As we've said, evolutionary theory seeks
to explain the origin of species and biodiversity: Why are there
so many species? After all, why doesn’t natural
selection produce a “super species” that does all things well,
out-competing all others? To answer that question, we need to understand
tradeoffs and how they factor in evolution.
Suppose you are looking to buy a new vehicle,
and you want something with a powerful engine, for towing a trailer.
But you are mindful of the price of fuel and want something that
is fuel-efficient. You will soon realize that there is no such
thing as a vehicle that is both powerful and fuel-efficient. This
is because power comes at a price: powerful engines are big and
require large amounts of fuel. Smaller engines use less fuel,
but are not well-suited for towing because they lack horsepower.
Of course, you may compromise on both and settle for a vehicle
that is somewhere in between - moderate power and moderate fuel
efficiency.
This predicament illustrates the principle of tradeoffs: Specializing
in one thing means doing other things less well. In other words,
a super-species is impossible because of the metabolic, developmental,
and ecological tradeoffs inherent to living things, tradeoffs
that result from a diversity of habitats and a limited supply
of energy.
Evolutionary theory predicts that traits that result in the highest
number of offspring will become increasingly
common in a population. That is, populations will come to
be characterized by traits that maximize lifetime reproductive
success (fitness).
Exactly which traits are successful is determined by the conditions
in which the population lives (including its evolutionary history).
Remember, fitness is dependent on context. And tradeoffs
occur when a trait is beneficial under one set of conditions but
lowers fitness in other situations[#](Rosenzweig 1995)(Sih, et al. 2004).
A classic example concerns growth rate and resistance to desiccationThe drying out of a living organism
in two species of barnacles[#](Connell 1961b).
Free-swimming in their larval stages, adult barnacles
are usually fixed to a hard surface, where they feed by filtering
planktonAny drifting organism that inhabits the open waters of oceans,
seas, or bodies of fresh water.
and particles of organic matter from the water. Changes
in water levels leave many barnacles exposed to air at low tides.
Connell studied two sympatricOccuring in the same place
species of barnacle to understand why two species coexisted,
rather than one excluding the other via competition.
He showed that one species, Balanus balanoides, has a
faster rate of growth and can overgrow, smother and displace the
other, Chthamalus stellatus. But to fuel their rapid
growth, Balanus must eat, meaning they need to spend
more time underwater. By specializing on rapid growth, however,
Balanus have lost the ability to resist desiccation when
exposed to air. On the other hand, Chthamalus are able
to survive long periods out of water, but must therefore get by
on less food, meaning they grow more slowly[#](Connell 1961b).
A diversity of habitats thus leads to a diversity of species.
A species that can "make a living" in a variety of habitats
(a generalist, like our compromise vehicle above) may benefit
from the ability to exploit a variety of resources, especially
if habitat is lost due to disturbance (like fire or volcanic activity)
or if habitat quality changes with some frequency. On the other
hand, specialist species exploit a subset of resources or habitats
more effectively than generalist species. However, if a
species evolves into a specialist, it effectively abandons all
other resources or habitats, as well as the ability to exploit
them[#](Rosenzweig 1995).
This creates opportunities for other species, generalist
or specialist.
Reproductive strategies provide other examples of the effect
tradeoffs can have on the evolution of species. Because energy
is limited, organisms have limited resources that they are
able to devote to reproduction. One important tradeoff concerns
the balance between number of offspring vs. the size of each offspring[#](Berkeley 2007)(Kolm, et al. 2006).
Producing more offspring means producing smaller offspring;
producing larger offspring means producing fewer offspring. The
'strategy' that leads to the highest reproductive success for
that population in that environment will become increasingly
common.
Another common tradeoff occurs over the degree of parental care[#](Burley & Johnson 2002)(Satou, et al. 2001)(Rueber, et al. 2004).
Parental care costs both energy and time, which means
reproducing less often and having fewer offspring each time, but
it also means your offspring are more likely to survive. Another
involves age at sexual maturity, a compromise between reproducing
now and reproducing sometime in the future[#](Bielby, et al. 2007)(Gardmark, et al. 2003)(Day, et al. 2002).
Beginning to reproduce early in life can be advantageous
if mortality rates (from predation or death by other causes) are
high among older age classes. However, if mortality rates are
highest for juveniles and low for adults, reproductive success
is highest for individuals that reach sexual maturity later[#](Reznick, et al. 1996).
This is presumably because young individuals, being smaller,
have less energy to devote to offspring, which have high mortality
rates as a consequence.
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