The answer is stabilizing selection.
<span>Sickle-cell anemia is a recessive disorder caused by the presence of two recessive alleles "s", so genotype is "ss". This disorder is characterized by sickle hemoglobin. In an area with malaria, heterozygous individuals "Ss" (with one dominant allele and one recessive allele) have an advantage. These individuals will have both normal and sickle hemoglobin. But pathogen that causes malaria affect only normal hemoglobin, so heterozygous individuals will have half of the hemoglobin resistant to the pathogen and those individuals are resistant to malaria.</span>
Stabilizing selection favors heterozygotes Ss, disruptive selection favors dominant (SS) and recessive (ss) homozygotes, while directional selection favors dominant (SS) or recessive (ss) homozygote. Since in this example, people with genotype Ss (heterozygotes) are in advantage, then this is an example of stabilizing selection.
It all depends on the trait. If the dominant is more common that the recessive trait then it is more likely to a dominant trait in the gene pool. But if the recessive trait is more common in the gene pool then you will have a recessive trait. Does this make sense? Let me know if I'm right!
The scientists who provided this hypothesis had an idea that the species evolved in order to accommodate their needs on certain islands. For instance, an example is beak size of certain birds which Darwin thought that this was to eat certain foods.
Bacteria are a prokaryotic cell, did it in class the other day
