Chapt. 14a Macroevolution – The Long Run

Microevolution and Macroevolution

• Microevolution is defined as the change in allele frequencies within populations.

  • It often occurs quickly enough to be observed from one generation to the next.

  • Scientists can measure the strength of selection and conduct controlled experiments to test evolutionary hypotheses.

• Macroevolution occurs at the species level (speciation) and above.

  • This process often takes millions of years ($1.0 \times 10^6\,yrs$).

  • Evidence for macroevolution must be reconstructed from what is left behind, primarily through the following sources:

    • Fossils: These allow for the identification of large-scale patterns, reconstruction of past climates and environments, and the formulation of hypotheses regarding species distributions in space and time.

    • Living Organisms: Insights can be gained via molecular phylogenetics, genomics, and comparative developmental biology.

• Evo-Devo (Evolutionary Developmental Biology): This field compares the developmental processes of different organisms to determine ancestral relations between taxa and study the evolution of developmental processes.

  • Embryology: Involves the timing and pattern of gene activity, which is largely genetic in nature.

  • Example: Developing dolphin limbs provide a case study for patterns and timing of gene activity.

• High Diversity in Beetles: The high diversity rate observed in beetles is the result of an origination (speciation) rate that is significantly higher than the extinction rate ($\alpha > \Omega$).

Biogeography and Spatial Patterns of Diversity

• Biogeography is the study of geographic patterns of taxa and the processes that result in those observed patterns.

• Spatial Patterns: Diversity is not random; specific parts of the world exhibit higher diversity than others.

  • The Tropics: Generally have higher levels of biodiversity compared to other regions. Some scientists propose this is due to a higher origination rate, while others argue it is due to a lower extinction rate; it may be a combination of both.

• Environmental Influence: Environmental conditions alone cannot explain species distributions. Notable examples include:

  • Llamas: Found in the South American Andes Mountains but not in the North American Rocky Mountains.

  • Rodents: South American mountain rodents share more similarities with species in the South American plains than with species in the Rocky Mountains.

  • Island Species: Islands are dominated by species that are effective dispersers.

• Alfred Russel Wallace: He created the first global map of biogeographical regions, noting that each region has its own distinctive balance of species.

Dispersal, Disjunctions, and Vicariance

• Dispersal: Occurs when individuals colonize new areas, sometimes crossing barriers such as oceans, rivers, mountains, or deserts.

• Disjunctions: Two closely related species that are spatially (physically or geographically) separated.

  • This concept was originally pioneered by A. Gray with plants in the mid-$1800s$.

  • Disjunctions can reveal past land or water connections or long-distance dispersal events between regions.

• Vicariance: The formation of geographic barriers to dispersal and gene flow.

  • Plate Tectonics: The most common process driving vicariance.

  • Vicariants: Term used for disjunct species formed by vicariance.

• Relationship to Phylogeny: If a clade does not disperse after land breakup, the species' phylogeny should reflect the history of the landmass itself (Area Phylogeny). The most closely related clades are typically found on the most recently separated landmasses.

Biogeographic Examples: Marsupials and Camels

• Marsupials: Represent an example of both vicariance and dispersal.

  • Distribution: Most extant species are in Australia, yet the oldest fossils come from China and North America. The Virginia opossum is the only extant species in North America today.

  • Evidence Integration: To understand this pattern, scientists use molecular phylogeny of extant species, the fossil record, and tectonic reconstructions.

  • Key Findings:

    1. All Australian marsupials form a monophyletic group (a common ancestor and all its descendants).

    2. The Australian clade is nested within the South American clade.

    3. Tectonic history: North America separated from the Southern Hemisphere first; then South America separated from Australia and Antarctica; finally, Australia and Antarctica separated.

    4. Marsupials lived in Antarctica when it was warmer and were more similar to Australian species than South American ones.

• Camels: Originated in North America but are now extinct there.

  • Descendants in South America include llamas and alpacas.

  • Descendants in other regions include the Camelus bactrianus and Camelus dromedarius.

Drivers of Macroevolution: Speciation and Extinction

• Diversity (D): The number of species in a particular clade.

• Diversity Formula (Population Scale): $N_1 + \text{births} + \text{immigration} - \text{deaths} - \text{emigration} = N_2$.

• Diversity Formula (Macroevolutionary Scale): $D_1 + \text{originations} + \text{immigrations} - \text{extinctions} - \text{emigrations} = D_2$.

• Global Scale Formula: On a worldwide scale, immigration and emigration are excluded: $D_1 + \text{originations} - \text{extinctions} = D_2$.

• Rates of Change:

  • Origination Rate ($\alpha$): The rate at which new species evolve.

  • Extinction Rate ($\Omega$): The rate at which species go extinct.

  • Standing Diversity: The total diversity at any given time.

  • Turnover: The total number of originations and extinctions in a given time period.

• Example Calculation from Fossil Records:

  • If a stage lasts $6.0\,my$ and cumulative originations total $48$ and cumulative extinctions total $27$ over a total period of $24.0\,my$:

  • $\alpha = 48 / 24 = 2.0\,\text{new spp./million yrs}$.

  • $\Omega = 27 / 24 = 1.125\,\text{extinct spp./million yrs}$.

• Case Studies:

  • Mammals: Studies of four groups (primates, rodents, artiodactyls, and terrestrial carnivores) from $66.0$ to $2.6\,mya$ show $\Omega$ followed $\alpha$ with a short lag.

  • Artiodactyls: Defined as animals that walk on their $3rd$ and $4th$ toes (e.g., sheep, goats, pigs, giraffes).

  • Dinosaurs: Originated in the late Triassic ($230.0\,mya$) and went extinct at the end of the Cretaceous ($66.0\,mya$), except for birds. They had high turnover rates, but failed to produce new species ($\alpha$ dropped) at the end of the Cretaceous.

Charting Life's Rises and Falls

• Challenges: Distinguishing species by fossils only and the incompleteness of the record. Biologists have described $61,000$ extant vertebrate species, while paleontologists have described only $12,000$ fossil species.

• Marine Invertebrates: Frequently used in studies because they fossilize well. Their rises and falls often occur together.

  • The biggest mass extinction occurred at the end of the Permian ($252.0\,mya$), wiping out up to $96.0\%$ of marine life.

Environmental Drivers of Macroevolution

• Intrinsic Factors: Basic physiology within a clade that influences $\alpha$ and $\Omega$.

• Extrinsic Factors: Physical factors in the environment, such as seafloor composition and climate.

• Climate Drivers:

  • Radiation: Earth is warmed by solar energy; some is reflected (ice increases reflection) and some is absorbed (land increases absorption).

  • Particulate Matter: Volcanism and fires can block sunlight.

  • Milankovitch Cycles: Changes in Earth's orbit affecting incoming radiation.

    1. Eccentricity: Ellipticity of the orbit; period of $\approx 96,000\,yrs$.

    2. Obliquity: Tilt of Earth's axis (varies from $22.1^{\circ}$ to $24.5^{\circ}$); period of $41,000\,yrs$.

    3. Precession: Orientation of the axis (pole wandering); period of $\approx 22,000\,yrs$.

  • Plate Tectonics: Continental positions influence ocean circulation and ice formation.

• Oxygen Isotopes: Warm water has higher concentrations of $^{18}O$. Rock formed in warm water contains more $^{18}O$ than rock formed in cold water. (The most common isotope is $^{16}O$ at $99.76\%$).

• Correlation: Positive correlation exists between warmer ocean temperatures and higher standing diversity of marine invertebrates.

The Tempo of Evolutionary Change

• Punctuated Equilibrium Model: Proposes that most species undergo little change for most of their history (stasis), punctuated by brief periods of rapid morphological change, often associated with speciation and environmental shifts.

• Gradualism: Proposes that species transform gradually into others over time.

  • Anagenesis: The gradual transformation of a species lineage from one form to another.

  • This model is currently less supported than punctuated equilibrium as a primary source of speciation.

Adaptive Radiations

• Definition: Clades that diversify rapidly (much speciation) by adapting to a wide range of resources, ecological niches, or lifestyles ($\alpha \gg \Omega$).

• Triggers for Adaptive Radiation:

  • Empty Niches: Resulting from the formation of new islands/lakes or the extinction of other groups (e.g., mammals radiating after dinosaurs).

  • Key Innovations: New phenotypes that allow access to previously unreachable resources.

    • Nectar Spurs: In columbines (Aquilegia), these facilitated specialized pollinator relationships, leading to high diversification.

    • Enzymes in Beetles: Horizontal transfer of enzymes from bacteria/fungi allowed beetles to feed on hard plant tissues.

• Notable Examples:

  • Hawaiian Archipelago: Endemic swordtail crickets ($37\,spp$), Hawaiian honeycreepers ($50\,spp$) with diverse beaks, and silversword plants.

  • Cichlid Fishes: Radiation in East African lakes.

  • Cambrian Radiation: Triggered by environmental change and the genetic toolkit.

The Cambrian Explosion

• Timeline: Occurred at the dawn of the Cambrian period, approximately $541.0\,mya$.

• Characteristics: Near-simultaneous appearance of great animal diversity and diverse body plans over about $25.0\,my$.

• Burgess Shale: A critical fossil deposit in British Columbia, Canada (discovered $1909$); contains $65,000$ specimens, mostly soft-bodied, representing $\ge 93\,spp$.

• Hypothesized Causes:

  • Developmental Toolkit: Evolution of Hox genes, which assign sections of the developing body to specific parts. This allowed for dramatic new body plans with modest mutations.

  • Rise in Dissolved Oxygen ($O_2$): Rising oxygen levels in oceans may have enabled energetically demanding activities like swimming and burrowing.

  • Abiotic Factors: Changes in seafloor chemistry and rising sea levels creating shallow seas.

  • Ecological Change: The evolution of predators drove selection for better locomotion, sensory organs, protective structures, and grasping parts.

Extinctions: Background vs. Mass Events

• Background Extinction: The typical, continuous rate of extinction due to climate change, habitat loss, competition, predators, or disease. Clades survive if $\alpha > \Omega$.

• Mass Extinction: A statistically significant increase in $\Omega$, a decrease in $\alpha$, or both, resulting in permanently depressed diversity.

• The "Big Five" Mass Extinctions:

  1. End-Ordovician ($443.0\,mya$): Continental drift and glaciation; $86.0\%$ species extinct.

  2. Late Devonian ($372.0\,mya$): Global cooling and volcanism; $75.0\%$ genera extinct.

  3. End-Permian ("The Great Dying") ($252.0\,mya$): Largest extinction in history. Lasted $< 200,000\,yrs$. Caused by the Siberian Traps (a Large Igneous Province the size of the US). Resulted in $96.0\%$ species extinction due to global warming and ocean acidification ($\text{CO}_2$ lowering pH).

  4. End-Triassic ($201.0\,mya$): Likely extreme volcanism; $80.0\%$ species extinct.

  5. End-Cretaceous (K-T) ($66.0\,mya$): Caused by a giant asteroid impact (Chicxulub Crater in Yucatan, Mexico) and likely volcanism (Deccan Traps in India). Evidence includes high iridium concentrations and soot. Marine invertebrates and some dinosaurs were already declining due to sea-level regression.

• Large Igneous Provinces (LIPs): Extreme volcanism where Earth cracks open, releasing molten rock and massive amounts of $\text{CO}_2$, $\text{SO}_2$, methane, and toxic metals for thousands or millions of years. LIPs cause acid rain, ozone depletion, and global warming.

Questions and Discussion

• Question: Why is there no immigration or emigration in the global diversity formula?

  • Answer: On a worldwide scale, the scale is all-encompassing; species are either present or absent on the planet, so they cannot immigrate from elsewhere.

• Review Question: What is a clade?

  • Answer: A branch on a phylogenetic tree representing an ancestor and all its descendants.

• Study Hint: In Figure 14.7, if $\Omega > \alpha$ for an extended period, standing diversity will decrease. If $\alpha \gg \Omega$, the ecosystem would become swamped with the taxon.

• Review Question: Why are sea-level regression events important to marine life?

  • Answer: A drop in sea level results in less available continental shelf (shallow seas), which is the primary habitat for many marine species.

• Review Question: What gene family assigns different sections of the developing body to different body parts?

  • Answer: Hox genes.