L7 - Macroevolution

Variation, Heritability, Superfecundity

• Variation: Natural differences within a species population due to genetic mutations, influencing traits like height or intelligence, relevant to survival and reproduction.

• Heritability: The ability to pass genes and associated traits to offspring, ensuring trait inheritance across generations (lineage).

• Superfecundity: More individuals are born than can survive on available resources, leading to competition where only the best-adapted traits ensure survival and reproduction (“survival of the fittest”).

Species and the Fossil Record

• Defining Species: Biologically, species are populations capable of reproducing fertile offspring within themselves (reproductive isolation).

• Morphospecies: Identifying species in the fossil record based on morphology.

• Pseudo Extinction: The arbitrary nature of identifying species in the fossil record.

• Ghost Lineages: Assumed lineages with missing fossil records, inferred between two known points in time.

• Lazarus Taxa: Species thought extinct that are rediscovered (e.g., coelacanth).

Macroevolution and the Fossil Record

• Time Scale: The fossil record's primary advantage is its vast time scale, allowing observation of evolution over millions of years.

• Environmental Change: The fossil record helps understand the interplay between biological and environmental changes over geological time.

• Macroevolution: Large-scale evolutionary patterns, including diversification and species radiation.

• Brachiopods vs. Bivalves: Example of macroevolutionary patterns where brachiopod diversity decreased as bivalve diversity increased around 300 million years ago.

• Documenting Diversity: Quantitative paleontology emerged in the 1980s, shifting from specimen-based to data-driven analysis.

• The Sepkoski Curve:

  • Published in 1981, documenting marine diversity through time using families as a proxy for species.

  • Showed a significant diversity drop at the end of the Permian period.

  • Showed that there was an exponential diversity increase since then (with a smaller drop off at the end of the Cretaceous).

  • Suggested we live in the most biodiverse time ever.

  • Identified the "Big Five" mass extinctions.

• "Big Five" Mass Extinctions:

  • End Ordovician

  • Late Devonian

  • Permian-Triassic

  • Triassic-Jurassic

  • Cretaceous-Palaeogene

• Sepkoski's Three Evolutionary Faunas: Identified three main faunas:

  • Cambrian Fauna: Characterized by trilobites, monoplacophoran molluscs, and inarticulate brachiopods.

  • Paleozoic Fauna: Includes crinoids, corals, ostracods, and articulate brachiopods.

  • Modern Fauna: Dominated by bivalves, echinoderms (especially sea urchins), sponges, and gastropods.

Paleobiology Database

• Open Source Database: Paleontologists and museums contribute fossil data, accessible for analyses.

• Updating Sepkoski's Curve: Efforts to update the Sepkoski curve using genera instead of families revealed biases.

• Rock Volume Bias:

  • More recent rock formations are more abundant, leading to a sampling bias in fossil records.

  • Studies showed a strong correlation between rock volume and sample diversity.

• Addressing Bias: Statistical methods developed to correct for rock volume bias reveal a more complex diversity pattern through time.

• Revised Understanding: Questions the assumption that today's biodiversity is the highest ever, suggesting a possible carrying capacity for Earth's biodiversity.

• Mass Extinctions Remain Significant: Mass extinctions still drive macroevolutionary patterns despite bias corrections.

• Ongoing Progress: New databases with higher temporal resolution are being developed.

Measuring Macroevolution

• Diversity Measures:

  • Number of species

  • Ecological diversity

  • Morphological diversity

• Ecology Through Time: Analyzed using “ecology cubes” to map ecosystem changes.

  • Ecology cube has 3 axis

  • Tiering - Height in water column

  • Feeding - Method of gathering nutrients

  • Motility - Degree of mobility and method of movement

• Morphology Through Time: Quantitative analyses of morphological datasets to track evolutionary changes.

Radiations and Mass Extinctions

• Diversity Change Equation: $\Delta \text{Diversity} = \text{New Species (N}_s) - \text{Extinctions (N}_e)$

• Mass Extinction: \text N_e >> \text N_s

• Radiation: \text N_s >> \text N_e

• Stasis: $\text N_s \approx \text N_e$

Cambrian Explosion

• Timing: Occurred during the Cambrian period (541 to 485.4 million years ago).

• Not the Origin of Animals: Molecular clocks and Ediacaran biota fossils indicate earlier animal origins.

• Rapid Diversification: Rapid diversification of animal phyla, with most modern phyla appearing.

• Ecological Innovation:

  • Emergence of animal-dominated ecosystems.

  • Bioturbation (organisms churning sediments).

  • Predator-prey dynamics.

• Agronomic Revolution: Shift from Ediacaran “Garden of Ediacara” to complex Cambrian ecosystems with burrowing, swimming, and predation.

• The level of sediment and ground mixing greatly increased.

Drivers of the Cambrian Explosion

• Ecosystem Interactions: Development of new interactions within ecosystem may drive evolutionary changes.

• Oxygen Levels:

  • Hypothesis: Rising oxygen levels enabled more energy-intensive lifestyles, driving diversification.

  • Great Oxygenation Event led to initial oxygen increase.

  • Oxygen levels approached modern levels around the Cambrian explosion.

• Test Using Modern Analogs:

  • Studied modern oxygen minimum zones to examine animal distribution.

  • Found a correlation between oxygen levels and the presence of carnivorous organisms.

  • Limited oxygen restricts energy-intensive lifestyles like predation.

• Food Availability:

  • Increased organic carbon preservation in sedimentary rocks during the Palaeozoic.

  • Nutrient availability and tectonic activity may have influenced food supply.

• Genetic Factors:

  • Evolution of HOX genes in bilaterians.

  • HOX genes regulate body plan development, enabling diverse morphologies.

Fire Triangle Analogy for the Cambrian Explosion

• Combination of Factors: Genetics, evo-devo, ecology, and environment may have jointly driven the Cambrian explosion.

• Environment: Oxygen, food, and temperature.

Mesozoic Marine Revolution

• Timing: Mesozoic Era.

• Ecosystem Changes: Shift from Paleozoic benthic ecosystems to Mesozoic ecosystems with:

  • Epifaunal suspension feeders were replaced by infaunal burrowers.

  • Increase of burrowing depth.

• Shell Morphology:

  • Paleozoic gastropods had simple shells.

  • Mesozoic gastropods had complex, thicker shells with spines.

• Bivalves: Shift from epifaunal/semi-infaunal bivalves to infaunal bivalves.

• Crinoids: Crinoids retreat from shallow-water environments to deep-water environments.

• Gary Vermeil's Escalation Hypothesis:

  • Predator-prey dynamics drive evolutionary changes.

  • Comparison between modern Caribbean and Pacific ecosystems:

    • Caribbean: weaker shells and crushing strength

    • Pacific: stronger shells and crushing strength.

• Predator Evolution: Emergence of rays, shell-crushing lobsters and crabs, shell-drilling gastropods, and fish with teeth.

• The ability to eat hard prey is called durophagy

Size and Metabolic Rate

• David Jablonski's Hypothesis:

  • Increase in organism size through time, correlated to metabolic rate.

  • Higher metabolic rate requires more energy.

  • Reorganization of Prime Production: The reorganisation results in a higher level of energy production, therefore can sustain predatory life cycles

• Primary Producer Revolution: Shift from green algae to coccolithophores, dinoflagellates, and diatoms.

• Size Advantage: Larger primary producers reduce the number of steps in the food chain, reducing energy loss.

• Nutrient Availability: Increased nutrients from weathering may have fueled changes in primary production.

• Angiosperm Evolution: Flowering plants (angiosperms) evolved during the Cretaceous and have a higher vein density in their leaves.

• Leaf Vein Density: Related to leaf conductance of water vapor, potentially leading to increased rainfall and weathering.

Summary

• The fossil record serves as a unique, time-scaled view of the history of our planet. It captures the story of organisms interacting with the planet and the environment.

• The cambrian explosion, defined by the emergence of these animal-dominated ecosystems, the evolution of modern Fila, and the predator-prey ecosystem.

• These studies combine data from geological records, observations of organisms using fossil records, observations from modern biology, and observations from from the modern environment.