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I have looked online but still do not understand how two organisms can have the the same species names but be in different genera?

Do all genera share common species names?

Also which would be more closely related, two organisms with the same genus name but different species name or two organisms with different genus names but the same species name?

According to a textbook, the two with the same genus are more closely related, but I do not understand why?

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    $\begingroup$ For a very simple analogy, consider the Genus name to be like a person's last name and the Species name to be like their first name. John Smith is more closely related to his brother Bob Smith than he is to his neighbor, John Brown. $\endgroup$ – C_Z_ Jan 6 '16 at 15:24
  • $\begingroup$ @CactusWoman, good analogy, unless of course John Brown and John Smith's mom, well, you know... John Jr.... $\endgroup$ – AMR Jan 6 '16 at 20:13
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    $\begingroup$ I assume that you are talking about the second part of the binomial name when you write 'species name', which is in fact called 'species epithet' (or 'specific epithet'). 'Species name' is referring to the full binomial name in binomial nomenclature. $\endgroup$ – fileunderwater Jan 8 '16 at 20:46
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Two different species can have the same species epithet if they belong to different genera ('species name' is referring to the full binomial name). Consider for example Pinus glabra and Ilex glabra

P. glabra

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I. glabra

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Two species can be have the same genus name (meaning they belong to the same genus) and will therefore necessarily have different species epithets (otherwise we would name them exactly the same way which would be very confusing). Consider for example Pinus glabra (again) and Pinus resionosa

P.glabra

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P. resinosa

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As a consequence, if two species share the same species epithet then it tells you nothing about how closely related they are. If they share the same genus name, then they belong to the same genus (except exceptions probably!).

For the above examples, the term glabra comes from "glaber" (latin) which means "Hairless". So the only thing you can tell from P. glabra and I. glabra is that they are probably both hairless when you compare them with their respective closely related species.

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  • $\begingroup$ Could the analogy be used for two organisms with different family and same genus? (I am aware that this is no longer binomial nomenclature). So, they would be less closely related compared to two organisms with same family, different genus? In simple terms, the category that comes first should be prioritised when deciding how closely related they are? (if that makes sense). $\endgroup$ – city7lights Jan 6 '16 at 16:43
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    $\begingroup$ Yes indeed. Extending on @CactusWoman nice analogy (which is almost more than just an analogy): If the species is a first name, then genus is the family name and family is the ethnical origin and so on... A case could exist where two individuals of different ethnic groups would have the same family name but that would not be due to common descent but only to "chance". $\endgroup$ – Remi.b Jan 6 '16 at 17:09
  • $\begingroup$ I corrected your use of 'species name' to 'species epithet', since this is the term for the second part of the binomial name (also 'specific epithet' or 'specific name'). 'Species name' is referring to the full binomial name. $\endgroup$ – fileunderwater Jan 8 '16 at 20:54
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    $\begingroup$ And then there are cases when completely different organisms have the exact same genus and specific epithet, such as Orestias elegans Ridl. and Orestias elegans Garman, 1895. $\endgroup$ – Gaurav Jan 11 '16 at 3:05
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This is a widespread issue amongst binomial names. Only the full binomial name has to be unique in nomenclature, and therefore many genera contain different organisms with the same specific epithet. As Remi.b has already mentioned, two organisms with binomial names that have the same species but different genus are very likely more different from each other than two organisms that share a genus but have a different specific epithet.

For example, the Wikipedia page on the specific epithet "Officinalis" lists dozens of organisms with a variant of that specific epithet conforming to the Latin declension of its genus.

This list consists of organisms which are completely unrelated to each other, containing fungi, plants and animals.

Althaea officinalis (marshmallow)
Anchusa officinalis (bugloss)
Archangelica officinalis (angelica)
Asparagus officinalis (asparagus)
Avicennia officinalis (mangrove)
Bistorta officinalis (European bistort)
Borago officinalis (borage)
Buddleja officinalis (pole butterflybush)
Calendula officinalis (pot marigold)
Cinchona officinalis (quinine)
Cochlearia officinalis (scurvygrass)
Corallina officinalis (a seaweed)
Cornus officinalis (cornelian cherry)
Cyathula officinalis (ox knee)
Cynoglossum officinale (houndstongue)           
Euphrasia officinalis (eyebright)
Fumaria officinalis (fumitory)
Galega officinalis (goat's rue)
Gratiola officinalis (hedge hyssop)
Guaiacum officinale (lignum vitae)
Hyssopus officinalis (hyssop)
Jasminum officinale (jasmine)
Laricifomes officinalis (a wood fungus)
Lavandula officinalis (lavender)
Levisticum officinale (lovage)
Lithospermum officinale (gromwell)
Magnolia officinalis (magnolia)
Melilotus officinalis (ribbed melilot)
Melissa officinalis (lemon balm)
Morinda officinalis (Indian mulberry)
Nasturtium officinale (watercress)
Paeonia officinalis (common paeony)
Parietaria officinalis (upright pellitory)
Primula officinalis (cowslip)
Pulmonaria officinalis (lungwort)
Rheum officinale (a rhubarb)
Rosa gallica 'Officinalis' (apothecary rose)
Rosmarinus officinalis (rosemary)
Salvia officinalis (sage)
Sanguisorba officinalis (great burnet)
Saponaria officinalis (soapwort)
Sepia officinalis (cuttlefish)
Sisymbrium officinale (hedge mustard)
Spongia officinalis (bath sponge)
Stachys officinalis (betony)
Styrax officinalis (drug snowbell)
Symphytum officinale (comfrey)
Taraxacum officinale (dandelion)
Tormentilla officinalis (tormentil)
Valeriana officinalis (valerian)
Verbena officinalis (vervain)
Veronica officinalis (speedwell)
Zingiber officinale (ginger)
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