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For example,

say species A is common ancestor of B, and C. Species B is a common ancestor of D and E.

We would expect that there will be more genetic similarity between D and E than D and C. And those genetic similarity must exist in B.

In other word, we won't expect genetic similarity that don't "cross" the common ancestor or the evolutionary tree.

The exception is probably genetically engineered bacteria.

That being said, am I correct?

Some people say that we have similarity with pigs and chimps even though our common ancestors may be to far off. That won't happen right?

To summarize

I expect that evolutionary tree will form a well, tree. Genetic similarity would infect "nearby" trees and can't jump between trees without connectors, such as common ancestors.

Is that what we observe for ALL species?

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There's a weird debate going on in the comments of dmm's answer and there were several downvotes that I don't understand on my answer. Do you think my answer answers your question? I thought you were interested in understanding whether or not D might be more genetically similar to C than to E, is it correct? Or were you mostly interested in a debate of what is "believing", or about a science-religion debate? Or something else... –  Remi.b Dec 15 '13 at 8:57
    
Note: dmm's answer and the debate have been deleted. –  Remi.b Dec 15 '13 at 9:44
    
A reader might also want to have a look to this post on the same subject –  Remi.b Dec 15 '13 at 9:58
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2 Answers 2

up vote 3 down vote accepted

You have an excellent answer from Remi.b already but I just wanted to add/emphasise this (because there is always more than one way of explaining something and IMO the site benefits from having many answers to the questions)...

The tree we construct does not necessarily accurately reflect what happened in evolution. If B & C evolved from A, and D & E came from B, we would create this tree if we measured using the correct indicator. But the methods we have are not perfect. The first evolutionary trees were based on morphological descriptions etc. and clearly some of the classifications were going to be wrong. These days we use molecular methods, which are probably more accurate but could also be wrong some times. For example if we based our phylogeny on one SNP variant we could have some idea about the phylogeny between a few species, but if we based it on millions of SNPs we would have a much better idea - as technology & models improve that is becoming more realistic. The key point here being there is a difference between the trees we can draw from evidence, and the real evolutionary tree.

If, for example, D evolved from B but was more genetically similar to C it could be due to convergent evolution (if the same mutations are selected for in C and D but neutrally or negatively in B). This is because, if there is a shared standing genetic variation at the point of speciation or the same mutations occur after speciation, and then C & D are imposed under similar selection and B is not, the allele favored in that environment will spread in C & D but not B (it may drift in B).

Imagine this with humans, where populations split off around the globe. Two populations spread from Africa (A) in to Southern Europe (SE) and Western Asia (WA). Population WA then derived two more populations - Central Asian (CA) and Eastern European (EE). If the Eastern European population developed a habit for drinking cow milk, and that information passed to the Southern Europeans, then selection would favor the ability to digest lactose in those two populations. If we then used the lactase persistence SNP (which had standing genetic variation at the time of divergence) to derive a phylogeny the populations SE and EE would appear related because the allele for lactase persistence is common, but in A WA & CA it would be rare (neutrally evolved). In this case the phylogeny we construct would be different to the true phylogenetic tree.

Basically if there is genetic variation at the point of divergence, or convergent selection, or any gene flow occurs after divergence it makes assessing the phylogeny more difficult and prone to error.

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+1 Very good clarifications for important concepts I only wrote some lines about. –  Remi.b Dec 15 '13 at 10:21
    
Thanks @remi.b . –  GriffinEvo Dec 15 '13 at 10:26
    
Very good point in deciding where my simple theory would go wrong. –  Jim Thio Dec 15 '13 at 16:12
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Your question seems rather confused to me, I don't quite understand it. Below I try to explain some stuff, I am not quite sure it would help you finding the answer to your question. Let me know if it helps you and how I can improve this answer.

While natural selection selects for a given allele (gene variant), mutations create new variation. And because of this process of mutation there is genetic pattern in species D that would cannot be found in species B (assuming that we have the DNA of species D). But what is similar between B and D were probably also present in their common ancestor B. The exceptions are molecular convergent evolution and horizontal gene transfer (natural of artificial).

Because of common ancestry …

  • Humans have similarities with pigs
    • Example: endothermic amniotes
  • Humans have similarities with a bug
    • Example: bilaterial symmetry
  • Humans have similarities with jellyfish
    • Example: Heterotrophy
  • Humans have similarities with an oak tree
    • Example: We have mitochondrias
  • Humans have similarites with any living things on earth!
    • Example: Genetic information is hold into a DNA/RNA molecule.

enter image description here

If you might observe that D is genetically speaking more similar to C than to E… This can be due to:

  • Horizontal gene transfer (artifical or natural)
  • The mutation rate (or ability to fix mutations) in one lineage might be greater than in another lineage.
    • For example, if lineage E underwent many mutations, they might be genetically speaking have accumulated many difference compare to D, way more than the number of difference that D has with C.
  • The phylogeny might be wrong
  • There might have had some convergent evolution.
    • Think about fishes which opsins in the eyes need to be adapted to the amount of light which depends on the turbidity of the water. After the three lineages splitted, the change of envirronment might have occured the sequence AATGATCCT suddenly became beneficial. Only two lineages had the chance to produce the right mutation. Or maybe lineage E live at low depth and therefore it doesn't need this mutation because there is enough light at low depth anyway.

Therefore, one should not expect a common ancestor to be the arithmetic mean (supposing it means something) of the current species. And one should not expect that two closely related species on the tree of life should necessarily be more genetically similar to each other than any of the two to a third species.

Note that the phylogeny is constructed mostly based on genetic data. Which makes that two closely related species (on our tree which is not necessary the absolute tree of life) are (almost) necessarily more genetically similar (it is kind of a definition then).

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I hope you don't mind, I added an image to make it easier to follow the discussion. –  Alan Boyd Dec 13 '13 at 12:49
    
@AlanBoyd You did well! Thank you. –  Remi.b Dec 13 '13 at 13:05
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Are there genetic similarity between humans and pig that don't exist in chimps? –  Jim Thio Dec 13 '13 at 13:29
    
Yes there are. The reasons are found in the last part of my answer. The most important point concerns the mutation that occurred in the chimps lineage. –  Remi.b Dec 13 '13 at 13:59
    
Note that if the rates of evolution are considerably different in different branches, taxa from slow-evolving lineages may have smaller genetic distances from each other than to their respective fast-evolving sister groups. Although this is not expected on the level of species. –  har-wradim Dec 13 '13 at 18:27
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