How is a species defined?

A group of organisms with similar characteristics that can interbreed to produce fertile offspring.

Why does the species definition not work for bacteria?

They reproduce asexually, so they do not interbreed.

Why can the species definition not be tested on fossils?

Interbreeding cannot be observed.

Give an example of a hybrid that shows a limitation of the species definition.

Lions and tigers can interbreed in captivity; a horse and a donkey produce an infertile mule.

In gel electrophoresis, why do DNA fragments move towards the positive electrode?

DNA is negatively charged because of its phosphate groups.

How does a banding pattern show relatedness?

The more bands two species share, the more similar their DNA, so the more closely related they are.

What is bioinformatics?

The use of computers to store, compare and analyse biological data such as DNA sequences.

What does peer review involve?

Independent experts check the methods and conclusions of a paper before it is published.

Name three ways scientists validate new evidence.

Publishing in scientific journals, peer review and discussion at conferences.

Name the five kingdoms.

Prokaryotae, Protoctista, Fungi, Plantae and Animalia.

What evidence led Woese to propose three domains?

Ribosomal RNA sequences showed that archaea differ from bacteria as much as from eukaryotes.

Classification and the species conceptEdexcel A-Level Biology B: Flashcards

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List the classification hierarchy from highest to lowest.

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List the classification hierarchy from highest to lowest.: Domain, kingdom, phylum, class, order, family, genus, species.
How is a species defined?: A group of organisms with similar characteristics that can interbreed to produce fertile offspring.
Why does the species definition not work for bacteria?: They reproduce asexually, so they do not interbreed.
Why can the species definition not be tested on fossils?: Interbreeding cannot be observed.
Give an example of a hybrid that shows a limitation of the species definition.: Lions and tigers can interbreed in captivity; a horse and a donkey produce an infertile mule.
In gel electrophoresis, why do DNA fragments move towards the positive electrode?: DNA is negatively charged because of its phosphate groups.
In gel electrophoresis, which fragments travel furthest?: The smaller fragments.
How does a banding pattern show relatedness?: The more bands two species share, the more similar their DNA, so the more closely related they are.
What is bioinformatics?: The use of computers to store, compare and analyse biological data such as DNA sequences.
What does peer review involve?: Independent experts check the methods and conclusions of a paper before it is published.
Name three ways scientists validate new evidence.: Publishing in scientific journals, peer review and discussion at conferences.
Name the three domains.: Archaea, Bacteria and Eukarya.
Name the five kingdoms.: Prokaryotae, Protoctista, Fungi, Plantae and Animalia.
What evidence led Woese to propose three domains?: Ribosomal RNA sequences showed that archaea differ from bacteria as much as from eukaryotes.

Exam questions on Classification and the species concept

Researchers collected several specimens of an unfamiliar beetle from the canopy of a rainforest. All the specimens are female, no males have ever been found, and the beetles are thought to reproduce without mating.
Suggest how the researchers could use DNA to decide whether the beetle is a new species.2 marks
Gel electrophoresis was used to compare DNA from four related species of finch, A to D. DNA from each species was cut with the same restriction enzyme and the fragments were separated on an agarose gel. Species A and B shared five of their six bands, species A and C shared three bands, and species A and D shared only one band.
Explain why the smaller DNA fragments are found further from the wells at the end of the run.2 marks
Until the 1970s, living organisms were classified into five kingdoms: Prokaryotae, Protoctista, Fungi, Plantae and Animalia. In 1977 Carl Woese compared the base sequences of ribosomal RNA from many organisms. He found that the prokaryotes fell into two groups, bacteria and archaea, which differed from each other as much as each differs from the eukaryotes. Archaea also differ from bacteria in the lipids of their membranes and in their RNA polymerase.
Explain how Woese's evidence supports the three-domain model rather than the five-kingdom model.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).