What you need to know
- 4.14(i)speciesA group of organisms that can interbreed to produce fertile offspring. concept, phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. and genotypeThe alleles an organism has for a gene. in classificationSorting organisms into groups based on shared features and evolutionary relationships..
- 4.14(ii)Critical evaluation of new molecular data.
Historical hunting and the Quagga Project are enrichment, not extra 4.15 or 4.21 recall. Selective breeding for a stripe pattern is not the same as resurrecting an identical extinct genome.
By the end, you should be able to…
- Separate a striking phenotype from a species decision.
- Explain what museum DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. can and cannot show.
- Evaluate a breeding-back claim without treating it as de-extinction of identical animals.
Build your understanding
A zebra that seemed like another species
The quagga was a southern African equid with stripes on the head and neck that faded towards a plain brown rear. It was hunted in the nineteenth century; the last captive animal died in 1883. For a long time many naturalists treated it as a separate speciesA group of organisms that can interbreed to produce fertile offspring. because the phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. was so distinctive.
Appearance was the available evidence. That is a morphological species judgement. It is a reasonable starting hypothesis, not a proof. The biological species test could not be run after extinction, and living plains zebras were not compared genetically until much later.
Museum DNA changed the grouping
In 1984, Higuchi and colleagues recovered DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. from dried muscle of a museum quagga — among the first sequence data from an extinct animal — and compared mitochondrial fragments with a living zebra. Later studies, using more specimens and more of the genome, placed the quagga inside the plains zebra, Equus quagga, as the subspecies Equus quagga quagga.
The scientific community evaluated those data: independent laboratories, more loci, and comparison with other zebras. The phenotype (reduced striping) remains real. The classificationSorting organisms into groups based on shared features and evolutionary relationships. question is whether that phenotype marked a fully separate species or a distinct populationAll the individuals of one species living in one area at the same time. within a wider gene poolAll the alleles of all the genes in a population.. Current consensus is subspecies, not a third zebra species.
Breeding for a look is not a time machine
The Quagga Project in South Africa selectively breeds living plains zebras that already show reduced striping, aiming for animals that resemble historical quaggas. That uses remaining genetic variation in Equus quagga. It does not reconstruct the exact extinct genome, and it does not undo the loss of the original population.
The project is a useful discussion of phenotype versus genotypeThe alleles an organism has for a gene.. Success on appearance would not automatically mean the original subspecies had been restored in every genetic sense. Treat press claims of “bringing back the quagga” as a prompt to ask which definition of species or subspecies is being used.
Make your reasoning visible
Name the evidence type
Stripe pattern is phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment.. Museum sequences are genotypeThe alleles an organism has for a gene.. Extinction removes the breeding test.
Keep subspecies language
Equus quagga quagga is a trinomen for a subspecies. It is not a binomial for a separate speciesA group of organisms that can interbreed to produce fertile offspring..
Avoid this misconception
The claim: If it looks unique and is extinct, it must have been its own species.
The correction: Molecular evidence can place an extinct form inside a living species as a subspecies. Phenotype alone does not settle the rank.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Why could nineteenth-century naturalists not apply the biological species concept to the last quagga?
Show suggested answer
The wild population was gone and the last animals were in captivity. A test of interbreeding with plains zebras in nature was no longer available.
Practice question
Retrieval and explanation · suggested answer
What did museum DNA add to the classification argument?
Show suggested answer
Sequence comparison with living zebras provided genotypic evidence that the quagga grouped with plains zebras, supporting subspecies rank rather than a wholly separate species.
Practice question
Retrieval and explanation · suggested answer
Why is a plains zebra that looks like a quagga not automatically the original subspecies restored?
Show suggested answer
Selective breeding can reconstruct a similar phenotype from remaining variation without recovering the extinct population’s full genome or history.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A headline says “DNA proves the quagga never existed”. Rewrite the scientific claim and state one limit of the 1984 mitochondrial fragments.
Show a hint
Existence of the phenotype versus rank of the taxon.
Explore the reasoning
The animals existed; the claim should be that they are classified as a plains-zebra subspecies rather than a separate species. Early mtDNA fragments sample one locus from one or few specimens, so they cannot alone describe the whole genome or every population; later multi-specimen studies were needed to strengthen the conclusion.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare classification by phenotype with classification by genotype.
Classification and three domains
Practice question
Retrieval and explanation · suggested answer
Compare classification by phenotype with classification by genotype.
Lesson: 4B 1: Principles of Classification
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The first extinct-animal sequences
4B · Thinking Bigger: The QuaggaHiguchi et al. reported in Nature in 1984 that DNA could be recovered from dried museum muscle of a quagga and that mitochondrial sequences differed from a mountain…Specification 4.14(ii)Read & use the scienceClose article
Why this matters
Higuchi et al. reported in Nature in 1984 that DNA could be recovered from dried museum muscle of a quagga and that mitochondrial sequences differed from a mountain zebra at a limited number of sites. The paper is famous for showing that extinct-organism DNA can be studied. Later work revised the quagga’s relationship to the plains zebra. That revision is evaluation of new data, which is the heart of 4.14(ii). Ancient DNA studies work with short, damaged fragments and must control contamination. One mitochondrial sequence traces only one part of ancestry, so comparing more samples and more genomic regions can change the inferred relationship. The quagga story is a useful example of classification being revised through evidence. It also warns against treating a museum specimen’s appearance as the only criterion. This is a research extension to 4.14: ask which comparison taxa and data types were included before accepting a grouping.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.14(ii) understand the process and importance of critical evaluation of new data by the scientific community leading to new taxonomic groupings, based on molecular evidence, including the three-domain system (Archaea, Bacteria and Eukarya)
Use the science
Why was a comparison with only one living zebra species an incomplete classification argument in 1984?
Explore a possible response
Relatedness is judged among several groups. Without plains zebra and other equid sequences in the same analysis, you cannot tell whether the quagga sits inside Equus quagga or beside it as a sister species.
Explore the source
- Higuchi et al., DNA sequences from the quagga (Nature, 1984)Nature · Abstract
Look-alike bees, different species?
4B · What Is a Species?The Natural History Museum describes cryptic bumblebee species: organisms that look similar enough to be grouped under one name until other evidence reveals differences. DNA barcodes can help,…Specification 4.14(i)Read & use the scienceClose article
Why this matters
The Natural History Museum describes cryptic bumblebee species: organisms that look similar enough to be grouped under one name until other evidence reveals differences. DNA barcodes can help, but taxonomists compare several individuals and traits before redrawing a boundary. The biological species concept raises another question: do the groups exchange genes in nature? Conservation priorities may change when one apparent widespread species turns out to include several narrower populations. This illustrates why species is a scientific hypothesis supported by phenotype, genotype and reproductive evidence, not a label decided by colour alone.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.14(i) understand that classification is a means of organising the variety of life based on relationships between organisms using differences and similarities in phenotypes and in genotypes, and is built around the species concept
Use the science
If two bee populations look alike but have different DNA barcodes, what more would strengthen a claim that they are separate species?
Explore a possible response
Consistent differences in multiple samples, reproductive isolation or lack of gene flow, and relevant ecological or phenotypic evidence would strengthen it.
Explore the source
- Bumblebee specialist group and cryptic speciesNatural History Museum
A domain diagram can change with evidence
4B · Domains, Kingdoms or Both?The three-domain model in the Pearson specification separates Bacteria, Archaea and Eukarya using molecular evidence. The Natural History Museum notes that newer analyses may place eukaryotes within an…Specification 4.14(ii)Read & use the scienceClose article
Why this matters
The three-domain model in the Pearson specification separates Bacteria, Archaea and Eukarya using molecular evidence. The Natural History Museum notes that newer analyses may place eukaryotes within an archaeal lineage, leading some researchers to discuss two top-level domains instead. Students still need to know the required three-domain framework. The real research lesson is that a classification is evaluated and revised when new data improve the explanation of evolutionary relationships. A scientist must say which sequences were compared and why the resulting tree is credible. An updated diagram is not evidence that organisms themselves suddenly changed.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.14(ii) understand the process and importance of critical evaluation of new data by the scientific community leading to new taxonomic groupings, based on molecular evidence, including the three-domain system (Archaea, Bacteria and Eukarya)
Use the science
Why can two published domain trees differ even though the organisms did not change?
Explore a possible response
They may use different molecular datasets or analytical assumptions, producing different hypotheses about ancestry. Scientists compare the evidence and methods.
Explore the source
- What is taxonomy?Natural History Museum