POPULATION GENETICS

Our previous lecture and lab work on the biological basis of life and principles of inheritance focused on the genetic makeup of individuals and the transmission of genetic material from one individual to offspring.  Our focus now turns to the genetic properties of groups of individuals, or populations, which is the unit of analysis in modern evolutionary biology and biological anthropology.

A population is a group of exclusively and randomly interbreeding individuals of a species (so inheritance is presumed to be occurring).  Populations are also called breeding populations, local populations, Mendelian populations, or demes.  A species is a combination of one or more populations that can interbreed and produce viable offspring.

There are two important characteristics of a population: panmixia and endogamy.  Panmixia means that mates in a population are selected randomly, with respect to genotype.  Endogamy refers to mating only within the population.  So populations are, by definition, panmictic and endogamous.  If individuals of a group do not mate randomly or mate outside the group, that group of individuals is not a population.

Evolutionary change in a population occurs if there are genetic changes from one generation to the next.  This type of change is measured by changes in allele frequencies between generations. Evolutionary change that occurs on this scale is called microevolution and is distinguished from evolutionary change that leads to the development of new species (macroevolution) or the development of higher taxa like genera or families (megaevolution).  The study of modern human biological variation is concerned only with microevolutionary change.  Paleoanthropology deals with macro- and megaevolution based largely on the fossil record.

Microevolutionary studies seek to understand how and why changes in the genetic makeup of a population occur as well as the results of the genetic changes. The "how" refers to the principles of inheritance.  The "why" refers to the four forces of evolution (mutation, gene flow, random genetic drift, natural selection). The "results" refers to the genetic composition of the population.

There are three ways to measure the genetic composition of a population: gene pool, genotype proportions, and allele (gene) frequencies.  In class we studied how genotype proportions and allele frequencies are calculated.

• The gene pool is the frequencies of all alleles for all traits in a population.  Though this is the most complete way to describe the genetic makeup of a popution, it is very difficult to measure.

• Genotype proportions are the frequencies or proportions of individuals with each genotype for an individual trait in a population.

• Allele frequencies (gene frequencies) are the frequencies or proportions of all alleles for an individual trait in a population.  The symbol p refers to the proportion of the dominant allele and the symbol q refers to the proportion of the recessive allele (for a discrete trait with two alleles). In a three-allele system, p and q stand for the codominant alleles and r represents the recessive allele.
  

HARDY-WEINBERG THEOREM

Scientists determine if a population is microevolving or not by using the Hardy-Weinberg theorem. The theorem was developed independently by Godfrey H. Hardy (English mathematician) and Wilhelm Weinberg (German physician) in 1908.

A Hardy-Weinberg population is one in which there are no changes in allele frequencies or genotype proportions over time; therefore, there is no microevolutionary change.  Hardy-Weinberg equilibrium refers to the state of stasis or no change in the allele frequencies and genotype proportions of a population.  The Hardy-Weinberg law states that "the frequencies of p and q will remain the same throughout any number of generations given a stable, random-breeding population isolated from other populations" (Molnar 2002:58).

Three conditions must be met for a population to be in equilibrium:  (1) there is an infinitely large population; (2) the population is panmictic and endogamous; and (3) no forces of evolution are operating.

The Hardy-Weinberg theorem is used to determine if a population is microevolving or in equilibrium. This mathematical theorem is

 p² + 2pq +q² = 1

for a discrete trait with two alleles, where p² is the proportion or frequency of homozygote dominants, 2pq is the proportion or frequency of heterozygotes, and q² is the proportion or frequency of homozygote recessives;

or, the theorem is

 p² + 2pq + q² + 2pr + 2qr + r² = 1

for a discrete trait with three alleles, where r is the third allele, p² is the proportion or frequency of p homozygotes, 2pq is the proportion or frequency of p-q heterozygotes, q² is the proportion or frequency of q homozygotes, 2pr is the proportion or frequency of p-r heterozygotes, 2qr is the proportion or frequency of q-r heterozygotes, and r² is the proportion of r homozygotes.

The Hardy-Weinberg theorem is used in these three ways:

1.  It allows one to determine if a population is in equilibrium or evolving (does it diverge from the expected equilibrium condition predicted by the theorem?).

2.  The way a population deviates from the equilibrium condition may indicate which evolutionary forces are operating.

3.  It allows one to predict what will happen in terms of genetic change in a population.

OBJECTIVES

Upon successful completion of this lab, students will know how to

• calculate genotype proportions of a population.

• calculate allele frequencies of a population.

• apply the Hardy-Weinberg theorem.

• evaluate whether or not a population is in Hardy-Weinberg equilibrium.

• explain why a population deviates from Hardy-Weinberg equilibrium.

READINGS

Read pages 25-46 [except section on DNA typing] in France (2004).  Be sure to bring this material to the lab or you will be unable to complete the assignment.

Review Chapter 3 in the Mielke, Konigsberg, and Relethford text book

Review the Population Genetics lecture notes and handouts; bring the handouts to the lab sessions.
 

ASSIGNMENT

• Record all answers clearly and legibly in pencil on the answer sheet provided by the instructor.
  

• Complete all of Exercise 1.6 except question #5.
  

• Clearly label p, q and r and clearly label different genotypes (ex. TT, Tt, tt).

• All values for p, q and r and for genotype proportions must be expressed as decimals rounded to hundredths place. One exception is that the answers to question 6.c. should be rounded to thousandths place.
  

• You must show all of your calculations.

• For question 7, answer an additional question numbered 7.e. if you find a statistically significant difference between expected and observed genotype proportions: What possible explanations are there for the difference in expected and observed genotype proportions?

• For question 8, answer an additional question numbered 8.f. if you find a statistically significant difference between expected and observed genotype proportions: What possible explanations are there for the difference in expected and observed genotype proportions? Also note that question 8.e. concerns observed allele frequencies.

• Complete questions #1 through 7 in Exercise 1.7

• Clearly label p, q and r and clearly label different genotypes (ex. TT, Tt, tt).
  

• All values for p, q and r and genotype proportions must be expressed as decimals rounded to hundredths place.
  

• You must show all of your calculations.
  

• For question 1, answer an additional question numbered 1.b. if you find a statistically significant difference: What possible explanations are there for the difference in genotype proportions?

• Your answer to 7.b. should be as specific as possible.
  


• The lab is due at the beginning of class on Wednesday, October 31.
  

GLOSSARY

allele - alternate forms of a gene

allele frequencies - the frequency of each allele for a trait in a population

assortative mating - mating with non-randomly selected individuals

discrete trait - a trait controlled by one gene locus (a.k.a., simple trait, monogenic trait)

dominant - an allele that masks phenotypically other allele(s)

endogamy (endogamous) - mating within a population

gene - a portion of a chromosome that carries the genetic material coding for a trait(s)

gene flow - the exchange of genetic material from one population to another

gene frequencies - the frequency of genes (really, alleles) for a trait in a population

genotype - the genetic makeup of an individual, usually considered one trait at a time

genotype proportions - the proportions of different genotypes present in a population

Hardy-Weinberg equilibrium - the state of non-evolution in a population

Hardy-Weinberg law - states the relationship between genotype frequencies and evolutionary change

Hardy-Weinberg population - an infinitely large, panmictic, endogamous population on which evolutionary forces are not acting

Hardy-Weinberg theorem - the mathematical equation that expresses the relationship among/between genotype proportions in a population

heterozygote - an individual carrying two different alleles for a trait

homozygous dominant - an individual carrying two dominant alleles for a trait

homozygous recessive - an individual carrying two recessive alleles for a trait

macroevolution - evolutionary change resulting in the development of new species

megaevolution - evolutionary change resulting in the development of new super-species taxa

microevolution - evolutionary change resulting in changes in allele frequencies in a population over time

mutation - random alteration of genetic material resulting in the creation of new variation

negative assortative mating - mating with an individual who looks different

p - the frequency of one allele for a trait (the dominant allele in a two-allele system)

paleoanthropology - the multidisciplinary study of human origins and human evolution

panmixia (panmictic) - random mating with respect to genotype

population - a group of panmictic, endogamous interbreeding individuals of a species

positive assortative mating - mating with an individual who looks similar

q  - the frequency of one allele for a trait (the recessive allele in a two-allele system)

r - third allele in a three-allele system for a discrete trait

random genetic drift - microevolutionary changes in a population due to biological sampling

recessive - an allele that is masked phenotypically by another allele

species - an group of populations that can interbreed and produce viable offspring
 

REFERENCES

France, Diane L.
2004 Lab Manual and Workbook for Physical Anthropology (5th ed.). West/Wadsworth, Belmont, CA.

Mielke, James H., Lyle W. Konigsberg, and John H. Relethford
2006 Human Biological Variation. Oxford University Press, New York.

Molnar, Stephen
2002 Human Variation: Races, Types and Ethnic Groups (5th ed.). Prentice Hall, Upper Saddle River, NJ.