Concepts of Biology (BIOL115) - Dr. S.G. Saupe (; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321

More Evolution:  Population Genetics

I.  Some definitions.

  1. What is an individual?  
        Seems obvious - a separate organism with a unique genetic composition.   But what about things like aspen clones or mushrooms?  They tend to make our view of individuals a little "muddy."

  2. What is a species?  
        There are two general criteria: (i) individuals that look alike belong the same species; morphological species concept); and (ii) individuals that can interbreed and produce fertile offspring, and that are isolated from other individuals, comprise a species (biological species concept).  Click here for more on speciation

  3. Populations and gene pools
        A population is a group of individuals of same species that occupy an area.  They share a common gene pool, which refers to the collective group of alleles in the individuals in the population.

  4. Macro- vs. micro-evolution  
        Macro = big; micro = little.  Thus, macroevolution would refer to large scale evolutionary changes - such as the formation of new groups above the species level.   Microevolution would be smaller scale changes - such as changes within species. 

  5. Evolution
        Recall that we defined evolution as changes in species composition over time.  Using the population and gene pool model, another way to look at evolution is that it is a change in allele frequency in a gene pool over time.

  6. Adaptation
        Features that make an organism suited for its environment (Or, the process by which a species becomes suited to its environment).

II.  Variation
     Refers to differences in traits observed between individuals of a population.  Genetic variation is the raw material for evolution.  The SOURCES of variation include: (a) mutation, which creates NEW forms of genes; (b) recombination as a result of sexual reproduction.  This is caused by recombination of chromosomes during fertilization, and crossing over and independent assortment during meiosis; and (c) abnormal chromosomes (number and structure).

  1. Graphical Expression of variation.
        Plot a frequency distribution for a trait (i.e., graph of  # individuals vs range of variation)

  2. Analyzing variation.
        Easily observed at the molecular level (i.e., electrophoresis, DNA fingerprinting)

III.  Evolution Revisited.  
    Recall that we defined evolution as a change in gene frequency in a gene pool over time.  In class, we'll use the following model to represent this process.                     ****  insert diagram ****

IV.  Cause of changes in allelic frequency over time.

  1. Mutation.
    Refers to changes in DNA sequence of genes.  The rate is low.  Mutation results in the introduction of new genes and is the ultimate source of new genetic information.       ****  insert diagram ****

  2. Migration or Gene flow.
    Refers to gene movement into (immigration) or out of (emigration) a population.      ****  insert diagram ****

  3. Genetic drift.
    Changes in the gene pool due to chance.  This is important in small populations.  For example, it would not be too surprising to flip a coin 10 times and observe 3 heads (30%) and 7 (70%) tails.  But, it would be surprising to observe 30% heads if we flipped the coin 1000 times.  Thus, we expect chance errors to occur more frequently in smaller populations.  Genetic drift is essentially due to a sampling error - the small population doesn't reflect the allelic frequency of the whole.    ****  insert diagram ****  

        Drift can result from:  

1.  A Genetic "Bottleneck".  
    The original population size is greatly reduced, the allelic frequencies of the remaining individuals do not reflect that of the original population.  Imagine a bottle filled with an equal mix of red and white marbles.  Now tip it over and pour some out.  The chances that frequency of red and white marbles that you poured out reflects the genetic composition of the whole is small.  Example:  Northern elephant seals were hunted to 20 individuals in 1890's.  Population recovered to 30,000, however in 24 gene loci tested, there is no genetic variation.  Normally, you expect much variation between individuals.

2.  "Founder Effect".  
    A small population establishes itself in a new site, and it is not representative of the whole.  Important in the Galapagos and islands in general.  For example, Tristan de Cunha, a small island group in the Atlantic betwen SA and Africa was settled by 15 British.  Of the 240 descendents, 4 had retinitis pigmentosis and 9 were carriers for this progressive form of blindness.  This is a much higher frequency than the population at large.

  1. Non-random mating
    Non-random mating results in allelic changes.  Non-random mating can result from:  (a) Inbreeding - will increase the frequency of homozygous recessives; (b) assortive mating - likes mate.     ****  insert diagram ****

  2. Natural Selection.
        The environment favors one phenotype; not all the phenotypes are of equal fitness.  (As an aside, when term FITNESS is used in an evolutionary sense, it doesn't refer to how often you work out at the gym; rather it indicates the ability to contribute offspring to the next generation - REPRODUCTIVE SUCCESS).  There are three major types of natural selections:

  • directional - favors one extreme, changing environment; ****  insert diagram ****

  • stabilizing - favors the intermediate, eliminates the extremes, stable environment; ****  insert diagram ****

  • disruptive - favors both extremes, changing environment     ****  insert diagram ****l

V.   Allelic frequencies
    Allelic frequencies remain constant in a population over time unless acted upon by mutation, natural selection, drift, non-random mating or migration (gene flow).  In the absence of these factors, the population is said to be at EQUILIBRIUM.  If this is the case, then there will not be evolutionary change, since we defined evolution as a change in allele frequency over time.

****  insert diagram ****

A.  Example.
Consider a plant with either red (dominant) or white (recessive) flowers that is determined by a single gene with two alleles (A, a).   Thus, there are three possible genotypes:  AA, Aa, and aa.  Let's further assume that we have a population of 500 plants and that there are 320 individuals with the genotype AA, 160 Aa and 20 aa.   

Question:  What is the frequency of each genotype?  Simply divide the number of individuals of a particular genotype by the total number of indivuals.  Thus, for our example: AA = 320/500 = 0.64; Aa = 160/500 = 0.32; aa = 20/500 = 0.04.

Question:  What is the frequency of each allele?  This is also a simple c alculation.  Let's consider the "A" allele first.  We know that each of the 320 homozygous dominant individuals have two "A" alleles, and each of the 160 heterozygotes have a single "A".  Since there are a total of 1000 alleles (500 individuals x 2 alleles), we can calculate the number of "A" alleles according to the following equation:  "A" = ((320 x 2) + 160)/(500 x 2) = 0.8.   Similarly, to calculate the frequency of the "a" allele:   a = ((20 x 2) + 160)/1000 = 0.2.

Question:  What will the frequency be in the next generation?  Do a good, ole fashioned Punnett Square - and viola, it's the same!  No surprise...that's what we predicted - a population in equilibrium will show no change in gene frequency.  ****  insert diagram ****

B.  Hardy-Weinberg (1908).  
    These two d
eveloped a mathematical model to express allelic frequencies in a population at equilibrium.   They used symbols p and q (mind your p's and q's) where p = frequency of the dominant allele and q = frequency of recessive allele.  Thus, p + q must be 1 (p+q=1 or q=1-p or p=1-q).  They used the symbol p2 to represent the frequency of homozygous dominant individuals, q2 for homozygous recessive, and 2pq to represent the heterozygotes (ignoring the math and logic behind this,  p2 + 2pq + q2 =1.).  The beauty of this relationship is that it provides us a way to determine allelic/genotypic frequencies.  

    Let's consider our red and white flowers again.  To determine the frequency of individuals we simply plug the individual allelic frequencies into the Hardy-Weinberg equation.  Recall our red and white flowers that have a frequency of 0.8 A alleles and 0.2 "a" alleles.  The frequency of the homozygous dominant genotype is p2  which equals = (0.8)2 = 0.64.  The frequency of the homozygous recessive is q2 which equals (0.2)2 = 0.04.  Thus the frequency of the heterozygotes = 2pq = 2(0.8)(0.2) = 0.32.  Ok, now let's see how much you learned.

Problem 1:  The frequency of phenylketonuria (PKU, a recessive disorder) is 1 in 10,000.  What is the frequency of the heterozygotes in this population?  Click here for answer  Problem 2:  A large population of field mice have a dominant allele for large ears, E, that occurs in the gene pool with a frequency of 0.6 and its recessive counterpart, e, has a frequency of 0.4.  (a) What are the frequencies of genotypes EE, Ee, and ee? and (b) If there is a population of 10,000 individuals, what is the actual number of each genotype?  Click here for the answer.
Problem 3:  A natural population of frogs consists of 96% brown and 4% green animals.  Skin color is due to a single gene with green (g) recessive to brown (G).  (a) What is the frequency of the dominant and recessive alleles?  (b) What is the frequency of heterozygous & homozygous dominant frogs?  Click here for the answer. Problem 4:  Create your own :-)

C.  If a population is NOT at equilibrium, then allelic frequencies, i.e., evolutionary changes, will occur over time.  When would a population deviate from Hardy-Weinberg (HW) equilibrium ?  A population will deviate, evolve, if there is:

  1. Mutation - the HW model assumes no mutation.

  2. Migration or Gene flow - the HW model assumes no gene flow; i.e., that the population is isolated from others.

  3. Genetic drift  - the HW model assumes a large population.

  4. Non-random mating - the HW model assumes random mating.

  5. Natural Selection - the HW model assumes no selection; i.e., there is equal fitness of all phenotypes.

VI.  Cats Delight.....(partly a rerun)
Let's return to our cat model.  Now, let's assume that we want to open a restaurant in St. Joe serving our specialty, roast cat, to throngs of hungry students and others.  Wow! Business is booming and so we franchise (reproduce) the operation.  Soon there are Cats Delight Restaurants all over the Midwest (and very few stray cats).  Our restaurant is certainly "fit".   

      We'll assume that the recipe is kept in a locked vault in the office of each restaurant.  When a franchise opens, we go to the vault and make a duplicate copy of the recipe book  to pass to the next restaurant.  The cooks in the kitchen use a photocopy to work from to put together the tasty dish .

       Now, let's imagine that in one of the restaurants,  there is a slight error in the recipe book - when it was duplicated the amount of salt was changed from one teaspoon to one tablespoon.  This would represent a mutation in the instructions.  Any franchises that arise from this one would also have increased levels of salt in the recipe.  There is now some variation, regular and extra-salty, in our population of restaurants.  [note:  to make our model more realistic we should probably have lots of restaurants all using different amounts of salt, a "bell-shaped" curve of restaurants]

      If the extra salty-roast cat tastes better than regular, these restaurants will do a booming business.  The original restaurants may fold.  In the meantime, the extra-salty will thrive spawning franchises here and there.  Thus, the frequency of the extra-salty recipe would increase and the regular decrease - viola, evolutionary change via natural selection.  The consumer is the selective agent that is acting upon the individual (restaurant).  Those restaurants that are most fit (salty recipe) will do better than those without, and hence, lead to more franchises.  Over time we'll see the loss of the regular version restaurant and all salty ones.  This is a good example of directional selection. 

       This could also serve as an example of stabilizing selection.  Imagine that we have our two varieties of restaurants and a population of consumers who are rather finicky, uniform and unchanging in their preference for food selection. We will find that the restaurants that serve the best-liked recipe will thrive to the detriment of others.  Thus, there would be selection for the intermedate or average type.  Can this model be adapted to fit disruptive selection?

    Time for supper!

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Last updated: December 02, 2004
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