Chapter 23: Systematics, Phylogenies, and Comparative Biology
Introduction to Systematics
Definition: Systematics is a modern scientific challenge focused on understanding the history of ancestor-descendant relationships among all forms of life on Earth.
Challenges: The fossil record is often incomplete, leading to unanswered questions.
Solution: Systematists must rely on various types of evidence to establish the best hypotheses of evolutionary relationships.
Core Study: Systematics is the study of evolutionary relationships.
Key Tool: Systematists construct phylogenies.
Phylogeny: The evolutionary history of a group of organisms, tracing their ancestral lineage and relationships through time.
Data Used: Molecular data, morphological data, and fossil data are employed.
Visualization: Phylogenies are visualized through phylogenetic trees.
Phylogenetic Trees: These are branching diagrams that illustrate how species diverged from common ancestors.
Darwin's Vision: Charles Darwin theorized that all species descended from a single common ancestor.
He depicted the history of life as a branching tree, coining the term "descent with modification." This term refers to the evolutionary process where species change over time from a common ancestor.
Visual Representation: Branching diagrams depict these evolutionary relationships.
Understanding Evolutionary Relationships
Similarity vs. Evolutionary Relatedness: Similarity between species may not always accurately predict their evolutionary relationships.
Some species diverge more significantly than others.
Similarity is not necessarily a good predictor of the time elapsed since two species shared a common ancestor.
Evolution is Not Always Divergent:
Convergent Evolution: This occurs when two unrelated species independently evolve the same features or adaptations.
This typically happens in similar environments where similar adaptations are favored.
Consequently, two species that are not closely related may end up appearing more similar to each other than they are to their actual close relatives.
Evolutionary Reversal: This is a process in which a species re-evolves characteristics of an ancestral species that were previously lost, leading to superficial similarity with distant relatives.
Cladistics: Constructing Phylogenetic Hypotheses
Definition: Cladistics is an approach used by systematists to construct phylogenetic hypotheses based on ancestral and derived characters.
Characters in Cladistics:
Ancestral Characters (Plesiomorphies): Similar characters found among species that are inherited from the most recent common ancestor of an entire group.
Derived Characters (Apomorphies): Characteristics that represent a departure from the ancestral form; they are not inherited from the most recent common ancestor of the group being studied.
Synapomorphies (Shared Derived Characters): These are character states that are shared by two or more species and are different from the ancestral character state. Only shared derived characters are informative for determining evolutionary relationships.
Methodology:
Character Data Collection: Systematists first gather data on many characters for all species included in the analysis.
These characters can be almost any aspect of the phenotype, including morphology, physiology, or DNA sequences.
Useful characters exist in recognizable character states, which are one of two or more distinguishable forms of a character (e.g., presence or absence of a tail in vertebrates).
Determining Character States for Each Taxon: After gathering data, systematists determine the states for the characters for each taxon in the analysis.
Taxon: A species or a higher-level group, such as a genus or family.
Coding: Typically, character states are coded as numbers:
1 = \text{presence of derived character}
0 = \text{presence of ancestral character}
Examples:
Hair: A derived character for mammals. All mammal species share a common ancestor that existed more recently than the common ancestor of mammals, amphibians, and reptiles.
Lungs: An ancestral character. Lungs are also present in amphibians and reptiles, indicating they evolved prior to the common ancestor of mammals.
Determining Ancestral vs. Derived States:
To determine which character state was exhibited by the most recent common ancestor of the group under study, an outgroup is used.
Outgroup: A species or group of species that is closely related to, but not a member of, the group under study (the ingroup).
Purpose: The outgroup helps assign character states by providing a reference point for ancestral conditions.
Caveat: Outgroup species do not always exhibit the ancestral condition perfectly, as they also evolve. The determination is most reliable if the character state is exhibited by several different outgroups.
Cladogram Interpretation: Derived characters found between cladogram branch points are shared by all organisms above those branch points and are absent in any below them. The outgroup typically lacks the derived characters that define the ingroup.
Construction of a Cladogram:
A cladogram is a branching diagram depicting a hypothesis of evolutionary relationships.
Clades: Evolutionary units that refer to a common ancestor and all its descendants. Clades are indicated by the possession of synapomorphies (shared derived characters), which are informative about phylogenetic relationships.
Ancestral States: Shared ancestral characters, known as symplesiomorphies, are not informative for grouping organisms within a phylogeny because they are inherited from an older common ancestor shared by a broader group.
Complications: Homoplasy:
Phylogenetic studies are rarely simple due to homoplasy.
Homoplasy: A shared derived character state that has not been inherited from a common ancestor exhibiting that character state. Instead, it results from convergent evolution or evolutionary reversal.
Principle of Parsimony: To resolve ambiguities caused by homoplasy, systematists apply the principle of parsimony.
This principle favors the hypothesis that requires the fewest assumptions or evolutionary events.
A phylogeny that requires the fewest evolutionary changes (e.g., character state transitions) is considered the best hypothesis of phylogenetic relationships.
Example: If cladogram (a) requires 4 changes and cladogram (b) requires 5 changes, cladogram (a) would be favored based on parsimony.
Systematics and Classification (Taxonomy)
Taxonomy: The science of classifying organisms.
Binomial Nomenclature: Each species is given a unique scientific name, consisting of two words.
The first word is the Genus, and the second is the species (e.g., Homo sapiens, Sciurus carolinensis).
This naming system is universal and provides a consistent name for species across all scientists globally.
Hierarchical Classification: Life is classified into a nested hierarchy.
Domains (Most Inclusive): Bacteria, Archaea, Eukarya.
Linnaean Hierarchy (from most inclusive to exclusive):
Kingdom
Phylum
Class
Order
Family
Genus
Species
Flexibility: Categories at different levels may include a varying number of taxa.
Ideal vs. Reality: Ideally, taxonomic groups should reflect evolutionary relationships. However, traditional groups do not always align well with new evidence and understanding.
Grouping Organisms Based on Phylogeny
It is crucial for groups to be classified based on their phylogenetic relationships.
Monophyletic Group (Clade):
Includes the most recent common ancestor of the group and all its descendants.
These are considered valid evolutionary units.
Paraphyletic Group:
Includes the most recent common ancestor, but not all of its descendants.
These groups are not considered natural evolutionary units.
Polyphyletic Group:
Includes descendants from multiple lineages, but does not include their most recent common ancestor.
These groups typically result from convergent evolution and are also not considered natural evolutionary units.
Phylogenetic Species Concept
Definition: This concept defines species based on their phylogenetic relationships.
A species is defined as a group of populations that have been evolving independently of other groups of populations.
Phylogenetic analysis is the method used to identify such species.
A phylogenetic species is characterized by one or more shared derived characters (synapomorphies), which imply a period of separate evolution.
Advantages over Biological Species Concept:
Temporal Depth: It looks to the past to determine whether a population has evolved independently for a long enough time to develop its own derived characters.
Applicability: It can be applied to both sexually reproducing and asexually reproducing species (a limitation of the biological species concept).
Limitations:
Over-splitting: It raises questions about whether every slightly differentiated population constitutes a distinct species, or if each habitat contains its own species of organisms.
Ongoing Debate: Evolutionary biologists are actively working to reconcile issues with these and other species concepts to achieve a comprehensive understanding of species demarcation.
The Origin of HIV
The transcript briefly references "The Origin of HIV" by SciShow, suggesting a real-world example or discussion related to the application of phylogenetic principles to understand disease evolution. (No further details provided in the transcript).
More Related?
The final slide, "CC BIOLOGY MORE RELATED?" indicates a prompt for further discussion or an example comparing relatedness, likely using phylogenetic principles. (No further details provided in the transcript).