Describe a species as a group of organisms that can reproduce to produce fertile offspring.

Cambridge IGCSE Biology 0610 – 1.2 Concept and Uses of Classification Systems

Learning Objective

Describe a species as a group of organisms that can reproduce to produce fertile offspring in nature.

Key Concepts (aligned with the syllabus)

• Organisms can be classified into groups by the features they share (morphological, physiological or genetic).
• Definition of a species and the role of reproductive isolation.
• Binomial nomenclature – the two‑part scientific name of every species.
• Classification systems aim to reflect evolutionary relationships because taxa should represent groups that share a common ancestor.
• Use of DNA‑sequence similarity in modern taxonomy.
• Construction and use of dichotomous keys.
• Major classification systems – Linnaean and phylogenetic.

Definition of a Species

A species is a group of organisms that can reproduce and produce fertile offspring in nature. All members share a common gene pool and are reproductively isolated from members of other species.

Reproductive Isolation

Barriers that prevent gene flow between species can be:

1. Pre‑zygotic barriers – act before fertilisation (e.g., temporal, behavioural, mechanical, ecological).
2. Post‑zygotic barriers – act after fertilisation (e.g., hybrid inviability, hybrid sterility).

Examples of Species Boundaries

• Domestic dog (Canis lupus familiaris) – different breeds are varieties within the same species.
• Human (Homo sapiens) – all humans can produce fertile offspring with each other.
• Horse (Equus ferus caballus) × Donkey (Equus africanus asinus) → Mule – the hybrid is usually infertile, illustrating post‑zygotic isolation.

Binomial Nomenclature

Each species has a unique two‑part Latin name:

• Genus name – capitalised.
• Specific epithet – lower‑case.
• Both parts are italicised (or underlined when handwritten).

Examples: Homo sapiens, Canis lupus, Pan troglodytes.

Classification Systems

Linnaean System

A traditional hierarchical system that groups organisms by overall similarity. It uses seven main ranks:

• Kingdom, Phylum, Class, Order, Family, Genus, Species.

Phylogenetic (Evolutionary) System

Groups organisms to reflect common ancestry and evolutionary relationships. Modern taxonomy prefers this system because DNA data reveal true lineages.

Comparison

The Linnaean system groups organisms by overall similarity, whereas the phylogenetic system groups them by shared ancestry; the latter is increasingly used as DNA‑sequence similarity provides reliable evidence of evolutionary relationships.

DNA‑Sequence Similarity in Modern Taxonomy

Comparing DNA‑sequence similarity allows taxonomists to assess how closely related different organisms are. Species that share a recent common ancestor have a higher proportion of identical DNA sequences than those that diverged earlier.

Taxonomic Hierarchy – Example (Human)

Dichotomous Keys

A dichotomous key leads the user to an identification by choosing between two contrasting statements at each step.

Example – Simple key to identify three common garden plants:

1a. Plant has flowers ................. go to 2
1b. Plant has no flowers .............. go to 3

2a. Flowers are yellow ................. Taraxacum officinale (dandelion)
2b. Flowers are white .................. Bellis perennis (daisy)

3a. Leaves are needle‑like ............. Pinus sylvestris (Scots pine)
3b. Leaves are broad and lobed ........ Betula pendula (silver birch)

Activity Prompt: Ask students to construct a dichotomous key for a set of five locally‑found organisms (e.g., two flowering plants, a fern, a moss, and a conifer).

Assessment Questions

1. Define a species using the concept of fertile offspring.
2. Explain the difference between pre‑zygotic and post‑zygotic reproductive isolation, giving one example of each.
3. Provide an example of two organisms that can interbreed but produce infertile hybrids.
4. Write the binomial name for the domestic cat and explain the formatting rules.
5. Why do modern classification systems aim to reflect evolutionary relationships?
6. How can DNA‑sequence similarity be used to place organisms into taxonomic groups?
7. Using the key below, identify the plant described as “Leaves are broad and lobed”.
   

   1a. Plant has flowers ................. go to 2
   1b. Plant has no flowers .............. go to 3
   
   2a. Flowers are yellow ................. Dandelion
   2b. Flowers are white .................. Daisy
   
   3a. Leaves are needle‑like ............. Pine
   3b. Leaves are broad and lobed ........ Birch
           

Suggested Diagram