International A Level Biology / Classification
What is a species? revision notes
Edexcel International A Level Biology (YBI11), chapter 11: Classification, specification point 4.14. Free lesson.
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A species can be defined by whether its members produce fertile offspring, or by their shared ancestry. These ideas help classify organisms and use molecular evidence to identify the closest relatives.
What the biological species concept means
The biological species concept defines a species as a group of organisms that can interbreed with each other to produce fertile offspring. Fertile offspring can breed themselves. This is the key test: two different species may produce offspring together, but that alone does not make them one species. You need to consider whether the offspring can reproduce, not just whether it is born.
Why horses and donkeys are separate species
A horse and a donkey can mate, but their offspring, a mule, is almost always infertile. Horses are Equus caballus and donkeys are Equus asinus. They share a genus, but have different species names. A mule is healthy and can live a normal life, but its chromosomes usually cannot pair during meiosis, so it cannot make working gametes. A horse has 32 chromosomes and a donkey has 31, giving a mule 63 in total. Because a mule cannot usually produce fertile offspring, horses and donkeys are separate species under the biological concept.
When the fertile offspring test cannot be used
The biological concept does not work for every organism or situation. In some cases, biologists cannot test whether two groups would produce fertile offspring. In others, the result could misleadingly suggest that separate species are one. Asexual organisms, such as bacteria, reproduce by dividing and do not interbreed. Fossils cannot be bred because the species is extinct. Isolated populations, such as birds on separate islands, never meet, so it is unknown whether they could interbreed. Some separate species, especially plants, produce fertile hybrids, which could make the…
How the phylogenetic concept differs
The phylogenetic species concept classifies organisms by shared ancestry rather than by interbreeding. Phylogeny means the evolutionary history of a group. A group belongs to one species if its members share a common ancestor and can be distinguished from other groups. The biological concept uses breeding and fertile offspring as evidence. The phylogenetic concept uses DNA base sequences, protein amino acid sequences and anatomy. It can be used for bacteria, where the biological concept cannot be applied. To find how closely organisms are related, compare molecular evidence carefully.
How sequence differences show the closest relative
Compare the same molecule, such as the same gene, in each species. The species with the fewest differences is the most closely related because it shares the most recent common ancestor with the species being compared. For percentage similarity, work out how many bases match, then divide by the total and multiply by 100.