If the five aforementioned conditions aren't met, the population will change as a result of a change in the allele frequency.

Mutations, the first condition, must not occur to maintain genetic equilibrium (when a population doesn't change over time, but remains the same). A mutation is simply a change of genes, usually occurring during the DNA replication process, but sometimes due to radiation, electricity or toxic chemicals. Mutations can be extremely harmful, but can be beneficial as well; without mutations, new species would never evolve. The British naturalist Charles Darwin proposed that natural selection (survival of the fittest) favors mutations that make organisms stronger. Mutations, however, are rare.

If a population dwindles in size, the gene pool will be greatly affected because larger populations (with respectively larger and more diverse gene pools) are stronger. Smaller gene pools are more susceptible to alteration because of random events. A prime example of such alteration is the founder effect which occurs when a population is founded by only a few individuals. This effect is responsible for the high number of cases of Huntington's disease in western Ohio and southern Indiana where one of the original pioneering families carried the disease. It's also responsible for the number of seven-toed cats in New England, where one of the original population members was a mutant with seven toes. Another danger related to small populations is inbreeding. If two relatives mate, no new genetic material is introduced and the resulting offspring can suffer from physical deformities, a weak immune system, and even retardation among other things. For this reason, it is against the law for close relatives (such as siblings, first and second cousins, parents and children, etc.) to marry in the United States.

Mating must be random for genetic equilibrium to exist. This is usually impossible, though (especially with humans!), because some mates are more desirable than others. Courting rituals and appearance and smell are major factors. If mating is non-random, the allele frequency will change to reflect the new patterns.

If organisms leave or enter a population, equilibrium will be lost. Most animals display some sort of migration which causes the size and diversity of the population to constantly change.

No gene can have more significance over another (not dominant/recessive alleles, but genes); otherwise, this, too, will cause equilibrium damage. Sometimes, a single gene can cause damage or even death to an individual in a population. This gene can be transmitted through the generations to inflict many of the members. Hemophilia is a common damaging gene. In theory, harmful genes should disappear from the population over time because they are against the laws of natural selection and are not favorable to the organism.

Three generations of extra toed cats

Posted by permission Mark Parisi