Humans have the ABO and Rhesus blood typing systems. I have two questions about it:
- Why have we evolved these blood types?
- Do other animals have different blood types as well?
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For some blood types, evolution and environmental selective pressures are clearly important for their persistence. For example, the Duffy blood type includes a receptor that allows certain types of malarial parasites to enter the red cell. Thus, in some malarial areas of Africa, populations with Duffy-negative blood types have a distinct survival advantage because absence of the Duffy antigen provides a measure of protection against malaria. The percentage of people lacking the Duffy antigen is much higher in these locations than in areas not endemic for malaria.
Animals and bacteria have cell surface antigens referred to as a blood type. Antigens from the human ABO blood group system are also found in apes and old world monkeys, which have inherited the same system. Other animal blood sometimes agglutinates (to varying levels of intensity) with human blood group reagents, but the structure of the blood group antigens in animals is not always identical to those typically found in humans. The classification of most animal blood groups therefore uses different blood typing systems to those used for classification of human blood.
Blood group systems occur where there are multiple alleles of a gene which produces polymorphism in a surface component of the erythrocyte such that the different forms of the component display different antigenicities. This can cause haemolysis of transfused blood that has a different antigenicity to that of the erythrocytes of the recipient, whose immune system consequently regards it as foreign.
The major ABO blood group system of humans involves the gene for a glycosyltransferase which catalyses the addition of particular sugars to glycocoproteins or glycolipids. In addition to inactivation of the gene (giving blood group O in homozygotes) there are two forms of the active gene, encoding enzymes transfering either N-acetyl galactosamine or galactose (1):
The ABO system is found in non-human primates, although with AO, BO and AB as variants in different species (2). Various other animals have polymorphism in this system — pigs, for example, are AO (3). Most animals possess the ABO gene (4), although it may or may not be expressed, and, if it is, may or may not give rise to polymorphism.
Blood group systems exist in other animals that involve different genes. The confusingly-named AB system of cats, for example, involves the gene for an acetylneuraminic acid hydroxylase, which catalyses the formation of the activated precursor for addition of N-glycolylneuraminic acid to cell-surface glycans (5). This gene is inactive in humans (6).
Not all such blood group systems involve changes in the sugars of glycoproteins or glycolipids, but those involving mutations in proteins (e.g. in dogs (7)) tend to be less well characterized at a molecular level.
A common answer to the question “Why have we evolved these [ABO] blood types?” is that the ability to change the sugars on the erythrocyte membrane provides resistance to pathogenic organisms (parasites, bacteria and viruses). This could be because the pathogens bind the sugars to gain entry into the cells, or because they have similar sugars on their own cell surface and so do not provoke an immune response from the host. However the counter-argument of non-functional genetic drift (@CactusWoman) is difficult to exclude if one focuses solely on the ABO system.
The functional argument becomes more convincing if one considers that the ABO blood types are just one example of the ability of animals to vary the sugars on their cell by selecting from a large repertoire of genes with the potential to specify different glycosylating enzymes (8). This is shown by the fact that different animals exhibit different sugar antigens on the surface of their erythrocytes (whether or not they display polymorphism). There are over 100 genes for glycosylating enzymes, but one example of this diversity is instructive. The ABO gene has undergone duplication and, of perhaps seven members of this family (9), the products of three are well characterized (10,11,12), although their genes are either inactive in humans or not expressed in erythrocytes (i.e. in their precursors). Like the ABO gene product, the enzymes are α-1,3-glycosyltransferases, but differ in the acceptor carbohydrate:
To answer your first question; it is not necessary that every trait have an evolutionary advantage. Neutral traits can persist in the population without having any adaptive value, or they can persist as a byproduct of another trait that does have adaptive value. Even a slightly detrimental trait can become fixed due to genetic drift. Take a look at this rather famous paper critiquing the "adaptionist programme" if you're interested.
That being said, I personally don't know for sure whether or not different blood groups have an adaptive advantage. Someone else can probably provide a better answer to this portion of your question.
However, to answer your second question: yes, yes they do. Indeed the Rhesus blood typing system was named after the rhesus monkey.