BIOB51 - Lecture 10 Notes
Models being Heterozygote superiority, frequency dependent selection, etc. Do those really
matter? Can we find evidence for them in natural populations?
Cichlids are a diverse group of fish and a large amount of them live in Lake Tanganyika. There
was a book written on it called Darwin’s dreampond because there has been incredible adaptive
radiation of these fishes. They have different kinds of feeding, colours, sizes, etc. but one type
that’s there is called the scale-eating fish, which eats the scales of other fish.
These fish swim up to the side of the other fish and bite them. “Right-handed” fish have their
mouths twisted in that direction since they attack the left side. So they have been under
selection in order to get the best gulp of scales from their victims. So they swim to the left, grab
a bunch of scales and swim out. Same thing for the “left-handed” fish except their mouth is
twisted the other way since they attack the right side. Two different alleles.
There are two alleles in this population so how are these two being maintained? The idea was
maybe it’s being maintained by frequency dependent selection. They found some evidence that
the frequency of the right-handed fish to left-handed fish fluctuated at around 0.5, which is really
key and matches what we predict from our models of frequency dependent selection.
If you’re at a low frequency, you have a high fitness advantage. If you’re at a high frequency,
you have fitness disadvantage.
The fish that are commonly being side swiped by these scale-eating fish are more vigilant and
are looking for those predators more on one side of their body than on the other because on
one side, it’s much more common for an attack than on the other side. So you could see how
maybe this could illicit some kind of frequency dependent selection. If you’re attacking from the
less common side, you might be more successful because your victims are slightly less vigilant.
They’re expecting it slightly less.
Frequency of left-handed fish (frequency fluctuating) and they also looked at actively breeding
adults within the population. These orange squares represent the frequency of actively breeding
adults that are either right-handed or left-handed. It runs in the opposite pattern to our frequency
dependent selection curve. Example: Most common frequency at sample year 81 was rr, which was above 0 or above 0.5.
So rr is more frequent in the population and the breeding adults aren’t rr, they’re Rr.
So there seems to be a flip and a switch of breeding adults and frequency in the population. So
morph that’s in the higher frequency has fewer # of breeding adults and morph that’s in the
lower frequency has higher # of breeding adults.
There’s research and hypotheses on why handedness is maintained in our populations and why
it’s maintained at different frequencies. Left-handedness appears at different frequencies in
different areas of populations around the world. Not just one common frequency for human kind
for handedness; it kind of fluctuates.
Dawson 1970 experiment on plotting what happens when an organism has a lethal recessive
allele and watch it. He did this work in flour beetles, which have one lethal recessive allele. This
is an allele that causes a real loss in fitness if everyone dies. From initial population, they
wanted to see how the pattern will look like and how fast will this deleterious allele be lost in the
They were trying to match what they saw with a model prediction. The frequency of the lethal
recessive allele would decrease really quickly at first but then it becomes rarer and rarer in the
population; its decline would slow down. So it becomes less pre