Macroevolution and the Sixth Mass Extinction

Macroevolution and the Current Biodiversity Crisis

• The Three Major Threats to Biodiversity: Human activities are the primary drivers of the current decline in global biodiversity. The three most significant threats are:

  • 1st: Human Alteration of Habitats: The outright destruction, fragmentation, or degradation of natural environments.

  • 2nd: Invasive Species: Species introduced to new environments, primarily by humans, that outcompete or prey upon native species.

  • 3rd: Overharvesting: The unsustainable removal of individuals from a population (e.g., overfishing or poaching).

• Quantifying Recent and Current Extinction Rates:

  • The current rate of extinction is significantly higher than the extinction event observed at the end of the Pleistocene epoch (approximately $11,000$ years ago).

  • Scientific Determination of Extinction: A species is officially determined to be extinct if no individuals have been seen in the wild for a period of $50$ years.

  • Calculation Challenges: Current rates are calculated from known species recorded as going extinct. It is likely that many more species have become extinct but remain unrecorded because they have been missing for less than the mandatory $50$-year threshold.

  • Record Completeness: The most comprehensive and reliable records of extinction currently exist for birds and mammals.

  • Historical Observations: Biogeographer Charles Elton (1958) echoed earlier observations, stating: "We must make no mistake; we are seeing one of the greatest historical convolutions of the world’s flora and fauna… a wilderness in retreat."

• Comparative Extinction Rates:

  • 1850–2000: During this period, the extinction rate was over $100 \times$ higher than the historical background extinction rate.

  • Current Global Estimates: Estimates based on global data suggest current extinction rates are $100 \times$ to $1,000 \times$ higher than the background rate.

  • Future Projections: Some scientists estimate that $50\%$ of current species could be extinct by the year $2100$.

  • The Sixth Mass Extinction: Many scientists conclude that the "Sixth Mass Extinction" is already underway due to human influence.

• Vertebrate Extinction Trends (1500–2014):

  • Cumulative extinctions, measured as a percentage of IUCN-evaluated species, have risen sharply since the 19th century.

  • Extinction rates for Mammals and Birds have significantly outpaced those for Reptiles, amphibians, and fishes, though all vertebrate groups show a cumulative rise far above the steady, low-level background extinction rate.

Human-Caused Habitat Alterations

• Direct Habitat Destruction: The total loss of habitat areas. As an example, the endangerment of tigers is largely attributed to the loss of their natural habitat.

• Habitat Fragmentation: The breaking up of large, continuous habitats into smaller, isolated patches.

  • Example: The fragmentation of forest ecosystems impacts species like the Northern Spotted Owl. In some regions, bare ground cleared of trees allows snow to be visible on slopes, indicating a loss of canopy cover.

• Eutrophication of Water:

  • Definition: A process occurring when a body of water becomes overly enriched with minerals and nutrients (excess nitrogen and phosphorus), typically from fertilizer runoff or sewage.

  • The Process of Dead Zone Formation:

    1. During spring, sun-heated freshwater runoff (e.g., from the Mississippi River) creates a barrier layer in bodies of water like the Gulf of Mexico, cutting off saltier water below from oxygen in the air.

    2. Nutrients from fertilizer and sewage ignite massive algae blooms.

    3. When the algae die, they sink and decompose, a process that consumes the available oxygen in the deeper water.

    4. Hypoxia (Oxygen Deprivation): The water becomes a "dead zone." Fish must flee the area or die in massive numbers. Tiny organisms at the base of the food chain also perish.

  • Gulf Dead Zone: This region of low-oxygen water in the Gulf of Mexico covers nearly $5,800\,sq\,miles$ and is an annual occurrence.

• Ocean Acidification:

  • Increased levels of $CO_2$ dissolve in ocean water, making it more acidic.

  • This leads to the destruction of coral reefs, which are critical biodiversity hotspots, housing approximately $1/4$ of all ocean biodiversity.

Climate Change and the Greenhouse Effect

• The Role of Milankovitch Cycles:

  • The Earth's orbit and tilt cycle through three intervals: $96,000$, $42,000$, and $22,000$ years. These cycles can reinforce or offset each other.

  • Historical data from ice cores shows that Earth’s actual temperature correlates well with these cycles.

  • Current data indicates that based on Milankovitch cycles alone, Earth should have been gradually cooling over the last $9,000$ years and entering a mild glacial cycle. Instead, temperatures remain high due to human activity.

• Principle Greenhouse Gases (GHGs):

  • There are approximately $8$ GHGs, with three being the primary drivers of human-induced warming:

  • Carbon Dioxide ($CO_2$):

    • Concentration: $280\,ppm$ in year $1800$; risen to $400\,ppm$ in $2015$ and $405\,ppm$ in $2018$.

    • Sources: Primarily the burning of fossil fuels.

    • Removal: Photosynthesis is the only natural process to remove $CO_2$ from the atmosphere, but deforestation is drastically reducing this capacity.

  • Methane ($CH_4$):

    • Potency: Approximately $21$ to $23$ times more effective at trapping heat than $CO_2$.

    • Sources: Livestock and landfills.

    • Trend: An abrupt increase in the last $50$ years, especially since $2000$.

  • Nitrous Oxide ($N_2O$):

    • Sources: Burning fossil fuels and the application of nitrogen fertilizers to agricultural crops.

    • Trend: Significant increase since $2000$.

  • Water Vapor: Increasing temperatures lead to increased evaporation, creating more water vapor, which also acts as a greenhouse gas.

• The Greenhouse Effect Mechanism:

  • Greenhouse gases act like the glass in a greenhouse on a global scale. Light energy from the sun enters the atmosphere; much of it is transformed into heat energy. Greenhouse gases prevent this heat from passing back out into space, effectively trapping it and heating the planet.

Projections and Biological Impacts

• Measuring Ancient Atmospheric Data:

  • Ancient levels of $CO_2$ are measured from ice cores in glaciers. Air trapped in snow is compressed into bubbles within ice, preserving atmospheric records for over $400,000$ years.

• Future Temperature Projections (RCP Scenarios):

  • Computer models use Representative Concentration Pathways (RCP) to project warming based on $CO_2$ emissions:

    • RCP 2.6: Emissions peak by $2020$ and kemudian substantially drop.

    • RCP 4.5: Emissions peak around $2040$ before declining.

    • RCP 8.5: Emissions continue to rise throughout the $21st$ century.

• Biological Survival and Adaptation:

  • The primary concern is whether organisms can survive rapid climate change through two main mechanisms:

    1. Adaptation: Evolving fast enough to keep pace with environmental changes.

    2. Movement: Migrating to suitable habitats (e.g., shifts in the geographic ranges of the Sugar Maple or American Beech).

• Case Study: The Tawny Owl:

  • The Tawny Owl population contains alleles for both grey and brown feathers.

  • As climate change reduced snow cover, the frequency of the brown-feathered allele increased through natural selection, as brown owls were better camouflaged and thus better adapted.

  • Phenotypic Plasticity: This refers to the ability of an organism to change its phenotype in response to environmental changes (represented by the term "plastic" in relevant biological contexts).

  • Significance of Genetic Variation: Without the pre-existing brown allele, the species might have declined or gone extinct. While mutations can produce new adaptive alleles, they are too rare to be relied upon during periods of rapid climate change.