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Club EvMed: Why is human childbirth so difficult? Obstetrics and the evolution of labor - Shared screen with speaker view
Meredith Spence Beaulieu, PhD (she/her/hers)
21:45
Feel free to post perspectives or questions in the chat at any point!
Joseph Graves Jr
23:45
Africa is not one place, and has populations displaying the greatest genetic variation within our species, plus considerable environmental variance, so no surprise on the range of rates.
Elizabeth Reifsnider
24:23
Is CPD lower in areas of malnutrition/food insecurity? With less food, perhaps smaller infants and less CPD.
Joseph Graves Jr
25:35
Of course, malnutrition could have proportional effects on adult females, so no reason to suppose that malnutrition reduces this problem.
Gillian Bentley
26:25
@Elizabeth Often lower birthweights but malnutrition during development can "flatten" the pelvis and leads to greater problems esp in younger women (e.g., teens) who haven't stopped growing
Adam Hunt
26:44
I wonder whether you can predict problems by disparity between parent size: if the father is significantly larger as an adult than the mother, is there more likely to be a too-large baby? And, vice versa, do more similar sized females/males (or couples where the female is larger than the male) have less problems? Or do foetus sizes not reflect adult sizes?
Gillian Bentley
27:21
Jonathan Wells wrote an interesting paper about this looking at ethnically-mixed partnerings: South Asians and Europeans
Ge Zhang
28:04
disparity between parent size: This might be a reason behind assortative mating.
Elizabeth Reifsnider
28:30
I agree about the impact on malnutrition on growth of pelvis during female development. Best solution is infants who are not LGA and well-nourished mothers.
Adam Hunt
29:17
It's interesting, because if there's selection on larger mothers to prevent problems in childbirth, there must be stronger selection on larger males (e.g. sexual competition)?
Joseph Graves Jr
29:56
“Ethnically-mixed” pairings in this regard would only have relevance if the populations in question are different in their mean sizes. Also, as ethnicity refers to common national origin and culture, that is not the appropriate term to use here when the real issues is potential biological variation between populations. For example, a pairing between a German of European descent and French person of European descent is an ethnic mixture as well.
Vinny Lynch
30:05
There is some genetic data supporting an association between maternal height (body size) and birth size (and gestational age): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4540580/
Gillian Bentley
30:55
The Wells et al paper about S Asians and Europeans was predicated on the info that birthweights were v different between the two populations so not done randomly as I understand it
Oscar R. Sánchez
36:37
Does the model take into account the difference in size between boys and girls?
Joseph Graves Jr
41:32
So this would mean sexually antagonistic genes.
David Haig
42:15
I would frame it in terms of parent-offspring conflict rather than sexual antagonism
Joseph Graves Jr
44:29
If genes favoring larger D are selected for in males, but selected against in females, seem to fit the sexual antagonism definition.
David Haig
45:32
But the fetuses are both males and females. The selection is not in the father but in the fetus.
Joseph Graves Jr
49:21
Agreed that selection acts on the fetuses, but those fetuses contain genes that came from their parents (female/male), so antagonism is not acting differentially in development, but on parental fitness. However, I think this distinction may really be a minor point.
David Haig
50:38
There are two paternal genes in the father but only one in the fetus. It is not the same thing. I have distinguished between parental antagonism and sexual antagonism.
Joseph Graves Jr
50:49
So does “White” here mean persons of European descent, if so best to say that than use social definition.
Philipp Mitteroecker
53:53
I agree that this is some kind of parent-offspring conflict, but only for those individuals above the “fitness cliff”
David Haig
54:26
Agreed. Going over the cliff is in nobody’s interest
Esha Bhattacharjee
54:49
Hi everyone. Can anyone please tell me what is ultra-male and ultra-female?
David Haig
55:20
ultra is simply an exaggeration of the difference to make it more obvious
Gillian Bentley
55:23
@Philipp, given that many women in the global north have unnecessary C-sections and many women in the global south who might need one can't get one, how does that affect your model that C-sections might affect selection in the future?
Joseph Graves Jr
55:38
I think this is simply referring to the most exaggerated forms of male and female pelvis.
Esha Bhattacharjee
56:15
Thanks
Meredith Spence Beaulieu, PhD (she/her/hers)
56:51
We’ll be opening up for discussion soon! Feel free to virtually raise your hand at any point to join the queue. (Click “Reactions” at the bottom of your screen, then “Raise Hand.”)
Gillian Bentley
57:47
Meredith, for some reason my Zoom is not showing the Reactions for this session but I would like to ask a question
Philipp Mitteroecker
58:11
For the evolutionary dynamics, only the C-sections that did save lives matter, not those for other reasons.
Gillian Bentley
58:27
Thank you
Gillian Bentley
58:42
@Philipp: in reality that might be very few then...
Philipp Mitteroecker
58:54
agreed
Leithen M'Gonigle
58:54
Philipp, do you expect that c-sections are a temporary medical solution and that relaxed selection will lead to a new equilibrium where there is a new cliff?
Philipp Mitteroecker
01:01:50
Fetal size cannot increase a good deal more because of the metabolic capacity of the mother, among other reasons. Also, medical advancements in neonatology and pelvic floor treatment reduce selection for large newborns and a narrow pelvis. Hence, the trend we described certainly has a limit and may have lowed or stopped already.
Meredith Spence Beaulieu, PhD (she/her/hers)
01:08:53
We have time for a couple more questions before we close! Feel free to post in the chat or raise your hand.
Holly Dunsworth
01:09:06
One idea: male pelves aren't packed with as much stuff and that the pelvis is more like the cranium than we realize. Big brain = big skull. Presence of uterus, vagina, clitoris = big pelvic canal
Vinny Lynch
01:10:00
I’m curious if and how much of the variation in pelvis shape between males and females is a byproduct of other developmental differences between Ms and Fs rather than a direct product of selection (ie., a spandrel)?
Holly Dunsworth
01:10:31
I wrote about that, Vinny: https://onlinelibrary.wiley.com/doi/abs/10.1002/evan.21834