The allele S for sickle cell anemia is a harmful, autosomal recessive allele. It is caused by a mutation in the normal allele A for hemoglobin the oxygen-carrying protein on red blood cells.
Malaria is a deadly tropical disease that is common in many African populations. Heterozygotes AS with the sickle cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. The sickle cell example shows that fitness depends on phenotypes and also on the environment. What do you think might happen if malaria were to be eliminated in an African population with a relatively high frequency of the S allele?
How might the fitness of the different genotypes change? How might this affect the frequency of the S allele? The sickle cell trait is controlled by a single gene. Natural selection for polygenic traits, which are controlled by multiple genes, is more complex, although it is less complicated if you consider just phenotypes for polygenic traits rather than genotypes.
There are three major ways that natural selection can affect the distribution of phenotypes for a polygenic trait. Recently reported research may help solve one of the most important and long-lasting mysteries of human biology. The mystery is why people with the AS genotype for sickle cell hemoglobin are protected from malaria. As you read above, their sickle cell hemoglobin gives them higher fitness in malaria areas than normal homozygotes AA who have only normal hemoglobin.
The malaria parasite and its mosquito vector were discovered in the late s. The genetic basis of sickle cell hemoglobin anemia and the resistance to malaria it confers were discovered around Since then, scientists have assumed, and some evidence has suggested, that the few sickle-shaped red blood cells of heterozygotes make them less hospitable hosts for the malaria parasite than the completely normal red blood cells of AA homozygotes.
This seems like a reasonable hypothesis, but is it the correct one? The new research suggests a different hypothesis. Working with genetically engineered mice as model organisms, researchers in Portugal discovered that an enzyme that produces the gas carbon monoxide is expressed at much higher levels in the presence of sickle cell hemoglobin than normal hemoglobin.
Furthermore, the gas seems to protect the infected host from developing the lesions and symptoms of malaria, even though it does not seem to interfere with the life cycle of the malaria parasite in red blood cells. These findings may lead to new therapies for treating malaria, which is still one of the most serious public health problems in the world.
The findings may also shed light on other abnormal hemoglobin variants that are known to protect against malaria. School Days Except for their plastic lunch coolers, you might think that this picture of children on their way to school came from the s.
Genes in Populations Individuals do not evolve because their genes do not change over time. Forces of Evolution The factors that cause allele frequencies to change are called the forces of evolution. Genetic Drift Genetic drift is a random change in allele frequencies that occurs in a small population. The bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire or disease epidemic. By chance, allele frequencies of the survivors may be different from those of the original population.
The founder effect occurs when a few individuals start or found a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left.
The Amish population in the U. The population has grown to almost , individuals who rarely interact with people outside the Amish community. One of the founders carried a recessive allele for a rare condition called Ellis-van Creveld syndrome - a type of dwarfism that results in extra fingers and short limbs as seen in this image.
Today the Amish population has far more cases of this syndrome than any other population in the world. Mutation Mutation creates new genetic variation in a gene pool. Gene Flow Gene flow occurs when individuals move into or out of a population.
Natural Selection Natural selection occurs when there are differences in fitness among members of a population. 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 a sexual dimorphism. The distribution of those phenotypes typically forms a normal distribution. The effect of the three types of natural selection have different effects on this normal distribution. When one extreme phenotype is favored by natural selection, the distribution of the phenotype shifts in that direction.
This is responsible for the increase in antibiotic resistant bacteria because the bacteria with the most resistance are most likely to survive when antibiotics are used. This is the most common type of selection because it is associated with the adaptation of an organism to the environment. This creates a increasing division within the population which may ultimately lead to two different phenotypes. The preservation of variation in a population is important because it provides a foundation on which natural selection can act.
Natural selection can only cause evolution if the different alleles produce different phenotypes. Because many organisms are diploid, heterozygotes are carriers of recessive alleles, preserving them in the population. If G is dominant over g, the genotypes GG and Gg would have an equal chance of survival. This keeps the g trait in the population even though it is not the good trait. In the case of the sickle cell allele, heterozygotes can be the most fit when malaria is prevalent because malaria is less severe in people who are carriers of the sickle cell allele.
Diversifying selection is the type that is most likely to result in a new species. True False. All Rights Reserved. Date last modified: July 12, Created with SoftChalk LessonBuilder. Directional Selection 2. Stabilizing Selection 3. There are a few basic ways in which microevolutionary change happens. Mutation, migration, genetic drift, and natural selection are all processes that can directly affect gene frequencies in a population.
Imagine that you observe an increase in the frequency of brown coloration genes and a decrease in the frequency of green coloration genes in a beetle population. Migration or gene flow Some beetles with brown genes immigrated from another population, or some beetles carrying green genes emigrated. Genetic drift When the beetles reproduced, just by random luck more brown genes than green genes ended up in the offspring.
0コメント