Sci 103 Spring 2006 – Outline 18
Reading:
Darwin, Terminology, Evidence for Evolution, Population Evolution, and Speciation
Chapter 20
EVOLUTION
Evolution and Religion
Evolution does not preclude religion (could be the mechanism used by a creator,
but this cannot be tested, therefore is not scientific). Religion is dependent on faith,
which is separate from science. Therefore, religion does not have a place in the science
classroom. This does not preclude it from being taught in other courses, however.
Please consult the following website for information regarding this controversy
and guidelines for teaching science in schools. This site states the opinions of scientists
and religious clergy. The website includes a downloadable guide for teachers.
http://www.aaas.org/programs/centers/pe/evoline/index.shtml
Charles Darwin
(1809-1882)
1. Theory of Evolution
(a) Origin
Proposed by Darwin in 1859 (date of publication, written in 1842) in “The Origin
of Species”.
(b) Description
Darwin suggested that evolution occurs by natural selection (he did not
challenge the existence of a divine creator – possible that the creator used natural
methods to produce change over time = evolution).
2. Darwin’s Evidence
(Sailed on HMS Beagle for 5 years and was therefore able to study natural history
in a number of locations. He made comparisons – giving him the evidence required to
explain evolution by natural selection).
Figure
(a) Fossils
Observed that extinct species resemble living ones – did one give rise to the
other?
Figure
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(b) Geographical Distribution
The organisms in one geographical area tend to be distinctive – suggesting
common lineage – whereas those in areas with similar climates that are separated
geographically (different continents) tend to be dissimilar/unrelated.
(ie. similar organisms in one location descended from the same ancestors).
(c) Islands
Organisms on islands often are generally similar to those on the mainland, but
have slight variations – suggesting that they are descended from the same ancestors but
have changed slightly as they have become adapted to their new habitat.
3. Darwin’s Finches
Figure
(a) Collection
(i) Darwin collected 31 individual finches from the Galapágos Islands.
(ii) These represented 13 species, all similar to one another, apart from their bills.
(b) Beak Size and Shape reflects Food Source/Use
Finches with large beaks crush seeds.
Finches with narrow beaks eat insects.
Finches with sharp beaks drink seabird’s blood.
Therefore, their bills are adapted to function – evidence for natural selection (ancestor
modified to form new species adapted to a particular habitat).
Terminology for Evolution
1. Evolution
The continuous genetic adaptation of a population of organisms to its
environment over time.
2. Natural Selection
(a) Description
The process by which individuals (that have characteristics that are advantageous
in a particular environment) produce more offspring than other individuals (that do not
have these advantageous traits).
(b) Mechanism
These characteristics are selected for by the environment, eg. Snowy
environment will select for white individuals (will live longer and therefore produce more
offspring and dominate environment).
Figure
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3. Macroevolution
The large-scale patterns, trends, and rates of change over long periods of time,
eg. generation of new species.
What is a species (The Biological Species Concept)?
Groups of actually or potentially interbreeding populations which are
reproductively isolated from other such groups.
Members of a species have the ability to produce fertile offspring with other
members of the species.
Figure 20.12
4. Microevolution
Changes in allele frequencies ie. within a species (at population level) (more
offspring of a certain type in the population selected for).
5. Adaptation
(a) Description
Process of becoming more adapted to surviving a particular set of environmental
conditions.
Eg. Sickle cell anemia – heterozygote advantage to malarial infection.
Figure
(b) Mechanism
Natural selection (organisms become increasingly adapted to their environment
as advantageous alleles are selected for).
6. Darwin’s Theory of Evolution in Modern Terms
Evolutionary changes within a species (microevolution) occur via natural
selection so that the species becomes increasingly adapted to its environment.
These changes accumulate and eventually result in new species
(macroevolution).
N.B. The changes within species occur as a result of mutation. These mutations are not
created as a result of selection pressure, but are random.
Evidence for Evolution
1. Fossil Record
(a) Description
Fossils are the preserved remains, tracks, or traces of organisms.
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(b) Successive Evolutionary Change
Seen in the fossil record – supports evolution (change in phenotype over
generations).
Eg. Oyster shell shape and horse evolution.
Figures
N.B. Fossilization is a rare event, therefore there ARE gaps in the fossil record. This is
not, however, evidence against evolution. For instance, in the past, creationists have used
gaps in the fossil record of whales as evidence against evolution. New fossils have
recently been found that fill these gaps.
Figure
2. Molecular Record
(a) Sequence Divergence
(i) Each evolutionary change occurs by substituting new alleles for old ones (occurs by
mutation).
(ii) Therefore, a series of evolutionary changes occurs by changing the nucleotides of
DNA. This means that more distantly related organisms have more differences in their
DNA sequences than closely related organisms.
(iii) By comparing DNA sequences (align sequences of 2 organisms and count how
many bases are different) biologists can determine how closely related organisms are
(their phylogeny).
Eg. DNA Sequences:
Species 1
Species 2
AAACGTAGCC
AAAGGTAGCC
 1 nucleotide difference out of 10 nucleotides = 10% sequence divergence
(b) Phylogenetic Trees
(i) Use
Represent (and understand) the evolutionary history of a group of organisms.
(ii) Significance
Phylogenetic trees for many genes support the anatomical record.
Figures
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3. Anatomical Record
(a) Development
Embryonic development of vertebrate embryos is similar and therefore suggests
evolution from a common ancestor.
Eg. All at one stage have gills and a tail.
Figure
(b) Sharing the Same Parts
(i) Homologous Structures
Structures that (although they may differ in form and function) are derived from
the same body part of the common ancestor.
Eg. Bones of vertebrate limbs (same basic bones used in different ways).
Figure
(ii) Analogous Structures
Features that resemble one another as a consequence of the same types of
selective pressures, but were formed from different structures. (Evolution by natural
selection, but NOT a common ancestor).
Eg. Wings in birds, reptiles, and mammals.
Convergent evolution – Australian marsupials and placental mammals occupy
similar niches.
Figure
(c) Vestigial Structures
Structures that are diminished or had a greater physiological significance in the
ancestor than the descendent.
Eg.
Appendix – Has no known function in humans, but in apes it is used to house
bacteria that digest the cellulose cell walls of the plants they eat. The presence of an
appendix in both groups suggests a common ancestor.
Whale pelvic bones – Reduced.
Figure
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Population Evolution (Microevolution)
How do allele frequencies within populations change?
Figure
1. Mutation
The source of new alleles, but occurs at a very slow rate. Therefore, other factors
are more important in the evolution of populations.
2. Gene Flow (Migration)
Movement of alleles from one population to another.
Maintenance of new allele frequencies are dependent on whether the migrating
individual can adapt to the new environment.
3. Non-random Mating
Eg. Selective mating.
4. Genetic Drift
Random changes in allele frequencies of a population as a result of chance events.
Eg. A freak storm removing most individuals in a population. Only a few individuals
remain.
5. Selection
(a) Natural Selection
Selection by the environment.
Eg. Peppered Moth
Birds = Selecting agents
Figure
(b) Artificial Selection
Breeder = Selecting agents
Figure
Process of Speciation
Species formation occurs over a period of time and involves a number of steps:
1. Adaptation of Populations to their environment or random changes may occur in
separate populations.
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2. Formation of Ecological Races
Allele frequencies change enough within each population that they are considered
ecological races.
3. Isolating Mechanisms Prevent Population Hybridization
(Occurs when there is no gene flow between populations).
Examples of isolating mechanisms:
(i) Geographic Isolation
Physical separation (eg. 2 populations isolated by a mountain range).
(ii) Temporal Isolation
The 2 populations reproduce in different seasons or different times of day.
4. Two Races become Incapable of Interbreeding
 separate species.
Figure 20.13 and 20.14
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