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Evolutionary Processes
Accumulating Evolutionary Changes
Terms to know…
Population - all the members
of a single species that can
interbreed in the same area
Gene Pool – the genes that
are present in a population
Population genetics – the
study of variations in gene
pools
MICROEVOLUTION – small changes in the gene
pool/future population. (example: wolf ancestors
evolving into dogs)
*Basically, microevolution
studies small changes in
alleles that occur within a
population. Over time, these
small changes can
accumulate, resulting in
major differences within the
population. Dogs have been
artificially selected for certain
traits, resulting in a wide
range of breeds with very
different characteristics*
The Peppered Moth (microevolution example)As trees became darker due to industry, darker moths had
a better chance of survival. Over time, more darker alleles
were present in the population.
MACROEVOLUTION – when the accumulation of small
changes over extended periods of time lead to BIG
changes in the gene pool/population (example:
manatees and elephants descending from the same
ancestral species)
Since we can’t see macroevolution occur in our
lifetime, is it even possible?
A. Causes of Evolution
1. Genetic mutations or variation – differences either already
occur in the population (e.g. color variations) or they appear
through random mutation. Different environments lead to the
increase or decrease of these traits through natural selection.
Note: Some mutations may at
first appear harmful, but give
an advantage if the
environment changes, (an
increase in FITNESS)
2. Gene Flow- as members of one population leave an area
and move to another, they sometimes bring new genes with
them. This increases variation and can lead to natural
selection. It also may prevent two populations from becoming
separate species (speciation).
Does gene flow increase or decrease diversity?
Example of GENE FLOW
●Each rat snake represents a separate population
of snakes
●These snakes remain similar and can interbreed
●This keeps their gene pools somewhat similar
●They are considered subspecies
Figure 18.3
Which populations do you
think are the LEAST similar?
Why?
3. Nonrandom Mating- being the fittest means reproducing,
not necessarily living longer. If certain individuals reproduce
more than others (regardless of how long they live), then
natural selection is occurring and their traits will become
more common in future generations. Also called “sexual
selection”
a. Random mating is pairing by chance
b. Nonrandom mating – individuals choose their mates
Which method of
mating do humans
use?
Can you think of any
organisms that mate
randomly?
Sexual selection occurs when
males compete for the right to
reproduce and the female selects
males of a particular phenotype.
(guppies, lions)
This results in some extreme
characteristics.
Sexual
dimorphism when males and
females of a
species look
different.
4. GENETIC DRIFTchanges in the gene pool due to random chance. This really only
occurs in very small populations.
Bottleneck Effect – a type of genetic drift caused by a
sudden and severe reduction in population. It reduces
overall diversity, and can lead to extinction. Ex Cheetah
Cheetahs have very little diversity in their gene pool due
to bottleneck
http://evolution.berkeley.edu/evolibrary/news/070701_cheetah
FOUNDER EFFECT
Another example of
genetic drift where rare
alleles or combinations
are found in the original
individuals establishing a
small and isolated
population
Dwarfism is really
common in Amish
communities due to the
higher than average rate
of this allele (by random
chance) that was found in
the original German
founders.
Five in Five, Five Words or Less
1. Gene Pool
2. Microevolution
3. Gene Flow
4. Phenotype
5. Genetic Drift
Those can cause Natural
Selection…
So what happens next?
A. Results of Natural Selection
1. Directional Selection
One phenotype is
favored over another
and gradually becomes
more common; Causes
a shift in one direction of
the overall appearance
of the species over
many generations
Ex: horses get larger,
peppered moth
2. STABILIZING SELECTION
occurs when all
extreme
phenotypes are
eliminated and the
average phenotype
is favored; causes
the population to
become more
similar over many
generations
Ex: number of eggs
per nest
Human babies have an average size due to
stabilizing selection. Babies that are too big can't
get through birth canal while babies that are too
small have low survivability after they are born. Too
big and too small have been selected against for
millennia, leading to most babies weighing between
5 and 9 pounds at birth.
How do you think the future of baby birth weight will
change thanks to modern medicine? Are we still
experiencing stabilizing selection?
3. DISRUPTIVE SELECTION -- occurs when extreme
phenotypes are favored and the middle trait is selected against;
can lead to a population splitting and forming two separate
species.
Imagine this scenario....
Sleebos come in many sizes, the most common Sleebo is 4
inches long, but some can be 10 inches and others can be as
small as 1 inch. A new predator is introduced to the Sleebo
island. Small sleebos are able to hide under rocks and avoid
being eaten. Large sleebos are too big for the predator to eat.
What will happen to the Sleebo population?
Types of Selection
Back to MACROEVOLUTION
All of these evolutionary
changes accumulating over time
can result in so many
differences that scientists elect
to reclassify the future
generations as a new species.
Speciation – the splitting of one
species into two or more
species
Scientists decide when speciation occurs, so it’s important to know how
we define a species. Surprisingly, this is not actually an easy idea to
define…
A species is a group of similar individuals who can SUCCESSFULLY
and NATURALLY interbreed (However, this is one of MANY definitions)
When species collide…
Hybrids occur when members of
different (but very closely related)
species produce viable offspring.
However, hybrids are rare in nature
and often are not very healthy.
Lion + Tiger = Liger
Horse
+
Donkey
=
Mule
Zebra
+
Donkey
=
Zonkey
Example of SPECIATION
Flycatcher species
●Empidonax minimus
●Empidonax virescens
●Empidonax tralli
These birds all live in the
same area. In order to be
separate species, they cannot
interbreed. Any idea what
stops them from mating with
each other?
Figure 18.10c
Each species
has a unique
song and each
species occupies
a different
habitat during
their mating
season.
What would
happen if two of
these birds did
mate?
How to make a new
species…
In order for speciation to occur, a
population must be split
reproductively and prevented from
interbreeding. Here is how this
happens…
REPRODUCTIVE ISOLATING MECHANISMS
A. PREZYGOTIC - before zygotes
1. Habitat Isolation – different
locations
2. Temporal Isolation – different
reproductive timing
3. Behavioral Isolation – not
attracted to one another (i.e.
different songs)
4. Mechanical Isolation – the
anatomy doesn’t fit
5. Gamete isolation – the sperm
and egg don’t work together
Damselfly penises
REPRODUCTIVE ISOLATING MECHANISMS
B. POSTZYGOTIC – after
the zygote
1. Zygote mortality –
miscarriage
2. Hybrid sterility – the
offspring can’t reproduce
3. F2 fitness – the hybrids
are selected against
because they are not
healthy
MODES OF SPECIATION
1. Allopatric SpeciationPopulations are
physically separated by
geographic barriers and
genetic variations
accumulate over time
2. SYMPATRIC SPECIATION
Members of a population
stop interbreeding, but
without any physical
barrier, usually caused by
mutation or sexual
selection
Ex. A mutation changing
chromosome number
Ex. Mate preference
Sympatric vs Allopatric
Speciation can occur through splits or just the simple (linear)
accumulation of differences.
ADAPTIVE RADIATION
speciation in multiple
directions; a single ancestral
species becomes several
different (but similar) species