5. Genetic Drift:
random fluctuations in allele  frequencies due to chance events

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a. founder effect: a small group of members of a population not possessing the same/original allele frequencies establish a new population.  

b. bottleneck: when a natural disasters greatly reduces the size of a population. Random, surviving individuals exhibit as a group different allele frequencies then the population prior to the disaster. 

          let's look again at what causes allele frequencies to change...

let's explore in today's lab how allele frequencies may change due to chance...
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                populations, evolution and 

            the Hardy Weinberg equilibrium

4. non random (assortative) mating:
occurs when mate selection is influenced by differences in phenotypes/genotypes of potential mates

                 what do you think the likelihood would be that:
      1. members of a population did not experience any mutations in their DNA
      2. members of a population selected their mates completely at random
      3. a population had an infinite number of mating individuals 
​      4. a population did not experience immigration OR emigration
      5. a population was not subjected to natural selection 
                                                            ????????????

Let's try a sample problem to see if we can determine the change in allele frequencies in a population over time...
In bears, the allele that controls coat color exhibits two alleles.
​Black fur is dominant over brown fur. 
In 1960, a population of 500 bears contained 100 brown bears. By 2010, after the encroachment of humans, that population had been reduced to only 80 bears with 5 bears exhibiting the brown phenotype.     

3. mutation:
changes in DNA sequences may lead
to novel phenotypes

2. gene flow:
migration either in or out of a population may alter the frequencies of alleles

                  1. natural selection:
     nature determining the fitness of a                              phenotype

                if these conditions were met, then the allele frequencies in the population would remain constant...
let's say that a population of unicorns exhibited two alleles for a gene
that controlled their horn color:
H coded for silver horns
h coded for gold horns
The frequency of the "A" (silver) allele
and the frequency of the "a" (gold) allele combined would equal 100%.
So if 60% of the alleles for that gene were A,
the a allele would account for 40% of the alleles in the population for that gene. 
60%+ 40% = 100%
or
.6 + .4 = 1

what about the frequency of individuals in that population that exhibit  
either the gold or silver phenotype?

the silver phenotype could be either AA or Aa
homozygous dominant   AA  or .6 x .6 = .36            36%
        heterozygous As  or  2(.6 x .4) = .48                  48%

the gold phenotype could only be 
or homozygous recessive  aa   .4   x  .4  =  .16           16% 
                                                                                         _______
                                                                                          100%
Do you think these numbers would remain constant in a real population?
If not, then microevolution would be occurring...
a change in the frequency of alleles in a population over time.