What Is a Species? The Biological Species Concept
Biological Species Concept: A species is a group of organisms that can potentially interbreed with one another to produce fertile offspring. Members of different species are reproductively isolated from each other.
The biological species concept is the cornerstone of how we classify and count biodiversity. It places reproductive compatibility , not just appearance , at the heart of species identity.
Horses (Equus caballus) and donkeys (Equus asinus) can interbreed to produce mules. However, mules are infertile , they cannot reproduce. Because the offspring are not fertile, horses and donkeys are classified as separate species, despite their close relationship.
Binomial nomenclature is the universal system for naming species. Always write the name in italics: the Genus is capitalised, and the specific epithet (species name) is lowercase. For example: Homo sapiens, Canis lupus.
The biological species concept has limits: it cannot easily be applied to asexually reproducing organisms (such as bacteria or many plants), since these species do not rely on interbreeding to reproduce. It also cannot be applied to fossil organisms where breeding behaviour cannot be observed.
Alternative species concepts exist to address these limitations:
- Morphological species concept , organisms are classified as the same species based on shared physical (morphological) characteristics. This can be applied to fossils and asexual organisms, but relies on subjective judgements about which traits matter.
- Phylogenetic species concept , a species is the smallest group of organisms that shares a common ancestor and can be distinguished by unique derived characteristics. This is based on evolutionary history rather than breeding behaviour.
Reproductive Isolation, Speciation, and Variation Within Species
Even within a recognised species, populations can become reproductively isolated , meaning they stop interbreeding with other populations. This is the first step towards the formation of new species.
Types of speciation:
- Allopatric speciation , populations become geographically separated (e.g., by mountains, rivers, or oceans), preventing gene flow. Over time, independent mutation and natural selection cause the populations to diverge until they can no longer interbreed.
- Sympatric speciation , new species arise within the same geographic area, typically through mechanisms such as polyploidy (especially in plants) or the exploitation of different ecological niches.
Three main types of reproductive isolation:
- Geographic isolation , physical barriers such as mountains, rivers, or oceans prevent populations from meeting.
- Temporal isolation , populations breed at different times of year (different seasons or times of day).
- Behavioural isolation , differences in mating rituals, signals, or preferences prevent interbreeding even when populations are in contact.
The Galápagos finches are a classic case of allopatric speciation. Finches colonising different islands became geographically isolated from each other. Over generations, beak shapes diverged in response to the food sources available on each island , seeds, insects, or cactus nectar , producing distinct species. The significance of this beak variation was later studied in depth by researchers such as Peter and Rosemary Grant, whose long-term fieldwork demonstrated natural selection acting on beak morphology in real time.
Within a species, individuals show variation, which comes in two main types:
- Discrete variation , controlled by one or a few genes; produces distinct categories (e.g., blood groups: A, B, AB, O).
- Continuous variation , controlled by many genes (polygenic); produces a range of values (e.g., height, skin colour).
This variation within a species is the raw material upon which reproductive isolation and natural selection act: individuals with advantageous traits survive and reproduce more successfully, and if isolated populations experience different selective pressures, they diverge , potentially becoming new species.
The primary sources of new genetic variation are mutations (changes to DNA sequences) and recombination during meiosis (crossing over and independent assortment). Sexual reproduction and gene flow (movement of alleles between populations) redistribute existing variation rather than generating new variants. Natural selection then acts on this variation, favouring advantageous traits in a given environment.