Macroevolution Overview

Macroevolution Definition

• Refers to evolutionary changes on a large scale that occur over long periods, typically at or above the species level.
• Encompasses the origin of new species, mass extinctions, adaptive radiations, and major evolutionary innovations.

Scope of Macroevolution

• It seeks to explain the "big picture" of evolution.
• It seeks to determine why and how life has changed over billions of years.
• Includes the change from single-celled organisms to the diversity of life forms observed today.

Distinguishing Macroevolution from Microevolution

• Microevolution involves evolutionary changes within a species.
• Microevolution includes changes in allele frequencies over time.
• Microevolution is driven by mutation, natural selection, genetic drift, and gene flow.
• Macroevolution involves evolutionary patterns observed across species and higher taxonomic groups.
• Macroevolution involves processes like speciation, extinction, and the origin of major clades.
• Both macroevolution and microevolution rely on similar mechanisms like natural selection.
• Macroevolution operates on a much larger temporal and spatial scale.

Mechanisms of Macroevolutionary Change: Speciation

• Speciation is the process by which one species splits into two or more distinct species.
• Allopatric Speciation occurs when a population is geographically isolated, leading to reproductive isolation and divergence.
• Sympatric Speciation happens within the same geographic area.
• Sympatric Speciation is often due to ecological or behavioral factors that prevent interbreeding.

Mechanisms of Macroevolutionary Change: Extinction

• Extinction is the permanent loss of a species.
• Background Extinction is the normal rate of species extinction.
• Background Extinction is due to environmental or ecological factors.
• Mass Extinction involves large-scale extinction events that wipe out significant portions of life.
• Mass Extinction is associated with catastrophic events like asteroid impacts and volcanic activity.

Mechanisms of Macroevolutionary Change: Adaptive Radiation

• Adaptive Radiation involves the rapid diversification of a single ancestral species into many different forms.
• Adaptive Radiation often follows a mass extinction or colonization of new environments.
• An example of adaptive radiation is the diversification of mammals after the extinction of non-avian dinosaurs 66 million years ago.

Mechanisms of Macroevolutionary Change: Convergent Evolution

• Convergent Evolution is the independent evolution of similar traits in different lineages.
• Convergent Evolution is due to similar environmental pressures.
• An example of convergent evolution is the evolution of wings in birds, bats, and insects.

Mechanisms of Macroevolutionary Change: Punctuated Equilibrium

• Punctuated Equilibrium was proposed by Stephen Jay Gould and Niles Eldredge in 1972.
• This theory suggests that species remain relatively unchanged for long periods (stasis).
• Stasis is punctuated by brief, rapid periods of evolutionary change.
• Punctuated Equilibrium contrasts with Darwin's gradualism.
• Darwin's Gradualism states evolution occurs slowly and continuously.

Patterns in Macroevolution: The Cambrian Explosion

• The Cambrian Explosion occurred around 540 million years ago.
• The Cambrian Explosion was a relatively short evolutionary event.
• Most major animal phyla appeared in the fossil record during this time.
• The causes of the Cambrian explosion are debated.
• The reasons likely include increased oxygen levels, developmental innovations (e.g., the evolution of hox genes), and ecological factors (predator-prey dynamics).
• This event highlights the rapid diversification of life forms, and is a hallmark of macroevolutionary change.

Patterns in Macroevolution: Mass Extinctions

• The "Big Five" Mass Extinctions
  • Ordovician-Silurian (443 mya)
  • Devonian (359 mya)
  • Permian-Triassic (252 mya)
  • Triassic-Jurassic (201 mya)
  • Cretaceous-Paleogene (66 mya)
• The Cretaceous-Paleogene Extinction is attributed to an asteroid impact and volcanic activity.
• This event led to the extinction of non-avian dinosaurs and allowed mammals to dominate.
• Mass extinctions act as reset buttons in evolution.
• Mass extinctions pave the way for new groups to evolve and diversify

Patterns in Macroevolution: Gradualism vs. Punctuated Equilibrium

• Gradualism: The slow, steady accumulation of small evolutionary changes, as originally proposed by Darwin.
• Punctuated Equilibrium emphasizes rapid bursts of change interspersed with long periods of stasis.
• Both patterns are observed in the fossil record, and the debate continues over which one dominates the evolutionary history of life.

Patterns in Macroevolution: Coevolution

• Coevolution is the process by which two or more species reciprocally affect each other's evolution.
• An example of coevolution is the coevolution of flowering plants and pollinating insects.

Major Macroevolutionary Innovations: Evolution of Multicellularity

• It is one of the most significant evolutionary transitions.
• It leads to the rise of complex organisms.
• The first multicellular organisms appeared around 1.6 billion years ago.
• This allowed for specialization of cells and complex body structures.

Major Macroevolutionary Innovations: Evolution of Flight

• Flight evolved independently in several groups: insects, pterosaurs, birds, and bats.
• Flight offered significant adaptive advantages, including access to new habitats and escape from predators.

Major Macroevolutionary Innovations: Origin of Homo sapiens

• Human evolution can be used as an example of a macroevolutionary process.
• Human evolution has led to the development of a highly complex, social species capable of altering its environment.
• The genus Homo emerged around 2.5 million years ago.
• Modern humans (Homo sapiens) appeared around 300,000 years ago.

Macroevolution and the Fossil Record: The Importance of the Fossil Record

• The fossil record provides critical evidence for macroevolution.
• The fossil records document the rise and fall of species.
• The fossil records shows the appearance of new forms, and patterns of extinction.
• The fossil record is incomplete and biased toward species with hard parts that fossilize well (e.g., bones, shells).

Macroevolution and the Fossil Record: Transitional Fossils

• Transitional fossils show intermediate traits between ancestral and descendant species.
• They provide evidence for evolutionary transitions.
• An example of a transitional fossil is Archaeopteryx.
• Archaeopteryx shows characteristics of both dinosaurs and birds.

Macroevolutionary Theories in Current Research: Developmental Evolution (Evo-Devo)

• Evolutionary developmental biology (Evo-Devo) examines how changes in an organism's development (due to gene regulation) lead to macroevolutionary innovations.
• Changes in hox gene expression can lead to dramatic shifts in body plan structure.

Macroevolutionary Theories in Current Research: Genomics and Phylogenomics

• Advances in genomic sequencing have revolutionized our understanding of evolutionary relationships and divergence times.
• This allows for more precise reconstructions of macroevolutionary events.