Here are my calculations (I've calculated fitness on an 'egg to
egg' basis â how many next-generation eggs is each egg likely to
produce; multiply probability of survival to adulthood by average
fecundity to get a per egg absolute fitness... Again, you could
take somewhat different approaches). NOTE ALSO; in the 2-generation
examples, I've taken the average number of each offspring for each
phenotype as starting point for next generation -- in other words,
calculated number of grandchildren as absolute fitness. You could
do this differently, too (e.g., calculate fitness for each
generation separately and average them). Finally, I've essentially
assumed clonal reprod, which you wouldn't have to do with some
approaches...
SCENARIO 1
|
egg-adult |
|
absolute |
relative |
|
survival |
fecundity |
fitness |
fitness |
A1A1 |
0.9 |
50 |
45 |
0.86 |
A1A2 |
.85 |
55 |
46.75 |
.89 |
A2A2 |
.75 |
70 |
52.5 |
1 |
SCENARIO 2
|
adults |
ave |
absolute |
relative |
|
generation2 |
fecundity |
fitness |
fitness |
A1A1 |
|
70 |
2835 |
1 |
A1A2 |
39.74 |
65 |
2582.94 |
0.91 |
A2A2 |
39.38 |
55 |
2165.63 |
0.76 |
SCENARIO 3
|
|
ave.adult |
absolute |
relative |
|
survival |
fecundity |
fitness |
fitness |
A1A1 |
|
55 |
.49.5 |
|
A1A2 |
|
65 |
55.25 |
0.98 |
A2A2 |
.75 |
75 |
56.25 |
1 |
SCENARIO 4
|
adults |
ave |
absolute |
relative |
|
generation 2 |
fecundity |
fitness |
fitness |
A1A1 |
|
75 |
|
|
A1A2 |
|
65 |
55.25 |
0.91 |
A2A2 |
42.19 |
55 |
|
|
he first scenario shows directional selection for A2;
homozygote A2 has higher fitness than heterozygote and heterozygote
has higher fitness than other homozygote; under this scenario A2
goes to fixation; there's no indication of disruptive selection
that would allow A1 to persist.
In the 2-generation-per-year scenario (scenario 2), fitness
differences are reversed in the 2nd generation, so this suggests
some disruptive selection, which increases the likelihood of
maintained genetic variation. However, I've calculated a per
annum fitness (i.e., how many descendants at end of year per
individual at beginning of year, and the higher fitness of A1 in
2nd generation over-rides the higher fitness of A2 in first, so,
all else being equal, it looks like the net result would still be
directional selection, in the opposite direction, taking A1 to
fixation -- but it would take longer. (Also, of course, all else
might NOT be equal -- if, for example, population sizes were
different for the two generations, that could change
things...)
This isn't changed in third scenario with different
fecundities; even though fecundity contributions to fitness are
exactly symmetrical now, the survival differences still reduce A2's
advantage in first generation and enhance A1's advantage in second
-- so probably still goes to A1 fixation, and probably
faster.