Natural Selection and the Origin of Species
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Natural Selection and Sickle-Cell Anemia
| Genotype | Phenotype | Fitness |
|---|---|---|
| AA | 100% normal hemoglobin | Somewhat reduced fitness because of no resistance to malaria |
| AS | Enough normal hemoglobin to prevent sickle-cell anemia | Highest fitness because of resistance to malaria |
| SS | 100% abnormal hemoglobin, causing sickle-cell anemia | Greatly reduced fitness because of sickle-cell anemia |
This is how natural selection can keep a harmful allele in a gene pool.
- The allele (S) for sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation in the normal allele (A) for hemoglobin (a protein on red blood cells).
- Malaria is a deadly tropical disease.
- 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.
This example shows that fitness depends on phenotypes and also on the environment.
Three Ways Natural Selection Affects Phenotypes
- Stabilizing selection: Occurs when phenotypes at both extremes of the phenotypic range are selected against.
- Directional selection: Occurs when one of the two extremes is selected.
- Disruptive selection: Occurs when phenotypes in the middle of the range are selected against.
10.4 Origin of Species
A species is a group of organisms of the same species that can reproduce and create fertile offspring in nature. For a new species to arise, some members of the population must reproduce isolated from the rest of the species. But first, they must become geographically isolated.
Allopatric Speciation
If members of a species remain separated for too long from the original population, they may evolve genetic differences. If the differences prevent them from interbreeding with the original population, they have evolved into a new species.
Sympatric Speciation
This occurs when new species arise without geographic separation.
Coevolution
Species that are in symbiotic relationships tend to evolve together; when one species changes, the other must also change in order to adapt.
Timing of Evolution
- Gradualism: When geologic and climatic conditions are stable, evolution may occur gradually.
- Punctuated equilibrium: When geologic and climatic conditions change, evolution occurs more quickly. Long periods of little change may be interrupted by a quick change.