Chapter 24: Species Concepts
Biological Species Concept (BSC)
Definition: A population (or group of populations) that are reproductively isolated from other groups. The key testable idea is that members have the potential to interbreed in nature to produce viable, fertile offspring.
Core criterion: Reproductive isolation separates species from one another; gene flow is limited or prevented between populations of different species.
Testable aspect: Can be evaluated by observing interbreeding and the viability/fertility of offspring in natural or controlled settings.
Examples used in lectures:
Western Meadowlark vs Eastern Meadowlark (often treated as different species in field identification)
Gaudy Commodore (Precis octavia) used to illustrate morphological similarity/divergence and species delimitation
Implications: Emphasizes reproductive compatibility as the defining feature of species boundaries.
Limitations of the BSC:
Not applicable to asexual species (e.g., Bdelloid rotifers are described as asexual; each individual is female).
Not easily applied to fossil species (no living interbreeding tests).
Not always clear when populations are geographically isolated but could interbreed if brought together; does not handle hybrids or hybrid zones well.
Ring species and ongoing hybridization can blur boundaries.
Example illustrating limitation: Bdelloid rotifers are asexual; thus the BSC cannot be used to delimit species in this group.
Morphospecies Concept
Definition: Species are identified by differences in size, shape, or other morphological features.
Visual/structural criteria: Species boundaries are inferred from observable morphological differences (e.g., body size, shape, coloration, skeletal features).
Examples used in lecture:
Large range of bear morphologies depicted (e.g., polar bear, brown bear, Asiatic black bear, sun bear, etc.) to illustrate how fixed morphological differences can define taxa.
Hypothetical or representative specimens showing 6 ft, 8–10 ft sizes, etc., to illustrate variation among potential species.
Advantages: Widely applicable, simple to apply in the field, useful across many taxa where genetics is unavailable.
Limitations:
Polymorphism within a species can be mistaken for separate species.
Cryptic species (genetically distinct but morphologically similar) may be overlooked.
Subjective thresholds for what counts as a sufficient morphological difference.
Not ideal for asexual or fossil taxa where morphology alone may be insufficient.
Misidentification risk when populations are highly variable morphologically (e.g., the R. variabilis highland vs lowland morphs; R. imitator with multiple morphs; R. fantastica as an associated morph).
Example caveats from lecture:
R. variabilis (highland morph) vs R. variabilis (lowland morph)
R. imitator (striped vs varadero vs spotted morphs)
Takeaway: Morphological distinctness can indicate species, but it may over- or under-estimate diversity depending on polymorphism and cryptic diversity.
Phylogenetic Species Concept (PSC)
Definition: Identifies species based on the evolutionary history of populations; species are the smallest monophyletic groups that share a common ancestor.
Monophyly: A monophyletic group is an evolutionary unit that includes an ancestral population and all its descendants, but no others.
Formal idea: A clade consisting of an ancestor and all of its descendants.
Synapomorphy: A shared, derived trait that is found in certain groups and their common ancestor but is absent in more distant ancestors. Synapomorphies define clades.
Practical approach: Use genetic data (e.g., DNA sequence data) to identify monophyletic groups and shared derived traits.
Case study workflow (elephants):
Sample tissue from multiple individuals across populations (e.g., 7 Asian elephants and 195 African elephants, including forest and savanna).
Sequence multiple nuclear genes (e.g., four genes) from each individual.
Use sequence data to estimate a phylogenetic tree.
Interpret monophyletic groups as distinct evolutionary lineages (potential species).
India/Sri Lanka/Sumatra/Borneo (distribution data) helps contextualize geographic lineages within PSC.
Key idea: PSC emphasizes evolutionary independence and historical lineage separation rather than current reproductive compatibility or morphology alone.
Synapomorphy and Evolutionary History ( PSC-related)
Synapomorphy is a trait shared by two or more groups and their most recent common ancestor, but not present in distant ancestors.
Role in PSC: Helps define clades and assess whether populations form monophyletic groups consistent with species boundaries.
Case Study: How many species of elephants?
Applying Biological Species Concept (BSC): Do forest and savanna elephants interbreed? If yes, are the offspring viable and fertile?
Applying Morphospecies Concept: How much morphological difference is required to reject a two-species hypothesis?
Applying Phylogenetic Species Concept (PSC): How many monophyletic groups emerge across sampled populations?
Data collection (illustrated in lecture):
Asian elephants and African elephants sampled; tissue collected; DNA sequenced for multiple loci.
Phylogenetic analysis used to infer lineage relationships and assess monophyly of forest vs savanna elephants.
Expected outcomes (as discussed):
BSC may support two species if forest and savanna elephants do not interbreed or produce viable, fertile offspring.
Morphospecies assessment depends on whether observed morphological differences consistently delineate the groups.
PSC often reveals two distinct monophyletic lineages corresponding to forest and savanna elephants, supporting species status under PSC.
Geographic context: Distribution in Africa (forest, savanna, Central, Eastern, Southern Africa) and Asian populations (India, Sri Lanka, Sumatra, Borneo) provide a framework for interpreting lineage splits.
Speciation: A Process Overview
Speciation is a process by which one species splits into two or more distinct species.
Timeframe: It occurs over evolutionary timescales rather than instantly (depicted as Time 1 -> Time 2).
Two core steps:
Genetic isolation: A barrier to gene flow isolates populations from one another.
Genetic divergence: Mutation, natural selection, and genetic drift in isolated populations drive divergence.
Result: The accumulation of lineage-specific differences leads to reproductive isolation and the emergence of distinct species.
Mechanisms of Reproductive Isolation
Broad division: Prezygotic Isolation vs Postzygotic Isolation
Prezygotic Isolation (no mating or no mating success):
Temporal isolation: Populations breed at different times (e.g., Bishop pines and Monterey pines flowering/pollen release timing).
Habitat isolation: Populations breed in different habitats (e.g., mainland mice vs beach mice).
Behavioral isolation: Different courtship displays prevent interbreeding (e.g., songbird species-specific songs attract different females).
Mechanical isolation: Mating fails due to incompatible reproductive structures (e.g., genitalia morphologies in some insects).
Gametic barrier: Eggs and sperm are incompatible (e.g., binding proteins in sea urchin sperm and eggs).
Postzygotic Isolation (mating occurs, offspring viability or fertility is compromised):
Hybrid inviability: Hybrids do not develop normally and die early (e.g., ring-necked doves vs rock doves eggs hatch rates < ~6%).
Hybrid sterility: Hybrids mature but are sterile (e.g., horses × donkeys producing sterile mules).
Summary: Prezygotic barriers reduce or prevent mating; postzygotic barriers reduce fitness of hybrids after mating, contributing to reproductive isolation between populations.
Allopatry and Allopatric Speciation
Allopatry: Geographic separation of populations (“different homeland”) leading to distinct evolutionary trajectories.
Biogeography: Study of how populations are distributed geographically and the historical factors shaping those patterns.
Allopatric speciation by dispersal (a) Dispersal and colonization:
Geographic isolation occurs when individuals disperse to a new habitat.
Genetic isolation arises due to lack of gene flow.
Genetic divergence accumulates via mutation, drift, and selection.
Allopatric speciation by vicariance (b) Geographic isolation due to a barrier that splits a population:
Chance event physically separates populations.
Genetic isolation arises due to lack of gene flow.
Genetic divergence accumulates via mutation, drift, and selection.
Case study: Allopatric species by vicariance (Ribas, Cracraft, et al.)
Hypothesis: The Amazon River system's history is key to diversification of trumpeter birds.
Findings:
Distribution of trumpeter species aligns with geological events (river formation and fragmentation).
Rivers such as the Amazon have subdivided ancestral populations, reducing gene flow.
Result: Eight species isolated by vicariance were identified.
Case Study: Trumpeter Birds and Vicariance
Key insights:
Geological events (formation and changes of major rivers) act as barriers that fragment populations.
Vicariance-driven speciation can rapidly generate multiple closely related species.
Case study supports the idea that historical landscape features shape current biodiversity patterns.
Comparison of Species Concepts: Criteria, Pros, and Cons
Biological Species Concept (BSC)
Criterion: Reproductive isolation between populations; no interbreeding or viable, fertile offspring across species boundaries.
Advantages: Links to evolutionary independence through isolating mechanisms; widely taught and used.
Disadvantages: Not applicable to asexual or fossil species; difficult when populations do not overlap geographically; potential misclassification of polymorphic species; ambiguous in hybrid zones.
Morphospecies Concept
Criterion: Morphologically distinct populations as species.
Advantages: Widely applicable; practical for field work and fossils (to some extent); testable with morphology.
Disadvantages: Polymorphism can mislead; cryptic species may be overlooked; subjective thresholds for differentiation; not well-suited for asexual/fossil taxa.
Phylogenetic Species Concept (PSC)
Criterion: Smallest monophyletic group on a phylogenetic tree; defined by unique evolutionary history and shared derived traits (synapomorphies).
Advantages: Widely applicable; based on objective, testable criteria; explicitly links to evolutionary history.
Disadvantages: Requires well-estimated phylogenies; can over-split taxa if lineages are shallow or incomplete; may rely on limited data in some groups.
Practical takeaway: In many cases, multiple species concepts are used together to triangulate species boundaries; each concept has strengths and weaknesses depending on the taxon and data available.
Key Terms and Concepts to Remember
Monophyletic group: An ancestral population and all its descendants, with no non-descendant members.
Synapomorphy: A shared derived trait that defines a clade.
Isolating mechanisms: Factors that prevent gene flow between populations, either before (prezygotic) or after (postzygotic) mating.
Allopatry: Geographical separation leading to speciation.
Dispersal vs Vicariance: Two mechanisms driving allopatric speciation; dispersal involves movement to a new area, vicariance involves geographic barriers splitting populations.
Hybrid inviability and hybrid sterility: Postzygotic barriers reducing fitness of hybrids.
All three species concepts (Biological, Morphological, Phylogenetic) provide complementary perspectives on what constitutes a species.
Notable Illustrative Details from the Transcript
Species examples for concept illustration: Western Meadowlark vs Eastern Meadowlark; Gaudy Commodore (Precis octavia); R. fantastica, R. variabilis, R. imitator morphs (illustrating morphological diversity and mimicry).
Bdelloid rotifers as an explicit example of asexual organisms challenging the Biological Species Concept.
The elephant case study demonstrates a real-world application of three species concepts to a single taxon, highlighting potential discrepancies and convergence among concepts.
The Trumpeter birds case study demonstrates vicariant speciation driven by riverine barriers, aligning species boundaries with historical geology.
Equations and Quantitative Details (LaTeX)
Monophyly definition in evolutionary terms: A group containing a common ancestor and all of its descendants, i.e., a clade. ext{Monophyletic group} = ext{Ancestral population}
ightarrow ext{All descendants}Temporal/space scales cited: 37 ext{--}5 ext{ million years ago} (mid-Miocene changes referenced in the context of mammalian evolution and biogeography).
Population sampling scales (case study): Example sampling scale in the elephant PSC case: 7 ext{ Asian elephants} \ 195 ext{ African elephants} ext{ (including forest and savanna)}$$
A general structure for allopatric divergence: three-stage model per dispersal or vicariance.
Dispersal: Geographic isolation → Genetic isolation → Genetic divergence, via mutation, drift, and selection.
Vicariance: Geographic isolation → Genetic isolation → Genetic divergence, via mutation, drift, and selection.