Chapter 14: Speciation and Extinction
Chapter 14: Speciation and Extinction
Causes of Species Extinction
The causes of species extinction are varied, ranging from individual fitness differences to catastrophic global events, and are currently being driven significantly by human activities.
Models of Speciation
Speciation Pace: Evidence from the fossil record supports two distinct models for how speciation occurs:
Gradualism ():
Definition: Suggests that evolutionary change proceeds in small, continuous, incremental steps over long periods.
Example: The fossil record showing gradual evolution in microscopic ocean protists.
Punctuated Equilibrium ():
Definition: Proposes that the pace of evolution varies, characterized by long periods of little or no change (stasis) interrupted by short, rapid bursts of significant evolutionary change.
Evidence: The fossil record indicates that many animals have evolved in short bursts, followed by extended periods of stasis.
Adaptive Radiation
Definition: Bursts of speciation, characteristic of punctuated equilibrium, often occur during a process called adaptive radiation.
Mechanism: This happens when a population inhabiting a patchy or heterogeneous environment rapidly gives rise to multiple specialized forms in a relatively short amount of time.
Example (Anolis Lizards - ): Each Caribbean island hosts a unique collection of Anolis lizard species, illustrating how adaptive radiation has led to a diversification of forms adapted to different niches across distinct island environments.
Speciation and Extinction: A Dynamic Interplay
Biodiversity on Earth is fundamentally shaped by the continuous and dynamic interaction between the emergence of new species (speciation) and the disappearance of existing species (extinction).
Types of Extinction
Background Extinction ():
Definition: The ongoing, normal rate of species disappearance due to localized factors and fitness differences among individual species.
Causes: These extinctions are typically driven by factors such as:
Competition for resources
Predation or disease
Gradual environmental change
Characteristics: At any given moment, these extinctions are considered relatively random in terms of which individual species are affected and are often offset by new speciation events.
Mass Extinction ():
Definition: Significant, widespread, and rapid departures from the typical background extinction rates, resulting in a substantial and non-random loss of taxonomic diversity.
Characteristics: Involve the loss of huge numbers of species, and the species lost are not random (i.e., certain taxa are disproportionately affected).
Nature: These events are often caused by large-scale catastrophes for which natural selection does not adequately prepare species.
Causes of Mass Extinctions
Impact Theory ():
Hypothesis: Suggests that large meteorite or comet impacts can trigger mass extinction events.
Example: The extinction of non-avian dinosaurs at the end of the Cretaceous period is widely attributed to such an impact.
Mechanism: The impact would have ejected massive amounts of debris into the atmosphere, leading to a dramatic and global alteration of the environment (e.g., blocking sunlight, causing climate shifts), which ultimately led to the extinction of numerous species.
Speciation After Mass Extinction ()
Opportunity for Diversification: Major environmental changes that cause mass extinctions can paradoxically create new evolutionary opportunities.
Process: Surviving organisms are able to exploit newly available resources and ecological niches in the fundamentally altered environment.
Outcome: This leads to periods of rapid diversification and adaptive radiation among the surviving lineages, filling the ecological voids left by the extinct species.
Example: The adaptive radiation of mammals occurred after the extinction of the non-avian dinosaurs, allowing mammals to diversify and occupy a vast array of ecological roles previously held by dinosaurs.
The Sixth Mass Extinction
Current Event: The Earth is currently experiencing what is often referred to as the sixth mass extinction event.
Accelerating Rates: Current extinction rates are accelerating at an alarming pace, primarily due to human activity.
Vulnerable Areas: This acceleration is particularly pronounced in vulnerable ecosystems, such as islands, which host many endemic species with limited ranges.
Human Impacts: Humans are profoundly altering the global environment through several key activities:
Habitat Loss and Fragmentation: Destruction and division of natural habitats, reducing biodiversity and isolating populations.
Pollution: Introduction of harmful substances into ecosystems, impacting species health and survival.
Introduction of Nonnative Species: Invasive species can outcompete native species, disrupt food webs, and introduce diseases.
Overharvesting: Unsustainable exploitation of resources, leading to population declines and extinctions of species.
Recent Vertebrate Extinctions (): Historical and ongoing examples demonstrate human-caused extinctions across various vertebrate groups:
Fishes: Chinese paddlefish, Las Vegas dace (causes: habitat destruction).
Amphibians: Jalpa false brook salamander (cause: habitat destruction), Southern day frog (cause: undetermined).
Reptiles (including birds): Yunnan box turtle (causes: habitat destruction, overharvesting), Martinique lizard (cause: undetermined), Dodo (causes: habitat destruction, overharvesting), Laysan honeycreeper (cause: habitat destruction), Passenger pigeon (cause: overharvesting), Great auk (cause: overharvesting).
Mammals: Steller’s sea cow (cause: overharvesting), Javan tiger (causes: habitat destruction, overharvesting), Caspian tiger (causes: habitat destruction, overharvesting), Yangtze River dolphin (causes: habitat destruction, overharvesting).