Phylogenies and the History of Life: Study Notes

Ch 25: Phylogenies and the History of Life

Phylogenetic Trees and Their Interpretation

Phylogenetic trees are diagrams that represent the evolutionary relationships among organisms. They are essential tools in biology for visualizing how species are related through common ancestry.

• Phylogeny: The evolutionary history of a group of organisms.

• Branch: Represents a population through time.

• Node: A point where a branch splits, representing the most recent common ancestor.

• Tip: The endpoint of a branch, representing a living or extinct group.

• Root: The most ancestral branch in the tree.

• Outgroup: A taxon that diverged before the taxa being studied, used as a reference point.

Example: A phylogenetic tree showing the relationships among primates, with humans and chimpanzees sharing a recent common ancestor.

Reading and Comparing Phylogenetic Trees

Phylogenetic trees can be drawn in different ways but may represent the same relationships. The order of branching is what matters, not the orientation or shape of the tree.

• Different tree shapes can represent the same evolutionary history.

• Key is to identify which taxa share common ancestors and the sequence of branching.

Example: Several trees with different layouts but identical branching patterns for a set of species.

Principle of Parsimony in Phylogenetics

Scientists use the principle of parsimony to infer phylogenetic trees. Parsimony suggests that the simplest explanation, requiring the fewest evolutionary changes, is preferred.

• Parsimony: The tree with the least number of changes is considered most likely.

• Used to select among possible trees when reconstructing evolutionary relationships.

Example: If two trees explain genetic data, the one with fewer mutations is favored.

Monophyletic Groups

Monophyletic groups, or clades, include an ancestor and all its descendants. Identifying these groups is crucial for understanding evolutionary relationships.

• Monophyletic group: Consists of a common ancestor and all its descendants.

• Can be identified by tracing all branches from a single node.

Example: The group containing all primates descended from a common ancestor forms a monophyletic clade.

The Fossil Record

The fossil record provides evidence of past life and evolutionary events. Fossils form when organic remains are preserved under specific conditions.

• Fossil formation: Occurs when remains are buried rapidly and mineralized.

• Types of fossils include intact fossils, cast fossils, and permineralized fossils.

Example: A leaf fossil preserved in sedimentary rock.

Type of Fossil	Description
Intact fossil	Preserved remains with little decomposition
Cast fossil	Remains decay, leaving a mold filled by minerals
Permineralized fossil	Minerals infiltrate and harden tissues

Limitations of the Fossil Record

The fossil record is incomplete and biased due to several factors affecting fossilization.

• Habitat bias: Organisms living in areas with sedimentation are more likely to fossilize.

• Tissue bias: Hard parts fossilize more readily than soft tissues.

• Temporal bias: Recent fossils are more common than ancient ones.

• Abundance bias: Common species are more likely to be found as fossils.

Example: Marine organisms with shells are more frequently found as fossils than soft-bodied terrestrial species.

Life's Timeline

Major events in the history of life are recorded in the fossil record and depicted in geologic time scales.

• Life originated over 3.5 billion years ago.

• Major radiations and extinctions mark the boundaries of geologic eras.

Example: The Cambrian explosion marks a rapid diversification of animal life.

Adaptive Radiation

Adaptive radiation occurs when a single lineage rapidly diversifies into many descendant species, often following the evolution of a key trait or the opening of new habitats.

• Adaptive radiation: Rapid production of many species from a single lineage.

• Triggered by ecological opportunity or evolutionary innovation.

• Observed in both fossil record and phylogenetic trees.

Example: Darwin's finches diversified to fill different ecological niches in the Galápagos Islands.

The Cambrian Explosion

The Cambrian explosion was a period of rapid evolutionary change, resulting in the appearance of most major animal groups.

• Occurred about 541 million years ago.

• Triggered by increased oxygen levels, evolution of predation, and new ecological opportunities.

• Led to the development of hard body parts and complex body plans.

Example: Fossils from the Burgess Shale show diverse animal forms from the Cambrian period.

Mass Extinctions

Mass extinctions are events in which large numbers of species are lost rapidly. These events have shaped the course of evolution.

• There have been five major mass extinctions since the Cambrian explosion.

• The End-Cretaceous extinction (about 66 million years ago) led to the demise of the dinosaurs.

• Causes include asteroid impacts, volcanic eruptions, and climate change.

Example: The asteroid impact at the end of the Cretaceous period is linked to global cooling and decreased photosynthesis.

Mass Extinction Event	Approximate Time (mya)	Main Cause
End-Permian	252	Volcanism, climate change
End-Cretaceous	66	Asteroid impact, volcanic activity

Evidence for Asteroid Impact

Geological and chemical evidence supports the hypothesis that an asteroid impact caused the End-Cretaceous extinction.

• Iridium layer: High levels of iridium found worldwide, rare on Earth but common in asteroids.

• Impact crater: Chicxulub crater in Mexico matches timing of extinction.

• Shocked quartz and microtektites: Formed by high-energy impacts.

Example: The presence of iridium and shocked quartz in sediment layers dating to 66 mya.

Consequences of Mass Extinction

Mass extinctions disrupt ecosystems and reduce biodiversity, but also create opportunities for surviving lineages to diversify.

• Global cooling and decreased photosynthesis followed the End-Cretaceous impact.

• Species capable of surviving in harsh conditions persisted and diversified.

Example: Mammals diversified after the extinction of dinosaurs. Additional info: These notes expand on the original content by providing definitions, examples, and tables for clarity and completeness.