Biological Evolution Comprehensive Lecture Series

Introduction to the Study of Evolution

• The study of evolution in this module covers a vast array of life forms, including microbes, protists, plants, invertebrates, and vertebrates.

• Every module in the course follows a structured approach:

  • Defining characteristics and features.

  • Evolution and origins of the specific group.

  • Diversity (the different organisms comprising the group).

  • Lifestyles, including form and function (feeding, reproduction, movement, etc.).

• Understanding evolution is critical to answering how life transitioned from single cells to millions of complex species and why certain groups share features.

• Evolution provides the mechanisms behind biological diversity.

Historical Context and Timeline

• Scaling the history of the universe ($13.8 \times 10^9$ years) into a single calendar year provides perspective on human history:

  • The present day would represent only the last $0.1$ seconds of that year.

  • Dinosaurs would have gone extinct yesterday.

  • Charles Darwin would have been born less than $1$ second ago.

• Scientific evolution involves observation, forming hypotheses, and testing those hypotheses via rigorous methods.

Evolutionary Thought Before Darwinism

• Aristotle ($384-322$ BC): Developed the 'scala naturae' or scale of nature. This was a naturalistic model based on logic and philosophy rather than supernatural intervention. He viewed each form as perfect, permanent, and fixed in a specific space on a scale of increasing complexity. This model heavily influenced Western science but is not supported today.

• Islamic Golden Age Contributions: Textbooks often omit the historical continuity of evolutionary ideas from Muslim scholars between the $8\text{th}$ and $13\text{th}$ centuries.

  • Scholars postulated ideas such as adaptation, survival of the fittest, the origin of humans from apes/monkeys, evolutionary constraints, extinctions, and hereditary variability.

  • Nasģr al-Dģn Tţsģ ($1201-1274$ AD): This Persian polymath proposed basic evolutionary concepts approximately $600$ years before Darwin. In his work Akhlaq-i-Nasri, he noted that organisms gaining new features faster are more variable and gain advantages. He stated that bodies change as a result of internal and external interactions.

Development of Evolutionary Theory and Influential Figures

• Carl von Linn (Linnaeus) ($1707-1778$): A Swedish botanist known as the founder of taxonomy. He developed the binomial system for naming species and a nested system of classifications where similar species are placed in a genus, similar genera in a family, and so on (published in $1735$).

• James Hutton ($1795$): Proposed the principle of gradualism, suggesting that geological features are the product of slow, continuous processes.

• Thomas Malthus ($1798$): A British political economist who wrote "Essay on the Principle of Population." He observed that plants and animals produce more offspring than can survive, leading to a struggle for survival. This was a response to declining living conditions in $19\text{th}$-century England.

• Jean Baptiste de Lamarck ($1809$): Published a hypothesis of evolution suggesting that species change over time in response to their environment.

  • He suggested species are plastic and can be modified; characteristics acquired during an organism's life are passed to offspring (e.g., giraffes stretching their necks).

  • He promoted the idea of gradualism but included the idea that organisms strive for perfection through "will," which is rejected by modern science because acquired characteristics are not inherited.

• George Cuvier ($1769-1832$): A French paleontologist who developed the study of fossils. He recognized fossils as evidence of earlier life and observed changes in rock strata. He advocated for Catastrophism, the idea that successive catastrophes (like floods) periodically wiped out most life forms.

• Charles Lyell ($1797-1875$): A British geologist who promoted Uniformitarianism in his work Principles of Geology ($1830$). He argued that geological features result from slow, gradual changes and that the laws of nature remain constant, allowing modern phenomena to explain the past.

• Alfred Russel Wallace ($1858$): While studying in the Malay Archipelago, Wallace independently arrived at the hypothesis of natural selection and sent his ideas to Darwin, prompting Darwin to publish his own work.

Charles Darwin and the Theory of Natural Selection

• Early Life: Born in Shrewsbury, England ($1809-1882$). He dropped out of medical school at Edinburgh University and later studied religion at Cambridge, where he met botanist John Henslow.

• HMS Beagle Voyage ($1831-1836$): Darwin served as an unpaid naturalist on a $5$-year global voyage. He collected plants, insects, animals, and fossils, and studied geological formations.

  • Argentina: Found fossils of the Glyptodont and noted their similarity to living armadillos. He also found Megatherium (giant ground sloth) fossils, noting the relationship between extinct ground-dwellers and living tree sloths.

  • Andes Mountains: Found marine invertebrates and seashells high in the mountains, concluding that gradual geological processes caused the uplift.

  • Cape Town ($1836$): Visited the Sea Point contact zone where igneous granite intruded into the Malmesbury group rocks.

  • Gal
    pagos Islands: Located $950\,km$ off South America. Darwin observed that species varied by island.

    • Finches: $14$ species existed on the islands compared to $1$ on the mainland. They exhibited diverse beak sizes and shapes suited to specific food sources (seeds, cactus, insects). This is a hallmark of Adaptive Radiation.

    • Tortoises: Differed in shell shape (saddle-backed for reaching tall cacti vs. dome-shaped for ground vegetation).

    • Scalesia: Daisy/sunflower relatives that grew to tree size, with different species on different islands.

• The Theory (1859): Darwin published On the Origin of Species.

  • The central idea is Descent with Modification: all life shares a common ancestor and species accumulate differences over generations as they adapt to environments.

  • Natural Selection is the mechanism. It involves differential reproductive success: organisms with traits better suited to their environment produce more offspring.

Darwin's Postulates and Observations

• Observation 1: More offspring are born than can survive to reproduce.

• Observation 2: Variety exists within all species.

• Inference 1: Individuals better adapted to the environment are more likely to survive and reproduce ("Survival of the Fittest").

• Inference 2: If adapted characteristics are inherited, the nature of the species change slowly over time.

• Darwin could not explain how organisms change or how traits are inherited; these questions remained until the advent of genetics.

Clarifying Common Misconceptions

• Evolution is not a linear "march of progress" (e.g., from ape to human). It is a branching process illustrated by evolutionary trees.

• Natural selection does not have foresight and does not create variants; it selects among existing variations.

• Acquired characteristics (e.g., a bodybuilder's muscles) are not passed to offspring.

Evidence for Evolution

1. The Age of the Earth

• The Earth is approximately $4.6 \times 10^9$ years old.

• It formed from a solar nebula that contracted due to gravity, with remaining material forming planetesimals through accretion.

• Radiometric Dating: Based on the fixed decay rate (half-life) of radioactive elements.

  • Uranium-238 ($U^{238}$): Decays to Lead-206 ($Pb^{206}$) with a half-life of $4.5 \times 10^9$ years.

  • Carbon-14 ($C^{14}$): Decays to Nitrogen-14 ($N^{14}$) with a half-life of $5730$ years.

• Stratigraphy: The study of rock layers (strata). For example, the Grand Canyon is $1900\,m$ deep with strata dating back $1.2 \times 10^9$ years.

2. The Fossil Record

• Taphonomy: The study of changes to an organism from death until discovery, including decomposition, burial, and fossilization.

• Key Transitions and Examples:

  • Oldest life: Microfossil traces in the Barberton Greenstone Belt (South Africa) and Greenland date to $3.5-3.75 \times 10^9$ years ago.

  • Stromatolites: Colonies of cyanobacteria appearing $3.5 \times 10^9$ years ago.

  • Multicellularity: Emerged $2.1 \times 10^9$ years ago (Gabon fossils).

  • Early Animals: Otavia antiqua ($760\,mya$ sponge-like fossils from Namibia).

  • Ediacaran Fauna: Soft-bodied organisms from $575\,mya$.

  • Cambrian Explosion ($545\,mya$): A rapid diversification (over $5-10$ million years) giving rise to all extant phyla. Found in the Burgess Shale.

  • Transitional Forms:

    • Archaeopteryx ($150\,mya$): Intermediate between dinosaurs and birds. Shared dinosaur features (teeth, long tail, claws) and bird features (feathers, fused clavicle, wings).

    • Tiktaalik roseae ($375\,mya$): Intermediate between fish and tetrapods. Had scales and fins but also a mobile neck, wrist joints, and ribs.

    • Diarthrognathus ($250\,mya$): Intermediate between reptiles (cynodonts) and mammals, specifically regarding the two-jointed jaw.

    • Pakicetus ($50\,mya$): Intermediate between hoofed mammals (artiodactyls) and whales.

3. Morphology and Cladistics

• Cladogram: A visual hypothesis representing how taxa are related through common ancestry.

• Monophyletic Clade: A group consisting of an ancestor and all its descendants.

• Paraphyletic Group: Consists of an ancestor and some, but not all, descendants.

• Polyphyletic Group: A grouping that lacks the common ancestor of all species in the group.

• Homology: Shared traits inherited from a common ancestor (e.g., four limbs in tetrapods).

• Synapomorphy: A newly derived shared trait defining a specific subgroup.

• Analogy / Convergent Evolution: Superficially similar traits that evolved independently to serve the same function (e.g., wings of birds vs. bats; body shapes of sharks vs. dolphins).

• Vestigial Structures: Remnants of features that served functions in ancestors (e.g., limb bones in pythons/whales, human tailbone, wisdom teeth used for grinding plant tissue human ancestors could not digest easily).

4. Molecular and Biochemical Evidence

• All life shares a genetic code ($DNA$).

• Genetic Similarity: Humans share $99\%$ genes with chimps, $66\%$ with daffodils, and $40-50\%$ with bananas.

• Specific Human Genes:

  • HAR1 (Human Accelerated Region 1): An $118$ letter sequence that changed significantly since humans/chimps diverged; involved in cerebral cortex folding.

  • ASPM: Mutations in this gene can reduce brain size.

• Biochemical pathways: Myoglobin (oxygen storage in muscle) and Hemoglobin (oxygen transport in blood) show common ancestry. The cytochrome c oxidase gene allows for comparison based on the proportion of bases shared.

5. Embryology

• Ontogeny Recapitulates Phylogeny: Although simplified, embryonic development often mirrors evolutionary history (e.g., human embryos having tails and pharyngeal pouches).

6. Biogeography

• Continental drift explains the distribution of species.

  • Pangaea began as a supercontinent that split into Laurasia and Gondwana.

  • Australia’s isolation led to the adaptive radiation of marsupials, which show convergent evolution with placental mammals in similar ecological niches.

Mechanisms of Evolution

1. Genetic Variation

• Mutation: Random changes in DNA.

  • Point mutations: Change in one base (e.g., Sickle cell disease caused by a substitution in hemoglobin).

  • Alterations: Duplications or deletions (e.g., Down's syndrome, Wolf-Hirschhorn syndrome).

  • Only germ-line mutations are passed to offspring.

• Gene Flow: Movement of genes between populations (e.g., high in fruit flies, low in wind-pollinated corn). It increases variation within a population but reduces differences between populations.

• Sexual Reproduction: Results in genetic shuffling through meiosis (recombination and crossing over).

2. Natural Selection Types

• Stabilizing, Diversifying, or Directional selection.

• Direct Observations: Peppered moths (Biston betularia) changing color due to industrial soot; antibiotic resistance in HIV ($3TC$ drug selects for resistant strains); insecticide resistance in insects.

3. Sexual Selection

• Intrasexual Selection: Competition within one sex (usually males) for mates (e.g., male-male combat, territories, sperm competition).

• Intersexual Selection (Mate Choice): One sex (usually females) chooses a mate. Females often have higher "cost of reproduction" (expensive eggs, gestation, lactation) and choose based on "showiness" of males (leks, dances, ornamentation).

• Trade-off: Showiness can increase predation risk.

4. Genetic Drift

• Random fluctuations in allele frequencies, most impactful in small populations.

• Founder Effect: A few individuals isolate from a large population (e.g., Retinitis pigmentosa on Tristan da Cunha).

• Bottleneck Effect: A sudden event drastically reduces population size (e.g., Northern elephant seals hunted to $20$ individuals; Cheetahs with high inbreeding/abnormalities due to loss of range).

5. Co-evolution

• Species reciprocally affect each other's evolution (e.g., predator/prey, plant/pollinator).

• Orchid and Hawk Moth: Darwin predicted an insect with a $30\,cm$ proboscis to match the Angraecum sesquipedale orchid’s nectary; it was discovered $60$ years later.

• Co-speciation: Parallel speciation in interacting species (e.g., gophers and their lice).

Speciation and Reproductive Isolation

• Species Definition: A group of individuals that can interbreed in nature and produce viable, fertile offspring.

• Prezygotic Barriers (Impede mating/fertilization):

  • Habitat: Different areas.

  • Temporal: Different breeding times.

  • Behavioral: Unique courtship (e.g., Blue-footed boobies).

  • Mechanical: Incompatible anatomy.

  • Gametic: Sperm/egg cannot fuse.

• Postzygotic Barriers (Prevent hybrid success):

  • Reduced hybrid viability: Hybrids are frail.

  • Reduced hybrid fertility: Hybrids are sterile (e.g., Mules).

  • Hybrid breakdown: Future generations are sterile or feeble.

• Modes of Speciation:

  • Allopatric: Geographically isolated (e.g., Grand Canyon squirrels).

  • Sympatric: Occurs in the same area (e.g., fruit flies specializing on hawthorns vs. apples; polyploidy in plants).

  • Parapatric: Continuous population with a geographic gradient (e.g., heavy-metal tolerant plants near mines).

  • Peripatric: Small population isolated at the edge of a larger one.

Macroevolutionary Patterns

• Macroevolution: Large-scale trends over billions of years, reconstructed via geology and fossils.

• Stasis: Lineages that change little (e.g., cockroaches, horseshoe crabs).

• Character Change: Directional or rapid changes (e.g., complexity in ammonite shell sutures).

• Lineage Splitting:

  • Anagenesis: Gradual transformation of one species into another.

  • Cladogenesis: Splitting of a gene pool into branches, creating diversity.

• Tempo:

  • Gradualism: Slow, constant change.

  • Punctuated Equilibrium: Rapid changes followed by long periods of stasis.

Mass Extinctions

• Life has experienced $5$ mass extinctions.

• End-Permian Extinction ($252\,mya$): The largest event; $96\%$ marine and $70\%$ land species died. Causes include Pangaea formation (loss of coastline), extreme volcanism (Siberian Traps), and a possible $480\,km$ asteroid crater in Antarctica.

• Cretaceous-Tertiary (K-T) Extinction ($65\,mya$): Also known as K-Pd. Wiped out non-avian dinosaurs and ammonites.

  • Iridium Anomaly: High iridium concentrations found in the K-T boundary clay layer globally (Alvarez Hypothesis).

  • Asteroid Impact: Chicxulub crater ($180\,km$ diameter) in Mexico. Evidence includes glass spherules (tektites) and shocked quartz.

  • Deccan Traps: Massive volcanic lava beds in India ($500,000\,km^2$).

Evolutionary Developmental Biology (Evo-Devo)

• Heterochrony: Changes in the rate and timing of development.

  • Allometric Growth: Different growth rates for different body parts (e.g., bat finger elongation).

  • Paedomorphosis: Retention of juvenile traits in adults (e.g., axolotl gills, human cranial shape).

• Homeotic Genes: Master regulatory genes that control spatial organization of body parts.

  • Hox Genes: Subset in animals. They occur in the same linear order on chromosomes as the body regions they control. Duplications in Hox clusters are linked to major transitions (Invertebrate to Vertebrate, Fish to Tetrapod).

  • MADS-box Genes: Subset in plants controlling flower parts.

• Developmental Cascade: Begins with Maternal Effect Genes (e.g., Bicoid genes determining anterior-posterior axis via morphogen gradients) $\rightarrow$ Segmentation Genes (Gap, Pair-rule, Segment-polarity) $\rightarrow$ Hox Genes.

Evolution of Complex Structures

• "Irreducible Complexity": The fallacy that certain systems (like the eye) cannot function if any part is missing.

• Eye Evolution: Progression from light-sensitive pigments (rhodopsin) $\rightarrow$ eyecups (direction sense) $\rightarrow$ fluid-filled eyecups (focus) $\rightarrow$ complex camera-type eyes with lenses (independently evolved in insects, molluscs, and vertebrates).

Practical Relevance of Evolution

• Agriculture: Genetic variation prevents total crop loss. Clonal crops (e.g., Irish potato famine) are vulnerable to single diseases. Use of "refugia" can slow pesticide resistance.

• Medicine: Understanding pathogen evolution (zoonoses like Rabies, Malaria, COVID-19) assists vaccine development. Managing Antibiotic Resistance is a global crisis; resistance often appears within years of discovery (e.g., Penicillin discovered $1928$, resistance identified $1940$).

• Conservation: Knowledge of bottlenecks and inbreeding allows for Genetic Rescue—introducing immigrants to an isolated population to increase viable offspring (e.g., isolated Vipers in Sweden, Scandinavian Wolves).