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Natural selection is the evolutionary process which
selects the variation(s) of organisms best suited for a particular
environment. Natural selection and
its evolutionary consequences provide a scientific explanation for
the fossil record of ancient life, as well as for the molecular and structural
similarities observed among the diverse species of living organisms.
The degree of kinship between organisms or species can be
estimated from
the similarity of their DNA sequences; this similarity often
closely matches organisms' or species' classification based on anatomical
similarities.
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DNA Sequence Similarities in Some Primates |
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The
graphic above shows that:
- All of these primates had a common ancestor, the ancestral
primate.
- The human and chimpanzee have the closest evolutionary
relationship as their DNA is the most similar.
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Theory
of Natural Selection
- Overproduction: Within a population more offspring are born than can
possibly survive.
-
Competition: Since the number of individuals in a population tends to
remain constant from generation to generation due to limited resources, a
struggle for survival occurs.
- Survival
of the Fittest: The individuals who survive are the ones
best adapted to exist in their environment due to the possession of variations
that
best suit them to their environment.
This genetic variability within a species is chiefly due to
mutation and genetic recombination.The
variation of organisms within a species increases the
likelihood that at least some members of the species will survive under changed environmental conditions.
- Reproduction: Variations assist or hinder individuals in their struggle
for survival.
The best adapted individuals survive and reproduce,
passing on the favorable variations to their offspring.
- Speciation:
As time and generations continue, adaptations are passed
on and new species may evolve from a common ancestor.
Small
differences between parents and offspring can accumulate in
successive generations so that descendants become very different from
their ancestors.
An adaptation is a variation which assists an
organism or species in its survival. Biological adaptations include include
changes in structures, behaviors, or physiology that enhance survival and reproductive success in a
particular environment.
Some
characteristics give individuals an advantage over others in
surviving and reproducing, and the advantaged offspring, in turn, are more
likely than others to survive and reproduce. The proportion of individuals that have
advantageous characteristics will increase.
Behaviors
have evolved through natural selection. The broad patterns of
behavior exhibited by organisms have evolved to ensure reproductive
success.
Modern Examples of
Natural Selection
 Peppered
moth:
- two varieties of peppered moth existed, a light colored
and a dark colored one
- as industrialization and coal burning increased, the
environment in England where these moths lived became dirtier
- the dark colored variety of the moth blended into the trees
and increased in numbers, while the light colored moth was
less adapted and decreased in numbers
Insect resistance to insecticides:
- Insecticides
kill insects not resistant to the insecticide, while insects resistant to the
insecticide live to reproduce. The insecticide acts as a selecting
agent.
Bacterial resistance to antibiotics:
- Bacteria not resistant to an antibiotic are killed by it, while resistant bacteria live
to reproduce. The antibiotic is a selecting agent for these bacteria.
Evolution and
Extinction
Evolution
does not necessarily mean long term progress is going to go in
a certain direction. Evolutionary changes often
appear to be like the growth of a bush. Some branches survive
from the beginning with little or no change, many die out
altogether, and others branch out repeatedly, sometimes giving rise to more complex organisms.
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Direction of Evolution |
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.
Note the divergence of the various groups
from a common ancestor and the fact that some branches
became extinct. |
Extinction of a species occurs when the environment
changes and the adaptive characteristics of a species are insufficient to allow its
survival. The fossil record indicates that many organisms that lived long ago are extinct.
Extinction of a species is common; most of the species that have lived on
earth no longer exist.
The Fossil Record
Fossils are direct or indirect remains of organisms
preserved in media such as sedimentary rock, amber, ice, or tar. Fossils have been
found that indicate organisms existed well over 3 billion years ago.
These organisms were simple, single-celled organisms.
About a billion years ago, increasingly complex multi-cellular organisms began to evolve.
The higher up you go in an undisturbed rock stratum (rock
layer), the younger the rock layers become and therefore it is believed the fossils
within these layers, as compared to lower rock layers.
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Relative
Dating of Undisturbed Sedimentary Rock and its Fossils |
 Upper strata generally contain fossils of
younger, more complex
organisms, whereas, the lower strata contain fossils of
simpler life forms. This means there is a tendency toward
increasing complexity in life forms over time. |
When comparing fossils in undisturbed strata, fossils can
be found in
upper strata which, although different from fossils in lower
strata, resemble
those fossils. This suggests links between modern forms and
older forms, as
well as divergent pathways from common ancestors.
Classification
Biological
classification is based on how organisms are related.
Organisms are classified into a hierarchy of groups and subgroups based
on structural similarities and evolutionary relationships.
The
species
is the most fundamental unit of classification. This is
a group of organisms which are close enough in their evolutionary
relationship to be
capable of successful reproduction and having fertile
offspring.
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