[Popgenlunch] Robin Waples in PopGenLunch Friday

Joe Felsenstein joe at gs.washington.edu
Tue Apr 17 20:56:16 PDT 2018

Folks --

Thanks to Sharon Browning for giving last week's talk.

This week, another interesting topic, from a real expert on effective
population number:

Robin Waples (Senior Scientist, NOAA Fisheries, Northwest Fisheries
Science Center)

Title: Consequences of sex reversal for effective population size

He has provided us with a very detailed Abstract:

Sex reversal (sequential hermaphroditism) is common in plants, some
invertebrates, and several marine fish families. Protandrous species
start life as males and later change to females, while protogynous
species do the opposite. Evolutionary theory indicates that sex
change can be favored when fecundity increases with age faster in one
sex than the other, in which case the sex with the faster rate of
increase becomes the terminal sex. Conversely, the adult sex ratio is
skewed toward the initial sex—sometimes very highly so. For a given
set of vital rates (age-specific survival and fecundity for both
sexes), there exists an optimal age for sex change (ESS age) that is
evolutionarily stable. These ESS analyses, however, do not account
for indirect effects on another key evolutionary parameter, effective
population size (Ne), which is reduced when sex ratios are skewed.
Wright’s famous sex-ratio adjustment for Ne is not directly applicable
to sequential hermaphrodites, because it is necessary to consider
lifetime variance in reproductive success by the same individuals
operating as both males and females. This raises the following
question: Do species that exhibit sex change incur an evolutionary
cost in terms of reduced Ne caused by skewed sex ratios? If so,
evolutionary tradeoffs must be involved, at least at the population
level. I evaluated this issue using both hypothetical vital rates and
empirical data for 8 marine fish species that are sequential
hermaphrodites. Results: 1) In all cases, Ne at the ESS age at sex
change was as high or higher than for a gonochoristic (fixed sex)
population with a 1:1 sex ratio. This occurred in spite of the skewed
adult sex ratio in the sex-changing population, which indicates that
the ability of individuals to operate as both male and female allows
the population to avoid some of the evolutionary constraints imposed
by fixed sexes (i.e., that half the genes for the next generation must
come from each sex). 2) However, Ne at the ESS age at sex change was
always lower than it could have been if sex change had occurred at an
earlier age. This implies an opportunity cost for the population in
terms of a missed chance to further reduce effects of genetic drift.
3) Using a simple transformation of vital rates, I develop a novel
method to quantify the strength of selection for
sex reversal in a population.

As usual, PopGenLunch this quarter will be on Fridays from
12:30-1:30pm in our usual room, S110 Foege Building.

The schedule for the rest of the quarter is:

April 27 (*)
Marshall Abrams (Philosophy, University of Alabama - Brimingham)
Deemphasizing individuals: Population genetics and the metaphysics of
evolutionary processes

May 4

May 11

May 18 (S)

May 25 (S)
reserved, see below

June 1 (S)
reserved, see below

(One of the last two sessions will actually have Bruce Weir, speaking on
"Estimating the inbreeding coefficient", and the other will have Elizabeth
Thompson, speaking on a closely related topic. They will be working
out who goes first.)

Note that we have three open slots. For those sessions, volunteers are,
of course, welcome.

This quarter I will be away for about 4 of the sessions, 3 of them in Friday
Harbor, from where I will try to attend by Skype. We will try to have sessions
anyway, with others in charge. I marked the session I have to miss by
(*) above,
and those which I will attend by Skype by (S).

As usual, bringing and eating lunches is welcome (though food is not provided).

Joe (CEO, PopGenLunch)
Joe Felsenstein joe at gs.washington.edu
Department of Genome Sciences and Department of Biology,
University of Washington, Box 355065, Seattle, WA 98195-5065 USA

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