There was very little assimilation of Orma, Boran or Arsi and you can see that by looking at the FST (
measure of genetic distance between populations) between Ethiopian Somalis and Somalis from Somalia. The FST between the two populations is extremely low at 0.0008.
If the Ogaden had absorbed more Arsi, Boran and Orma in their expansion, the FST would be much higher than 0.0008 given how distant these groups are genetically to Somalis.
https://www.biorxiv.org/content/10.1101/2020.10.21.348599v2.full
Northern Somalis generally abhor assimilation. You can see this in that the Wardai Orma (now Known as Warday Cali) still exist as a stand alone clan in Jubaland and have an MP in the Somali parliament. Sheekhaal and various Dir clans (Bajamaal, Gaadsan etc..) have lived among the Ogaden for centuries without being assimilated at all.
View attachment 217901
https://anthrosource.onlinelibrary.wiley.com/doi/pdf/10.1525/aa.1961.63.1.02a00060
The demographic expansion of the Ogaden is due to fast population growth as they conquered new grazing land. More land means more livestock which means more milk & meat which mean more children survive into adulthood which means rapid population growth.
Clans on the periphery, I guess, will have a greater chance of having segments of its demographic blurring the lines (not assuming it is the rule, but we know it took place, with Oromos as well). Often when sampling certain measures are controlled for, like being asked if your last 4 grandparents were full Somali, to get a better reading of ethnic non-admixed Somalis in studies related to nutritional science, pharmacogenomics, etc (same samples are often used across several research aims, and there are other sampling considerations, such as, to what degree the sample-set is representative). Then the statistical distance measure will become similar across the board. In fact, I would go as far as saying that if we included the "peripheral" clans, the FST would still be relatively low. But this, I believe, will not rationally explain away the tiny minority with substantial gene flow. And would further grant that, although admixture will probably occur more at the ends, it does not necessarily mean a natural gradient. That is not a given factoring in conditions that limit this. There are aspects of social and cultural barriers for such a phenomenon.
Taken from the study:
(A higher proportion of the Amhara-Tigrye_Oromo majority’s admixtures were observed in Ethiopian Somali than the Somali from mainland Somalia..)
^Although this can be one of the studies with a low sample size, and a couple of highly mixed individuals that carry Ethiopian-related genetics that skew. But in the practical sense, this skew will statistically more likely occur for Ethiopian Somalis and increase in significance as you move toward the edges. This is not supposed to mean there is a rule.
Despite the fact that Oromos share long geographic boundaries with the Ethiopian Somali, the results in this study don’t show genetic relationship of the two better than that each has with others in the major Group 1.
^This is very impressive, but it is followed by:
This might be due to the
isolation of the sampling areas of the two communities.
We still believe a strong genetic contact of the two along their boundaries. However, the contact between the two people is likely during the post 16th-century Oromo migration (
Lewis 1966).
Hence, their genetic relation may quickly decay as we go away from their current shared areas.
With the last sentence, in particular, I don't think there is a gradient thing going on. There can be containment that can present itself in abrupt forms, but there are also possibly higher admixture rates as well overall, that might not change things to a great deal in the statistical sense, but somehow change the conversation into accepting such things takes place (to a relatively minor degree).