BIO220H1 Study Guide - Final Guide: Lactase Persistence, Lactose Intolerance, Lactase

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19 Apr 2012
Lecture 12: Human-Agricultural Coevolution
Coevolution: That reciprocal evolutionary response between species as a consequence of
natural selection having been imposed on each other, by each other
One example: wolves and moose
Wolves prey on moose and impose selection, and the subsequent generation of moose
due to the wolves’ selection, which selections for wolves with certain attribute.
It can also happen within a species between the sexes
One sex can impose changes in the other sexual dimorphism and sexual antagonism
How do humans fit in?
Humans have imposed a selection on cows, is it reciprocal?
Natural Selection: Interaction between ecology and genetic
The ecology is what generates natural selection
The genetics provides the materials to be passed throughout the generations
BUT culture comes into play as well
Cattle Pastoralism
Unique to humans
Milk and Humans
Humans need milk, especially breast milk at infancy
The baby drinks Lactose, and needs Lactase into Glucose and Glactose
Lactase production normally declines in adulthood, with the exception of people of northern
European origin
This decline leads to “lactose intolerance”
Lactose intolerance is the “Wild Type”
Origins in Europe, lactase production persists throughout adulthood
Allows digestion of milk
Has this had any effect on the cattle?
The substitutions are the causal variants (happens to be linked but not causal)
They were favoured by natural selection (were the adaptations favoured?)
If substitutions in the lactase genes causes lactase persistence, then we can predict:
If other human populations with lactase persistence should have the same substitutions, or
changes with similar consequences
But if it was the dairy agriculture that imposed evolution by natural selection on substitutions
that cause lactase persistence, then we can predict:
Other human populations practicing dairy agriculture should have lactase persistent
We should see genetic evidence of past selection at the lactase gene
Expanded Sampling of Lactase persistence
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Many African cultures practice dairy farming
Do they show lactose tolerance?
Do adults have lactase persistence?
Lactase Digestion
African cultures with dairy farming also have distinctive forms of the lactase gene
DNA sequences differences correlate significantly with the ability to digest lactose
Strength inference that these molecular variants actually affect the phenotype
Independent evolution in different population supports driving force of natural selection
Chance vs. Determinism
Single events can be due to chance alone or to deterministic processes
Example: A single coin flip: could be a simple probability or could be a biased coin
Independent lines of evidence can confirm deterministic explanations by rejecting chance
e.g., a chance process predicts:
Heads 2x in a row: 0.5 * 0.5 = 0.25 = 25%
Heads 3x in a row: 0.5 * 0.5 * 0.5 = 0.125 = 12.5%
Heads 4x in a row: 0.5*0.5*0.5*0.5 = 0.0625 = 6.25%
Convergent Evolution
Independent evolution of the same trait in different groups
Groups can refer to populations or species [ex: bats and birds]
Allows more confident determination that selection is at work
Does convergent evolution use the same or different genes?
In this case, it appears to use the same genes
Summary: Lactase persistence evolution
Studies of Neolithic human remains (10,000 years BCE) [at the onset and before agriculture]
suggest that they were unable to digest lactose as adults
Cattle domestication did not occur until approximately 7,500 to 9,000 BCE
Studies in African cultures also show mutations in lactase associated with lactase persistence
into adulthood
Strong evidence of replicated association between genotype and phenotype
Independent derivation supports milk digestion as an example of convergent evolution
toward adult digestion of milk
Predictions we haven’t touched upon yet
We should see some genetic evidence of this historical, natural selection affecting this Particular
part of our genome
Can we detect an imprint of such coevolution in our genomes?
Turns out we can try to do that by applying the principles of population genetics :
To determine how natural selection can affect the genomes of an organism, we can:
We should see differences in the fate of those mutations that change amino
acids (replacement = non-synonymous) versus those that do not (synonomous)
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