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A3.1 Diversity of organisms

Diversity of organisms links variation, species concepts, classification tools, chromosome evidence, and genome comparisons to explain biological similarity and difference.

Syllabus
First assessment 2025
Topic
A3.1
Level
SL

Learning objectives

A3.1.1Variation as defining feature of life• Variation occurs between individuals within every species• Continuous variation often results from polygenes and environmental effects• Darwin's observations of variation supported natural selectionA3.1.2Species as groups with shared traits• Species can be grouped by shared morphology and structural traits• Morphological species concept is useful in fieldwork, fossils, and many plants• Sexual dimorphism and within-species variation can make morphology difficultA3.1.3Binomial system• Binomial nomenclature gives each species a universal two-part Latin name• Genus is capitalized; species is lowercase; both are italicized or underlined• Species in the same genus share similar traitsA3.1.4Biological species concept• Biological species concept defines species by interbreeding and fertile offspring• It emphasizes gene flow between members of the same species• Limits include extinct organisms, asexual species, and cases with gene flow between distinct formsA3.1.5Difficulties in distinguishing species• Speciation is gradual as isolated populations diverge• Populations may still be theoretically able to interbreed but cannot meet• Species boundaries can therefore be partly arbitraryA3.1.6Diversity in chromosome numbers• Chromosome number is usually constant within a species but varies between species• Diploid chromosome numbers are even because chromosomes occur in homologous pairs• Examples include fruit fly 8, rice 24, human 46, chimpanzee 48, and dog 78A3.1.7Karyotyping and karyograms• Karyotypes show chromosome number and structure in a cell• Karyograms arrange chromosomes by length, banding pattern, and centromere position• Human chromosome 2 supports fusion from two ancestral ape chromosomesA3.1.8Unity and diversity of genomes within species• A genome is all genetic information in an organism• Members of a species share most of their genome but differ at variants• SNPs are common single-base differences used as biological markersA3.1.9Diversity of eukaryote genomes• Eukaryote genomes vary in size and base sequence• Between-species variation is much larger than within-species variation• Not all DNA codes for proteinsA3.1.10Comparison of genome sizes• Genome size is the DNA amount in one chromosome set• Larger genome size does not necessarily mean greater organism complexity• Polyploidy can greatly increase genome size, especially in plantsA3.1.11Whole genome sequencing• Whole genome sequencing began at scale with the Human Genome Project• Current uses include evolutionary relationships, gene discovery, and pathogen research• Future uses include personalized medicine, diagnosis, disease prevention, and vaccine targets

Variation is the pattern that selection can act on

Pattern What the data look like Common biological basis
Continuous variation A measured range with intermediate values Often many genes plus environmental effects
Discontinuous variation Distinct categories with no intermediates Often one or a few genes with large effects
Individuals of one species arranged along a continuum of the same varying trait.

Variation occurs among individuals in every species. Darwin recognized that, when some inherited variants affect survival or reproduction, their frequencies can change across generations by natural selection.

Morphology is useful evidence, not an automatic verdict

The morphological species concept groups organisms by a shared set of diagnostic structures. It is fast in fieldwork and can be applied to fossils, plants and asexual organisms when breeding evidence is unavailable.

A male and female lion look different but belong to the same species, Panthera leo.

Appearance varies within species. Sexual dimorphism can make males and females look unlike, while unrelated species can converge on similar forms. A boundary should therefore use several traits and, where possible, independent evidence.

A binomial gives every species one universal name

A scientific species name has two parts: the genus followed by the specific epithet. In Panthera leo, Panthera names the genus and leo distinguishes the species within that genus.

  • Capitalize the genus; write the specific epithet in lowercase.
  • Italicize both words when typed; underline each word separately when handwritten.
  • After the first full use, the genus may be abbreviated: P. leo.

The shared genus indicates a group of similar, closely related species. The binomial prevents ambiguity between local common names, but the name records a classification—it is not itself evidence that the species boundary is correct.

Gene flow defines the biological species concept

Under the biological species concept, members of one species can interbreed and produce viable, fertile offspring. Successful reproduction lets alleles move through a shared gene pool.

Question What failure would show
Can mating and fertilization occur? A pre-zygotic reproductive barrier
Does the hybrid survive? Reduced hybrid viability
Can the hybrid produce functional gametes? Hybrid sterility

This concept cannot be tested directly for fossils, and it does not fit organisms that reproduce only asexually. Distinct forms may also retain limited gene flow, so interbreeding evidence must be interpreted with ecology and evolutionary history.

A gradual process creates a fuzzy species boundary

1

Populations begin within one species and exchange genes, so their members remain genetically connected.

2

A geographic or ecological barrier reduces gene flow. Mutation, selection and genetic drift can then change the populations independently.

3

Differences accumulate gradually; there is no single moment at which every trait changes from ‘same species’ to ‘different species’.

4

Separated populations may still be theoretically capable of interbreeding even though they never meet. Scientists therefore infer a boundary from converging reproductive, morphological, ecological and genetic evidence.

Chromosome number is characteristic, not a complexity scale

Species Diploid chromosome number
Fruit fly 8
Rice 24
Human 46
Chimpanzee 48
Dog 78

In a typical diploid cell, chromosomes occur as homologous pairs—one member of each pair inherited from each parent—so the diploid number is usually even and normally constant within a species.

Chromosome number varies widely between species but does not rank organism size or complexity. Closely related species can differ by a chromosome fusion or split while retaining many homologous genes.

A karyogram turns chromosome structure into evidence

A karyotype is the chromosome number and structural features of a cell. A karyogram is an arranged image in which homologous chromosomes are paired using length, banding pattern and centromere position.

Three independent matches support ancestral fusion: human chromosome 2 follows the combined band order of chimpanzee 2A and 2B, contains telomere-like sequence internally, and retains a vestigial second centromere. Together these are stronger than chromosome count alone.

Human chromosome 2 aligned with chimpanzee chromosomes 2A and 2B, including a functional centromere, an internal telomere sequence and a vestigial centromere.

One species combines genomic unity with variant sites

A genome is all genetic information in an organism: nuclear DNA plus mitochondrial DNA and, in plants, chloroplast DNA. Messenger RNA and transfer RNA are products of genes, not additional parts of the genome.

Scale Pattern Meaning
Most sites within one species The same base sequence is shared Common ancestry and shared biological organization
Variant sites Alleles differ among individuals Heritable diversity within the species
Single-nucleotide polymorphism (SNP) One base differs at a particular position A marker that can be tracked through populations or families

Most SNPs are in non-coding DNA and have no known phenotypic effect. Some lie in genes or regulatory regions and can affect function, drug response or disease risk; association alone does not prove causation.

Genome size and genome sequence answer different questions

Measure What it records What it does not tell you alone
Base sequence The order of nucleotides Whether every difference changes phenotype
Genome size DNA in one haploid chromosome set Gene number or organism complexity

Eukaryote genomes vary in both sequence and total DNA. Sequence variation between species is much larger than variation within one species, while repeated and other non-coding DNA can contribute greatly to genome size.

Whole-genome duplication produces polyploid organisms with several chromosome sets. This can greatly enlarge plant genomes without making the organism proportionally more complex, so a genome-size comparison is not a complexity ranking.

Sequencing produces a dataset; validation produces a conclusion

Whole-genome sequencing determines the order of bases across an organism’s complete DNA. The Human Genome Project established large-scale reference sequencing and accelerated bioinformatics for storing, comparing and interpreting sequence data.

Current and developing uses include:

  • comparing evolutionary relationships
  • locating genes and variants associated with disease
  • identifying and tracking pathogens
  • finding possible diagnostic, treatment or vaccine targets
  • guiding prevention or treatment using an individual’s genetic profile

A sequence match or statistical association generates a hypothesis. Clinical effect, causation and safety require independent evidence; sequencing alone does not diagnose every disease or guarantee that a proposed target will work.

Summary: match the species question to the evidence

Question Useful evidence Main limit
How do individuals vary? Trait distributions, genes and environment A pattern does not identify its cause by itself
Where is a species boundary? Morphology, gene flow and fertile offspring Speciation is gradual and some cases cannot be crossed or observed
What changed in chromosome history? Number, banding, centromeres and sequence landmarks One similarity alone is weak evidence
How do genomes differ? Sequence variants and genome size measured at the right scale More DNA does not mean greater complexity

Start with the biological question, choose evidence at the organism, reproductive, chromosome or sequence scale, then state what that evidence cannot decide alone.

Variation as defining feature of life

2 marks

State, giving a reason, the type of variation shown by shell length.

Species as groups with shared traits

2 marks

List two anatomical features of humans that are characteristic of primates.
1.
2.

Binomial system

4 marks

Outline the binomial system of classification.

Biological species concept

3 marks

Outline the criteria that should be used to assess whether a group of organisms is a species.

Difficulties in distinguishing species

1 mark

State with a reason whether the genetic evidence shows that the western coyote and the grey wolf have overlapping ranges.

Diversity in chromosome numbers

1 mark

State the chromosome number in this gorilla.

Karyotyping and karyograms

8 marks

Explain the use of karyotyping in human genetics.

Unity and diversity of genomes within

7 marks

Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species.

Diversity of eukaryote genomes

1 mark

The table shows the estimated total number of genes in several organisms.

SpeciesEstimated number of genes
Saccharomyces cerevisiae (a yeast)6000
Escherichia coli (a bacterium)3200
Drosophila melanogaster (fruit fly)14000
Canis familiaris (domestic dog)19000
Oryza sativa (rice)51000
Homo sapiens (human)25000

What can be deduced from the information in this table?

Comparison of genome sizes

2 marks

Using the data provided in the table, discuss whether genome size positively correlates to organism complexity.

Whole genome sequencing

3 marks

Discuss the current and potential future uses of whole genome sequencing.