Evolution, 4th Edition

How To BE FiT 287

obtain enough energy to develop more seeds. A female slipper shell (Crepidula for-

nicata) carries a stack of smaller males; when she dies, the biggest male changes

sex, having become large enough to produce abundant eggs (see Figure 11.14C).

Conversely, many sex-changing fishes are females first. In the bluehead wrasse

(Thalassoma bifasciatum), some individuals start life as females and later become

brightly colored “terminal-phase males” that defend territories and achieve

high reproductive success by mating with many females (see Figure 11.14B) [60].

Almost all species of sex-changing fishes, as well as other sex-changing animals,

change sex when they have reached about 70 per cent of their maximum size [1],

as predicted by mathematical theory [12].

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_11.13.ai Date 11-21-2016

Number of species

0

2

1

6

5

4

3

7

9

8

(A)

0.02 0.14 1 7.4

Seed mass (g)

Light-gap species

Shade species

Seed mass (mg)

0.001

0.01

100

10

1

0.1

1000

100,000

10,000

(B)

Graminoid Forb Shrub Tre e Vine

Au: Add “(log scale)” to part (A) x-axis label?

FIGURE 11.13 The size (mass) of individual seeds is correlated

with the microhabitat in which the seedlings grow. Larger seeds

are thought to be adaptive for species whose seedlings grow

slowly because of dim light. (A) Among 40 tree species from

tropical moist forests, those that germinate in light gaps have

smaller seeds than those that germinate in subcanopy shade.

(B) Seed mass of more than 7200 species of plants. Forest species

(trees and vines) have larger seeds, on average, than do the other

growth forms, which predominate in open environments. The

median of each sample is surrounded by a box representing the

25th to 75th percentiles, and lines indicating the 10th and 90th

percentiles. Dots show outliers. (A after [18]; B after [39].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_11.14_v2.ai Date 11-21-2016

Small Large

Female

Male

(A) No sex change (B) Protogyny (C) Protandry

Male

Male

Female Female

Small Large Small Large

Number of eggs

produced or fertilized

Size of adult

FIGURE 11.14 A model for the evolution of sequential her-

maphroditism. (A) When reproductive success increases equally

with body size in both sexes, there is no selection for sex change.

(B) A switch from female to male (protogyny) is optimal if male

reproductive success increases more steeply with size than female

reproductive success does. This is the case in the bluehead wrasse

(Thalassoma bifasciatum) shown here: females are yellow; males

are blue, white, and green. (C) The opposite relationship favors

the evolution of protandry, in which males become females when

they grow to a large size. For example, a female slipper shell

(Crepidula fornicata) carries a stack of males; when she dies, the

lower-most male becomes female. (After [60].)

11_EVOL4E_CH11.indd 287 3/22/17 1:11 PM