5 + 0.25 = 1

  • AA = 25%, Aa = 50%, aa = 25%

• If these frequencies are seen in the next generation, the trait is in equilibrium, or not currently evolving.

• If these frequencies are not seen in the next generation, the trait is evolving.

Another Example

• If New genotype frequency is AA = 49%, Aa = 42%, aa = 9%, then:

  • p^2 + 2pq + q^2 = 1 which means 0.49 + 0.42 + 0.09 = 1

  • (0.7)^2 + 2(0.3 * 0.7) + (0.3)^2 = 1

  • p = 0.7 = A allele

  • q = 0.3 = a allele

Non-Random Mating

• How do mating patterns factor in?

• Non-random mating doesn’t alter allele frequencies, so not a force of evolution.

• But can change the genotypic proportions, then natural selection can act.

• Is mating ever really random?

• Positive Assortative Mating (+/+)

• Negative Assortative Mating (+/-)

• Inbreeding: An extreme case of positive assortative mating.

  • Results in a loss of variation, frequent expression of recessive phenotypes.

  • Can have health implications.

  • All societies have cultural taboos, non-human primates have behavioural strategies (shaped by natural selection).

• Example: reduction in the population size of the Florida Panther has led to inbreeding, which is evident in the expression of recessive phenotypes like Kinked-tail.

• Hemophilia: Blood clotting disorder caused by a mutation in Factor VIII.

  • X-linked recessive trait, so XX individuals can be carriers; XY individuals more likely to express.

• Many male members of the European royal families have hemophilia due to inbreeding, resulting in increased expression of recessive traits.

Next Steps

• Quiz #2 (closes Wednesday at 9pm)

• Friday: Tutorial #3: more allele frequency simulation

• Next week: Microevolution II