Lecture on Macroevolution and Extinction Events

Macroevolution

I. What Is Macroevolution?

  • Definition: Macroevolution refers to evolutionary processes and patterns occurring over extensive time frames, typically millions of years, that result in significant changes, including:

    • The origin of major groups.

    • Adaptive radiations.

    • Mass extinctions.

  • Key Themes:

    • How lineages diversify.

    • The influence of new ecological opportunities on biodiversity.

    • Patterns of stasis and rapid change observed in the fossil record.

    • Factors that trigger large extinction events.

  • Scope of Study: Macroevolution looks beyond individual populations to examine changes across entire clades throughout deep time.

II. Adaptive Radiation

  • Definition: Adaptive radiation is a process whereby a single ancestral lineage quickly diversifies into multiple descendant species occupying various ecological niches.

  • Characteristics:

    • Leads to significant morphological and ecological diversity.

  • Criteria for Adaptive Radiations:

    1. Common Ancestry: All species involved descend from a singular ancestral species.

    2. Trait Divergence Related to Ecological Performance: Species develop different morphological or physiological traits enhancing performance in distinct environments.

    3. Rapid Speciation: A swift emergence of new species appears over a relatively short evolutionary time frame.

  • Importance of Adaptive Radiations: Many of Earth's diverse groups originated through adaptive radiations, including:

    • Mammals following the extinction of dinosaurs.

    • Flowering plants (angiosperms).

    • Desert lizards.

    • Possibly the Cambrian explosion.

III. Examples of Adaptive Radiation

  1. Darwin’s Finches (Galápagos Islands):

    • Descended from a single ancestral species and diversified into multiple forms with varied beak shapes and diets.

  2. Cichlid Fishes:

    • Repeatedly radiated in African lakes, with many lakes generating similar ecological types (ecomorphs) independently.

  3. Anolis Lizards (West Indies):

    • A combination of adaptive radiation and convergent evolution, each Caribbean island hosts species evolving into similar ecological ecomorphs (e.g., trunk-ground, trunk-crown, twig, grass-bush). These forms evolved independently on each island, showcasing repeated evolution of similar adaptations to analogous environmental challenges.

IV. Convergent Evolution

  • Definition: Convergent evolution occurs when unrelated lineages independently develop similar traits due to occupying comparable ecological niches.

  • Commonality in Adaptive Radiations:

    • Similar environments create analogous ecological opportunities.

    • Some trait combinations are favored repeatedly due to ecological advantages.

  • Example: Different islands produced nearly identical Anolis ecomorphs despite their separate evolutionary histories, indicating strong ecological selection favoring similar forms.

V. Causes of Adaptive Radiation

  • Major Processes Driving Adaptive Radiation:

    1. Ecological Opportunity:

    • Arises when new habitats or resources become accessible, leading to rapid species diversification due to limited competition.

    • Sources of ecological opportunity include:

      • Colonization of unoccupied habitats (e.g., Darwin’s finches, Anolis lizards, Hawaiian honeycreepers, Hawaiian Drosophila).

      • Young islands or isolated ecosystems lacking established species.

      • Mass extinctions, allowing survivors to diversify into the newly available ecological niches (e.g., mammal radiation post-dinosaur extinction).

    1. Key Innovations:

    • Key innovations are novel traits enabling organisms to exploit previously inaccessible resources or environments.

    • Examples of key innovations include:

      • Jointed limbs in arthropods, facilitating diverse locomotion and feeding strategies.

      • Waxy cuticle and stomata in plants, allowing for land colonization.

      • Flight in birds, bats, and insects, which spurred considerable bursts of diversity.

VI. Punctuated Equilibrium

  • Definition: Punctuated equilibrium is a model describing a pattern in the fossil record characterized by:

    • Abrupt appearance of new species.

    • Extended periods of morphological stasis.

    • Concentration of most morphological change during speciation events.

  • Contrast with Darwinian Gradualism: Unlike the gradualism proposed by Darwin which suggests evolution is a slow, continuous process, punctuated equilibrium posits real and common stasis.

  • Foundational Ideas:

    • Proposed by Niles Eldredge and Stephen Jay Gould in 1972.

    • Stasis is real and frequent.

    • Rapid evolutionary change correlates with speciation events.

  • Testing Punctuated Equilibrium: Requires:

    • A thoroughly resolved phylogeny.

    • A well-documented fossil sequence.

    • Evidence showing that ancestral and descendant species coexisted long enough for researchers to assess how change occurred.

  • Case Study: Caribbean Bryozoans demonstrated:

    • Species sustained minimal changes for 2–6 million years.

    • New species emerged relatively quickly (within approximately 100,000–160,000 years).

    • Evident patterns strongly supported the theory of punctuated equilibrium, contrary to the researcher's initial aims to disprove it.

    • Meta-analyses reveal both gradual and punctuated modes of evolution in various lineages.

VII. Stasis and Living Fossils

  • Definition of Stasis: Stasis refers to extended periods where species exhibit minimal morphological changes.

  • Examples of Living Fossils:

    • Ginkgo Trees: Leaves resemble fossils found 40 million years ago.

    • Coelacanths: Considered extinct for 65 million years until rediscovered.

    • Horseshoe Crabs (Limulus): Display extensive genetic diversity yet minimal morphological alteration.

  • Implications of Stasis: The presence of living fossils illustrates that stasis is not due to a lack of genetic variation. Instead, factors such as stabilizing selection, developmental constraints, or ecological stability likely contribute to the maintenance of long-term morphological consistency.

VIII. Extinction

  • Definition: Extinction is the eventual fate of nearly all species that have ever lived.

  • Types of Extinction:

    1. Background Extinction: Ongoing, normal extinction rates resulting from ecological and evolutionary processes.

    2. Mass Extinction: Catastrophic events resulting in the disappearance of over 60% of species within a few million years, characterized by being rapid, global, and affecting numerous lineages.

  • Major Mass Extinction Events: Five critical mass extinction events have been identified in Earth's history.

IX. The K–Pg Mass Extinction

  • Key Features:

    • Occurred approximately 66 million years ago.

    • Led to the extinction of 60–80% of species, including dinosaurs (excluding birds), many marine reptiles, and several plant groups.

    • Following extinction, fungi and ferns proliferated rapidly.

  • Evidence for Asteroid Impact:

    • A global spike of iridium found at the boundary layer, with iridium being rare on Earth yet abundant in meteorites, at more than 100 global locations.

    • A meteor with a diameter of 10–15 km necessary to create this signature.

    • The Chicxulub crater off Yucatán correlating with this timing and size.

  • Consequences of the Impact:

    • Triggered earthquakes (up to a Richter scale of 13).

    • Induced massive volcanic activity.

    • Resulted in global fires.

    • Caused acid rain.

    • Led to global cooling due to atmospheric debris.

    • Generated a tsunami approximately 4 km high that struck North America.

    • This event significantly reshaped Earth's biosphere and created ecological space for mammal radiation.

X. The End-Permian Extinction

  • Key Features:

    • The most severe extinction event in Earth’s history occurring around 252 million years ago.

    • Resulted in 96% of species and over half of all families disappearing.

    • Devastated many marine invertebrates, reef builders, various reptiles, and amphibians.

  • Possible Causes of the End-Permian Extinction:

    1. Sea Level Changes:

    • Significant sea-level drops led to the elimination of most shallow seas.

    1. Changes in Ocean Chemistry:

    • Deep oceans transitioned to anoxic (oxygen-depleted) conditions, potentially due to disrupted circulation from Pangea's formation.

    1. Climate Instability:

    • Enormous volcanic eruptions (Siberian flood basalts) covered an area of 1.5 million km².

    • Alternating periods of cooling and intense warming occurred due to greenhouse gas accumulation.

    1. Synergistic Effects:

    • Interactions among sea-level change, anoxic conditions, and climate variations likely intensified the magnitude of the extinction event.

      • This extinction dramatically altered the course of multicellular life and evolutionary history.