Environmental Engineering Reference
In-Depth Information
change in the mean trait value or mean strategy in the population.That is,
biological differences alone do not lead to the evolution of a trait or a strat-
egy.The reason for this is that if no relationship existed between the trait
value of a parent and those of their offspring, there would be no way that a
parent could pass on the benefit of a particular trait to its offspring. So, the
next reproductive period would produce offspring with the same degree of
biological difference in traits as observed in the parental population.
Evolution via natural selection comes about when three conditions are
fulfilled (Endler 1986). First, there must be biological differences among in-
dividuals in a trait that influences their capacity to cope with prevailing en-
vironmental conditions. Second, individuals with a particular trait must pass
on that trait to their offspring.That is, offspring must inherit a particular
trait from their parents.This will come about by passing a genetic code to
offspring—called genetic transmission of a trait. Finally, there has to be a
consistent relationship between a trait and fitness.Together, these three cri-
teria can be used to explain how different traits or strategies come about
in a population as a consequence of environmental change.
An example of this process is revealed in the evolution of beak depth
among seed-eating finches (
Geospiza
species) of the Galapagos Islands.These
are the celebrated group of finches that, in part, inspired Darwin to for-
mulate his theory of evolution by natural selection. In this group of finches,
beak thickness is important because it determines what kinds of seeds each
species is adapted to utilize. Birds with thick, fat beaks are able to crush thick
seeds that have a hard seed coat. Birds with narrower, nimble beaks are bet-
ter suited to gathering and consuming thin seeds.
Gibbs and Grant (1987) conducted a long-term study during which time
they made careful measurements on beak depths of birds within a popula-
tion under different environmental conditions determined by a cyclic phe-
nomenon of global weather change called El Niño.The measurements were
made during El Niño years when there was plenty of rainfall and during
drought conditions in years between El Niño events. Plants on the Gala-
pagos produce seeds in all kinds of environmental conditions. However, the
seeds of different plant species dominate in different conditions because dif-
ferent plant species do well in different environmental conditions. Drought
conditions favor those plants that produce seeds with thick seed coats; thus
there is a surplus of those seed types in the environment.Alternatively, rainy
conditions favor those plants that produce thin seeds. In drought years then,
there is abundant food for those individuals within a species that have com-