286 CHAPTER 11Number of offspring
All else being equal, a genotype with higher fecundity has higher fitness than one
with lower fecundity. Why, then, do some species, such as humans, albatrosses,
and kiwis, have so few offspring?
The answer, again, is trade-offs. The British ecologist David Lack proposed that
the optimal clutch size for a bird is the number of eggs that yields the greatest
number of surviving offspring [33]. The number of surviving offspring from larger
broods may be lower than the number from more modest clutches because parents
are unable to feed larger broods adequately. This decrease in offspring survival
has proved to be one of several costs of large clutch size in birds. Excessively large
clutches may also reduce the parents’ subsequent clutch size and survival [55]. A
modest number of eggs per clutch may therefore result in higher fitness than a
greater number.
The great tit (Parus major) has been the subject of many ecological studies
because it is abundant and will use boxes provided for nesting, which enables
researchers to monitor the birds’ lives. A long-term study of survival and repro-
duction was performed in the Netherlands that involved changing brood size by
moving some hatchlings among nests [57]. The researchers estimated the effects
of these treatments on fitness. Artificially increasing brood size decreased fitness
because it lowered survival in the nest, survival from fledging to the next breed-
ing season, and the probability that the parents would lay a second clutch in the
same year. Decreasing brood size also reduced fitness, simply because the nests
produced fewer fledglings. From these data, the clutch size that would maximize
fitness was estimated to be 8.9 eggs, close to the natural mean of 9.2.
At a given level of reproductive effort, there must be a trade-off between the
number of offspring and their size, ranging from many but small to large but few.
Relative to adult body size, the largest offspring among birds and mammals, or
those that require the most care, are in those species that have only a single off-
spring at a time, such as kiwis, albatrosses, elephants, and humans. Larger off-
spring are advantageous in species in which, because of their habitat or mode of
life, starting life at a large size greatly enhances the chance of survival. Among
plants, the wind-dispersed seeds of orchids are microscopic, and can grow only if
the embryo becomes associated with a mycorrhizal fungus. At the other extreme,
the water-dispersed seed of a coconut palm (Cocos nucifera) can weigh up to 20
kg (see Figure 11.2). Seeds are typically larger in plants that germinate in the
deep shade of closed forests than in species that germinate in well-lit sites, such
as early-successional disturbed environments or light gaps formed by treefalls in
forests, because the survival and growth of a seedling under adverse conditions
are enhanced by the food stored in a large seed’s endosperm (FIGURE 11.13A)
[18]. Perhaps for this reason, trees and vines generally have larger seeds than forbs
(herbs) and grasses, most of which grow in open habitats (FIGURE 11.13B) [39].Life histories and mating strategies
Males as well as females are subject to costs of reproduction, and that fact under-
lies some interesting variations in life histories. For example, some plants, annelid
worms, fishes, and other organisms change sex over the course of the life span
(a phenomenon called sequential hermaphroditism). In species that grow in size
throughout reproductive life, a sex change can be advantageous if reproductive
success increases with size to a greater extent in one sex than in the other (FIG-
URE 11.14). For example, the pollen required to fertilize many ovules requires
much less energy to produce than an equivalent number of seeds. Many species
of squashes (Cucurbitaceae) and other plants produce male flowers when they are
small, and switch to producing female flowers when they become larger and can11_EVOL4E_CH11.indd 286 3/22/17 1:11 PM