Class Notes (806,856)
Canada (492,490)
Biology (2,220)
BIO220H1 (238)
Lecture 13

Lecture 13.docx

4 Pages
Unlock Document

University of Toronto St. George
John Stinchcombe

BIO220: Lecture 13 Human—Agricultural Coevolution What is Coevolution?  Coevolution reciprocal evolutionary responses in a pair of species, caused by selection imposed by each other o Cycle of each act of selection leads to a response and in turn imposes selection on the other partner  Sexual coevolution Reciprocal evolutionary responses in the two sexes, caused by selection imposed by each other  Human agricultural coevolution  reciprocal evolutionary responses in humans and their agricultural species How do humans coevolve with agricultural species?  Evolution by natural selection is a genetic and ecological process o Ecology produces selection o Genetics provides the material that is transmitted across generations  For humans, ‘ecology’ includes culture and the organisms we interact with  Human infants depend on milk (provided by breast feeding) but what about adults?  Humans digest milk using the lactase enzyme  But most of us continue to consume milk past our need  Lactase production normally declines in adulthood o Exception: people of north European origin  Lactose intolerant is the wild type  Humans across globe has long history of evolution with cattle  lots of different breeds of cattle (so a cow is a valuable and useful animal in most parts of the world) o They provide labour, milk, meat  Observation: In European populations, changes in the lactase gene strongly correlated with the ability to digest lactose as an adult  Hypotheses: o The substitutions are the causal variants (that allow those individuals to digest milk) o And they were favoured by natural selection (not due to genetic drift)  Predictions: o If substitutions in the lactase gene cause lactase persistence, then other human populations with lactase persistence should have the same substitutions, or changes with similar consequences. o If daily agriculture imposed evolution by natural selection on substitutions that cause lactase persistence, then other human populations practicing dairy agriculture should have lactase persistent phenotypes and we should see genetic evidence of past selection at the lactase gene  Experiment: Besides Northern European cultures, there were other cultures in Africa that practice dairy farming. Do they show lactose tolerance?  Results: African cultures with dairy farming also have distinctive forms of the lactase gene o The DNA sequence differences correlate significantly with the ability to digest lactose  Conclusion: o These are actually the variants affecting the phenotype o Because we saw the same pattern of evolution (also at the genetic level) we’re more confident that is due to natural selection Why?  Natural selection driving lactase to high frequencies  If we’re flipping a coin, how many times would someone had to flip heads in a row until you become suspicious that it was not a fair coin?  We can use the same line of argument for genetic drift (a random process) vs. natural selection o So the odds of lactase persistence would randomly lead to high frequencies in Northern European and African populations. So the more we observe this in independent draws, the less likely it is random chance alone o So likely that natural selection drove lactase persistence in East African population  Convergent evolution: independent evolution for the same trait in different groups (populations or species) o Does convergent evolution use the same or different genes? In that lactase example, it appears to be the same genes but it’s not always the case  Looked at fossil records of Neolithic human (10 000 BC) remains suggest that they were unable to digest lactose as adults  this makes sense because our best estimate of domestication of cattle was around 7500-9000 B.C. o Suggests that this gene (at least in Europeans) evolved in the last 5000-7000 years)  Studies in African cultures that also showed these mutations, we see a replicated association between genotype so the mutations in phenotype we see in African populations AND European populations. The fact that these mutations seem independently derived supports that it could be an example of convergent evolution.  One prediction we haven’t considered yet we should see genetic evidence of past selection at the lactase gene  Answer this by seeing what imprint does coevolution leave in genomes What imprint does coevolution leave in genomes? How does selection affect the genome? We should expect: 1. Differences in the fate of those mutations that change amino acids (replacement= non-synonymous) vs. those that do not (synonymous=silent) a. If you can change stretches of DNA without changing protein, that won’t have much effect on phenotype b. Can divide mutations in 3 categories: i. Neutral (do not affect fitness) ii. Deleterious (negative effects on fitness) we should see lower population frequencies for replacement mutations iii. Beneficial (improve fitness)  these changes, natural selection favours then we expect to see more replacement or non-synonymous mutations than synonymous 2. Lower genetic variability in regions of the genome that experience selection a. If there’s one vast variant that allows you to digest milk and that variant rises in frequency where it spreads to the whole population, then everybody in that pop should have the same mutation and there shouldn’t be much variability  As beneficial mutations fix, they ‘drag’ along with them nearby mutations= Genetic hitchhiking  Complete sweep: everyone in population get the mutation  Incomplete sweep: only a fraction of the population get the mutation If there is recombination, we should see in both cases, a dramatic reduction in variation at the site of beneficial mutation. As you get farther away in either direction along the chromosome, the level of variation should start to recover (whether or not it’s a steep or gradual reduction in variation depend
More Less

Related notes for BIO220H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.