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Lecture 5

Lecture 5 Notes

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Biological Sciences
Maydianne Andrade

BIOB51 SEP 28 , 2010 Lecture 4: Evidence for Evolution CONTINUED - Human populations have also been exposed to agents of selection during their history - In some cases  the agents of selection are very strong! [strong effects on populations] - This study looks at populations in couple of high land areas in Tibet - Tibetan plateaus = very high altitudes - Air you breathe is very thin  very little oxygen available to extract from air [partial pressure very low] - This can impose very strong selection on people [we need oxygen to survive!] - Our physiology is presumably [according to theory of evolution] adapted to low-land life (which is where we started out as a species) - BUT about 25, 000 years ago = people colonized the high lands or the plateaus of Tibet - This study that suggests that there has been selection on them imposed by this sort of rarefied oxygen situation to change the way in which they physiologically process oxygen - The study measured women  they wanted to measure the effects of natural election in terms of offspring, childbirth and survivorship - So they measured the oxygen content of their blood  they found quite a lot of variation - Physiological changes: if you go up to high altitude there’s going to be induced changes in your body that allow you to deal with that low oxygen content:  You increase the # of capillaries in your body  Your lung volume increases  You increase the haemoglobin and the amount of red blood cells that you produce - That’s what you call ‘adaptation’ in physiology [not in terms of evolution or Darwin] - But we’re talking about those changes that are independent of the short-term changes if you’re exposed to high altitudes - We’re talking about heritable or inherited changes in your ability to use oxygen - They’ve [the study] demonstrated using population genetics method that it is actually due BIOB51 SEP 28 , 2010 - due to a dominant allele (B) - So, That means that their oxygen profile looks the same as if you take someone from sea-level (who hasn’t evolved in this context) and bring them up there and they get that physiological change: if you compare it to people with the AA genotype and who live on the plateau [and whose ancestors lived on the plateau] = THEY ARE THE SAME! BUT if you look at people with AB or BB genotype they’ve  This is actually important: b/c even though we show physiological acclimatization to high altitudes: even after those changes, there are still significant differences in the functioning of people living at high altitudes versus low altitudes  So if you look at people with the AA genotype: they’ve a harder time performing strenuous activity and cognitive tasks that are related to oxygen availability & there tends to be a higher rate of miscarriages So, there is a strong selection imposed by the need to oxygenate your blood essentially! [Even not including these physiological changes] - So the study first demonstrated that it was heritable! [Using family line analysis] - And then they looked if there were differences in the reproductive success of those individuals as a function of which genotype they have! - b/c if natural selection is operating on this heritable trait  then what we should see is the differences in individual reproductive output as a function of their genotype - what the study found was that there were no differences in how frequently the women conceived or actually the frequency of BIOB51 SEP 28 , 2010 childbirth  BUT what they found was the difference in offspring mortality - so women with AA genotype: their offspring more likely to die during infancy than the other genotype - we don’t know if it’s directly b/c infants have a harder time processing oxygen or if it’s something about the mothers  the point is that there is a difference that is related to which allele you posses in your reproductive success so selection is being imposed here [big sample size! –> significant difference] Note: high-altitude hypoxia = low oxygen availability - so what it means is: natural selection is operating on a trait that they’ve shown to be heritable - so allele frequency is ~0.78 [pretty close to 1 = high frequency] for those with dominant B allele [high altitude ppl] while it’s ~0 for low-landers  so over the course of 25,000 years  if their inferences of colonization by
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