Why does reproduction decrease as population size increases?

There are two kinds of answers to these questions . . .

• proximate (physiological) explanation -- females have not reached full size and their ovaries do not mature before age 3, so they do not ovulate or copulate before then

• ultimate (evolutionary) explanation -- earlier reproduction results in higher mortality of mothers or young, so alleles associated with earlier reproduction do not spread under conditions that Red Deer experience

populations can respond to increased chances of mortality with changes in the frequencies of alleles

a full understanding of density-dependent changes in populations (or anything else about animals and plants) requires both kinds of explanation!

as populations of Red Deer increase, age of first reproduction increases, fewer females have calves two years in succession, males have smaller antlers . . .

these changes are a result of physiological changes (a proximate explanation) -- as populations increase there is less food for each incdividual on average -- experiments with captive deer show that reduced food intake results in postponed reproduction and lower fecundity of females and smaller antlers of males

are these changes also a result of natural selection (an ultimate explanation)? -- how could we find out?

an evolutionary (ultimate) explanation focuses on changes in frequencies of alleles in a population

two terms are important in discussing these genetic changes . . .

evolution

natural selection

natural selection thus occurs when alleles are associated with phenotypes that differ in age-specific fecundity or survival -- alleles spread faster when associated with higher rates of increase of descendants

only rarely is it possible to do an experiment to show that a change in a population's reproduction or survival has resulted from natural selection . . . but there is one such experiment for vertebrates . . .

wild populations of Guppies Poecilia reticulata in Trinidad (studied by David Resnick and John Endler) show that natural selection can produce different balances of growth, reproduction, and survival

guppies inhabit shallow streams, females give birth to live young, males develop bright colors and court larger females -- in lowland streams a large predatory fish (Crenicichla)preys primarily on adult guppies -- in mountain streams Crenicichla is absent but a small predator (Rivulus)preys on juvenile guppies

we can compare populations of guppies in lowland and mountain streams . . .

	Lowland	Mountain	Ratio (L/M)
Age at sexual maturity (days)
Males	52	59	0.88
Females	72	83	0.87
Size at sexual maturity (mg)
Males	88	98	0.90
Females	218	270	0.81
Brood size (number of young)
	5.2	3.2	1.62
Brood mass / female mass
	0.25	0.19	1.32

* Figures in the table are averages for each population.

in lowland populations both sexes mature earlier at a smaller size -- also a female's first brood is larger -- although each baby is smaller, the total mass of each brood is a larger proportion of the female's own mass (see table below) -- guppies in lowland streams devote proportionately more resources to reproduction than do those in mountain streams

note that, when predation decreases survival of adults, individuals reproduce more at early ages -- when predation decreases survival of juveniles, individuals do just the opposite (consequently they grow faster, escape predation, and survive longer)

are these differences genetic?

the clearest evidence for genetic influences on any trait (behavior, physiology, or morphology) comes from breeding experiments in the laboratory (cross-breeding individuals from within and between populations) -- breeding experiments with guppies show that the differences in growth and reproduction are genetic

how long does it take such genetic differences to evolve?

Resnick and Endler transplanted guppies from a lowland stream to highland streams that lacked guppies -- 11 years later (30-60 generations of guppies) guppies there had changed genetically (as shown by more breeding experiments) to resemble natural highland populations in most ways

. . . in other words, they had evolved . . . they had evolved lower reproductive rates!

studies like these lead us to conclude . . .

life histories (age-specific patterns of reproduction and survival) are evolutionary adaptations -- they evolve in response to particular environments just as morphology and physiology do

any environment (including other species) favors a particular balance of growth, reproduction, and survival -- alleles spread if (and only if) they are associated with phenotypes that have a favorable balance -- in other words, these alleles spread as a result of natural selection

an adaptive balance between growth, reproduction and survival is often constrained by unavoidable trade-offs -- most organisms must make trade-offs between survival, reproduction, and growth because . . .

• individuals have a limited supply of resources (time, shelter, energy from food)
• reproduction, growth, and survival are not fully compatible (in other words, resources used for reproduction often cannot also be used for growth or survival -- and vice versa)

. . . and if the environment is likely to vary in the lifetime of an individual, then alleles spread fastest when they are associated with phenotypes that can adjust the balance between growth, reproduction, and survival to the current environment (as do red deer and great tits)

life tables are the key to understanding natural selection -- alleles spread in a population when they are associated with phenotypes with life history strategies (as summarized in life tables) that result in higher per capita rate of increase of descendants