Extinction and Evolutionary Trends

Key Topics Discussed:
- Patterns and trends in evolution
- Nature and types of extinction
- Contemporary vs. evolutionary extinction
- Threats to biodiversity
- Definition and causes of mass extinction events
- The "Big 5" mass extinction events
- Discussion of the 6th extinction
- Conservation strategies and considerations

Trends or patterns indicate directional change in clades; these can manifest as:
- Body size increases in mammals over time
- Increasing complexity of species
Example of Cope's Rule:
- Observes that mammals generally increase in body size over evolutionary time.

These trends may hold along certain branches of evolution for limited time spans; they are not universal throughout all segments of the evolutionary tree.
The direction of diversification may exhibit constraints.

No Constraints:
- Evolution can occur in either direction freely.
Constraints:
- Passive Constraints:
  - A tendency to follow a particular direction; being further from boundaries associated with constraints leads to reduced pressure.
- Active Constraints:
  - Follows a directional evolution without reliance on boundaries; determined by underlying selective conditions.

Definition of Extinction:
- A gradual evolutionary process where all individuals of a species die out, leaving no living descendants.
- Functional extinction can occur when only one sex or a few individuals of a species remain.
Example of Functional Extinction:
- As cited in Ryder et al. (2021), in California condors exhibiting facultative parthenogenesis.
Current Extinction Status:
- As reported, more than 99% of all species that have ever existed are now extinct.
- Notable extinction records from 2021 (source: Mashable).

Contemporary Time:
- Active search for living species and understanding current factors leading to extinction.
Evolutionary Time:
- Utilizing fossil evidence to understand historical extinctions, including analysis of when and how extinction events occurred.
Background Rate of Extinction:
- The historical rate at which species become extinct, often measured as the number of taxonomic families that go extinct per million years (my).
Likelihood of Extinction:
- Remains constant and is largely independent of the time a species exists.

Overview of Biodiversity Threats:
- Various types of threats exist affecting biodiversity, which can be both direct and indirect.
Ecological Impacts:
- Impacts compound, where one eco-factor may exacerbate another.
- The phrase "too little too late" summarizes the consequences when conservation efforts are not timely.

Defined as significant decreases in biodiversity characterized by:
- A sharp decline in species across multiple geographic and taxonomic groups within a relatively short time span.
Background Rate Comparison:
- Mass extinctions occur when the rate of extinction exceeds approximately 2-5 families per million years.

End Ordovician Extinction (± 445-435 mya):
- Significant loss of approximately 60% of marine invertebrate genera and 12-25% of all biological families due to causes potentially including global cooling and anoxic events.
- References: Harper et al. (2014).
End Devonian Extinction (± 365-362 mya):
- Affects 70-72% of all species; caused possibly by global cooling, anoxic events, impacts, and supernova events.
- References: Fields et al. (2020).
End Permian Extinction (± 251 mya):
- The largest extinction event with a devastating 53% of marine families, 84% of marine genera, 96% of marine species, and 70% of land species lost; multiple causes such as global cooling, anoxic events, and impact events.
- References: Benton & Twitchett (2003).
End Triassic Extinction (± 210-208 mya):
- Resulting in the loss of 12-20% of marine families and 65% of terrestrial species; causes include anoxic events, impact events, and flood basalt activities.
- References: Davies et al. (2017).
End Cretaceous Extinction (± 65 mya):
- Affected 50-62% of species (primarily dinosaurs) and 11% of families; potential causes consist of flood basalt events, global cooling, and impact events leading to ecological upheaval.
- References: Petersen et al. (2016).

Mass extinction events typically arise from a combination of factors operating synergistically, resulting in cascading effects, including but not limited to:

Supernova or Gamma Ray Burst: Cosmic events with potential ecological impacts.
Sustained Global Cooling: Results in habitat loss and significant climatic shifts.
Sustained Global Warming: Affects species adaptability and ecosystem viability.
Anoxic Events and Oceanic Overturn: Alters marine ecosystems drastically.
Impact Events: Large celestial objects colliding with Earth, causing significant ecological repercussions.
Flood Basalt Events: Major volcanic eruptions releasing vast volumes of rock, leading to climate changes.
Clathrate Methane Eruptions: Sudden release of methane hydrates leading to climate shifts.
Formation of Supercontinents: Alters global ecosystems through continental drift and environmental changes.

Holocene Extinction:
- Accelerated extinction rates observed in the last 50,000 years resulting in over 150 genera of megafauna disappearing by 10,000 years ago.
Causative Factors:
- Includes human impacts (especially on islands) and environmental changes.
- Current extinction rates are estimated to be 1000 times higher than historical background rates, roughly translating to 2.74 species lost daily.
Primary Causes of Current Extinction Rates Include:
- Hunting, habitat destruction (both direct and indirect), climate change, and invasive species.

Historical Approach:
- Focus was on preserving the greatest number of species.
Current Approach:
- Emphasis has shifted towards conserving phylogenetic diversity, which is determined by the total length of the branches on a phylogenetic tree, recognizing the importance of maintaining genetic variation and evolutionary history.

Trends and patterns of directional change, outlined by Cope’s rule and types of constraints.
Definitions of extinction and functional extinction; background extinction rates characterized as the number of taxonomic families extinguished per million years.
Various threats to biodiversity that compound extinction risk such as predation, competition, and disease, often resulting in too little, too late responses.

Clear definitions and causative factors relating to mass extinctions, including eight main drivers of these events.

Five major historical mass extinction events; the relationship between the Pleistocene megafauna extinction, the Holocene extinction, and human-related causes; highlights the importance of conserving phylogenetic diversity over mere species conservation.