3.4.5 Species and taxonomy

A species is a group that can interbreed to produce fertile offspring. Taxonomy organises species into nested groups, names them with a universal Latin system, and uses molecular evidence to reveal who descends from whom.

A species is a group that can interbreed to produce fertile offspring.

A species is a group of organisms that can interbreed and produce fertile offspring. The fertile-offspring part of the test is what does the work. Some inter-species pairs can mate and produce hybrid offspring, but if the hybrids are themselves infertile, the two parent groups remain separate species; the gene pools never merge.

Write interbreed AND fertile offspring. Both are required. Viable offspring is not equivalent. When you describe a non-species pair, state the reproductive consequence: gametes cannot fuse, or offspring are themselves infertile.

Every species carries a two-part Latin name: the genus (capitalised) followed by the specific epithet (lower case). Both parts are italicised in print, underlined in handwriting. Every species has one scientific name recognised across all languages, which removes the ambiguity of common names that vary between regions. When two competing binomial names differ in the first word but share the second, the dispute is at the genus level: biologists disagree about which broader group the species belongs to.

The definition has practical limits. Bacteria, many protists, and some plants reproduce asexually, so the interbreeding criterion cannot be applied. For fossil organisms, reproductive behaviour is unobservable. In both cases, morphological and molecular methods substitute for the interbreeding test.

Courtship behaviour acts as a species-recognition mechanism.

Courtship behaviour is any species-specific signal or response between potential mates before reproduction. Because the rituals (calls, dances, displays, chemical signals) are species-specific, they let an individual identify a mate of its own species. Responding only to its own species' signals avoids wasted reproductive effort on cross-species pairings that would produce no fertile offspring.

Every courtship answer runs through the same species-specificity chain. Courtship is species-specific, so it enables mates to recognise their own species, so recognition synchronises mating and pair-bonding, and so disrupting the signal reduces successful matings within the species. Skipping the species-specificity step loses the conceptual anchor.

To raise mating frequency in a laboratory population, add sexually active females. Don't write add males. Mating rate is typically limited by female receptivity, not by male number; add males is an explicit reject in mark schemes.

Taxonomy organises organisms in a nested hierarchy of eight ranks.

Classification organises organisms from the most inclusive group to the most exclusive, in a fixed sequence. Each rank is nested inside the rank above. Two organisms sharing the same genus also share the same family, order, class, phylum, kingdom, and domain. Two organisms in the same domain but different kingdoms share only the broadest features in common.

Class comes before Order. In a taxonomy-fill question, all five missing ranks must be correct for the single mark; any error scores zero.

The mnemonic Dear King Philip Came Over For Good Soup captures the order and is a recognised learning aid. Confusing Class and Order is a recurring error. The older five-kingdom system was replaced by the three-domain system (Bacteria, Archaea, Eukaryota) when molecular evidence revealed two prokaryote groups as different from each other as either was from eukaryotes.

Modern phylogeny uses molecular evidence to reveal evolutionary relationships.

Phylogeny is the reconstruction of the branching history of life: who diverged from whom, and when. Classical taxonomy used morphological features. Modern phylogeny uses molecular evidence. The principle is straightforward: organisms sharing a more recent common ancestor have had less time for their sequences to diverge through mutation, so their sequences are more similar.

Name all three molecular methods with base for DNA and mRNA. DNA sequencing alone is rejected. Genome sequencing is the only bare sequencing term AQA credits without a base qualifier.

Every evolutionary-relationship answer is a comparison, and the comparison names a third species. The mark-credited form is species A and species B are more closely related to each other than either is to species C. A claim about one pair in isolation earns nothing. A claim that two species are unrelated contradicts the assumption that all living organisms share common ancestry.

Every evolutionary-relationship answer is a triangle. Name the third species: A and B are more closely related to each other than either is to C. A pair alone scores nothing.

A phylogenetic tree is a branching diagram of inferred relationships. Time runs from the root to the tips. Two species branching from a node closer to the tips share a more recent common ancestor and are more closely related. Two species branching from a node closer to the root share a more ancient common ancestor. Vertical position on the page carries no meaning.

Pitfall — Read the phylogenetic tree forwards

The most recent common ancestor sits at the node closest to the tips, not the root.

Reading the direction backwards — treating a deep node as recent, or a tip-adjacent node as ancient — is the single error mode behind a notorious 16% mastery on the 2022 tree-reading question. The fix is mechanical: trace each species back from its tip to find the most recent shared node, and read that node as the common ancestor of those two species.

Reclassification is always a method story. The organisms have not changed; a named technique revealed relationships the previous method could not see. DNA base sequencing, genome sequencing, and electron microscopy with greater resolution are the techniques AQA credits. New technology and organisms evolved are both rejected.

Name the technique. DNA base sequencing, genome sequencing, or electron microscopy with greater resolution. New technology and modern methods are rejected. Organisms evolved is rejected; the organisms did not change, the method did.

Microscope resolution, not magnification, is the limiting property when distinguishing fine detail.

Two microscope types appear in 3.4.5 contexts: the optical microscope and the electron microscope. The property that distinguishes them is resolution, not magnification. Resolution is the ability to distinguish two adjacent points as separate. Magnification only increases image size; resolution determines whether the detail at that size is actually visible.

Write resolution, not magnification. Magnification increases image size; resolution distinguishes fine detail. The optical microscope's limit is resolution, not magnification.

When a question asks why a structure is difficult to study under an optical microscope, the credited answer pairs the difficulty with its consequence: difficult to stain, so not visible. The difficulty alone, without the consequence, is ignored by the mark scheme.

Pair difficult to stain with its consequence: difficult to stain, so not visible. The difficulty alone is ignored; the mark requires the consequence clause.

Key terms

• resolution
• evolutionary relationship
• common ancestor
• more closely related
• more recent
• phylogenetic tree
• DNA base sequence
• genome sequencing
• electron microscope
• genus