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UV Radiation and Skin Color
UV intensity varies by latitude; highest at low latitudes, correlates with darker skin in those regions.
Dark skin (more eumelanin) absorbs UV and protects circulating folate, which is important for healthy embryonic development and sperm production.
Skin Color as a Polygenic Trait
Polygenic trait: phenotype determined by alleles at more than one gene.
6 primary genes are involved in skin color; as many as 169 genes may contribute.
Demonstrated variation can be shown with a model using 3 genes with 2 alleles each.
Vitamin D and Folate Trade-Off
High UV radiation: less vitamin D production, BUT folate destruction increases; selection favors darker skin to protect folate (makes a balance between vitamin D synthesis and protects folate).
Low UV radiation: folate destruction is reduced, but vitamin D production is limited; selection favors lighter skin to increase vitamin D production.
Folate protection and vitamin D production influence skin-color evolution; vitamin D deficiency can lead to rickets and neural tube defects.
Evolutionary Forces: Natural Selection, Genetic Drift, and Gene Flow
Natural selection, genetic drift, and gene flow shape allele frequencies and genetic diversity.
Positive selection increases rare advantageous variants; negative selection removes harmful variants; they are not generally separable concepts.
Types of Natural Selection
Directional selection: shifts mean trait value in one direction.
Stabilizing selection: favors average trait values; disfavors extremes.
Disruptive selection: disfavors the mean; favors extremes.
Frequency-dependent selection: fitness of a trait depends on how common it is.
Positive frequency-dependent selection: common phenotypes have higher fitness.
Negative frequency-dependent selection: rare phenotypes have higher fitness; helps maintain diversity.
Directional Selection Example
Beak depth in finches during drought: larger beaks favored to eat bigger seeds; population mean beak size increases over time.
Stabilizing Selection Example
Human birth weight: too small or too large reduces survival; optimal weight lies between extremes.
Disruptive Selection Example
Flies with life cycles aligned to apple vs hawthorn fruit: those synchronized with a specific fruit have higher fitness; extremes favored over the mean.
Frequency-Dependent Selection Examples
Positive: Heliconius butterflies — many morphs are toxic; common morphs are learned by predators to avoid; rare morphs suffer higher predation.
Negative: Grove snails — common shell types are preyed upon more; rare shell types gain a fitness advantage, maintaining variation.
Example: Natural Selection Pattern Questions (hornbill and others)
Climate change can shift selective pressures (e.g., pigment in male hornbills) toward different optimum values; the pattern depends on how pigment affects fitness under new conditions.
Genetic Drift: Random Change in Allele Frequencies
Genetic drift is random change in allele frequencies due to sampling; it does not adapt populations.
Occurs in all finite populations; effects are stronger in small populations.
Bottleneck Effect
Sudden reduction in population size changes allele frequencies and reduces genetic diversity (e.g., northern elephant seals).
Founder Effect
New population started by a few individuals leads to loss of genetic variation; Amish show high frequency of rare recessive alleles due to founder effects.
Gene Flow (Migration)
Transfer of alleles between populations can introduce new alleles or alter existing frequencies.
Gene flow can constrain local adaptation and prevent genetic divergence.
Factors: habitat fragmentation, species mobility, location (islands), and corridors enabling movement.
Phylogeny and Classification
Taxon: named group; may or may not be a clade.
Clade (monophyletic group): an ancestor and all its descendants.
Phylogenetic trees depict evolutionary relationships and common ancestry.
Homologous vs Analagous Traits; Convergent Evolution
Homologous traits: shared due to common ancestry; used to build phylogenies (e.g., arm bones across vertebrates).
Analogous traits (convergent evolution): similar traits arising independently due to similar selection pressures (not from a common ancestor).
To determine homology, examine the trait in the common ancestor.
Ancestral vs Derived Traits
Ancestral trait: present in the common ancestor of a group.
Derived trait: different from the ancestral trait in descendants; used to define clades.
Species Concepts and Speciation
Morphological species concept: based on appearance and physical traits.
Lineage (phylogenetic) species concept: species as branches on the tree of life.
Biological species concept: populations that are actually or potentially interbreeding and reproductively isolated from others.
No single species definition fits all organisms.
Speciation: evolution of reproductive isolation within a population.
Allopatric speciation: geographic isolation leads to genetic isolation and divergence.
Sympatric speciation: divergence without geographic barriers; often via polyploidy in plants.
Polyploidy: extra chromosome sets; autopolyploidy (same species) vs allopolyploidy (hybridizing species).
Prezygotic and Postzygotic Barriers
Prezygotic barriers: habitat, temporal, behavioral, mechanical, and gametic isolation.
Postzygotic barriers: hybrid inviability, hybrid infertility, and hybrid breakdown.
Allopatric Speciation Details
Geographic barriers (dispersal or vicariance) separate populations.
Divergence via drift or selection leads to reproductive isolation when barriers persist.
Sympatric Speciation and Polyploidy
Speciation without geographic separation; polyploidy can cause immediate reproductive isolation (common in plants).
Evolution of Genes and Genomes
Gene duplication provides raw material for new functions; four possible fates:
One copy becomes nonfunctional (pseudogene).
One copy acquires new function (neofunctionalization).
Both copies retain original function but diverge in expression (subfunctionalization).
Both copies retain original function.
De novo genes and orphan genes arise from non-coding DNA.
Horizontal (lateral) gene transfer moves genes between distant species.
Antifreeze proteins in Antarctic icefish arose via gene duplication and neofunctionalization; in icefish, globin genes were disrupted, removing hemoglobin.
Notothenioids illustrate genome evolution: antifreeze proteins evolved from originally unrelated genes; some lineages lost previous gene function entirely.
Molecular Signatures of Evolution
Synonymous (S) vs nonsynonymous (N) substitutions reveal selection patterns:
N/S = 1: neutral evolution
N/S > 1: positive selection for change
N/S < 1: purifying (negative) selection
Molecular clock: DNA/protein sequences accumulate changes at a relatively constant rate; divergence time can be inferred by comparing sequence differences across species.
Calibration of clocks requires independent data (fossils, known divergence times, biogeography).
Practical Genomics: Interpreting Selection and Divergence
Higher N/S suggests adaptive changes in protein sequence.
Pseudogenes and histone genes show different selective pressures; pseudogenes drift neutrally.
Genome size and gene number vary widely; not strictly tied to organismal complexity.
Gas Exchange, Size, Scale, and Temperature
Size and SA/V ratio: smaller, thinner objects have larger SA/V; larger, thicker objects have smaller SA/V. For similar shapes, SA/V ∝ 1/size.
Ventilation know the definition
Diffusion drives gas exchange; diffusion path length (L) and surface area (A) matter.
Fick’s law (gas exchange): Q = rac{D A (P1 - P2)}{L}
Gases diffuse along concentration gradients; diffusion is passive.
Respiratory media: air vs water differ in O2 partial pressures, density, and viscosity; air generally supports easier diffusion than water.
Fish maximize O2 uptake with countercurrent flow across gills; water flows over lamellae where gas exchange occurs; lamellae minimize diffusion distance (L).
Hemoglobin, Oxygen Transport, and the Bohr Effect
Oxygen dissociation curve: Hb saturation vs PO2 shows Hb affinity changes with pH and temperature.
Left shift (higher Hb-O2 affinity): lower PCO2, higher pH, lower temperature.
Right shift (lower Hb-O2 affinity): higher PCO2, lower pH, higher temperature.
Bohr effect: lower pH (more acidic conditions in respiring tissues) lowers Hb affinity, promoting O2 release.
Endotherms vs Ectotherms and Thermoregulation
Endotherms: rely on metabolic heat to regulate body temperature (mammals, birds).
Ectotherms: rely on external heat sources (reptiles, most fish, invertebrates).
Heterotherms: switch between endothermic and ectothermic strategies.
Countercurrent heat exchange and specialized vasculature help conserve heat in endotherms.
Thermoneutral zone and metabolic rate relationships vary between endotherms and ectotherms.
Integrating Concepts for the Exam
Evolutionary forces produce changes in allele frequencies over generations via mutation, drift, selection, and gene flow.
speciation arises when gene flow is reduced and genetic divergence increases; allopatric vs sympatric pathways are common routes.
Phylogeny and classification rely on homologous traits and ancestral/derived character states to identify clades.
Molecular data (DNA sequences) provide insight into selection pressures, divergence times, and genome evolution.
Quick Reminders for the Exam
Distinguish between types of selection using changes in trait distributions and allele frequencies.
Be able to explain SA/V trade-offs, diffusion principles (Fick’s law), and how diffusion limits size and metabolism.
Understand how gene duplication and HGT contribute to genome novelty.
Be able to read a phylogenetic tree: root, nodes, clades, and the meaning of ancestral vs derived traits.
Know prezygotic vs postzygotic barriers and examples of allopatric vs sympatric speciation.
purifying selection
Note: LaTeX-ready formulas included where relevant. For quick review, focus on the core relationships and definitions above.