Module 8 Lesson 3 - 2

Five Conditions of Evolutionary Change

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Read pages 689 - 695

What mechanisms lead to gene pool change and micro-evolution? If the conditions of Hardy-Weinberg equilibrium are the conditions that keep allele frequencies from changing and prevent evolution, then the reverse of the list suggests the conditions that lead to changing allele frequencies and evolution.

The conditions that lead to microevolution include the following:

1. An open population with gene flow in and out through immigration and emigration


2. A small population whose frequencies can be affected greatly by chance events (genetic drift)


3. Mate selection - choosing mates on the basis of preferred genotypes or phenotypes


4. Mutation - creation of either new alleles or switching from one allele to the other


5. Natural selection - in a given environment, individuals with certain alleles are better suited for survival and reproduction than others are

Therefore, these five mechanisms for changes in allele frequencies are the tools used for species survival on an ever-changing earth.

Gene Flow

The gene flow that results from immigration and emigration decreases variation between populations, but it  increases diversity within populations. If the organisms new to the population are able to survive and adapt to the new environment, they could mate successfully and introduce new genes to the existing population.

Genetic Drift

Genetic drift occurs when small populations experience major genetic change due to chance events. Because smaller populations are more susceptible to genetic drift, a population must be large to be in Hardy-Weinberg equilibrium. Genetic drift occurs when sub-populations become isolated either by migration (founder effect) or natural disaster (bottleneck effect). If genes from a smaller subset of population are passed to the next generation, the allele frequencies of the next generation will vary greatly from the parental generation.

• The founder effect is a change in allele frequencies due to the separation or the migration of a small population that is not genetically representative of the original population. Darwin's finches are an example of founder effect. A small number of finches arrived in each isolated island and became founders of a new population. This led to about fifteen species of finches living in the Galapagos Islands. The Amish population in the United States with Ellis-Van Crevald syndrome is another example of founder effect. 


• The bottleneck is similar to the founder effect in that the new population is not representative of the original population.  However, the separation is due to a catastrophe such as drought or an epidemic, leaving few survivors. Unless the survivors had selective advantage against the changes in the environment, their genetic makeup is random and will not represent the gene pool of the original population.
  

Both result in the increased frequency of a rare allele but a loss of genetic variability. The controlled breeding in agriculture (crops and livestock) purposely sets conditions of genetic drift to increase the frequency of rare but desirable alleles.

Mate Selection

In most species, the process of mate selection is anything but random. The complex courtship rituals, dances, and displays of animals result in sexual selection. If one phenotype or genotype is very popular, the frequency of the involved alleles increases, and thus diversity decreases.

Inbreeding is a type of non-random mate selection. Although inbreeding does not change the allele frequencies within a population, it increases the frequency of homozygous recessive individuals. The offspring will be more likely to have two copies of the recessive alleles associated with genetic disorders.

Mutation

Random copying and transcription-translation errors commonly cause dominant alleles to mutate to recessive alleles and vice versa. In a previous lesson, you learned that spontaneous mutation rates are very low. Because most environments change constantly, for a population to accumulate enough mutation to yield a micro-evolution is difficult. However, mutation provides genetic variation within a population and makes possible evolution. If the environment changed to favour a specific mutation, the population with the mutated gene has selective advantage for survival and change of the gene pool of subsequent generations.

For example, warfarin is an anticoagulant used as rat poison to control rat populations in the 1950s. A small number of rats already had a mutation that made them resistant to warfarin; this gave them a selective advantage. These rats survived the warfarin applications and reproduced. Subsequent generations of rats inherited the warfarin-resistant gene. Currently, warfarin is no longer used as rat poison because most of the rat population has developed warfarin resistance.

Natural Selection

Natural selection is entirely dependent on the environment. Those with suitable alleles survive to reproduce and pass their genes to the next generation. For natural selection to occur and to result in evolution, the genetic variation or mutation must pre-exist in the population. If competition is fierce under conditions of rapid environmental change, the frequency of favoured alleles will increase because those individuals will be more successful in producing offspring. The individuals with the genetic variation will pass their traits to the next generation.

Remember that natural selection does not always result in evolution. Natural selection is one of many processes that can lead to an evolutionary change. Evolution is the result of the natural selection process.