1. natural (or artificial) selection 
     nature (or humans) determining the                    fitness of a phenotype

       1. members of a population did not experience 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 never experienced immigration OR emigration
      5. a population was never subjected to natural selection 

   let's say that a population of unicorns exhibited two alleles

              for a gene that controlled their horn color.           

   allele frequency differences?
   _____ brown
   _____ yellow
   _____ red
   _____ orange
   _____ green
   _____ blue

            population genetics
  evolution and the Hardy Weinberg equilibrium

bottleneck effect

   vs   

  allele frequencies for
​  original population:

  30% or  .30 brown
  20% or  .20 yellow
  20% or  .20 red
  10% or  .10 orange
  10% or .10 green
  10% or .10 blue

                                                "A" coded for silver horns
                                                "a" coded for gold horns
                Frequency refers to what % a given allele represents for that gene.
                                    The frequency of the "A" (silver) allele
                                  and the frequency of the "a" (gold) allele
                    combined would equal 100% since all of the alleles for that gene
                    must add up to 100% of the alleles and this gene has two alleles.

                                 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%
​                                                        A + a = 100%
                                                                 or
                                                           .6 + .4 = 1

         what about the frequency of individuals in that population that exhibit  
                                  either the gold or silver phenotype?
                                                 We follow the equation:
                                                  A²   +  2Aa  +  a²   =   1  

                          the silver phenotype could be either AA or Aa
            homozygous dominant   AA  or .6 x .6 = .36            so 36%   =     A²   
                             heterozygous    Aa  or  2(.6 x .4) = .48      so 48%   =    Aa

                                      the gold phenotype could only be 
            homozygous recessive     aa   .4   x  .4  =  .16            so  16%   =   a²
                                                                                                       _________
                                                                                                       100%     = all unicorns
     
           Do you think these numbers would remain constant in a real population?
                        If not, then microevolution would be occurring...
     microevolution = a change in the frequency of alleles in a population over time. 

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

founder effect

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

                                     we'll try a sample problem:
​ A population of cats exhibits black or white phenotypes;
 the black allele (A) is dominant over the white allele (a).
 Given a population of 1,000 cats, with 840 black and 160 white, determine:
1. the allele frequencies
2. the frequency of individuals per genotype
3. the number of individuals per genotype.
We'll use the formula:  A²   +  2Aa  +  a²   =   1  

                      This formula was derived from (A + a)(A + a) = A²   +  2Aa  +  a² 

Step 1:
The frequency of white cats = a² = 160 cats of a total 1000 cats = 160/1000 = 0.16

Step 2:
To find the allele frequency = a, take the square root of a² so √(a²) = √(0.16) = 0.4

Step 3:
Since A + a must equal 1, and a = .4    then A + .4 = 1
                                                                              A = 1 - .4                                                                                                                                                              A = .6
So the frequency of the dominant allele = .6 when the frequency of the recessive allele = .4

Step 4:
Now that the allele frequencies in the population are known, solve for the remaining frequency of individuals by using the formula A²   +  2Aa  +  a²   =   1  
1. Square A to find the percent of homozygous individuals in the population
       A = .6        A²  = A x A  =  .6 x .6  = .36

2. Multiply A x a to find the percent of heterozygotes in this population:
     2( A x a) = 2(.6 x .4) = .48

Let's check out math by plugging these numbers into the Hardy Weinberg equation:
                                                              A²   +  2Aa  +  a²   =   1 

               all of the                               all of the                             all of the  
 homozygous dominant cats + heterozygous cats + homozygous recessive cats = 100%  = 1
                         .36                     +             0.48             +                      0.16                                     = 1

the math checks out..
​If none of the 5 influences described above alter these numbers, this population will not change at allllll.

Highly unlikely for sure!
so monitoring populations and recalculating their numbers indicates how a population is changing over single generations which is called microevolution.
 

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. 

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

          Heterozygosity is the cornerstone for a healthy population.

                  Reductions in heterozygosity reduces adaptability.
Population Biologists/Ecologists will monitor the heterozygosity of a               population to determine if it is in danger of extinction.  

           what is the liklihood that...

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

   if these conditions were met, then the allele frequencies in the population               would remain constant...and that population would never evolve