Lesson 23: Systematics

23.1 Reconstructing Evolutionary Relationships

Phylogenetic systematics, or simply systematics, is the field dedicated to the reconstruction and study of evolutionary relationships. Scientists in this field are called systematists. They construct evolutionary trees, known as phylogenies (sometimes called cladograms), to represent hypotheses about the relationships among species.

Historically, Charles Darwin used branching tree diagrams to illustrate species descending from a single ancestor. A phylogeny depicts evolutionary relationships, where the interpretation should focus on how recently sets of species shared a common ancestor, rather than the arrangement of species at the top of the tree, as this can change without altering the actual relationships.

Phylogenies are hypotheses based on the best available data, which can include:

  • DNA and protein sequence data (most common)

  • Morphology

  • Physiology

  • Behavioral data

Terminology Associated with Phylogenies

  1. Rooted Phylogeny: To be meaningful, phylogenies must be rooted with an outgroup.

  2. Outgroup: A species outside the group being studied, relatively closely related, used to determine ancestral character states (polarizing characters). For example, a lamprey as an outgroup for jawed vertebrates establishes jawlessness as an ancestral trait.

  3. In-group: The group of organisms or taxa being studied.

  4. Ancestral State: A character state that arose prior to the common ancestor of the in-group.

  5. Derived State: A character state that is inherited from the most recent common ancestor.

  6. Clade: A group of species that share a common ancestor and include all of its descendants (e.g., tiger, gorilla, and human).

  7. Synapomorphy: A shared derived character state that defines a clade.

  8. Node: A point of divergence or fork in the tree, representing a common ancestor. Numbers at nodes can indicate the strength of data support for the depicted evolutionary relationships.

23.2 Approaches to Building a Phylogeny

Several approaches are used to build phylogenies, including cladistics, statistics, and the molecular clock.

1. Cladistics Approach

Cladistics relies on the principle of parsimony, which posits that the phylogeny requiring the fewest evolutionary events (e.g., character changes) is the best hypothesis of relationships.

  • Shared Derived Characters (Synapomorphies) are considered informative for establishing phylogenies. Shared ancestral states are not used.

  • Morphological Data: Characters are often represented by states (e.g., 00 for absence, 11 for presence). Synapomorphies are marked on the phylogeny to show how the tree is constructed (e.g., presence of hair groups tiger, gorilla, and human).

  • Complicating Factors: Similarity alone may not accurately predict evolutionary relationships due to:

    • Homoplasy: A shared character state not inherited from a common ancestor.

    • Convergent Evolution: Similar traits evolving independently in different lineages (e.g., wings of birds and bats).

    • Evolutionary Reversal: A trait reverting to an earlier state (e.g., loss of tails in adult frogs).

  • Resolving Homoplasy: The principle of parsimony is used, favoring the hypothesis that requires the fewest evolutionary assumptions or events.

  • Molecular Data: DNA sequence data provides a larger number of characters and is generally favored over morphological data. Computer programs are essential for analyzing large molecular datasets to find the most parsimonious relationships. Nucleotide changes at specific positions serve as synapomorphies.

2. Molecular Clock Approach

  • Estimates the timeframe of species divergence events based on rates of DNA mutations.

  • Most reliable when calibrated with data from the fossil record.

  • While generally reliable, some data suggest that evolutionary rates are not always constant; promising methods model situations with variable rates.

3. Statistical Methods

  • Allow independent assumptions about the rates at which different characters evolve.

  • Very useful when evolution has proceeded rapidly or when dealing with high rates of homoplasy (more effective than maximum parsimony in such cases).

  • These methods fit data to a model based on assumed character evolution rates to produce the best phylogeny.

23.3 Systematics, Classification, and Applications

Systematics focuses on the reconstruction and study of evolutionary relationships, while classification is the practice of placing species into a taxonomic hierarchy (e.g., phyla, classes, genera).

Types of Taxonomic Groups

  1. Monophyletic Group: Includes the most recent common ancestor and all of its descendants (e.g., Archosaurs, containing crocodiles and all descendants).

  2. Paraphyletic Group: Includes the most recent common ancestor but not all of its descendants (e.g., traditional "Reptilia" excluding birds).

  3. Polyphyletic Group: Does not contain the most recent common ancestor of all members; often grouped due to convergent evolution (e.g., "flying vertebrates" like bats and hawks).

Traditional classification systems do not always align with new phylogenetic understandings. For instance, recent phylogenetic advances show that birds evolved from dinosaurs, making them a type of reptile, challenging the traditional separation of Aves from Reptilia.

Applications of Phylogenetics (Comparative Biology)

  1. Illustrating Convergent Evolution: Phylogenies can show how similar traits (homoplasies), like sieve tubes in distantly related plants or wings of birds and dragonflies, evolved independently from different ancestral sources, as opposed to homologous structures derived from the same body part in a common ancestor (e.g., dolphin flipper and horse front leg).

  2. Revealing Sequences of Evolutionary Change: Complex characters evolve gradually over time, and phylogenies can illustrate these step-by-step changes (e.g., evolution of modern bird characteristics).

  3. Explaining Species Diversification: Phylogenies can test hypotheses about species richness (number of species per clade). For example, beetle specialization on angiosperms has independently occurred multiple times and is linked to high species richness, suggesting opportunities for adaptation promoting divergence and speciation.

  4. Illustrating Patterns of Dispersal: Biogeographical data can be integrated into phylogenetic analyses to study the center of origin and dispersal patterns of taxa (e.g., fish flies distribution in North America).

  5. Studying Disease Evolution and Transmission:

    • HIV/SIV Evolution: Phylogenetics showed HIV evolved multiple times from SIV (Simian Immunodeficiency Virus) in primates, transmitted to humans via blood-to-blood contact. It helps pinpoint the general time and location of initial human infection.

    • Identifying Infection Sources: Due to rapid mutation rates, phylogenies can trace how diseases like HIV and SARS-CoV-2 spread between individuals or populations, identifying the source of infection.