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Evolutionary Types and Models
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Convergent evolution
Convergent evolution occurs when organisms
that are NOT closely related live in the SAME
environment. They independently evolve
similar traits or structures that are adapted to
that shared environment.
Structures that are the result of convergent
evolution are termed ANALAGOUS
structures.
What similarities are shared by the following animals found in
similar environments all over the world that all like to eat ants and termites?
Anteater, South America
Sloth bear, South east Asia
Aardvark, Africa
Numbat, Australia
Echidna, England
Armadillo, USA
All of the animals on the previous slide have
developed powerful fore claws and long,
sticky tongues (most housed in long snouts)
that allow them to open the homes of social
insects (ants and termites) and eat them.
These include the four species of anteater, more than a
dozen armadillos, eight species of pangolin (plus fossil
species), the African aardvark, one echidna (an egglaying monotreme), the Australian marsupial known as
the numbat, the aberrant Aardwolf, and possibly also the
Sloth Bear of South Asia, all not related.
This is an example of CONVERGENT evolution,
common adaptations to similar environments.
What common traits do the
following animals seem to
share?
The North American kangaroo rat
(top), Australian hopping mice
(middle) , and North African and
Asian jerboa (bottom) have
developed convergent adaptations
for hot desert environments; these
include a small rounded body
shape with very large hind legs
and long thin tails, a characteristic
bipedal hop, and nocturnal,
burrowing and seed-eating
behaviors. These rodent groups fill
similar niches in their respective
ecosystems.
Convergent Evolution in Plants
Prickles, thorns and spines are all
modified plant tissues that have
evolved to prevent or limit
herbivory, these structures have
evolved independently a number
of times.
The aerial rootlets found in ivy
(Hedera, see left) are similar
to those of the climbing
hydrangea (Hydrangea
petiolaris, see right) and
some other vines. These
rootlets are not derived from a
common ancestor but have
the same function of clinging
to whatever support is
available
Divergent Evolution
Divergent evolution occurs when…
related species evolve different traits to the point where
they actually become a new species
Divergent evolution can result in the formation of….
HOMOLOGOUS structures, structures that are similar in
appearance but serve different functions
The most well know example of divergent evolution is with
Darwin’s finches
Remember: when Darwin was on his voyage on the HMS
Beagle he spent much time in the Galapagos islands. He
noticed that on each island there were finches but the
finches on each island, or group of islands, differed in
the size and shape of their beaks.
Why might the finches below,
found on different islands,
have developed differently
shaped beaks?
The birds were all about
the same size (10–20
cm).
The most important
differences between
species of finch were in
the size and shape of
their beaks, and the
beaks were highly
adapted to different
food sources.
The birds were all
brownish or black.
Their behavior differed,
and they had different
song melodies
Darwin theorized that at one
time there must have been
a common ancestor to the
finch species. That
common ancestor slowly
was dispersed as the
Galapagos islands were
formed and broke away
from one another.
Genetic variations amongst
the finches beaks were
then selected for by the
environment over a period
of many years, resulting in
the formation of new
species.
Speciation
Speciation is the
evolutionary process by
which new biological
species are formed.
Speciation can occur
in a variety of different
methods, termed
allopatric, peripatric,
parapatric and sympatric
speciation
During allopatric speciation, a
population splits into two
geographically isolated allopatric
populations (for example, by habitat
fragmentation due to geographical
change such as mountain building or
social change such as emigration).
The isolated populations then
undergo changes in their genetic
makeup as they (a) begin to adapt to
different environments or (b) they
undergo mutations. When the
populations come back into contact,
their genetic material has evolved so
much that they are no longer
capable of exchanging genes
In peripatric speciation,
new species are formed
in isolated, small
peripheral populations
which are then
prevented from
exchanging genes with
the main population.
Genetic drift is often
proposed to play a
significant role in
peripatric speciation.
In parapatric speciation,
the zones of two
diverging populations are
separate but do overlap.
There is only partial
separation afforded by
geography, so individuals
of each species may
come in contact or cross
the barrier from time to
time.
However, heterozygous
(one dominant, one
recessive allele) traits are
selected against by the
environment to the point
where the two species
can no longer produce
offspring together
anymore.
In sympatric
speciation,
species
diverge while
inhabiting the
same place.
Often cited
examples of
sympatric
speciation are
found in
insects which
become
dependent on
different host
plants in the
same area.
Rate of Evolution
There are two theories that are used to explain
how fast evolution is believed to occur.
#1. Gradualism
• States that evolution of species is slow, continuous and
gradual, subtle changes occur over million of years
• This theory would explain the adaptations observed in species
that are very well adapted to their environments such as
sharks, cockroaches and crocodiles.
#2. Punctuated Equilibrium
• States that during times of climactic or geographic changes in
environment organisms evolve more rapidly
• Some changes occur in rare, rapid events after a period of little
or no change
• This is the more accepted theory for most plant and sexually
reproducing land animal species.
#
of
C
H
A
N
G
E
S
Millions of Years
Decide which of the line in the graph above portrays the rate of gradualism
and which portrays the rate of punctuated equilibrium?
Do the theories of Gradualism and Punctuated
Equilibrium seem to oppose one another?
Punctuated equilibrium is often mistakenly thought to oppose the
concept of gradualism, when it is actually a form of gradualism. This
is because even though evolutionary change appears instantaneous
in fossils found in neighboring layers of the sediment of the earth,
change is still occurring incrementally, with no great change from
one generation to the next. Even though the graph lines on the
previous slide look to change quickly, we are still talking about
changes occurring slowly over millions of years. The changes are
simply occurring in intervals rather than continuously.
The relationship between punctualism and gradualism can be better
appreciated by considering an example. Suppose the average length of a
limb in a particular species grows 50 centimeters (20 inches) over 70,000
years—a large amount in a geologically short period of time. If the average
generation of that species is seven years, then our given
time span corresponds to 10,000 generations. It is therefore
reasonable to conclude that if the limb size in our
hypothetical population evolved in the most conservative
manner, it need only increase at a rate of 0.005 cm per
generation (= 50 cm/10,000), despite its abrupt appearance
in the geological record.