Convergent yDNA STR match values (or Haplotypes) are believed to be introduced over time as STR values may diverge in given sub-clades of a root clade and then converge back to the same values in later sub-clades. It is believed that STR values have the propensity to change back to their original values; possibly as frequently as they change further away from the original value. The accepted way to prove there are convergent values is when the haplotypes match but the haplogroups do not.
It could also be the case that two more distant haplogroups have (near-)identical haplotypes by simply exhibiting the same changes but in a different order over time. That is, no necessarily "change-back" in values but diverged in a similar way but not in the same order over time.
Convergence is an oft referenced idea that is unique to genetic genealogy. But a real understanding has not been studied and reported on specifically in a scientific way. There are some issues purely due to the test measurement technique that crop up (for example, in half-identical matching). But this one has not been studied enough to understand it properly.
Convergence is blamed for cases exhibiting Ornery STR Matching that are studied here and elsewhere. It may also be explaining cases of known same haplogroups where more distantly related members (in a genealogical time frame) are appearing closer (more identical) in their haplotypes today.
What we do know is that SNP values that determine Haplogroups change much more infrequently, in general, and possibly only once in the measurable human existence. This as opposed to STR values that determine haplotypes that seem to change much more frequently. The repeat pattern in the STR marker values can increase or decrease. Remember that STR markers are simply noticed repeat patterns in the base-pair sequence. One that can be measured and are noticed to change. The change may be due to the repeat adding or deleting; or because a true SNP (even an InDel) occurred to break the pattern within one of the repeats.
It is not clear if this convergence is an actual occurrence in the real DNA of the population or simply introduced as an artifact of testing only a few STR values during the test process. Even y111 STR markers, the "gold" standard of the last decade, may not be enough or the right mix for all populations. And this appearance of convergence is more an artifact of testing the particular set of markers. A more detailed study of testers and their known genealogy has not been done.
We have used the term reconvergent in the past. Simply because we assumed that they initially started out with the same values with some likely very distant ancestor. And so diverged and then re-converged. But there is simply not enough known to even apply that label. So we capitulate and stick with the more common term convergence.
It could also be the case that two more distant haplogroups have (near-)identical haplotypes by simply exhibiting the same changes but in a different order over time. That is, no necessarily "change-back" in values but diverged in a similar way but not in the same order over time.
Convergence is an oft referenced idea that is unique to genetic genealogy. But a real understanding has not been studied and reported on specifically in a scientific way. There are some issues purely due to the test measurement technique that crop up (for example, in half-identical matching). But this one has not been studied enough to understand it properly.
Convergence is blamed for cases exhibiting Ornery STR Matching that are studied here and elsewhere. It may also be explaining cases of known same haplogroups where more distantly related members (in a genealogical time frame) are appearing closer (more identical) in their haplotypes today.
What we do know is that SNP values that determine Haplogroups change much more infrequently, in general, and possibly only once in the measurable human existence. This as opposed to STR values that determine haplotypes that seem to change much more frequently. The repeat pattern in the STR marker values can increase or decrease. Remember that STR markers are simply noticed repeat patterns in the base-pair sequence. One that can be measured and are noticed to change. The change may be due to the repeat adding or deleting; or because a true SNP (even an InDel) occurred to break the pattern within one of the repeats.
It is not clear if this convergence is an actual occurrence in the real DNA of the population or simply introduced as an artifact of testing only a few STR values during the test process. Even y111 STR markers, the "gold" standard of the last decade, may not be enough or the right mix for all populations. And this appearance of convergence is more an artifact of testing the particular set of markers. A more detailed study of testers and their known genealogy has not been done.
Converge or Reconverge?
Converging is a defined concept in evolutionary biology. When looking at STR value signatures (or haplotypes) as here, we really are talking of diverging away then converging back. Hence, we prefer the term reconvergence here to reflect that they all started from the same place originally. But that does not really matter. What matters is that a common value set is observed in the present time and that not to long in the past, the value sets were likely different.We have used the term reconvergent in the past. Simply because we assumed that they initially started out with the same values with some likely very distant ancestor. And so diverged and then re-converged. But there is simply not enough known to even apply that label. So we capitulate and stick with the more common term convergence.
External References
- Gleason's tutorial
- ISOGG Wiki definition