BIOB51 – Lecture 9 Notes
Change of allele frequency over time.
When the assumptions are met, there’s no evolution occurring.
Important: Random Mating is occurring!
The reason why we measure allele frequencies and not genotype frequencies: so if we’re
thinking about our pile of shoes as our available alleles, we might pick two out, repair them all
and the amount of pairs that are white/white, black/white or black/black might change from one
generation to another. From one set of pairing time to another. But the number of black shoes
and the number of white shoes stays the same. When there`s no evolution, you always have the
same number of black shoes and the same number of white shoes.
We can see fluctuations in genotypes across generations even though no evolution has
occurred. Even though our allele frequencies are the same. Just due to the random chance of
pairing of individuals and the alleles they happen to have.
That’s why we can’t measure genotypes in each generation and look to see if something has
changed. That's why we have to measure allele frequencies because allele frequencies tell us
that something has actually changed.
That’s why we need Hardy Weiniburg to help us convert our gene frequencies into our allele
frequencies back into our gene frequencies back into our allele frequencies to look across
You can look at each and every sperm and each and every egg in the whole population. But this
is impossible because you`d be destroying them. So we cannot measure allele frequencies
directly but we can measure genotype frequencies directly.
Must know the answers to these questions to check with yourself if you understand.
Slide 12: We had a population and we applied a mutation rate to that population, A turning into a at a rate
of 1/10,000 individuals worth carrying this mutation.
Then we calculated, we had our initial frequencies in the population of A and a. We recalculated
them based on the mutation rate. So our new frequency of p was our old frequency minus the
mutation rate times p (p* = p - μp). It’s minus because we have A’s going into little a’s. For q, we
are adding so big A’s turning into small a’s.
Without mutation, evolution would not be possible. We need these new alleles/variances to
come forward/to constantly be generated.
If mutation creates a new allele, there are three directions it could go:
-Neutral: doesn’t change anything in the coding, doesn’t change the protein
-Beneficial: increase in fitness; favours mutant alleles, which will survive better, have more
offspring and will be more likely to produce offspring, be present in greater numbers and greater
frequencies in subsequent generations if it makes an increase in fitness
-Deleterious: decrease in fitness; selection for normal allele, which is less likely to put their
genes in the next generation, less likely to have offspring, their alleles/genes becomes less
represented in the next generation. That allele will be lost and not represented anymore by the
Started with a single cell of E.coli. Within this E.coli strain, there was no recombination, which
means that mutation is the only source of genetic variation. Any changes we know are occurring
is because of mutation.
They put this E.coli in a nutrient-limited medium so there was some sort of selection acting upon
this E.coli. Selection is going to favour any individual cell that is better at exploiting food
resources than its competitors.
Started with a single cell placed in a low nutrient medium and let it replicate over and over again
till they had a full vial full of cells (5 x 10 ). Skim off some of them, dilute this proportion of cells
and place them in a low nutrient medium. They’re given a chance to grow and proliferate for a
day. E.coli generally takes a few hours to have a new generation so over 1 day, there were
about 7 generations. Another subsection was skimmed out, diluted again and placed it back into
another low nutrient medium. This was repeated for 1,500 to make 10,000 generations with 12
different things (?).
Slide 22: Higher cell size = higher fitness, smaller cell size = lower fitness, cells that are bigger have been
better at exploiting the food resources.
Compared to the C. elegans experiment, they were left to accumulate mutations but here since
there’s some sort of selection pressure, those deleterious mutations are being sifted out of the
population so all we’re left with are these points where either nothing happens because