Why would a species from the Homo genus have overwhelmingly B negative blood type?

Question

In my story (go to see What evolutionary pressures would lead to Ogres?), there is a massive species from the Homo genus called ogre. They mostly have B negative blood type (I do not mean Rhesus null, but the simple absence of D antigen).

Precisely, 60 % of ogres are Rhesus negative, and 40 % are Rhesus positive. Also, 35 % of ogres are B, 25 % are A, 30 % are O, and 10 % are AB. Finally, 20 % of ogres are B negative, 15 % are B positive, 16 % are A negative, 9 % are A positive, 18 % are O negative, 12 % are O positive, 6 % are AB negative, and 4 % are AB positive.

In real life, most humans are O positive. That said, it depends of the ethnicity, and the region. For example, according to Héma-Québec (sorry, I live in Quebec), 85 % of Quebec's human population are Rhesus positive, and at the opposite, 15 % are Rhesus negative. Also, 46 % of Quebec's population are O, 42 % are A, 9 % are B, and 3 % are AB. Finally, 39 % of Quebec's population are O positive, 36 % are A positive, 7.5 % are B positive, 7 % are O negative, 6 % are A negative, 2.5 % are AB positive, 1.5 % are B negative, and 0.5 % are AB negative.

Also, according to Wikipedia, at 46.3 % of its human population, Armenia wins the world record of the country with the most citizens who are A positive. At 33.12 %, Bangladesh is the only country I know with humans who are mostly B positive. But, in India, there is an almost even number of humans who are O positive and humans who are B positive, at 32.53 % and 32.1 % respectively.

So, I wonder why would a species from the Homo genus benefit evolutionary speaking to mostly have B negative blood type.

Answer 1

Antigens and Antibodies:

Blood type is about antigens on the red cells (glycoproteins). The glycoproteins happen to match those on bacteria in the gut that are regularly exposed to the immune system. Your body produces antibodies to these antigens unless your body recognizes those antigens as self (i.e. you have the A and/or B antigens) So for the A&B antigens, You are a B because you don't produce an A antigen but do produce a B antigen.

Let's say your ogres have an enzyme in the blood that has a different primary function, but degrades the A antigen. All A antigen is immediately broken down, before the cells even get to the blood stream. This way, even even many ogres with the gene for the A antigen don't type as A. Some ogres have lost this enzyme, or have a different allele for the enzyme that doesn't degrade A, and can type as A (or have reduced enzymes and have a weaker expression of the A type).

• If you need a reason, let's say that as part of their stress response they can dump large numbers of immature nucleated red cells into the circulation. But these cell are only desirable in a crisis. Since they still have A antigen, the immune system rapidly removes them from circulation, and the A antigen is a marker for this. Those without This antigen clear the cells slower.

Part of the evolution of these ogres is to have a specific symbiotic bacteria that is taken up in the muscles and functions like an organelle, providing enhanced energy processing to optimize strength. These bacteria are coated heavily in glycoproteins corresponding with the B antigen. During the evolution of this symbiosis, it was beneficial for individuals with the B antigen who already had a lower immune response to the bacteria. So the starting population was enriched in B individuals. Ogres today treat the bacteria as "self" immunogenically, but those who have lack the B antigen are more prone to a muscle-debilitating autoimmune disorder. akin to the anti-mitochondrial auto-antibodies that can cause primary biliary cholangitis.

• An optional consequence of your ogres needing a new organelle to utilize enhanced strength might be that only the half-ogre children of female ogres inherit the organelles, while the sperm of male ogres does not carry the organelle.

If your population has a high number of Rh negative individuals, there can be a selective pressure to select Rh negative. Rh negative females who mate with Rh positive males can develop antibodies against the Rh factor if they have an Rh positive child. So after the first Rh positive child, their immune system attacks and often destroys subsequent Rh positive children. They can have more children with Rh negative males. So an initial founder effect enriching for Rh negative can become self-sustaining.

Answer 2

Founder effect

In genetics founder effect refers to lasting genetic oddities that relate directly to the narrow genetic make-up of an original population. The isolation of the first generation of a new colony acts as an artificial bottleneck making certain alleles more, or less, common in their descendants that in the parent population. Apart from slightly reduced clotting for O types, and the slight reduction of their survival rate in extreme trauma cases (a purely modern issue because those kinds of injuries have only become survivable for anyone in the last 40 years or less) there is little to no selection pressure effecting blood type. Once fixed at foundation the blood type percentages won't budge much unless the colony goes through a new bottleneck, severe genetic drift occurs, or an influx of new blood exerts a major influence.

To reiterate there is no evolutionary advantage to a particular blood type, but there is no disadvantage either so normal evolutionary processes don't really have any bearing here.