18.1.117-18 · Selection, Evolution, Classification and Conservation

Definitions of Species — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 36 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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18.1.1 Definitions of Species — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: State and evaluate the biological, morphological and ecological species concepts.Organisms have been classified into species for hundreds of years to investigate the diversity of life now …

Biological species
Morphological species
Weaknesses
Binomial name
Reproductive isolation
Sexual dimorphism

Part 1 of 7Why species are defined

Why species are defined

Organisms have been classified into species for hundreds of years to investigate the diversity of life now and in the past.

It is difficult to decide whether newly discovered organisms belong to an existing species or a new one.

Each organism gets a binomial name of two Latin words, the genus first with a capital and the species in lower case.

It is the same in every country and avoids confusion from common names.

Part 2 of 7Biological species concept

Biological species

Biological species a group of organisms with similar morphological and physiological features that can breed together and produce fertile offspring.

This is the most widely accepted definition.

Species are reproductively isolated from other species, so gene flow is blocked.

Fertile offspring matter: a horse and a donkey produce an infertile mule with 63 chromosomes that cannot pair at meiosis.

Limitations:

It is difficult and time-consuming to test interbreeding in practice.
It cannot be applied to fossils or to asexual organisms.
Some closely related species hybridise in nature.

Part 3 of 7Morphological species concept

Morphological species

Morphological species a group of organisms sharing many physical features that distinguish them from other species.

It was used by most scientists in the past because physical features are easy to observe.

Strengths

Strengths works for fossils, asexual organisms and museum specimens, and needs no breeding experiments.

Weaknesses

Weaknesses it is subjective, and sexual dimorphism or juveniles can look different while cryptic species look alike.

Part 4 of 7Ecological species concept

Ecological species

Ecological species a population of similar organisms living in the same area at the same time, occupying the same niche.

It is useful for asexual organisms such as bacteria, whose lineages specialise on different foods or habitats.

Weaknesses

Weaknesses niches overlap and change with season or life stage, and are hard to measure fully.

Part 5 of 7Comparing the concepts

Concept	Key criterion	Fossils?	Asexual organisms?
Biological	Interbreed to give fertile offspring	No	No
Morphological	Similar physical features	Yes	Yes
Ecological	Same niche, same area and time	Hardly	Yes

Part 6 of 7Combining evidence

Scientists combine anatomy, behaviour, ecology, chromosome number and DNA evidence.

Comparing DNA and protein sequences (e.g. cytochrome c) shows how recently species shared an ancestor.

Molecular evidence can show two look-alike populations are separate species, or different-looking forms are one species.

Species are snapshots of a gradual process, so boundaries can be fuzzy.

Part 7 of 7More from the specification

Taxonomic hierarchy and Linnaean ranks

Taxonomic hierarchy and Linnaean ranks grouping organisms into nested kingdoms, phyla,
- and species establishes universal biological classification across Linnaean ranks.

Three-Domain Classification System

Cellular life is classified into three distinct domains: Archaea, Bacteria,
and Eukarya, based primarily on molecular differences in ribosomal RNA sequences and membrane lipid structures.

Domain Eukarya Characteristics

Organisms in the domain Eukarya possess eukaryotic cells containing membrane-bound nuclei and linear DNA associated with histone proteins, divided into kingdoms including Protoctista, Fungi, Plantae,
and Animalia.

Viruses in Biological Classification

Viruses are acellular infectious entities consisting of genetic material enclosed in a protein capsid;
- lacking cellular organisation and independent metabolism, they are excluded from the three-domain system.

Three-Domain Classification System

Cellular life is classified into three distinct domains: Archaea, Bacteria,
and Eukarya, based primarily on molecular differences in ribosomal RNA sequences and membrane lipid structures.

Domain Eukarya Characteristics

Organisms in the domain Eukarya possess eukaryotic cells containing membrane-bound nuclei and linear DNA associated with histone proteins, divided into kingdoms including Protoctista, Fungi, Plantae,
and Animalia.

Viruses in Biological Classification

Viruses are acellular infectious entities consisting of genetic material enclosed in a protein capsid;
- lacking cellular organisation and independent metabolism, they are excluded from the three-domain system.

Protoctista

Kingdom of eukaryotes that are not fungi, plants or animals.

Eukaryote

A cell with a nucleus and membrane-bound organelles.

Archaea

Prokaryotes with ether-linked membrane lipids and no peptidoglycan.

Eukarya

Organisms with cells that have a nucleus and organelles.

Domain

The highest taxonomic rank.

Classification

Grouping organisms by shared features and relationships.

Kingdom

One of the five largest groups into which living organisms are divided.

Lipids

Concentrated energy, insulation, cell membranes.

Ribosome

The site where proteins are made.

Phylum

Rank below kingdom.

Key terms

Term	Meaning
Biological species concept	Group with similar features that can interbreed to give fertile offspring.
Morphological species concept	Group sharing physical features that distinguish it from other groups.
Ecological species concept	Population of similar organisms living in the same area at the same time.
Binomial name	Two-word Latin name: genus then species.
Reproductive isolation	Inability of populations to interbreed.
Sexual dimorphism	Difference in appearance between males and females of a species.

Avoid these mistakes

Saying a mule shows horses and donkeys are one species — the offspring are infertile, so they are separate species.

Using the biological concept for bacteria or fossils — it cannot be tested there.

Writing binomial names without a capital genus or with both words capitalised — genus capital, species lower case.

Assuming only one definition is ever used — scientists combine several kinds of evidence.

Last updated: 18 October 2026