Variation and species
Variation between organisms is a defining feature of life: no two individuals are identical in all traits.
Patterns of variation are the basis of naming, identifying and classifying organisms.
Morphological species concept: a species is a group of organisms with shared observable traits.
Biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring.
Be ready to explain limitations of the biological species concept: it does not always work cleanly for gradually diverging populations, asexual organisms or bacteria.
During speciation, populations become increasingly different over time, so the boundary between population and species can be arbitrary.
Binomial nomenclature
Use the binomial system for naming organisms.
First name = genus; second name = species.
Species in the same genus share similar traits.
Formatting rule: Genus starts with a capital letter; species is lowercase.
In formal writing, binomial names should be italicized.
Chromosomes, karyotypes and karyograms
Chromosome number varies between species.
Example required: humans = 46 chromosomes, chimpanzees = 48 chromosomes.
Diploid cells have an even number of chromosomes.
Karyotyping = arranging chromosomes using banding patterns, length and centromere position.
A karyogram is the organized visual display of chromosomes.
Evidence for human chromosome 2 fusion:
human chromosome 2 is unusually large
banding patterns match two ancestral ape chromosomes
evidence supports fusion from a shared primate ancestor
In NOS-style questions, distinguish testable hypotheses from non-testable statements.
This karyogram shows the full human chromosome set, arranged by size, pairing and banding pattern. It is useful for practising how chromosomes are classified in a karyotype/karyogram. Source
This diagram shows the proposed fusion of two ancestral chromosomes to form human chromosome 2. It directly supports the exam point about evaluating evidence for the chromosome 2 fusion hypothesis. Source
Genomes within and between species
The genome is all the genetic information of an organism.
Organisms in the same species share most of their genome.
Variation within a species includes differences such as single-nucleotide polymorphisms (SNPs).
Eukaryote genomes vary in:
overall size (total amount of DNA)
base sequence
Variation between species is much greater than variation within a species.
Do not assume larger genome size means greater organism complexity.
Whole genome sequencing
Whole genome sequencing is becoming faster and cheaper.
Current use: investigating evolutionary relationships.
Potential future use: personalized medicine.
Exam skill: compare genome sizes across taxonomic groups using database information, then judge whether genome size matches complexity.
Checklist: can you do this?
Define a species using both the morphological and biological species concepts.
Write a correct binomial name with proper capitalization and italics.
Identify and classify chromosomes in a karyogram using banding pattern, length and centromere position.
Interpret evidence for the fusion origin of human chromosome 2.
Compare genome size data and explain why genome size does not always equal complexity.
HL only: limits of the biological species concept
The biological species concept works poorly for asexually reproducing organisms.
It also works poorly in bacteria because horizontal gene transfer can move genes between lineages.
Therefore, interbreeding is not always a useful criterion for defining species.
HL only: chromosome number and fertility
Chromosome number is usually a shared trait within a species.
Cross-breeding between closely related species is less likely to produce fertile offspring if parent chromosome numbers are different.
This helps explain why chromosome number can contribute to reproductive isolation.
HL only: dichotomous keys
A dichotomous key identifies organisms by a sequence of paired, contrasting statements.
Each step gives two choices only.
Good keys use clear, observable characteristics.
In practical work, students may need to develop a dichotomous key for local plant or animal species.
This resource shows a real dichotomous key built from paired choices used to identify organisms. It is useful for seeing the structure you would copy when constructing your own exam or practical identification key. Source
HL only: environmental DNA and barcodes
Environmental DNA (eDNA) is DNA collected from environmental samples such as water, soil or air.
DNA barcodes are short standard DNA sequences used to help identify species.
Combining eDNA with barcoding allows rapid detection of species present in a habitat.
This method is useful for investigating biodiversity without directly observing or capturing organisms.
This figure shows how environmental DNA metabarcoding can be used to monitor ecosystems and species diversity. It links sample collection, DNA analysis and biodiversity assessment, matching the syllabus point on identifying species using eDNA barcodes. The visible title/description is “Global ecosystem and biodiversity monitoring with environmental DNA metabarcoding”. Source
High-yield exam links
Species definitions are useful, but all have limitations.
Speciation is gradual, so species boundaries are not always perfectly clear.
Chromosome number, karyotype evidence and genome data are all used to compare organisms.
Genome similarity within species and greater genome divergence between species explain both unity and diversity.
New genetic tools such as whole genome sequencing, DNA barcoding and eDNA are making biodiversity studies faster and more precise.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.