What is a Species?
The biological species concept — and its limitations
Before understanding how species form, we need to define what a species is. This is harder than it sounds — biologists have proposed over 20 different species concepts, each with advantages and limitations for different groups of organisms.
BSC
The biological species concept — Ernst Mayr, 1942
The biological species concept (BSC) defines a species as a group of populations whose members actually or potentially interbreed and are reproductively isolated from other groups. Two populations belong to the same species if they can interbreed and produce fertile offspring; they are different species if reproductive barriers prevent gene flow between them.
The BSC is the most widely used concept and maps well onto our intuition about what a species is. However, it has important limitations: it cannot be applied to fossils (we cannot test interbreeding), cannot be applied to asexual organisms (bacteria reproduce without sex), and breaks down at geographic edges of species ranges where hybridization between normally distinct species occurs (ring species). Many plant species hybridize freely and are still considered distinct.
Memory trick: Biological species = reproductively isolated from other groups. If they can't interbreed (or don't), they're different species. The reproductive barrier is the key.
RB
Reproductive barriers — what keeps species apart
Reproductive barriers are the mechanisms that prevent gene flow between species. They fall into two categories:
Prezygotic barriers (before fertilization):
• Habitat isolation: species use different habitats in the same area (garter snakes — some aquatic, some terrestrial in the same forest)
• Temporal isolation: species breed at different times (spotted and Thomas's skunk breed in different months; two oak species release pollen at different times)
• Behavioral (ethological) isolation: species don't recognize each other as potential mates (different bird songs, different courtship dances, different pheromone blends) — most important in animals
• Mechanical isolation: incompatible anatomy prevents copulation or pollen transfer
• Gametic isolation: sperm cannot penetrate egg (molecular incompatibility of surface proteins)
Postzygotic barriers (after fertilization):
• Hybrid inviability: hybrid embryos develop abnormally and die before reproduction
• Hybrid sterility: hybrids survive but are sterile (mule = horse × donkey — healthy but sterile)
• Hybrid breakdown: F1 hybrids are fertile but F2 offspring have reduced viability or fertility
Memory trick: Pre-zygotic = before the sperm meets the egg. Post-zygotic = the hybrid forms but doesn't work. Pre-zygotic barriers are more common and more efficient at preventing gene flow.
Modes of Speciation
Allopatric, sympatric, and other routes to new species
Speciation can occur through several different geographic and genetic mechanisms. The most important distinction is whether speciation requires geographic separation of populations.
💡 Speciation Rates — Gradualism vs Punctuated Equilibrium
How fast does speciation occur? Two competing models:
Phyletic gradualism: Species change slowly and continuously over time — the fossil record should show smooth gradations between ancestor and descendant species. Darwin's original view.
Punctuated equilibrium (Gould and Eldredge, 1972): Species remain largely unchanged for long periods (stasis) punctuated by rapid bursts of evolutionary change, often associated with speciation events. The fossil record more commonly shows abrupt transitions between species with long periods of stasis — supporting this model. Rapid speciation may occur in small, peripherally isolated populations undergoing rapid selection or drift.
Most evolutionary biologists now accept that both patterns occur — some lineages evolve gradually, others show punctuated equilibrium. The debate is about relative frequency, not about which is universally correct.
Allo
Allopatric speciation — geographic isolation drives divergence
Allopatric speciation ('different country') occurs when a geographic barrier physically separates a population into two or more isolated subpopulations. With gene flow blocked, the isolated populations diverge independently — accumulating different mutations, experiencing different selection pressures, and drifting in different genetic directions — until reproductive barriers evolve and the populations can no longer interbreed even if the barrier is removed.
Allopatric speciation is by far the most common mode. Examples: Darwin's finches (ancestral population from mainland South America colonized the Galápagos → each island subpopulation diverged in isolation → 13+ species evolved with different beak shapes and sizes adapted to different food sources). Squirrels on opposite rims of the Grand Canyon (Kaibab squirrel on north rim vs. Abert's squirrel on south rim — isolated by canyon, diverging but not yet fully reproductively isolated).
Memory trick: Allopatric = geographically separated = diverge in isolation = reproductive barriers evolve = new species. Geographic barrier first, then biological divergence. Most common mode.
Sym
Sympatric speciation — speciation without geographic isolation
Sympatric speciation ('same country') occurs when new species arise within the same geographic area, without physical separation. This requires some mechanism other than geography to reduce gene flow between subpopulations.
Polyploidy — the most common mechanism, especially in plants: a spontaneous doubling (or more) of chromosome number produces an individual that cannot mate with the parent species (different chromosome number → sterile offspring) but can potentially mate with other polyploids. Allopolyploidy (chromosome doubling after hybridization between two species) is particularly important — bread wheat (Triticum aestivum) is a hexaploid (6N = 42 chromosomes) formed by hybridization and chromosome doubling involving three different ancestral grass species. About 70% of flowering plant species are polyploid or descended from polyploid ancestors. Polyploidy can produce a new species in a single generation.
Ecological speciation — populations using different resources in the same area diverge due to disruptive selection + assortative mating. Apple maggot fly (Rhagoletis pomonella) — originally used hawthorn; 150 years ago began using apple trees (introduced to North America). Hawthorn and apple-race flies are reproductively isolated because they mate on their host plant and have different host preferences. A possible sympatric speciation event in progress.
Memory trick: Sympatric = same place, no geographic barrier. Polyploidy = instant new species in plants (most common). Ecological speciation = different niches in same area lead to divergence.
🔬 Applied Scenario — Speciation in the Modern World
Speciation is not just a historical process — it is happening right now, sometimes driven by human activity:
A
Darwin's finches — adaptive radiation in action. The 13+ species of Darwin's finches in the Galápagos are the textbook example of allopatric speciation followed by adaptive radiation. A small founding population from South America colonized the islands → colonized different islands (allopatric isolation) → diverged in beak morphology adapted to available food sources (insect-eating, seed-cracking, cactus-probing, even tool-using) → when populations re-encountered each other on the same island, character displacement intensified beak differences → ecological and behavioral barriers reinforced reproductive isolation.
B
Human-caused habitat fragmentation as speciation driver. Human activities that fragment habitats into isolated patches can initiate allopatric speciation in isolated subpopulations. The Florissant valley in Colorado has isolated populations of Pikes Peak sculpin (a fish) in disconnected stream segments — they are already diverging genetically. Urban environments are creating isolated populations of birds, mammals, and insects in city patches separated by 'seas' of concrete. The long-term result could be new species — but on a timescale of thousands to millions of years.
C
Polyploidy in crop plants. Many of our most important crops are polyploids, often formed by human-mediated hybridization and selection: bread wheat (hexaploid, 6N), cotton (Gossypium hirsutum, tetraploid), tobacco (Nicotiana tabacum, tetraploid), strawberry (octoploid, 8N), and bananas (triploid, sterile — which is why seedless). Understanding polyploidy is essential for plant breeding — polyploid crops often have larger fruit, higher yield, and more disease resistance than their diploid ancestors.
D
Cichlid fish — explosive speciation in African lakes. Lake Victoria in East Africa contains ~500 species of cichlid fish — all derived from a common ancestor within the last 15,000 years (after the lake refilled following a drought that eliminated most fish). This is one of the most rapid adaptive radiations ever documented. Cichlids rapidly occupied different ecological niches (algae scrapers, invertebrate pickers, fish predators, scale-eaters) through a combination of rapid morphological evolution, sexual selection on color, and ecological speciation. The introduction of Nile perch and eutrophication have since driven >200 cichlid species to extinction.
📌 Exam Application
Speciation questions test the biological species concept, reproductive barriers, and speciation modes:
1. Biological species concept: Reproductively isolated from other groups. Cannot apply to asexuals, fossils, or ring species.
2. Reproductive barriers: Prezygotic (before fertilization) = habitat, temporal, behavioral, mechanical, gametic isolation. Postzygotic (after fertilization) = hybrid inviability, hybrid sterility (mule), hybrid breakdown.
3. Allopatric speciation: Geographic isolation → divergence → reproductive barriers. Most common mode. Darwin's finches = classic example.
4. Sympatric speciation: No geographic isolation. Polyploidy = most common in plants (instant speciation). Ecological speciation = habitat/resource divergence within same area.
5. Polyploidy: Common in plants (~70% of flowering plants). Allopolyploidy = hybridization + chromosome doubling. Bread wheat = hexaploid. Can produce new species in one generation.
⚠️ The Most Common Speciation Mistakes
The mule is an example of POSTZYGOTIC reproductive isolation — not prezygotic. A mule (horse × donkey) is a hybrid that forms successfully — fertilization occurs, the embryo develops, and a healthy animal is born. The reproductive barrier is that mules are sterile — hybrid sterility, a postzygotic barrier. Students sometimes classify the horse-donkey incompatibility as prezygotic because 'horses and donkeys don't usually mate in the wild.' The classification is about the mechanism, not the frequency of mating.
Polyploidy produces instant speciation in ONE generation. Students think speciation always takes thousands of generations. Allopolyploidy can produce a new species in a single generation — a hybrid between two species doubles its chromosomes → cannot interbreed with either parent species → new species. This is why polyploidy is considered 'instantaneous speciation' and is the exception to the general rule that speciation is slow.
Allopatric isolation initiates speciation but doesn't guarantee it. Geographic isolation prevents gene flow, allowing divergence. But if the isolated populations come back into contact before strong reproductive barriers have evolved, they may interbreed and merge back into one species. Geographic isolation is the initiating condition, not the sufficient condition for speciation — reproductive barriers must evolve during the period of isolation.
✓ Quick Self-Test
1. What is the biological species concept and what are its limitations?
2. Distinguish between prezygotic and postzygotic reproductive barriers with two examples of each.
3. What is allopatric speciation and what is the most classic example?
4. How can polyploidy produce a new species in a single generation?
5. What is the difference between gradualism and punctuated equilibrium?
Answers:
1. The biological species concept defines a species as a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups. Limitations: cannot apply to asexual organisms (bacteria), cannot test interbreeding in fossils, breaks down at range edges where hybridization occurs between normally distinct species (ring species), and many plant species hybridize freely yet are considered distinct.
2. Prezygotic (before fertilization): habitat isolation (species use different habitats — garter snakes in water vs land), temporal isolation (breeding at different times — spotted vs Thomas's skunk). Postzygotic (after fertilization): hybrid inviability (hybrid embryo dies before reproducing), hybrid sterility (mule is healthy but sterile).
3. Allopatric speciation occurs when a geographic barrier (mountain range, ocean, river) physically separates a population into isolated subpopulations → gene flow blocked → populations diverge independently through mutation, selection, and drift → reproductive barriers evolve → new species. Classic example: Darwin's finches, where an ancestral South American species colonized the Galápagos Islands and diversified into 13+ species through isolation on different islands.
4. Polyploidy (chromosome number doubling) can produce a new species instantly because the polyploid individual has a different chromosome number from its parent species → meiosis in hybrids with the parent species produces non-viable gametes (sterile hybrids) → the polyploid is reproductively isolated from parents → if it can self-fertilize or find another polyploid, it establishes a new species in one generation.
5. Phyletic gradualism: evolution occurs slowly and continuously, with smooth transitions between ancestor and descendant forms. Punctuated equilibrium (Gould & Eldredge): species remain largely unchanged (stasis) for long periods, punctuated by rapid evolutionary change associated with speciation events — consistent with the abrupt transitions commonly seen in the fossil record. Both patterns occur in nature; they differ in relative frequency across lineages.
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