Environmental Engineering Reference
In-Depth Information
Evolution and Natural Selection
Biodiversity on Earth has been created through the process of
evolution,
the variation in genetic makeup of a
population of organisms through generations. Genetic changes take place in organisms within a population
over many generations, occurring randomly or through the process of natural selection.
Natural selection
res-
ults when genetic traits that strengthen an organism's chance of survival and reproduction are passed on from
generation to generation, ultimately altering the genetic makeup of a population, creating a new species. In the
process of natural selection, some organisms in a population are better suited to survive than others; therefore,
the organisms that survive are those that have a reproductive advantage and are more likely to pass on their
genes. Across time, entire populations contain traits that increase the likelihood to adapt, survive and repro-
duce.
The traits that lead to success are
adaptive traits,
or
adaptations.
For these traits to be passed on to offspring,
genes in an organism's DNA must code for that trait. During DNA replication, which occurs in an organism's
cells millions of times throughout its life, errors can arise. These errors lead to accidental alterations in DNA,
called
mutations.
Most mutations don't have an effect, but some can be favorable and some can be fatal. The
positive mutations can lead to a better ability to survive and reproduce in an environment, leading to natural se-
lection. These mutations also can occur through mating, when genetic material combines during sexual repro-
duction.
If not enough individuals in a population have traits that help them survive and reproduce in a changing envir-
onment, then the chance is higher that the species will dwindle in numbers and possibly go extinct. The Mon-
teverde golden toads are one example. (See the Case Studies in Appendix B for additional information about
the Monteverde golden toad.)
The environment plays a key role in determining which traits are beneficial; thus, an organism's environment
as well as genetic makeup affect natural selection and ultimately the evolution of a species.
Artificial selection
occurs when humans have an impact on which traits are selected during breeding.
Examples of species that have been artificially selected include dogs, cats, horses, cows, and many types of
flowers.
Types of Selection
Three models of natural selection have been described:
•
Stabilizing selection
occurs when a population's characteristics stay within a moderate range and neither
extreme is dominant. If individuals fall away in either extreme, their chances of survival are reduced.
This structure reduces evolutionary change and diversity. For example, birds are important to flower pol-
lination. If a bird's beak is too short or too long, it will not effectively reach and transfer pollen between
flowers; therefore, a medium-length bill is most effective and most common.
•
Disruptive selection
discriminates individuals with characteristics at the extremes, so fewer individuals
fall within the average. With this model structure, evolution does occur, as individuals fall to either ex-
treme in the population. As a general example, in an area that has small and large seeds for the bird pop-
ulation to eat, but no medium seeds, birds with small and large beaks will dominate while birds with
medium-sized beaks will be present in low numbers.
•
Directional selection
favors one extreme of the population, so the opposite extreme and the average in
the model are not where the majority of a population falls. As with disruptive selection, evolution does
occur. An example is the peppered moth. At one time, most of these moths were light in color, but fol-
