Three domains. Millions of species. A common ancestor. The shape of life on Earth.
Every sub-topic below feeds at least one of these questions.
What is a species?
What patterns are seen in the diversity of genomes within and between species?
The required syllabus content for A3.1, in order. Each card is one lesson-sized checkpoint.
Variation between organisms as a defining feature of life
This is the original morphological concept of the species as used by Linnaeus.
Binomial system for naming organisms
Biological species concept
Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation
Diversity in chromosome numbers of plant and animal species
Karyotyping and karyograms
Unity and diversity of genomes within species
Diversity of eukaryote genomes
Comparison of genome sizes
Current and potential future uses of whole genome sequencing
Variation is the raw material of evolution — and the basis on which we name and classify organisms.
No two individuals of any species are identical in all their traits. Variation is universal, complex, and arises from the interaction of genetics (inherited differences in DNA sequence) and environment (developmental conditions, nutrition, exposure). Patterns of variation — what's continuous, what's discrete, what segregates with reproductive groups — are the basis on which biologists name species in the first place.
Within a species, variation exists but is constrained. Between species, variation is much larger. The discontinuity is what makes "species" a useful category.
In the 1700s Carolus Linnaeus founded taxonomy by grouping organisms according to observable physical traits — the morphological species concept.
The morphological species concept defines a species as a group of individuals that look similar to each other and different from other groups. It worked well for visibly distinct organisms — and still does for most fossils, where DNA is unavailable. But it struggles with cryptic species (look identical, don't interbreed) and with sexually dimorphic species (look very different, do interbreed).
Linnaeus's other lasting contribution: every species gets a two-part scientific name. Homo sapiens. Panthera leo. Escherichia coli.
The first part of the name. Capitalised. Species within the same genus share many traits and have a relatively recent common ancestor.
The second part. Lowercase. Distinguishes this species from others in the same genus.
Scientific names are written in italics (or underlined if handwritten). Genus capitalised, species lowercase. Homo sapiens ✓ homo Sapiens ✗
Quick test: Periophthalmus darwini and Periophthalmus barbarus are close — same genus. Periophthalmus darwini and Ogcocephalus darwini are not closely related — different genera, despite the shared species epithet.
The modern, dominant definition: a species is a group that can interbreed in nature to produce fertile offspring.
The biological species concept (BSC) replaces the Linnaean morphological concept for sexually reproducing species. Two animals that look similar but cannot interbreed and produce fertile offspring are different species. Two animals that look very different but can interbreed (think Great Danes vs Chihuahuas) are the same species.
The BSC can't be applied to asexual organisms (bacteria, many plants), to fossils (no breeding tests), or to organisms that don't meet in nature (allopatric populations). For these cases other species concepts are used — phylogenetic, ecological, genetic.
If one species splits gradually into two over thousands of generations, when exactly does it become "two species"? Often the answer is arbitrary.
When a population becomes reproductively isolated into two non-interbreeding groups, the groups gradually accumulate different mutations and may eventually diverge into separate species. But "separate species" is not a sharp event — it's the endpoint of a long process. Whether to call two diverging populations the same species or two species can be an arbitrary judgement, especially when they don't meet to test the question. Ring species (where geographically adjacent populations interbreed but distant ones don't) are the classic example.
Diploid eukaryotes always have an even number of chromosomes (two of each kind). The exact number varies enormously across species.
Myrmecia pilosula — the lowest known diploid number.
22 pairs of autosomes + XX or XY sex chromosomes.
Our closest relative — two more than us. The difference matters.
Morus nigra — extreme polyploidy in a flowering plant.
A karyogram is a photo of an organism's chromosomes arranged in homologous pairs and ordered by length. It reveals chromosome number, sex, and visible abnormalities.
Chromosomes are sorted by:
Reading karyograms: two X chromosomes = female. One X and one Y = male. Three copies of any autosome (e.g. chromosome 21) = trisomy (Down syndrome for trisomy 21; trisomy 18 = Edwards; trisomy 13 = Patau).
Humans have 23 chromosome pairs. Chimpanzees and other great apes have 24. The leading hypothesis: two ancestral ape chromosomes fused in our lineage to form human chromosome 2. Four lines of evidence support this:
Members of a species share most of their genome but vary in detail — especially at single-nucleotide polymorphisms (SNPs).
The genome is the full set of genetic information in an organism. In humans, that's 23 pairs of nuclear chromosomes plus mitochondrial DNA. Members of the same species share the same gene set, in the same order, on chromosomes of the same length and structure. But within those constraints, individuals differ at millions of sites — most commonly at single-nucleotide polymorphisms (SNPs), where a single base differs between people.
A polymorphism is the existence of multiple forms of a trait within a species. SNPs are the most common kind. ABO blood group, eye colour and many disease susceptibilities ultimately come from polymorphisms.
There is a clear increase in genome size from viruses to bacteria to eukaryotes — but within eukaryotes, size and complexity are not well-correlated.
~48,500 bp. About the smallest known genome.
Typical bacterium.
3 billion base pairs across 23 chromosomes.
A small Japanese flowering plant — 50× the human genome.
Paris japonica isn't 50× more complex than a human — much of its genome is repetitive non-coding sequence. The "C-value paradox" is that genome size doesn't predict organismal complexity within eukaryotes.
Databases like the NIH genome browser let you extract genome sizes across the tree of life for direct comparison.
Working through a comparison reveals the broad pattern: viruses (kilobases) < bacteria (megabases) < eukaryotes (gigabases). Within eukaryotes, however, the spread is enormous — from a few Mb in some yeasts to over 100 Gb in some plants and amphibians.
In 2003 the first human genome cost ~$3 billion and took 13 years. Today, a complete human genome can be sequenced for hundreds of dollars in days.
Comparative genomics — comparing genomes across species — reveals when lineages diverged, what genes are conserved, and how new functions evolve. The basis of much modern phylogenetics.
Tailoring treatment to an individual's genome. Identifying genetic disease risk early; selecting drugs that will work for a specific patient's genetic profile; avoiding side effects from drugs metabolised differently by different genotypes.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Difficulties applying the biological species concept to asexually reproducing species and to bacteria that have horizontal gene transfer
Chromosome number as a shared trait within a species
Engagement with local plant or animal species to develop a dichotomous key
Identification of species from environmental DNA in a habitat using barcodes
The BSC is useful but not universal. Two contexts where it fails outright.
Many bacteria, some plants, some animals reproduce only asexually. They don't interbreed at all — so "interbreed to produce fertile offspring" can't define a species. Other concepts (phylogenetic, genetic similarity thresholds) must be used.
Bacteria routinely swap genes across species boundaries — via conjugation (plasmid transfer), transformation (DNA uptake from environment) or transduction (virus-mediated transfer). Genes flow between "species" without sex, blurring boundaries the BSC depends on.
Cross-breeding between species with different chromosome numbers rarely produces fertile offspring — the chromosomes can't pair up correctly during meiosis.
Members of a species share the same diploid chromosome number. Humans: 46 (gametes 23). Chimpanzees: 48 (gametes 24). Even if a human-chimpanzee hybrid embryo could form, during meiosis the chromosomes would fail to pair properly — homologous pairs would have no partners — and the gametes produced would be unbalanced. Hence the karyotype itself is a powerful reproductive barrier between closely related species.
A dichotomous key identifies an unknown organism through a series of two-option steps. Each choice leads to an identification or another pair.
You should develop your own dichotomous key for a group of local species, using observable characteristics that are unambiguous.
A DNA barcode is a short, standardised DNA sequence that uniquely identifies a species. By sequencing DNA shed by organisms into the environment, biologists can rapidly catalogue who lives there.
eDNA barcoding has transformed biodiversity surveys: it's much faster than traditional capture-based methods, picks up cryptic and rare species, and can detect non-native invasives early.
If you can't define one of these in a sentence, that's where to revise next.
“What might cause a species to persist or go extinct?”
“How do species exemplify both continuous and discontinuous patterns of variation?”