Macroevolution: The Long Run

Macroevolution

Macroevolution is defined as evolutionary processes occurring above the species level. It encompasses the origination, diversification, and extinction of species over extended periods of time.
In contrast, microevolution refers to evolutionary changes occurring within populations, specifically the adaptive and neutral changes in allele frequencies from one generation to the next.
Key concepts include:
- Macroevolution: Evolutionary changes that transcend the boundaries of a single species over long evolutionary periods.
- Microevolution: Small-scale evolutionary changes occurring within populations, such as an increase in the allele frequency for dark wings in a beetle population over generations versus the evolution of the dinosaur lineage on a larger scale.

Macroevolution vs. Microevolution:
- Both forms of evolution rely on the fundamental mechanisms of mutation, migration, genetic drift, and natural selection.
- Microevolution examples are observed through shifts in allele frequencies, while macroevolution involves larger evolutionary patterns and mechanisms.
A guiding question in macroevolution: What drives some clades to diversify significantly while others stagnate? Example: 1.5 million species of beetles.

Macroevolution encompasses:
- The origination, diversification, and extinction of species.
- Changes in allele frequencies within populations over generations.
- The independent evolution of similar traits across separate evolutionary lineages due to shared environmental pressures.

Biogeography: The study of how species are distributed across geographic space and time.
Important questions in biogeography include:
- Why do similar environments contain different species?
- How do good dispersers dominate island ecosystems?

Dispersal: The movement of populations from one geographic area to another, typically with minimal or no returns to the original location.
Vicariance: The emergence of geographical barriers that disrupt the connectivity of once continuous populations, leading to isolation and divergence.

Marsupials: A subclass of mammals recognized for giving birth to underdeveloped young that develop in an external pouch.
Evolutionary history reveals a mix of dispersal and vicariance:
- Most marsupials are currently found in Australia, with ancient fossils originating from China and North America.
- There’s only one extant marsupial species in North America, none in China today.
- Significance of the geographical shifts:
- 150 million years ago: Marsupial-like mammals existed in China.
- 120 million years ago: Dispersal to North America.
- 80 million years ago: Notable migration leading to the diversification of marsupials in Australia after the continent split.
This evolutionary pattern illustrates both dispersal (movement among continents) and vicariance (isolation through continental drift) within marsupial evolution.

Turnover ($T$): The rate at which new species originate ($eta$) and existing species go extinct ($ heta$).
- If the extinction rate $ heta$ exceeds the origination rate $eta$, the diversity within a clade decreases.
- Conversely, an increase in diversity occurs when $eta$ is greater than $ heta$.
Hypotheses for higher diversity in tropical regions compared to temperate zones:
1. Higher origination rate ($eta$) leads to faster creation of new species in the tropics.
2. Lower extinction rate ($ heta$) renders species in the tropics less prone to extinction.

Fossils are examined across four geological stages (A, B, C, & D) to calculate species originations and extinctions:
- Stage A: 24 species.
- Stage B: 10 new species originated, 6 extinctions occurred.
- Stage C: 12 extinctions, 8 originations.
- Stage D: 9 extinctions.
Overall, a total of 20 turnovers occurred throughout the stages, comprising 8 originations and 12 extinctions.

Mean origination rate for stages B, C, and D calculated as:
- ext{Mean } eta = (10 + 8 + 6)/3 = 8 ext{ new species per stage}.
Mean extinction rate for stages B, C, and D:
- ext{Mean } heta = (6 + 12 + 9)/3 = 9 ext{ species per stage}.
Based on a timescale of 6 million years per stage:
- Origination rate ($eta$) becomes eta = rac{8}{6} ext{ species per million years}, and extinction rate ($ heta$) becomes heta = rac{9}{6} ext{ species per million years}.

Adaptive Radiation: An evolutionary process wherein an ancestral species diversifies rapidly into various new species, each adapting to a unique ecological niche.
Example: The colonization of the Hawaiian archipelago by ancestors of modern silverwords leading to diversification into numerous species exhibiting distinct phenotypes.

Mass Extinction Events: When certain species become extinct, it opens ecological niches, allowing other lineages to adapt and diversify rapidly. E.g., After the extinction of large dinosaurs, large mammals adapted and diversified.
Emergence of New Traits: New traits within a clade known as key innovations can drive adaptive radiations. For example, the development of wings and exoskeletons in insects led to increased diversification.
- Elytra: Protective hardened forewings of beetles that shield delicate hindwings, enabling them to thrive and diversify in various environments.
Colonization Events: Species that migrate to distant islands often encounter abundant opportunities for adaptation and diversification due to the absence of competitors.

Beetles display unique adaptive radiation influences that distinguish them from other insect orders, primarily through key innovations, including the development of hindwings different from forewings, enabling a multitude of ecological adaptations.

Understanding macroevolution is essential for comprehending the vast diversity of life on Earth, underpinning phenomena such as adaptive radiation, vicariance events, and biodiversity metrics.