If Darwin is right, and natural selection drives evolution, then why has natural selection not acted against the defective allele in Africa and eliminated it from the human population there? Why is this potentially fatal allele instead very common there?
The defective allele has not been eliminated from Central Africa because people who are heterozygous for the sickle-cell allele are much less susceptible to malaria, one of the leading causes of death in Central Africa.
Examine the maps in figure The map on the right indicates the distribution of malaria in dark orange. Clearly, the areas that are colored in darker green on the left map overlap many of the dark orange areas in the map on the right.
Even though the population pays a high price—the many individuals in each generation who are homozygous for the sickle-cell allele die—the deaths are far fewer than would occur due to malaria if the heterozygous individuals were not malaria resistant.
Similar inheritance patterns of the sickle-cell allele are found in other countries frequently exposed to malaria, such as areas around the Mediterranean, India, and Indonesia.
Natural selection has favored the sickle-cell allele in Central Africa and other areas hit by malaria because the payoff in survival of heterozygotes more than makes up for the price in death of homozygotes. This phenomenon is an example of heterozygote advantage.
Sickle cell disease SCD is a major cause of death in young children in Africa. It is a recessive hereditary disease, meaning that sufferers have inherited the mutated form of the gene from both parents. The mutation causes red blood cells to take on an abnormal sickle like shape, and sufferers of the disease may experience severe infections, attacks of acute pain and stroke.
They also have an increased risk of death. Despite the existence of vaccines and therapeutics to manage the condition, , people died of the disease in Carriers have inherited the mutated form of the gene from only one parent and are said to carry the sickle cell trait SCT , but do not suffer from the disease.
Natural Selection and Polygenic Traits Sickle-cell trait is controlled by a single gene. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive.
This keeps birth weight within a relatively narrow range. Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. Disruptive selection occurs when phenotypes in the middle of the range are selected against.
This results in two overlapping phenotypes, one at each end of the distribution. An example is sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. Population bottleneck. Species temporarily reduced to very low number lose genetic diversity. Examples: cheetahs--low population during Pleistocene; elephant seals--hunted to near extinction during 19th century. Founder effect. Populations founded by just a few individuals have unusual gene frequencies.
Founder effect may start a new population with unusual gene frequencies which become the basis of new adaptations. Bottleneck causes reduced genetic diversity. For neutral alleles , genetic drift occurs in all populations and species. As a consequence, separated populations and species accumulate genetic differences. Gene flow means the movement of individual organisms from one population to another, or simply the movement of gametes e. Gene flow brings the gene frequencies of adjacent populations closer together.
Gene flow has the opposite effect of the founder effect: if it occurs, it prevents the accumulation of genetic differences. If it occurs , gene flow keeps adjacent populations tied together. If populations are to separate enough to be considered separate species, there must be barriers to prevent any significant gene flow. Mutations are spontaneous changes in the genetic material. These changes include:. Point mutations : changes in a single base pair in the DNA. Chromosomal changes : duplication, deletion, inversion, translocation.
Mutations introduce new alleles. Usually , the new alleles are deleterious. Some few , in a new environmental context, turn out to be beneficial. Some chromosomal mutations e. The Hardy-Weinberg principle assumes random mating : mate selection without regard to genotype.
Non-random mating means that mate selection is influenced by phenotypic differences based on underlying genotypic differences. In some species, males acquire harems and monopolize females. Elk, elephant seals, horses, lions, etc. Commonly, the males of such species are much larger than the females. In some species, females choose more attractive mates.
Peacocks, Woodducks, Picture-wing fruit flies, etc. Significance of non-random mating. Sexual dimorphism conspicuous differences between the two sexes result from non-random mating.
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