15.2 The Organism Classification Process

Prior to 1942, species classification relied heavily on appearance and structure.
However, visual similarities can be misleading, necessitating new classification methods.
In 1942, Ernst Mayer introduced the biological species concept.
Biological Species Concept:
- Definition: A species is a group of natural populations that interbreed or could interbreed and are reproductively isolated from other groups.
- Reproductive Isolation: Barriers prevent interbreeding between groups.
- Hybrids: Offspring resulting from interbreeding of different species.
  - Example: Wolves and dogs (Canis genus).
Evaluating the Biological Species Concept:
- Works well for the Animalia kingdom due to hybridization barriers.
- Limitations: Inapplicable to organisms that reproduce asexually (e.g., certain protists, fungi, animals, and all bacteria).
- Modern Approach: Biologists now study an organism's features to recognize species.

Discovery Rate: Hundreds or thousands of new species are discovered annually.
Known Species: Approximately 2,000,000 species have been described.
Estimated Total Species: Scientists estimate 8 to 10 million species (or more) remain undiscovered.

Linnaeus's System: Based on observed similarities among organisms.
- Example: Lions resemble chimpanzees more than they resemble fish.
Darwin's Contribution: Similarity indicates descent from a recent common ancestor.
Phylogeny: Classifying organisms based on similarities should reflect their evolutionary history.

Not all similar characteristics are inherited from a common ancestor.
Convergent Evolution: Organisms may share similar features without a shared common ancestor.
- Example: Birds and insects both have wings, but wing structure differs greatly.
Analogous Characters: Similarities arising through convergent evolution.

Cladistics: A method of analysis for reconstructing phylogenies based on shared characters.
- Goal: Determine the sequence of evolution in different groups.
Types of Characters:
- Ancestral Character: Evolved in a common ancestor.
  - Example: Backbone in birds and mammals.
- Derived Character: Evolved in the ancestor of one group but not the other.
Main Principle: Shared derived characters indicate close relationships; ancestral characters do not.
- Example:
  - Dogs and iguanas have limbs; whales do not appear to have limbs.
  - All three share the ancestral character of limbs, because whales descended from an ancestor with limbs.
  - Dogs and whales share mammary glands (shared derived character), not found in iguanas or their ancestors.
  - Therefore, dogs and whales share a more recent common ancestor than either shares with iguanas.

Cladogram: A branching diagram used to visualize evolutionary relationships.
Construction: Organisms with shared derived characters are grouped together.
Evolutionary Progression: New derived characters appear on the cladogram as groups evolve.

Objectivity of Cladistics: Simply indicates whether a character exists, regardless of the degree of difference.
Equal Consideration: Each character is initially considered equally.
Modern Studies: Attempt to weigh the evolutionary significance of characters.

Subjectivity: A subjective analysis of evolutionary relationships.
Phylogenetic Tree: Branching diagram where taxonomists assign varying importance to different characters.
Example: Placing birds in a separate class from reptiles due to features like feathers.
Potential Bias: Allows biologist's observations but also potential biases to influence the process.

Cladogram and phylogenetic tree each represents a hypothesis of evolutionary history, which is inferred and not observed.