Formation of New Species

Core Concepts

The biological species concept — and its limits

The biological species concept defines a species as a group of interbreeding natural populations that produce viable, fertile offspring and are reproductively isolated from other groups. It is elegant for animals that reproduce sexually, but it fails in three situations worth remembering:

• Asexual organisms (many bacteria, some plants) never interbreed — the concept simply does not apply.
• Fossils cannot be observed breeding, so paleontologists rely on other criteria (body form, ecology, lineage).
• Ring species — adjacent populations interbreed around a circle, but the end populations do not — show boundaries can be graded rather than sharp.

Alternatives — morphological (appearance-based), ecological (niche-based), and phylogenetic (ancestry-based) concepts — fill these gaps, but each has trade-offs; the biological concept remains the standard baseline.

Reproductive isolation: the engine of speciation

Speciation requires barriers that reduce or eliminate gene flow. These fall into two classes:

Prezygotic barriers act before fertilization:

• Habitat isolation — populations live in different environments (e.g., fish in deep versus shallow water).
• Temporal isolation — breeding seasons or times of day differ (e.g., frogs breeding in different seasons).
• Behavioral isolation — courtship displays, songs, or pheromones differ (e.g., fireflies flashing different patterns).
• Mechanical isolation — reproductive structures are anatomically incompatible (e.g., flowers matched to different pollinators).
• Gametic isolation — gametes meet but cannot fuse (e.g., sea urchins spawning into the water).

Postzygotic barriers act after fertilization:

• Reduced hybrid viability — hybrid offspring do not survive to reproduce.
• Reduced hybrid fertility — hybrids live but are sterile (e.g., mules from horse × donkey).
• Hybrid breakdown — first-generation hybrids are fine, but their offspring are weak or sterile.

Allopatric speciation: separation by geography

In allopatric speciation, a physical barrier — a mountain range, river, glacier, or ocean — splits a population so that gene flow stops. The two daughter populations then accumulate differences through independent mutation, natural selection, and genetic drift. Two routes produce the split:

• Vicariance — a pre-existing population is divided by a new barrier (e.g., a river changes course, or a land bridge is flooded).
• Dispersal — a small group colonizes a new area and becomes isolated from the source population (e.g., a few birds blown to an island).

A commonly taught example is the Kaibab and Abert squirrels of the Grand Canyon — one population divided by the canyon's formation until recognized as separate species.

Sympatric speciation: divergence without separation

In sympatric speciation, new species arise within the same geographic range, with no physical barrier. This is rare in animals but common in plants, where it usually happens through polyploidy — having more than two sets of chromosomes:

• Autopolyploidy — an error in meiosis doubles the chromosome set within a single species; the resulting individual can self-fertilize or breed with other polyploids but not with the diploid parents — instant reproductive isolation.
• Allopolyploidy — two different species hybridize and the hybrid's chromosomes double, producing a fertile new species combining both parents' sets. Bread wheat and many cultivated plants arose this way (commonly taught).

In animals, sympatric speciation is harder but documented: cichlid fishes in African rift lakes diversified into many species within one lake through habitat partitioning and mate choice, and the apple maggot fly has host races that mate on different fruit trees.

Adaptive radiation

When a single ancestor rapidly produces many species, each adapted to a different niche, the result is adaptive radiation — speciation multiplied. It typically follows the opening of new habitat or the evolution of a key new trait. Classic examples: Darwin's finches, Hawaiian honeycreepers, and the silversword plants of Hawai'i — all radiating from one colonizing ancestor (commonly taught).

Common Confusions