Lecture 2 - Human-Agricultural Coevolution

coevolution

coevolution

reciprocal evolutionary responses in a pair of species interactions caused by selection imposed by each other

how is selection produced

by ecological processes

genetics

provides the material that is transmitted across generations

ecology

cultures and organisms that we interact with as humans

lactase production evolution

lactase production normally declines in adulthood meaning people become more lactose intolerant as they age

lactase

enzyme that humans use to digest milk

lactose

made of glucose and galactose

what region of the world is persistent to lactase decline in humans?

in European origin

what are the potential reasons why European populations are persistent to lactase decline?

substitutions are the causal variants, they were favored by natural selection or genetic drift

predictions concerning European lactase persistence?

• other human populations with lactase persistence should have the same substitutions or changes with similar consequences on the production of enzyme

• populations practicing dairy agriculture should have lactase persistent phenotypes

• see evidence of past selection at the lactase gene in the human genome

what results did expanded sampling of lactase persistent populations show?

African cultures with dairy farming also have distinctive forms of the lactase gene

what is ability to digest lactose correlated with?

DNA sequence differences concerning the enzyme lactase

lactase digestion results conclusions

• the inference that molecular variants affect the phenotype is strengthened

• independent evolution of the same phenotype in different populations supports the driving force of natural selection

chance vs determinism

the fact that single events can be due to chance alone or deterministic processes

evolutionary chance

genetic drift

evolutionary deterministic processes

natural selection

how can we confirm deterministic explanations?

with the use of independent lines of evidence like the odds of getting heads twice, 3 times, etc in a row

convergent evolution

independent evolution of the same trait in different groups using the same genes to be assured that selection is causing evolution of that trait

evolutionary groups

could be populations or species

what should we expect if selection affects the genome?

• natural selection should act differently on mutations that change amino acids from ones that don’t

• lower genetic variability in regions of the genome that experience selection

• more differentiation between populations at selected sites than the rest of the genome

replacement

non synonymous mutations that change amino acids

synonymous mutations

mutations that don’t change amino acid fate

neutral mutations

we should expect similar properties of both replacement and synonymous mutations because these mutations don’t affect fitness

deleterious mutations

we should expect lower population frequencies for replacement/non synonymous mutations because they have negative effects on fitness

beneficial mutations

we should see more replacement mutations than synonymous ones because they improve fitness

neutral mutation equation

dN/dS = (approx) 1, equal rates

deleterious mutation equation

dN/dS < 1 , substitution rate for synonymous sites exceed the replacement sites

beneficial mutation equation

dN/dS > 1, substitution rate for replacement exceeds synonymous sites

genetic hitchhiking

as genetic mutations fix they drag along with them nearby mutations

• genetic variation in the regions will go down and the size of the region is dependent on the recombination rate

incomplete sweep

only one of the orange stars is missing a grey one majority still has both in the recombination event and in the no recombination event ½ of the chromosomes have the mutation

complete sweep

no recombination : all the chromosomes have the mutation (orange star)

recombination event: mutation has fixed in the populations along with the left and right stars, narrower rectangle of identical genetics and can determine start and end point where mutation arose

recombination

has a dramatic effect on how much genetic diversity is lost in a population

recombination example in the lactase persistent Africans

lost genetic variation in genome region responsible for lactase activity, individuals are homozygous for the trait

what do the longer lines in the lactose-intolerant Africans chromosomes indicate?

longer sections of the genome that are totally homozygous, implicated that natural selection is the cause for evolution in lactase persistence cause there is lower variation in that area of the genome