chapter 18

COLLEGE PHYSICS & BIOLOGY 2E: EVOLUTION AND THE ORIGIN OF SPECIES

DARWIN AND NATURAL SELECTION

  • Charles Darwin and the H.M.S. Beagle: Darwin's voyages on the Beagle provided empirical evidence for his theories regarding evolution.

  • Natural Selection: The mechanism by which evolution occurs, relying on the differential survival and reproduction of organisms due to variations in their traits.

  • Darwin’s Postulates:

    • Heritability: Most characteristics are passed from parent to offspring, suggesting a genetic basis for traits.

    • Overproduction and Competition: More offspring are produced than can survive, leading to competition for limited resources.

    • Variation: Characteristics of offspring vary, and many of these variations are heritable.

    • Adaptation: Over time, favorable adaptations lead to better survival and reproduction in particular environments.

DARWIN’S OBSERVATION

  • Darwin observed variations in the beak shapes among finch species.

  • He hypothesized that the beak of an ancestral species adapted over time, facilitating access to diverse food sources, exemplifying natural selection in action.

DARWIN AND WALLACE

  • Both Charles Darwin and Alfred Wallace independently proposed the theory of natural selection.

  • Their scientific papers were presented jointly to the Linnean Society in 1858, establishing a foundation for evolutionary biology.

OVERVIEW: EVIDENCE OF EVOLUTION

  • Divergent Evolution vs. Convergent Evolution: Different evolutionary paths leading to distinct characteristics in related species versus similar adaptations in unrelated species.

  • Fossil Record: Provides chronological evidence of past life forms and their evolution over time.

  • Homologous Structures: Anatomical features in different organisms that have similar structures due to common ancestry.

  • Vestigial Structures: Features that have lost their original functions through evolution.

  • Biogeography: Distribution of species and ecosystems around the globe, influenced by historical events like continental drift.

  • Molecular Biology: DNA analyses reveal genetic similarities among closely related organisms supporting evolutionary connections.

NATURAL VERSUS ARTIFICIAL SELECTION

  • The primary focus is on natural selection, whereas artificial selection has been practiced by humans for thousands of years, particularly in agriculture and animal husbandry.

    • Crops: Various food plants selectively bred for desirable traits.

    • Domesticated Animals: Breeding selected for specific phenotypes.

  • Selective Breeding: A human-driven process that has led to the creation of hybrids and distinct breeds.

  • In his seminal work, Origin of Species, Darwin contrasts artificial selection with natural selection, emphasizing its groundwork in selective breeding practices.

HOMOLOGOUS STRUCTURES

  • Examples of homologous structures include:

    • Human: Arm adapted for manipulation

    • Dog: Forelimb adapted for running

    • Bird: Wing adapted for flying

    • Whale: Flipper adapted for swimming

  • The similarity in these structures indicates that these organisms share a common ancestor, despite their adaptations to different environments.

CONVERGENCE

  • Convergent evolution is illustrated by the adaptations of the arctic fox and ptarmigan, which have developed similar traits due to analogous environmental pressures, rather than shared ancestry.

  • This results in independently evolving similar phenotypes in distantly related species, driven by similar selection pressures such as predation.

BIOGEOGRAPHY

  • The study of biogeography reveals how species develop and are distributed based on geographic location:

    • Groupings of species that evolved prior to Pangaea are found globally but as different species.

    • Groups that emerged post-breakup appear only in certain regions, such as Australian marsupials and Madagascar lemurs.

    • Species separated by geographical barriers like mountains or rivers demonstrate reduced diversity.

MOLECULAR BIOLOGY

  • DNA analysis offers recent support for evolution by demonstrating that closely related species share genetic similarities.

  • Mutation-driven evolution can lead to the emergence of new protein functions, reinforcing the connection between genetics and evolutionary theory.

MISCONCEPTIONS OF EVOLUTION

  • Common misconceptions include:

    • Evolution is merely a theory, not a fact.

    • Individuals undergo evolution rather than populations.

    • Evolution is solely concerned with the origin of life.

    • Organisms evolve with intention or purpose.

FORMATION OF NEW SPECIES

  • Species: Defined as a group of organisms that can interbreed to produce viable, fertile offspring.

  • Hybrid: A result of breeding between two separate species.

  • Gene Pool: The total collection of genes and their variants in a species.

SPECIATION

  • Allopatric Speciation: Involves geographic isolation leading to reproductive isolation. Factors include:

    • Dispersal: Movement of organisms from one location to another.

    • Vicariance: Geographic barriers naturally arising, separating populations.

    • Adaptive Radiation: Rapid evolution of diversely adapted species from a common ancestor.

  • Sympatric Speciation: Occurs without geographic isolation. Mechanisms include:

    • Chromosomal Errors: E.g., aneuploidy, autopolyploidy, allopolyploidy.

    • Reproductive Isolation: Can be prezygotic (before fertilization) or postzygotic (after fertilization).

SPECIATION EXAMPLES

  • Illustration in Darwin's On the Origin of Species: A diagram showcasing various speciation events contributing to biological diversity, akin to modern phylogenetic trees.

  • Example: Modern elephants are descended from the Palaeomastodon, existing 35–50 million years ago in Egypt.

ALLOPATRIC SPECIATION

  • Example shown with the northern spotted owl and the Mexican spotted owl, illustrating geographic separation leading to varying adaptations.

  • Speciation occurs when populations become isolated over long periods, resulting in genetic divergence and reproductive incompatibility.

  • Dispersal vs. Vicariance: Dispersal happens when individuals relocate, while vicariance occurs when a physical barrier becomes established.

ADAPTIVE RADIATION

  • Honeycreeper birds: An illustration of adaptive radiation, where one ancestral species has given rise to multiple new species, each adapted to different ecological niche.

SYMPATRIC SPECIATION

  • Speciation taking place within the same geographical region.

  • Causes include chromosomal errors such as:

    • Aneuploidy: Having too few or too many chromosomes.

    • Autopolyploidy: Duplication of chromosome sets resulting from errors in meiosis.

    • Allopolyploid: Combination of gametes from two different species resulting in viable offspring.

SYMPATRIC SPECIATION EXAMPLES

  • Aneuploidy results from nondisjunction during meiosis, leading to gametes with abnormal chromosome counts (e.g., resulting offspring may have 2n+1 or 2n-1 chromosomes).

  • Autopolyploidy occurs when mitosis is not followed by cytokinesis, producing offspring with double sets of chromosomes (4n).

  • Alloploidy results from the fusion of gametes from distinct species, requiring multiple matings to produce viable offspring.

REPRODUCTIVE ISOLATION

  • Prezygotic Barriers:

    • Temporal isolation: Different breeding times.

    • Habitat isolation: Different habitat preferences despite proximity.

    • Behavioral isolation: Species-specific actions impact reproduction directly.

  • Postzygotic Barriers:

    • Hybrid inviability: Ability to conceive without successful development.

    • Hybrid sterility: Hybrid offspring may be born but cannot reproduce.

TEMPORAL ISOLATION

  • Illustrated through two related frog species, Rana aurora (breeding early in the year) versus Rana boylii (breeding later). Examples provided through image modifications.

HABITAT ISOLATION

  • Speciation can result from differing habitat preferences, as demonstrated by two cricket species: Gryllus pennsylvanicus (sandy soil) versus Gryllus firmus (loamy soil).

GAMETIC BARRIER

  • Male damselfly's reproductive organ shape varies among species; only compatible with corresponding female species.

  • Flowers adapt to specific pollinators; for instance:

    • Foxglove Flower: Wide shape, favored by bees.

    • Trumpet Creeper: Tube-shaped, adapted for hummingbirds.

RECONNECTION

  • Hybrid Zones: Regions where two closely related species interact and breed.

  • Reinforcement: Hybrid fitness leads to divergence of species, where hybrids are less fit, thus promoting greater genetic separation.

HYBRID ZONE

  • After speciation, closely related species may breed in hybrid zones, leading to either reinforcement, fusion, or stability based on their reproductive barriers and hybrid fitness profiles.

RATES OF SPECIATION

  • Gradual Speciation: Change occurs gradually over time, with small, cumulative differences leading to divergence.

  • Punctuated Equilibrium: Species undergo significant change within short bursts of evolution, followed by extended periods with little change.

RATES OF SPECIATION EXAMPLES

  • In Gradual Speciation, species diverge slowly as traits evolve incrementally. Conversely, in Punctuated Equilibrium, harmonic change occurs rapidly before entering lengthy stasis periods.