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

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

       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.           

            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

​
​
​
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...

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