Chapter 25: Phylogeny and Fossils

Introduction

This chapter explores the methods and significance of reconstructing evolutionary relationships among organisms, the interpretation of the fossil record, and the geological context of life's history on Earth. Understanding these concepts is fundamental to the study of evolutionary biology and systematics.

Phylogenetic Trees

Definition and Utility

• Phylogeny refers to the evolutionary history and relationships among species or groups of organisms.

• Phylogenetic trees are diagrammatic representations that depict hypotheses about the evolutionary relationships among taxa.

• These trees help biologists understand patterns of descent and the timing of evolutionary events.

Interpreting Phylogenetic Trees

• Evolutionary relationships are inferred from the branching patterns of the tree.

• Sister groups are two descendants that split from the same node and are each other's closest relatives.

• Groups can be classified as monophyletic (includes a common ancestor and all its descendants), paraphyletic (includes a common ancestor and some, but not all, descendants), or polyphyletic (does not include the most recent common ancestor of all members).

Cladistics and Parsimony

• Cladistics is a method of classifying organisms based on common ancestry and shared derived characteristics (synapomorphies).

• Parsimony is the principle that the simplest explanation (tree with the fewest evolutionary changes) is preferred.

• Cladograms are constructed using data matrices and analyzed for the most parsimonious arrangement.

Homology vs. Homoplasy

• Homology refers to similarity due to shared ancestry (e.g., vertebrate forelimbs).

• Homoplasy is similarity due to convergent evolution or evolutionary reversals, not common ancestry (e.g., wings in bats and birds).

Taxonomic Classification

• Taxonomic ranks include Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.

• Scientific names are written in binomial nomenclature (Genus species).

Fossil Record

Major Extinctions

• Mass extinctions are periods of rapid, widespread extinction that drastically reduce biodiversity.

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

• The sixth mass extinction refers to the ongoing loss of species due to human activity.

Strengths and Limitations of the Fossil Record

• The fossil record provides direct evidence of past life and evolutionary transitions.

• It is incomplete due to biases in preservation (e.g., abundance bias, habitat bias, taxonomic bias).

• Types of fossils include body fossils (actual remains) and trace fossils (evidence of activity).

Fossilization Conditions

• Fossilization is more likely when organisms are rapidly buried in sediment and have hard parts.

• Environmental conditions such as low oxygen and rapid mineralization favor preservation.

Geologic Time

Geological Timeline and Major Events

• Earth's history is divided into eons, eras, periods, and epochs.

• The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic eras.

• The Cambrian explosion (~541 million years ago) marks a rapid diversification of animal life.

• Adaptive radiations are periods when organisms diversify rapidly into new forms and ecological roles, often following mass extinctions or the evolution of key innovations.

Significance of the Cambrian Explosion

• The Cambrian explosion represents a pivotal event in evolutionary history, with the sudden appearance of most major animal phyla.

• Possible triggers include increased oxygen levels, genetic innovations, and ecological interactions.

Key Terms and Definitions

Example: Constructing a Phylogenetic Tree

• Suppose you have data on morphological traits for several species. You can use a data matrix to score the presence or absence of traits, then use parsimony to infer the most likely tree.

• For example, if three species share a derived trait not found in others, they likely form a clade.

Additional info:

• Radiometric dating equations often use the decay constant () and the ratio of parent to daughter isotopes: , where is age, is number of daughter atoms, and is number of parent atoms.

• Adaptive radiations can be triggered by ecological opportunity (e.g., colonization of new habitats) or evolutionary innovation (e.g., evolution of flight).