Notes on Species Concepts and Diversity

Species Diversity and Classification Challenges

Known species in 1990: ~1,400,000; current identified/cataloged: ~2,000,000; many still unclassified, especially bacteria and archaea.
Global estimates of total species: commonly 5–50,000,000; a frequent figure is ~15,000,000.
Bacteria/archaea diversity estimates: ~10,000,000 to ~1,000,000,000; many cannot be cultured in the lab.
Subspecies and echotypes: geographically restricted groups with distinct traits or locally adapted, genetically distinct populations.
Biologists often confront when differences between populations exist but do not clearly justify separate species.

A species is a group with a distinctive set of attributes; in sexually reproducing species, members usually cannot interbreed with members of other species; they also share a distinct evolutionary history.
Defining the precise boundary between species is difficult; questions include how many differences are needed to classify as separate species.

Criteria depend on the organism; insect traits differ from bacterial traits; horizontal gene transfer in bacteria complicates delimitation.
Common characteristics used: morphological traits, ability to interbreed, molecular features, ecological factors, and evolutionary relationships.

Morphology is a traditional basis for species identification, applying to both macroscopic and microbial organisms.
Drawbacks: how many traits to consider; continuous variation (size/weight) complicates analysis; dissimilar traits do not always map cleanly to species boundaries.
Polymorphism example: Anubates Timtorius (dying poison frog) shows many color morphs within one species.
Similar-looking species can be distinct due to reproductive isolation (e.g., Northern Leopard frog Ranapipiens vs. Southern Leopard frog Ranatricularia look similar but are treated as different species due to isolation in nature).

The idea of reproductive isolation as a defining feature of species (Gobzanski’s early idea; Mayr’s Biological Species Concept).
Biological Species Concept (1930s–1940s): a species consists of populations whose members can interbreed in nature to produce viable, fertile offspring but cannot interbreed successfully with members of other species.
Limitations:
- Overlapping geographic ranges can complicate assessment.
- Some species interbreed yet remain distinct (e.g., yucca plants with ongoing interbreeding but distinct characteristics).
- Not applicable to asexual species (e.g., many bacteria) or extinct species.
Practical emphasis: mainly applicable to sexually reproducing modern animals and plants.

Use DNA sequences, gene order, chromosome structure, and chromosome number to distinguish populations.
Example: compare 16S rRNA gene sequences among bacteria.
Ambiguity: how much genetic difference warrants placing populations in different species (e.g., 1% vs 5%).

Distinguish species by habitat use and ecological niches (e.g., warblers by foraging habitat).
Bacteria often categorized by ecological factors (same resources, growth conditions, etc.).
Drawbacks: similar growth characteristics can occur across different groups; species may tolerate a range of conditions, blurring boundaries.

Evolutionary trees (phylogenies) describe relationships among species.
Approaches include fossil record analyses and DNA sequence comparisons to infer ancestry and relatedness.

Biological Species Concept (Ernst Mayr): interbreeding potential in nature; separate species cannot interbreed with other species.
Evolutionary Lineage Concept (George Gaylord Simpson, 1961): species defined by separate evolutionary lineages.
Ecological Species Concept (Lee Van Valen, 1976): species defined by an ecological niche.
General Lineage Concept (Kevin De Queiroz, 1998): a single general framework; a population of independently evolving lineages; uses multiple criteria from other concepts to decide membership; broadly supported due to its generality.

A species is a group of related organisms that share a distinctive set of attributes in nature.
Speciation is the process by which new species are formed.
Macrovolution refers to evolutionary changes that produce new species and groups.