Chapter 22: Evolution by Natural Selection Vocabulary

Fundamentals of Evolution and Natural Selection

• Definition of Evolution: Evolution is defined as a change in gene frequencies in populations over time.

• Role of Natural Selection: Natural selection is the process that explains the overall pattern of evolution.

• Differential Survival and Reproduction:

  • Individuals possessing genes that confer better survival capabilities produce more offspring.

  • Individuals lacking the necessary survival genes produce fewer offspring or face death.

• Loss of Alleles: Deleterious alleles are expected to be lost from a population over time due to selection against them.

Historical Context and Evolutionary Thought

• Development of Evolutionary Theory: Modern evolution by natural selection is primarily attributed to Charles Darwin and Alfred Russel Wallace, building upon prior historical perspectives.

• Plato:

  • Proposed that every organism is perfect and unchanging.

  • Viewed species as static entities.

• Aristotle:

  • Formulated the "Great Chain of Being" (scale of nature).

  • Positioned organisms on a fixed scale from lower to higher forms.

• Duration of Static Views: The static perspectives of Plato and Aristotle dominated Western biological thought for approximately $2000$ years.

• Jean-Baptiste Lamarck:

  • Proposed one of the first formal hypotheses stating that species change over time.

  • Retained the concept of the scale of nature.

  • Hypothesized the inheritance of acquired traits, suggesting that individual organisms change in response to their environment and pass those modified traits to their offspring.

• Darwinian vs. Lamarckian Model:

  • Lamarckian Model: Spontaneous generation of simple organisms moving up a scale over time, with individuals acquiring physical modifications during their lifetime and passing them to progeny.

  • Darwinian/Wallace Model: Evolution occurs via variation among individuals within a population. Subtle individual variations lead to differential success in producing offspring at the population level. Emphasizes descent with modification from common ancestors.

Temporal Scale and Evolutionary Lineages

• Proboscidea Lineage:

  • Temporal Range: Paleocene to Holocene ($60.0$ to $0.0$\,million years ago, or $\text{MYA}$).

  • Represents a deep evolutionary temporal scale showing structural changes across taxa (including taxa labels such as Goron, Deve, P, PD, G, and Ba).

Evidence for Change Through Time

• The Fossil Record:

  • Comprises all identified fossils discovered on Earth.

  • A fossil is defined as a remnant or trace of an organism from the past.

  • Forms of Fossilization:

  • Hard substances: Bones, shells, branches, or dung.

  • Impressions: Tracks or prints left in soft clay and sediments.

  • Preserved specimens: Organisms trapped in air- or water-tight environments, such as amber and resins.

• Vastness of Geologic Time:

  • Sedimentary rocks form in layers (strata) over time from deposited sand and sediment, establishing relative dating categories (older versus younger layers).

  • Atomic dating combined with sedimentary layers establishes the absolute geologic time scale.

• Extinction as Evidence:

  • Extinction demonstrates that the species present on Earth change over time.

  • Fossils confirmed that extinct organisms were truly gone, rather than merely undiscovered extant species.

  • Proves that species are dynamic rather than static, and that change can occur gradually.

• Transitional Features:

  • Intermediate traits linking older ancestral species to younger derived species.

  • Supports the Law of Succession, which states that extinct fossil species in a geographic region are structurally similar to extant species in the same region.

  • Example: Fin-to-Limb Transition (older to younger fossils measured in million years ago, $\text{mya}$):

  • Eusthenopteron ($\sim 385$\,mya): Shows early finray structures and humerus.

  • Tiktaalik ($\sim 375$\,mya): Demonstrates transitional limb elements.

  • Acanthostega ($\sim 365$\,mya): Shows early digit formation and wrist structures.

  • Tulerpeton ($\sim 362$\,mya): Fully formed limb elements with distinct distal components, ulna, radius, and humerus.

• Vestigial Traits:

  • Incompletely developed or reduced structures that have no function or reduced function in a species.

  • Direct evidence against the concept of absolute structural perfection.

  • Examples include reduced human coccyx (tailbone) or vestigial hip bones in whales.

• Contemporary Observations of Evolution:

  • Evolution is observed in real-time within human lifespans.

  • Examples include the development of antibiotic resistance in bacteria, insecticide resistance in insects, and rodenticide resistance in pest populations.

Evidence for Common Ancestry and Homology

• Geographic Proximity of Similar Species:

  • Closely related species are routinely found in the same geographic region, a pattern particularly pronounced in island ecosystems.

  • Geographic distribution reflects underlying phylogenetic branching patterns.

• Homology:

  • Similarity among species resulting from shared common ancestry.

  • Three Structural Levels of Homology:

  1. Genetic Homology: Similarities in DNA, RNA, or amino acid sequences inherited from a common ancestor.

     • Example: Amino acid sequences from a portion of the human Aniridia gene product are $90\%$ identical to those encoded by the Drosophila eyeless gene product.

     • Human Aniridia Sequence: LQRNRTSFTQEQIEALEKEFERTHYPDVFARERLAAKIDLPEARIQVWFSNRRAKWRREE

     • Fruit Fly eyeless Sequence: LQRNRTSFTNDQIDSLEKEFERTHYPDVFARERLAGKIGLPEARIQVWFSNRRAKWRREE

  2. Developmental Homology: Structural or functional similarities observed in embryonic stages or developmental processes due to shared ancestry.

     • Example: Early embryonic stages of a chick, human, and house cat show striking structural resemblances, including pharyngeal pouches and embryonic tails.

  3. Structural Homology: Similarities in adult organismal morphology due to common ancestry.

     • Example: Vertebrate forelimbs across diverse organisms (Human, Horse, Bird, Bat, Seal) serve vastly different functions (grasping, running, flying, swimming) but share identical underlying bone arrangements: Humerus, Radius and Ulna, Carpals, Metacarpals, and Phalanges.

• Speciation:

  • The observable process by which new species arise from pre-existing common ancestral populations.

• Internal Consistency:

  • The independent agreement between distinct data sets—including the fossil record, comparative anatomy, genetic homologies, phylogenies of extinct/extant taxa, relative dating, and absolute radiometric dating.

  • Demonstrated in the transitional fossil lineage of whales (e.g., Pakicetus).

The Mechanism of Natural Selection

• Influencing Factors on Darwin's Theory:

  • Insights from artificial selection in agriculture and breeding.

  • Principles of uniformitarian geology formulated by Charles Lyell.

  • Population principles and competition models described by Thomas Malthus.

  • Botanical observations and classifications by Joseph Dalton Hooker.

• Darwin's Four Postulates:

  1. Variation: Individuals within a population differ in the traits they possess.

  2. Heritability: Some trait differences are passed down to offspring genetically.

  3. Variable Survival and Reproductive Success: In every generation, more offspring are produced than can survive; thus, survival and reproduction are competitive.

  4. Non-Random Mortality and Reproduction: Individuals with certain heritable traits are more likely to survive and reproduce. Natural selection is the differential reproduction of individuals based on heritable trait variation.

• Fitness and Adaptation:

  • Biological Fitness: The relative ability of an individual to produce viable, fertile offspring compared to other individuals in the population. Fitness is a quantifiable, measurable parameter.

  • Adaptation: A heritable trait that increases the biological fitness of an individual in a specific environment relative to individuals lacking the trait.

  • Clarification: Selection does not involve purposeful or conscious choice; it is strictly differential survival and reproduction.

Case Studies of Evolution in Action

• Evolution of Antibiotic Resistance (Mycobacterium tuberculosis):

  1. A patient suffers from a lung infection caused by a large population of Mycobacterium tuberculosis bacteria. A tiny fraction of these bacterial cells randomly carry a point mutation in the rpoB gene (a cytosine-to-thymine, or C-T, mutation).

  2. Antibiotic drug therapy is administered, killing almost all normal M. tuberculosis cells. The patient's symptoms clear, and therapy ends. However, the small proportion of surviving cells are drug-resistant mutants.

  3. The mutant, drug-resistant bacterial cells proliferate, resulting in a secondary, major lung infection. The patient becomes ill once again.

  4. A second round of drug therapy is initiated but is completely ineffective against the mutant drug-resistant strain, leading to patient death.

  • Resistance can also spread horizontally between species (e.g., Escherichia coli) via plasmid transfer.

• Evolution of Beak Morphology in Galapagos Finches:

  • Measurable shifts in average beak size and shape occur in response to changing environmental selective pressures across seasons.

  • Serves as natural field experiments demonstrating directional changes in population traits over time.

Common Misconceptions and Evolutionary Constraints

• Individual vs. Population: Individuals do not evolve during their lifetimes; natural selection acts on individuals, but evolution changes gene frequencies within populations over generations.

• Non-Lamarckian: Acquired characteristics during an organism's life are not incorporated into the genome or inherited by offspring.

• Absence of Teleology: Evolution is not goal-directed. Adaptations do not occur because an organism "wants" or "needs" them.

• Role of Chance: Evolutionary variation arises through stochastic mutations, not purposeful design.

• Non-Progressive: Evolution does not move toward a pre-determined endpoint or produce "higher" or "lower" organisms.

• Lack of Absolute Perfection: Evolution does not yield perfect organisms; it acts as a "tinkerer," modifying existing pre-existing structures.

• Traits Are Not Always Adaptive: Non-adaptive traits persist due to structural, genetic, or developmental linkage.

• Evolutionary Constraints:

  • Fitness Trade-offs: Selection favoring a trait in one context may compromise performance in another.

  • Genetic Constraints: Selection is limited by available genetic variation and pleiotropic effects.

  • Historical Constraints: All adaptations are modifications of pre-existing ancestral structures.

  • Environmental Constraints: Abiotic and biotic conditions limit the direction and extent of adaptation.

Key Terminology

• Evolution: Change in gene frequencies within a population over time.

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype/heritable traits.

• Artificial Selection: Selective breeding of plants and animals by humans to encourage desirable traits.

• Fitness: The relative reproductive success of an individual in producing fertile offspring.

• Adaptation: A heritable trait that enhances an organism's survival and reproductive success in a given environment.

• Extinct / Extant: Extinct refers to a species that no longer exists; extant refers to a species currently living.

• Transitional Feature: A trait in a fossil organism that is intermediate between ancestral and derived forms.

• Vestigial Trait: A reduced or rudimentary structure with little to no function, residual from an ancestor.

• Speciation: The evolutionary process by which populations evolve to become distinct biological species.

Overview of Phylogenies and the History of Life

• Phylogenetic Analysis: Explores the evolutionary relationships and historical lineages connecting extinct and extant organisms across deep time.