Chapter 14: Macroevolution: The Long Run

Chapter 14: Macroevolution: The Long Run

Overview

Douglas J. Emlen and Carl Zimmer authored this chapter, which explores macroevolutionary processes that shape the history of life over extensive timescales. It provides an in-depth examination of diversity, speciation, extinction, and their implications for understanding life's evolution.

Learning Objectives

The chapter aims to help students:

  • Review the drivers of diversity at varied scales.
  • Understand the interplay between speciation and extinction that influences overall species diversity.
  • Evaluate mass extinctions, their causes, and consequences.

Microevolution and Macroevolution

Definitions
  • Microevolution: Evolution that occurs within populations, manifesting as adaptive changes and shifts in allele frequencies leading to heritable differences.
  • Macroevolution: Evolution occurring on a scale above the species level, encompassing the origination, diversification, and extinction of species over time.
Key Considerations
  • Recognizing the complex nature of defining species adds challenges to studying macroevolution's processes.

Drivers of Diversity

Diversity and Clades

A significant inquiry is why certain clades diversify extensively while others do not, exemplified by the estimated 1.5 million beetle species.

Biogeography
  • Biogeography: The study of species distribution over time and space, highlighting how geographical barriers and history shape diversity.
  • Notably, tropical regions demonstrate higher species richness compared to temperate zones.

Biogeographical Provinces

Wallace's Provinces

Alfred Wallace identified six key biogeographical provinces, each distinguished by unique species compositions.

Vicariance and Dispersal
  • Vicariance: The process where geographic barriers form, dividing populations, contributing to speciation.
  • Dispersal: The movement of individuals or populations to new areas, often with limited return.

Speciation and Extinction

Dynamics of Diversity

The diversity dynamics within a clade can be illustrated mathematically:
D2=D1+extoriginations−extextinctionsD_2 = D_1 + ext{originations} - ext{extinctions}
Where:

  • D1D_1 = initial diversity
  • D2D_2 = new diversity after a period of origination and extinction.
Rates of Origination and Extinction
  • Alpha (α): Rate of origination
  • Omega (Ω): Rate of extinction
  • Turnover Rate: Defines the number of species eliminated and replaced per unit time.

The Fossil Record and Evolutionary Faunas

  • Research into fossils reveals three major evolutionary faunas over the past 540 million years, such as the species turnover caused by mass extinction events.
Historical Context
Mass Extinctions

Recognized mass extinction events include the end-Permian mass extinction (~252 million years ago) and the Cretaceous (K-T) extinction event (~65 million years ago).

  • Background Extinction: Normal rate of species loss.
  • Mass Extinction: A significant spike in extinction rates beyond typical background levels.
Causes of Mass Extinction Events
  1. Large Igneous Provinces (LIPs): Cause dramatic climate shifts via toxic gas release, acid rain, and other environmental stresses.
  2. Asteroid Impacts: The Chicxulub impact, which coincides with the K-T boundary, provides evidence for extraterrestrial factors contributing to mass extinction.
The K-T Boundary

Evidence for the asteroid impact includes elevated levels of iridium found in the K-T boundary layer, indicating high-energy events associated with mass extinction.

Adaptive Radiations

  • Adaptive Radiation: Rapid diversification of a lineage into various forms, each adapted to utilizing different ecological resources.
Examples of Adaptive Radiation

Some key adaptive radiations include:

  • Cambrian Radiation: Resulted from environmental changes and key innovations in genetic structures, enabling diverse body plans.
  • Devonian Plant Radiation: Innovations like seeds and vascular tissues allowed plants to colonize land effectively.
  • Darwin’s Finches and Hawaiian Applications: Demonstrating how key innovations lead to ecological success and diversification through the use of unoccupied niches.

Key Concepts

Extinction Patterns
  • Adaptive Radiations occur under conditions of unoccupied ecological niches, showcasing how speciation can thrive in synchronous extirpation events.
  • Fossil records often illustrate periods of stasis interspersed with rapid morphological changes linked to speciation events.

Current Context and Implications

Human Impact on Extinction Rates

Recent data indicates ongoing vertebrate extinctions exacerbated by human-induced habitat loss, contributing to a significant rise in extinction rates relative to historical background levels.

Climate Change Effects

Increased carbon dioxide levels are correlated with rising global temperatures and decreasing ocean pH, contributing to an ecological crisis that may herald the onset of a sixth mass extinction event.

Summary of the Big Five Mass Extinctions

Each of these events has distinct causes, including large igneous volcanism and extraterrestrial impacts, bringing about varied effects on different organisms. Evidence suggests another major extinction event may be in progress due to anthropogenic factors.