LAWS / RULES -

Dollo's Law                     :           Living organisms do exhibit evolutionary irreversibility or evolution is irreversible.

Williston's Law               :           During the evolution of a lineage, serially homologous parts tend to reduce in number but get more and more differentiated and advance in function.

e.g. Trilobites - were having large number of appendages and more or less identical in shape and structure.

Modern arthropods possess fewer appendages but specialized function.

In fish the maximum number of bones in the skull has been counted 150. whereas in mammals only about 28 bones are present.

Cope's rule                     :           Organisms have tendency towards increase in size during their evolution.

E.g., evolution of horse, camel, elephant and all other herbivorous mammals exhibit increase in size. Dinosaurs, Turtle and Tortoise.

Hardy-Weinberg's Law  :            The relative frequencies of various kinds of gene in a large and randomly mating population tend to remain constant from generation to generation in the absence of mutation, selection and gene flow, (i.e., factors affecting gene frequency).

Gloger's Rule                  :           Desert animals are heavily pigmented and are comparatively small in Size.

Rapoport's Rule              :           Species adapted to colder region have wider latitudinal distribution than the species adapted to warmed climates.

Jordon's Rule                 :           Fishes of colder water are larger in size with more vertebra then those living in warmer water.

Bergman's Law               :           Warm blooded animals are larger in size in the colder regions as compared to hotter parts.

Allen's Law                     :           Extremities of the body like tails and ears become smaller in colder areas.

Baldwin effect                 :           Natural selection acts to bring about adaptations, could arise as a direct response to the environment or to the needs of organisms.

Sewall - Wright effect     :           The random changes in gene frequency occurring by chance in small population and not under the control of natural selection are called Sewall-Wright effect.

Bottleneck effect              :           If small deme isolated and restricted in distribution are exposed to random genetic drift resulting in the fixation of certain genes. The changes occurring in the gene ratio is called bottleneck effect. If in this case the gene pool is already limited the population cannot regain its former richness, that may cause extinction.