Microevolution

Microevolution is a change in a population's alleles over a period of time.

It is very difficult to detect changes that occur on the microscopic level. These changes must manifest themselves in the organisms phenotype. Since individuals do not evolve, one must keep a close eye on the individuals population to detect any change in genotypic modification. Biologists have a way to help them detect such changes; it is called the Hardy -Weinberg theorem.

Hardy-Weinberg Theorem:

This idea was developed to determine if a population was evolving. The authors of the theorem set up a series of parameters, which do not exist in nature, to be followed when determining the allele frequencies of any population. These guide lines are as follows:

  • The population must be very large in size.
  • It must be isolated from other populations. ( no gene flow)
  • No mutations.
  • Random mating.
  • No natural selection.

Since these characteristics cannot be eliminated from nature what is the purpose of these restrictions? To answer this question let us take a look at the mathematics behind this idea.

Let us take a look at a population with alleles for large and small ears. Let us use L for the dominant allele, large ears, and l for the recessive allele small ears. If we look at that population of people, we can determine the number of individuals with large and small ears. Since each group is a result of the following genotypes: LL, Ll, and ll, we have three groups of possible combinations. These alleles make up the gene pool for that trait. The Hardy -Weinberg theorem states that : p + q =1

p = the dominant allele

q = the recessive allele

1 = the gene pool of that trait.

So all the L alleles added to all the l alleles = 100% of the genes for that trait in that population.

If we expand that formula by multiplying it by itself we produce the expanded formula:

p2 + 2pq + q2 = 1

 

The p2 = the genotype LL, the 2pq = the genotype Ll, and the q2 = the genotype ll

If the large ear allele has a .8 frequency in the population, the l allele must have a .2 frequency since .2 + .8 = 1. If we substitute these frequencies into the expanded formula we can determine the percent of each of the three phenotypes in a given population.

p2 = .64 , 2pq = .32, and q2 = .04 = 1

--64%-
----32%
---------- 4%
----------= 100%

------ ------------

If we know the percent of the homozygous recessive organisms, we can take the square root of the decimal value and determine the frequency. With that value we can determine the frequency of the dominant allele by subtracting it from 1. So as one can see,the formula can be used if the % is given or the frequency.

This first calculation will act as a base and any changes that occur to the frequencies of further calculations may indicate that evolution is occurring in that population. Click here for the AP Lab 8 : Population Genetics and Evolution to develop this concept.

There are 5 basic causes of microevolution:

1. Genetic Drift: This represents random changes in small gene pools due to sampling errors in propagation of alleles. The bottleneck effect and founder effect are prime examples of genetic drift. In either case the number of individuals in a population is drastically reduced distorting the original allelic frequencies.

2. Gene Flow: The movement of alleles into and out of a gene pool. Migration of an organism into different areas can cause the allelic frequencies of that population to increase. Most populations are not isolated, which is contrary to the Hardy-Weinberg Theorem.

3. Mutations: These changes in the genome of an organism are an important source of natural selection.

4. Nonrandom mating: Inbreeding is a popular form of nonrandom mating. Individuals will mate more frequently with close individuals than more distant ones. Assortive mating, is another form of nonrandom mating. Here the individuals will mate with partners that closely resemble themselves in certain characteristics.

5. Natural Selection: Populations vary in the types of individuals and their reproductive success. Those individuals who leave more offspring behind than others, pass on more of their alleles and have a better success rate in dominating the population. Link to Darwin's books.