BIOL 112 Lecture 6

« previous | Friday, September 7, 2012 | next »

Chapter 22 Homework and Learning objectives are available on eLearning → Homework Folder

Hardy-Weinberg Principle

Hardy-Weinberg principle states that evolution is not taking place if allele/genotypic frequencies remain constant over time.

i.e. only Mendel's laws of random segregation and random mating are at work

for example, a deck of cards stays the same (all cards have same frequency) no matter how they are shuffled.

$\begin{array}{rl}p+q& =1\\ p^2+2pq+q^2& =1=(p+q{)}^2\end{array}$

Uses of Hardy-Weinberg

1. Test if a population is evolving over time:
   • Measure frequency of alleles at a certain time
   • Compare with allele frequencies at a later time
   • "expected" result is no change → no evolution
   • Observed should contradict this (Proof by contradiction)
2. Calculating frequency of allele or genotype based on limited information
   • Diploid; single locus/gene with 2 alleles represented by frequencies $p$ (typically dominant) and $q$ (typically recessive)
   • total freq = 100%, so $p+q=1$
   • possible combinations are $pp$, $pq$, $qp$, and $qq$, so $(p+q{)}^2=p^2+2pq+q^2=1$

Example

PKU is a recessive genetic mutation found in 1 out of 10,000 babies. how many individuals are carriers?

$\begin{array}{rl}{q}^2& =\frac{1}{10000}\\ q& =0.01\\ p=1-q& =0.99\\ 2pq& =0.0198\end{array}$

Therefore, 1.98% of population are carriers (heterozygous)

Microevolution

Microevolution are small changes over generations (gradualism):

$\sum _{i=1}^n\text{microevolutionary events}=\text{macroevolution}$ for large $n$

Caused by anything that changes allele frequencies:

1. natural selection: "survival of the fittest" / undesireable traits literally die out
2. genetic drift (finite population size): random fluctuations in allele frequencies due to chance events
   • affects small populations the most (see Figure 23.8) because of more sampling bias (like flipping a coin 10× vs. 1000×)
   • bottleneck effect: few individuals of population survive and new allele freq's aren't good representation of original gene pool. Genetic variability at most loci is severely reduced
   • founder effect: few individuals of population become isolated and form new population.
3. gene flow: Allele exchange between populations (loss or gain); reduces genetic diffirences between populations.
4. mutation
5. non-random mating: bias in mate selection

5 Fingers of Evolution

Changing allele frequencies reject Hardy-Weinberg "null hypothesis"