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Species evolve with
significantly
different
morphological and
behavioural traits
due to genetic drift
and other selective
pressures.
Example –
Homologous
features such as
tailbones are
evident in the
embryos of these
organisms.
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Divergent evolution occurs very rapidly
and simultaneously in one species
resulting in the production of three or
more new species.
Ground finches migrated from South
America (where they ate medium-sized
seeds) to the Galapagos Islands.
The island did not exhibit much
competition, thus the founding
population was very successful.
Since there were many food sources
available, different seed sizes, insects,
and cactus, the ecosystem grew to
contain 13 different species which
descended from the founding
population.
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Selection patterns operate to produce striking similarities
among distantly related species.
Some organisms, such as dolphins and shark, are
morphologically similar, but their genetic history is different.
Both are streamlined, fast swimming, and carnivorous, but a
shark is a fish, and a dolphin is a mammal.
When traits are similar in appearance, but have different
evolutionary origins they are said to be homoplasies (a.k.a.
analogous features)
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Marsupial
mammals evolved
in isolation from
placental
mammals, yet in
Australia, natural
selection favoured
species with traits
from both groups.
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One species evolves in
response to the evolution of
another species – they are
dependent on one another for
survival (a.k.a. reciprocal
adaptation).
Yucca moths and yucca plants
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Yucca flowers are a certain shape
so only that tiny moth can
pollinate them. The moths lay
their eggs in the yucca flowers and
the larvae (caterpillars) live in the
developing ovary and eat yucca
seeds.
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Recall the classification of living things – Kingdom, Phylum,
Class, Order, Family, Genus, Species.
This system demonstrates how organisms are anatomically
related.
A phylogenetic tree demonstrates the evolutionary relationship
among organisms.
Cladistics is a method for determining the evolutionary histories
of organisms – a cladogram is created.
The construction is based on the analysis of shared derived
(apomorphic) traits, or features that are shared only by
members of the group with the common ancestor (these are
called synapomorphies).
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A phylogenetic tree is a cladogram to which
additional information, such as evidence of
the dates of separation of lineages, has
been added.
The trunk represents the point in the past
when the lineage consisted of only one
ancestor.
What evidence is used to infer how traits
change during evolution?
 Determine the original state of the trait
 Determine how the trait has been
modified
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Recall the difference
between homologous and
analogous features:
The supporting structures
of bat and bird wings are
derived from a common
tetrapod (four-limbed)
ancestor and are thus
homologous. Although
they are also used for
flight (and thus are
analogous), insect wings
evolved independently
and their supports are not
homologous with the wing
bones of bats and birds.
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Consider the following cladogram:
A – ancestor to all vertebrates had fins
B – ancestor of all non-fish vertebrates had evolved 4 limbs.
Remember that ancestral features are not used to determine
relationships (eg: presence of a tail)
The tree below provides information about the relative
sequence in which species diverged or split.
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The ingroup include species that are chosen for
studying.
The outgroup are similar, but more distantly related
(the first group to diverge from the ingroup).
Refer to Table 1 on the handout “Applying
Cladistics”.
Considering the previous cladogram, which pair of groups is
more closely related (i.e.: shares a more recent common
ancestor) ?
Fern and liverwort or wheat and pine?
A: If we trace back the pathways for each of these pairs, they
lead to a clade (a point of divergence). A clade represents
the point where to groups last shared a common ancestor.
Fern and liverwort: diverged at clade B
Wheat and pine: diverged at clade D
1.
2.
Which group is more closely related to liverwort:
moss or maple?
A: When we trace back the lineage for moss to see where it
diverged from liverwort, and then do the same for maple
and liverwort, we see that they diverge from the same
point, at clade B.
3. If a different synapomorphy was described and it was known
to occur in moss and in pine, in which other groups must it
also be found?
A: If this trait is seen in pine and moss, there are two
possibilities. It might have evolved twice, independently in
two groups. Or, it evolved once in history. The most likely of
these two possibilities is that it evolved one time, and so
must have developed before the divergence of moss.
Therefore, all groups after that point must also have the
same trait. So we would also see the trait in fern, spruce,
maple, wheat and orchid.
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Amino sequences can be analyzed to construct
phylogenetic trees. Eg. Amino acid sequence for
cytochrome C, found in mitochondria:
Red letters indicate differences
from the human sequence while
blue letters indicate similarities
common to all species studied.
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Sequences of DNA inserted by viral infection into a
species’ genome can be studied to construct phylogeny.
SINEs – Short interspersed nuclear elements: repeated
DNA sequences of 300 base pairs in length
LINEs – Long interspersed nuclear elements: repeated
DNA sequences between 5000-7000 bp in length
These sequences do not express any RNA or protein
Since these insertion events are incredibly rare, if 2
species have the same SINE or LINE located at the same
position in their DNA, it is assumed that the insertion
occurred only once in a common ancestor.