Phylogeny

Chapter 20: Phylogeny in Biology

Overview: Investigating the Evolutionary History of Life

This chapter introduces the concept of phylogeny, which is the study of the evolutionary history and relationships among species. It sets the stage for understanding how living organisms are classified and connected through ancestry.

Phylogeny and Related Concepts

Phylogeny Defined

  • Phylogeny refers to the evolutionary history of a species or group of related species.

Systematics

  • Systematics is the scientific discipline that classifies organisms and determines their evolutionary relationships, essential for organizing the diversity of life.

Taxonomy

  • Taxonomy is the ordered division and naming of organisms, providing a framework for identifying and categorizing life forms.

Concept 20.1: Phylogenies Show Evolutionary Relationships

Key Concepts

  1. Binomial Nomenclature: A two-part scientific naming system developed by Carl Linnaeus, which assigns each species a name comprising its genus and species identifiers.

  2. Hierarchical Classification: Organisms are categorized into successive levels of classification ranks (Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species).

  3. Linking Classification and Phylogeny: The relationship between taxation and evolutionary history, acknowledging that classifications can reflect phylogenetic relationships but may not always align perfectly.

  4. Phylogenetic Trees: Diagrams that depict evolutionary relationships; understanding what can and cannot be deciphered from these trees is critical.

  5. Applications of Phylogenies: The utility of phylogenetic insights in various fields such as conservation, ecology, and evolutionary biology.

Taxonomic Hierarchy Example

  • Example: Panthera pardus (Leopard)

    • Kingdom: Animalia

    • Domain: Eukarya

    • Phylum: Chordata

    • Class: Mammalia

    • Order: Carnivora

    • Family: Felidae

    • Genus: Panthera

    • Species: Panthera pardus

    • A taxonomic unit at any level of hierarchy is termed a taxon.

Depicting Evolutionary Relationships

  • Phylogenetic Trees: Used by systematists to illustrate evolutionary relationships in a branching format.

    • Branch Point: Represents the divergence of two taxa from a common ancestor.

    • Sister Taxa: Groups sharing a recent common ancestor.

    • Rooted Tree: A tree structure that includes a branch representing the most recent common ancestor of all taxa in the tree.

    • Basal Taxon: A lineage that diverged early in a group's history and is near the common ancestor.

    • Polytomy: A branch from which more than two groups emerge, indicating an unresolved pattern of divergence.

Parts of a Phylogenetic Tree

  • The root represents the common ancestor of all taxa in the tree.

  • Horizontal branches indicate evolutionary lineages with specific ancestors.

  • Sister taxa share a common ancestor distinct from all other groups.

Alternative Forms of Tree Diagrams

  • Vertical, Diagonal trees, and the importance of tree rotation in maintaining evolutionary relationships.

  • Rotation at Branch Points: Demonstrates that the order of taxa does not imply a sequence of evolution leading to the last taxon shown.

Limitations of Phylogenetic Trees

What We Can Learn

  • Phylogenetic trees depict patterns of descent rather than phenotypic similarity.

  • They do not typically convey the timing of evolutionary events or the extent of change.

  • It is incorrect to assume that neighboring taxa in the tree directly descended from each other.

Concept 20.2: Inferring Phylogenies from Data

Morphological and Molecular Data

  • To deduce phylogenies, systematists aggregate information regarding the morphology, genetics, and biochemistry of living organisms.

  • Homology: Refers to similarities resulting from shared ancestry; vital for building phylogenies.

Distinguishing Homology from Analogy

  • Homology: Similarity based on common ancestry.

  • Analogy: Similarity resulting from convergent evolution.

    • Example:

      • North American mole (eutherian) versus Australian “mole” (marsupial).

  • Homoplasies: Analogous structures or molecular sequences that evolved independently.

  • Complexity enhances the likelihood of similarity being homologous.

Molecular Homologies

  • Evaluating molecular homologies relies on similarity in nucleotide sequences.

  • Systematists employ software tools for analyzing comparable DNA segments from diverse organisms.

Concept 20.3: Characters in Constructing Phylogenetic Trees

Shared Characters

  • Once homologous characters are established, they are informative in phylogenic analysis.

  • Cladistics: Classification based on common ancestry.

  • Clade: A group that consists of an ancestral species and all its descendants.

Valid Clades

  • A valid clade is described as monophyletic, containing an ancestor and all its descendants, as opposed to other types that may include only parts of a lineage (paraphyletic, polyphyletic).

Shared Ancestral and Derived Characters

  • Shared Ancestral Character: Characteristics that originated in an ancestor of the taxon.

  • Shared Derived Character: Evolutionary novelties specific to a clade; context-dependent as characteristics can be both ancestral and derived.

Phylogenetic Tree Visualizations

Branch Lengths

  • In specific phylogenetic trees, branch lengths may correlate with the quantity of genetic changes within a lineage.

  • Branch lengths can also be indicative of chronological time, based on fossil evidence.

Maximum Parsimony Principle

  • Systematists utilize the principle of maximum parsimony to limit potential phylogenetic trees by choosing the simplest solution with the least evolutionary changes in shared derived characters.

Phylogenetic Hypotheses

  • The best phylogenetic hypotheses fit morphological, molecular, and fossil data while being amendable to new evidence.

  • Phylogenetic Bracketing: Predicts features of ancestors and their extinct descendants, based on traits present in closely related living descendants.

Concept 20.4: Molecular Clocks and Evolutionary Time

Molecular Clocks

  • Applications of molecular clocks allow scientists to estimate evolutionary change times based on consistent mutation rates in certain genes.

  • Variability exists among individual genes concerning their clock-like nature.

Problems with Molecular Clocks

  • Irregularities: Molecular clocks may not function seamlessly due to selective pressures that make some DNA changes more favorable.

  • Realizations of evolutionary divergences that predate the fossil record tend to be fraught with uncertainties.

Application: Dating the Origin of HIV

  • Phylogenetic analysis reveals that HIV arises from viruses infecting primates.

  • Analysis shows HIV spread to humans multiple times and that specific strains evolved in a clock-like manner, with some suggesting its spread into human populations occurred in the 1930s.

Concept 20.5: Revising Evolutionary Understanding

Insights from DNA Sequencing

  • Recent advancements in DNA sequencing have provided insights into the deepest branches of the tree of life, shifting classification from five kingdoms to a more contemporary three-domain system: Bacteria, Archaea, and Eukarya.

Horizontal Gene Transfer

  • The extensive role of horizontal gene transfer in evolutionary biology highlights the importance of genetic material moving across different genomes, challenging traditional views of evolutionary lineage due to processes such as viral infections and fusion of organisms.

Summary

  • Phylogenies elucidate evolutionary relationships, inferred from various morphological and molecular data, allowing for the construction of phylogenetic trees.

  • Molecular clocks aid in tracking evolutionary timelines, further highlighting that new research continually refines our understanding of the evolutionary history of life.