Life History in a Nutshell
Remember that natural selection results in adaptation, the evolution of characteristics that lead to
high survival and reproduction in the environment of a species. To understand why species differ
in life history, the schedule of survivorship and fecundity, we will consider how different aspects of
life history may have evolved in different environmental conditions. We might intitially expect that,
to maximize the number of offspring they leave, organisms should produce offspring often,
produce many offspring at at time, and live forever. Obviously, they don't, because organisms
have a finite amount of energy.
We can think of life as requiring energy for three basic functions: maintenance, growth, and
reproduction. Energy allocated to one of these functions is not available for others. Thus, putting
energy into growth, which may enhance survival, will decrease the energy available for
reproduction, and vice versa. Similarly, when we consider reproducing, we can note that putting
energy into making many offspring will take energy away from the size of each offspring -organisms can typically produce a few large offspring or many small offspring, but giving that
there is not inifinte energy will not be able to produce many large offspring.
One set of life history theories considers species to fall into one of two categories: r-selected or Kselected (the overall model is called r-K selection.) The terms r and K come from logistic growth:
r refers to the per capita rate of population increase and K refers to the carrying capacity. Here's
how they apply to life history evolution:
Some species will occur in environments that have some kind of unpredictable disturbance, from
a factor such as weather, causing unpredictable adult mortalities. Such species will rarely have
populations grow to carrying capacity, because disturbance will decrease population size
regularly. Such species are predicted by r-K selection to evolve traits that result in rapid
reproduction, and give a high value of the maximum instantaneous per capita population growth
rate (r). Such species are termed r-selected.
Other species will occur in constant environments, and will grow to the carrying capacity (K.) At
carrying capacity, there is a high level of intraspecific competition for resources. Species in such
conditions are predicted to evolve traits that result in high competitive ability of adults and
offspring, and are termed K-selected.
The following table shows how r and K selection are predicted to affect life history traits:
r-selected species
K-selected species
reproduce early, because
unpredictable disturbances cause
high mortality, so individuals who
do not reproduce early are likely to
die before reproduction
reproduce later in life because
early reproduction, before large
adult size is attained, will result in
production of small offspring that
are not competitive
produce many offspring at a
time, because since mortality is
high, individuals who save energy
to reproduce more later will
probably die before reproducing
any more, and will have fewer
offspring overall.
produce few offspring at a time,
because producing many offspring
would require producing small
offspring who would not be
competitive.
produce small offspring,
produce large offspring, because
because producing large offspring
would reduce the number of
offspring that can be produced
small offspring would not survive
well (would not be competitive.)
tend to be semelparous. This
means they have all offspring at
one time. This will evolve because
individuals who save energy to
reproduce more later will probably
die before reproducing more, and
will have overall fewer offspring.
tend to be iteroparous. This
means they have many
reproductive bouts during the life
(not just one.) This will evolve
because individuals can only have
a few offspring at a time, to make
them large and competitive, so to
have many offspring requires
having them over many different
reproductive seasons.
have small adults because they
reproduce as early as possible,
before attaining a large size.
have large adults so that they can
produce large, competitive
offspring.
r-K selection has been applied in two ways:


large groups of species have been categorized as r-selected vs. K-selected. For
example, insects have been called r-selected, mammals K-selected. A problem with this
is that there are other reasons for differences between species besides variability versus
constancy of the environment. Insects are all small because have exoskeleton; mammals
can not be that small because they are endothermic. These size differences affect other
aspects of life history (age at maturity, etc.)
closely related species, or populations of a species, have been categorized as relatively
more r or K selected. Since close relatives are likely to be similar in many ways, this can
allow better testing of whether the differences in life history really reflect differences in
environmental variability, as predicted by r-K selection.
Deermice provide an example of a comparison of closely related species. Peromyscus
maniculatus occurs in a more variable environment thant does P. leucopus, and has more
offspring at a time, smaller offspring, and matures at an earlier age. Thus, deermice fit the
predictions of r-K selection. Many other pairs of closely related species fit the predictions of r-K
selection.
Some species, however, do not fit the predictions of r-K selection. For example, herring live in an
unpredictable, variable environment but are long-lived and iteroparous and large compared to
related fish species.
Species such as herring have shown that the r-K selection model is not complete. Another life
history strategy exists. It is called bet-hedging. Bet-hedging is predicted in environments that
have unpredictable disturbances that increase the mortality of young individuals, but do not affect
adults. Such disturbances are fairly likely to occur since young are generally more vulnerable
than adults. Bet-hedging refers to avoiding the risk of losing all young, if all young were produced
at once and it turned out to be a bad year, by reproducing a few young in many different years.
Bet-hedging thus results in characters similar to K-selection, but for an entirely different reason.
So if we see groups of species, or populations, in which some are have individuals that are more
iteroparous, larger, and have fewer offspring at a time, it could be that those species are Kselected compared to other more r-selected species, or it could be that they are bet-hedging, and
that other species/populations are not.
The models presented so far are evolutionary models, and predict that the differences we see in
life history among species are evolved, genetic differences. An alternative hypothesis to this is
that life history traits show phenotypic plasticity, which occurs when there are different possible
life history responses to different situations. For example, in a good year, individuals may have
more offspring, and in a bad year, fewer. Differences among species or populations may NOT be
genetic; they may reflect phenotypic responses to different environmental conditions. Showing
phenotypic plasticity could be an advantage; it could allow individuals to take advantage of
different environmental conditions, rather than having some genetically determined, inflexible life
history.