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Fossil Record, Homologies,
& Evolutionary Mechanisms
Objectives: 7A - Analyze & evaluate how evidence of common
ancestry among groups is provided by the fossil record,
biogeography, and homologies, including anatomical, molecular, and
developmental; 7B - Analyze & evaluate scientific explanations
concerning any data of sudden appearance, stasis, and sequential
nature of groups in the fossil record; 7F - Analyze & evaluate that
effects of other evolutionary mechanisms, including genetic drift,
gene flow, mutation, and recombination
Scientists have used the
fossil record to construct a
history of life on Earth.
This is only a theory
Fossil record is not complete
Homologous Body
 Structures That Have Different
Mature Forms But Develop From The
Same Embryonic Tissues
A cladogram is a theoretical
evolutionary tree made using cladistics.
– A clade is a group of species that are thought to
share a common ancestor.
– Each species in
a clade shares
some traits with
the ancestor.
– Each species in
a clade has
traits that have
 Vestigial Structure - structures
that have lost some or all of
their ancestral function in a
given species, but have been
retained during the process of
Mechanisms of Evolution
There are several:
1. Natural Selection
2. Gene Flow
3. Genetic drift
4. Mutations
5. Non-random mating
1. Natural Selection:
 Affects variation in a population as
the better adapted (more fit)
individuals survive and reproduce,
passing on their genes to the
successive generations.
 Acts only upon an organism’s
phenotype (its physical
 If the phenotype is better suited for
a changing environment, the
individual can survive and pass on its
Natural Selection
2. Gene Flow:
 Is the movement of alleles into or
out of a population (immigration or
 Gene flow can introduce new
alleles into a gene pool or can
change allele frequencies.
 The overall effect of gene flow is
to counteract natural selection by
creating less differences between
 Example:
 Plant pollen being blown into a
new area
Gene Flow
3. Genetic Drift
 The change in allele
frequencies as a result of
chance processes.
 These changes are much
more pronounced in small
Examples of Genetic Drift
 A) The Founder Effect:
 Small population that branches
off from a larger one may or may
not be genetically
representative of the larger
population from which it was
 Only a fraction of the total
genetic diversity of the original
gene pool is represented in
these few individuals.
Examples of Genetic Drift
 B) Population Bottleneck:
 Occurs when a population undergoes
an event in which a significant
percentage of a population is killed
or otherwise prevented from
•The event may
alleles entirely
or also cause
other alleles to
be overrepresented in a
gene pool.
EX. Cheetahs
4. Mutations:
 Are inheritable changes in the
genotype (genes).
 Provide the variation that can be
acted upon by natural selection.
 Mutations provide the raw material
on which natural selection can act.
Exp. Frog mutation
5. Non-Random Mating:
 In animals, non-random mating can
change allele frequencies as the
choice of mates is often an
important part of behavior.
 Many plants self-pollinate, which is
also a form of non-random mating