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"Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time." (Wikipedia)

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[Credit:https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/05%3A_Evolution/5.12%3A_Phylogenetic_Classification (I added the letters to the common ancestor nodes)]

Does this mean that Species 1 and 2 have a common descent which is (A), but Species 1 and 3 do not, even though they share a common ancestor which is (B)?

I was thinking that "common descent" means the immediate previous ancestor. After all, it's a relative matter and there will be a common ancestor at some point. There has to be a term for that other than "just previous common ancestor".

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    $\begingroup$ Welcome to Biology.SE. You've done a good job with laying out a clear question, but the question itself seems quite naïve. In addition, the way you've worded this odd — common descent isn't an individual it is a relationship! (Please fix this.) I also encourage you to check out some of the online resources available for learning more about evolution. For example, this a useful introduction to evolutionary theory from UC Berkeley. ——— I'm going to edit your post, but please check and make more changes as needed. $\endgroup$
    – tyersome
    Aug 6, 2022 at 0:19

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The Wikipedia definition is suitable to say that species 1 and 3 share common descent.

Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time.

Any node in a (fully bifurcating) tree that is an ancestor to more than two tips will necessarily contain tips that are also descended from some other node.

OP is correct that "there is a common ancestor at some point," and there is strong evidence that all cellular life arose by common descent from our most recent common ancestor.

In practice, the term is usually limited by context. For example, many more relevant comparisons can be made between roses and apples, than between roses and dogs, even though the ancestor of animals and plants did once exist. In a conversation about energy procurement it wouldn't make sense to reference the common descent between roses and dogs; but in a conversation about, say mitosis, it might.

The term "related" suffers similar ambiguity.

As the figure shows, two species can share many "common ancestors," so it may be necessary to specify the "most recent common ancestor." In the figure, both A and B are common ancestors of 1 and 2, but A is their most recent common ancestor.

B is the most recent common ancestor of 1,3, 2,3, and 1,2,3.

The thick black line coming down from B in the figure represents the connection to B's ancestors. All of those ancestors are common ancestors of 1,2,3, but none are the most recent common ancestor of 1,2,3.

The most recent common ancestor of cellular life is the last universal common ancestor.

OP may want to review the term clade for a slightly more precise way to talk about related species. A clade is a group composed of an ancestor plus all and only its descendants.

In the figure:

  • 1 and 2 form a clade with A.
  • 1, 2, 3, A, and B form a clade.
  • 1 and 3 (excluding 2) do not form a clade.
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    $\begingroup$ Indeed, I think it's easiest to think about using family relatedness as an analogy; you're related both to your parents and to your 8th cousins four times removed. In the context of life on earth, it's relevant that people are related to fruit flies; in the context of social behaviors it may only be relevant to consider primate relatives. $\endgroup$
    – Bryan Krause ♦
    Aug 5, 2022 at 22:39
  • $\begingroup$ Thanks for your help, in "clade" wiki page, i found this "last common ancestor". Please edit your answer to include how last common ancestor can be applied to the figure in my example. Is it true to say that Species 1 and 2 have the same last common ancestor but Species 1 and 3 do not have the same last common ancestor $\endgroup$
    – Freezing Soul
    Aug 5, 2022 at 22:45
  • $\begingroup$ en.wikipedia.org/wiki/Most_recent_common_ancestor $\endgroup$
    – Freezing Soul
    Aug 5, 2022 at 22:46
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    $\begingroup$ This is good, but I think it would be helpful if you addressed the OPs misuse/misunderstanding of "common descent". They seem to think this term refers to an individual species (i.e. that it is synonymous with some type of ancestor) rather than referring to relationship or process ... $\endgroup$
    – tyersome
    Aug 6, 2022 at 0:28
  • $\begingroup$ I figured it out my self from the answer, that all these terms: "common descent", "common ancestor" and "last common ancenstor" are being used in some abstract way or relative way, rather than to specify a single one. $\endgroup$
    – Freezing Soul
    Aug 6, 2022 at 4:40
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B is the ancestor of 1,2 and 3. Why do you think that is not an example of common descent between 1 and 3?

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    $\begingroup$ I was thinking that "common descent" means the just previous ancestor. After all, it's relative matter and there is a common ancestor at some point. There has to be a term for that "just previous common ancestor". $\endgroup$
    – Freezing Soul
    Aug 5, 2022 at 21:45
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    $\begingroup$ @Bowel Species are not well defined, especially in time, so "just previous common ancestor" is not a useful category and can't really be identified. $\endgroup$
    – Bryan Krause ♦
    Aug 6, 2022 at 13:31

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