POLYMOPHISM, BALANCE AND NON- BALANCE POLYMORPHISM

POLYMORPHISM : WHEN TWO OR MORE CLEARLY DIFFERENT PHENOTYPES EXIST IN THE SAME POPULATION OF A SPECIES.

IN OTHER WORDS, THE OCCURRENCE OF MORE THAN ONE FORM OR MORPH.

DARK-DARK-MORPH OR MORPH OR MELANISTIC MELANISTIC JAGUARJAGUAR

Light-morph Light-morph Jaguar (typical) Jaguar (typical)

POLYMORPHISM RESULTS FROM EVOLUTIONARY PROCESSES, AS DOES ANY ASPECT OF A SPECIES.

POLYMORPHISM IS HERITABLE, AND IS MODIFIED BY NATURAL SELECTION.

POLYMORPHISM AS DESCRIBED HERE INVOLVES MORPHS OF THE PHENOTYPE.THE TERM IS ALSO USED SOMEWHAT DIFFERENTLY BY MOLECULAR BIOLOGISTS TO DESCRIBE CERTAIN POINT MUTATIONS IN THE GENOTYPE.

POLYMORPHISM DEALS WITH FORMS IN WHICH THE VARIATION IS DISCRETE OR STRONGLY BIMODAL OR POLYMODAL.

THE SIGNIFICANCE OF GEOGRAPHICAL VARIATION IS IN THAT IT MAY LEAD TO ALLOPATRIC SPECIATION, WHEREAS TRUE POLYMORPHISM TAKES PLACE IN PANMICTIC POPULATIONS.

WITHIN POPULATION SEX-LINKED VISIBLE POLYMORPHISM

Stag beetles, Lucanus cervus

THE RELATIVELY RECENT ORIGIN OF SEX CHROMOSOMES IN THE PLANT GENUS SILENE PROVIDES AN OPPORTUNITY TO STUDY THE EARLY STAGES OF SEX CHROMOSOME EVOLUTION AND, POTENTIALLY, TO TEST BETWEEN

THE DIFFERENT POPULATION GENETIC PROCESSES LIKELY TO OPERATE IN NON RECOMBINING CHROMOSOMES SUCH AS Y CHROMOSOMES.

BALANCING POLYMORPHISM

The ability of natural selection to maintain stable frequencies of at least two phenotypes.

Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygote for the alleles under consideration have a higher adaptive value than the homozygote. In this way genetic polymorphism is conserved.

There are three main types of natural selection: In directional selection the allele frequency for a trait continuously shifts in one direction. In stabilizing selection the frequency of the alleles of lower fitness decreases until they vanish.

Balancing selection is similar but not identical to disruptive selection where individuals of extreme trait values are favored against those with average trait values. These terms are used for quantitative characters controlled by a number of genes.