Evolutionary trends Notes

Cambrian Explosion

• Significance: Represents a rapid diversification of life forms and animal lineages.

• Before the Cambrian: Few animal lineages were present (sponges, some worms).

• Cambrian Period: Explosion of diversity attributed to low genomic baggage and open ecological niches.

• Transitional community structure: Predominantly involved opportunistic scavengers.

• Predators were scarce, allowing for significant diversification.

Ecosystem Complexity

• Post-Cambrian communities developed more complex structures with varied interactions among species.

• Predation began to emerge, affecting organism interactions (e.g., sponges and smaller organisms).

Observations on Faunal Diversity

• Increase in number and variety of animal phyla throughout the Cambrian.

• Dominance of arthropods during this period, particularly trilobites.

• By the end of the Cambrian, most animal phyla representations existed.

Mass Extinction Events

• Defined as short periods with high extinction rates impacting numerous species.

• Contrasts with the continuous low-level extinction that occurs throughout time.

The Big Five Mass Extinctions

• A notable mass extinction (end of the KT boundary) is well known for eliminating dinosaurs.

• Graph Analysis: Demonstrates relationship between extinction rates and number of species through geological periods.

Permian Extinction

• Known as the Great Dying, leading to a 96% loss of marine species and significant land species loss.

• Hypothesized causes: Extreme volcanism, CO₂ increase affecting climate, and oceanic anoxia.

Cretaceous Extinction

• Approximately 65 million years ago, attributed to an asteroid impact near the Yucatan Peninsula.

• Evidence includes an iridium layer and geological disruptions.

Current Concerns

• Discussion on the potential sixth mass extinction event due to human activities.

• Extinction rates may be increasing dramatically due to habitat loss and climate change (
  100 to 1000 times the normal background rates).

Post-Extinction Dynamics

• After extinction events, seamless rediversification of surviving species (adaptive radiation).

• Example: Mammals diversified significantly after the dinosaurs’ extinction.

• Key adaptations of surviving organisms contributed to their success in new ecological niches.

Evolutionary Developmental Biology (Evo-Devo)

• New branch of evolutionary biology focusing on the genetic control of development.

• Understanding homeotic and regulatory genes essential for organismal development.

Morphological and Developmental Changes

• Key Adaptations: Traits allowing organisms to survive and thrive post-extinction.

• Heterochrony: Changes in the timing of growth can lead to morphological changes across species.

• Paedomorphosis: Retention of juvenile traits in adults (e.g., axolotls).

Examples of Developmental Changes

• Comparison of the growth rates in baby humans versus adult proportions leading to adult morphology.

• Changes in Hox genes illustrating how minor genetic variations result in significant morphological differences (e.g., number of insect legs).

• Developmental timing changes entre structural diversity among species (e.g., bird beak variations).

Adaptive Radiation

• After extinction, surviving organisms rapidly diversify into new niches.

• The speed of diversification depends on the extent of the extinction event, with quicker changes seen in less impactful events.