Why are populations polymorphic

In homogeneous populations the vast majority there is no seasonal variation in the flowering times of white-flowered bushes and pink-flowered bushes. Some of these triple polymorphic populations show heterogeneity of population structure, with seasonal differences of flowering time in the different morphs.

Polymorphic populations are generally intermediate between monomorphic populations of the different races of E. It is likely that they are of hybrid origin.

The hypothesis that polymorphic populations form a base for disruptive selection into monomorphic populations is also discussed. Southern hemisphere botanical ecosystems. Shopping Cart: empty. Search our journals. Eventually the entire population may become homozygous for the allele or - equally likely - the allele may disappear.

Before either of these fates occurs, the allele represents a polymorphism. Two examples of reduced polymorphism because of genetic drift:. The varying number of copies of the AMY1 gene in different human populations appears to have arisen from the evolutionary pressure of the differences in the starch content of their diet.

In regions of the world e. This is because children who inherit one gene for the "normal" beta chain of hemoglobin and one sickle gene are more likely to survive than either homozygote. Children homozygous for the sickle allele die young from sickle-cell disease but children homozygous for the "normal" beta chain are more susceptible to illness and death from falciparum malaria than are heterozygotes.

Hence the relatively high frequency of the allele in malarial regions. When natural selection favors heterozygotes over both homozygotes, the result is balanced polymorphism. It accounts for the persistence of an allele even though it is deleterious when homozygous.

All human populations are polymorphic for the prion protein PrP C. It is encoded by the prion protein gene PRNP. Two of the alleles have different codons at position - one encoding methionine ; the other valine.

Homozygosity for either allele increases the susceptibility to prion diseases. People who are heterozygous are more resistant.

A study of elderly women who had survived the kuru epidemic of the first half of the 20th century eating the tissues of the deceased was banned in showed that This table compares the gene frequencies in this population as well as in a population that never practiced mortuary feasts. A quick calculation will show that the gene pool of the exposed women deviates widely from what would be found if the population were in Hardy-Weinberg equilibrium. In this case, strong mortality selection is the cause.

The gene pool of the unexposed population is close to being in Hardy-Weinberg equilibrium. Here, again, natural selection has favored heterozygotes over both homozygotes and led to the speculation that cannibalism may have been common earlier in human history.

Natural vs. Sexual Selection: Balanced polymorphism in Soay sheep. Hirta is a tiny island in the North Atlantic miles off the northwest coast of Scotland.

In a small population of domestic sheep Ovis aries was introduced onto the island from the neighboring island of Soay. Since then these sheep have been allowed to run wild and, since , have been intensively studied.

The sheep have horns and, in males, these play an important role in competition for females. You can read about these findings in Johnston, Susan. John W. This content is distributed under a Creative Commons Attribution 3. Protein Polymorphisms All the examples above are of the protein products of alleles. Figure Courtesy of Susan McAlpine. The 4 alleles can be distinguished by the speed with which their protein product migrates: Fast F , moderately fast E , medium M , and slow S Electrophoresis of tissue extracts from 15 different green treefrogs Hyla cinerea reveals 4 allelic versions of the enzyme aconitase one of the enzymes of the citric acid cycle.

The results: Eight frogs 2, 3, 4, 6, 7, 9, 12, and 14 were homozygous for allele M. Frog 8 was homozygous for allele E. Three frogs 1, 11, 15 are heterozygous for the M and S alleles.

Two 5, 13 were heterozygous for M and E. Frog 10 was heterozygous for M and F. Single Nucleotide Polymorphisms SNPs Developments in DNA sequencing now make it easy to look for allelic versions of a gene by sequencing samples of the gene taken from different members of a population or from a heterozygous individual.

Populations whose diet is rich in starches e. Populations with low-starch diets e. How are polymorphisms useful? In population studies, it is used to assess the degree of genetic diversity in a population, including: The McAlpine study, which produced the photo above, found that the heterozygous frogs were more successful breeders than homozygous ones.

A search for polymorphisms in elephant seals and cheetahs has revealed that they have few or none. Determining whether two populations represent separate species or races of the same species.

This is often critical to applying laws protecting endangered species. Tracking migration patterns of a species e. How do polymorphisms arise and persist? Founder Effect: If a population began with a few individuals — one or more of whom carried a particular allele — that allele may come to be represented in many of the descendants. In the s Ariaantje and Gerrit Jansz emigrated from Holland to South Africa, one of them bringing along an allele for the mild metabolic disease porphyria. Today more than South Africans carry this allele and, in every case examined, can trace it back to this couple — a remarkable example of the founder effect.

Genetic Drift: An allele may increase - or decrease - in frequency simply through chance.