4- Phylogeny, Fossils, and the History of Life
FOSSILS AND EVOLUTIONARY HISTORY
Fossils represent a direct record of evolutionary history.
For instance, 50-million-year-old fish from Wyoming represent extinct relatives of modern herring.
PHYLOGENETIC TREES
22.1 READING A PHYLOGENETIC TREE:
A phylogenetic tree is a hypothesis of the evolutionary relationships among organisms.
22.2 BUILDING A PHYLOGENETIC TREE:
Phylogenetic trees are constructed using shared derived characters.
22.3 THE FOSSIL RECORD:
It offers direct evidence of evolutionary history.
22.4 THE HISTORY OF LIFE:
Phylogeny and fossils document life’s long evolutionary history.
Nature displays nested patterns of similarity among species, confirming evolutionary links.
Example: Humans share more similarities with chimpanzees than either does with monkeys, and all are more similar to each other than to mice or cats.
Carl Linnaeus classified biological diversity based on these patterns; Darwin later recognized them as an outcome of “descent with modification,” which refers to evolution.
EVOLUTIONARY PATTERNS
Evolution produces:
Nested Pattern of Similarities among species found on present-day Earth.
Historical Patterns recorded through fossils.
Life originated more than 3.5 billion years ago, with an estimated 10 million species currently inhabiting the planet.
EVOLUTIONARY HISTORY RECONSTRUCTION
To understand the evolutionary history over 3.5 billion years, nested similarities among species and the fossil record are essential.
PHYLOGENY AND DESCENT
Darwin noted that observed species are modified descendants of earlier species.
Distinct populations of ancestral species diverge through time, producing multiple descendant species.
This history of descent is termed phylogeny, showing evolutionary history and group relatedness.
Like genealogies, phylogenetic trees provide hypotheses about evolutionary relationships.
SPECIAITON AND PHYLOGENETIC ANALYSIS
Speciation is described as a branching process leading to distinct groups of organisms, which is illustrated using a phylogenetic tree:
Branches split at nodes, representing the most recent common ancestor of two descendant groups.
Phylogenetic trees reflect these events, indicating evolutionary relationships among groups such as vertebrate animals.
Example from Figure 22.2 shows relative relationships among vertebrates, proposing that birds are closely related to crocodiles and alligators.
PHYLOGENETIC TREE CONSTRUCTION
Building Phylogenetic Trees:
Phylogenetic trees can be constructed by comparing anatomical, physiological, or molecular features to infer relationships, based on hypotheses formulated from existing data.
New evidence may suggest alternate relationships, necessitating tree adjustments.
MONOPHYLETIC, PARAPHYLETIC, AND POLYPHYLETIC GROUPS
A monophyletic group consists of a common ancestor and all descendants (e.g., amphibians).
A paraphyletic group includes some but not all descendants (e.g., fish).
A polyphyletic group includes organisms from distinct lineages; traits evolved independently through convergent evolution.
Example classification can be misleading if based on polyphyletic arrangements (such as grouping bats and birds).
Taxonomy aims to recognize and name groups based on phylogenetic relationships, utilizing categories from species to domain.
CLASSIFICATION STRUCTURE
Taxonomical hierarchy:
Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.
Each taxonomic level represents a larger limb of the phylogenetic tree (e.g., the three domains are Eukarya, Bacteria, Archaea).
Taxonomy reflects evolutionary relationships and summarizes current scientific evidence.
PHYLOGENETIC TREES USING SYNAPOMORPHIES
Synapomorphies: Shared derived characters crucial for constructing phylogenetic trees; these help to establish sister-group relationships.
The concept of parsimony is utilized in phylogenetic construction, preferring simpler explanations that require fewer evolutionary changes.
Longer evolutionary paths with more changes are deemed less likely.
MOLECULAR DATA IN PHYLOGENY
Phylogenetic trees can be built from molecular data, such as DNA sequences, providing detailed characters for analysis.
Molecular comparisons have revolutionized the field, offering insights into relationship and evolution, and have been applied in various biological contexts.
FOSSILS AND THE FOSSIL RECORD
Fossils provide direct evidence of evolutionary history, documenting extinct species and allowing the dating of evolutionary events.
Example: Birds and crocodiles diverged from a common ancestor around 200 million years ago.
Fossils illustrate the dynamic history of life and environmental changes over geological time.
The fossil record is not complete; it primarily reflects organisms with durable hard parts that had a greater chance of fossilization.
Evolving environmental conditions, including geological events, influence fossil preservation.
Fossilization processes require rapid burial; for example, marine environments yield more complete fossil records.
TRANSITIONAL FOSSILS
Transitional fossils, like Archaeopteryx and Tiktaalik, provide evidence for significant evolutionary transitions, such as the shift from dinosaurs to birds or fish to tetrapods.
Understanding these fossils can clarify evolutionary relationships.
MASS EXTINCTIONS
Major extinction events have similarly shaped the evolutionary landscape, influencing the course of species evolution and what survived.
Five mass extinctions have occurred, with the most notable seen at the end of the Cretaceous, resulting in the extinction of dinosaurs and providing new evolutionary opportunities for survivors.
These extinctions reset ecological dynamics and engendered new groups to thrive post-extinction.
CONCLUSION
Evolution is evidenced through both fossil records and phylogeny.
Fossils provide context to current biodiversity and document the adaptations and transitions in life over time.
Examining both the fossil record and contemporary phylogenetic methods together forms a comprehensive understanding of life’s history on Earth.