EVOLUTION LESSON 4
Species & Speciation
Speciation
• Gradual process in which one population separates from
another.
• AND builds up enough genetic differences to become a
new species.
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Biodiversity threatened
Example: giant
panda
Protect some
species’ existence
Stop evolution of
new species (ex:
bacteria resistant)
Define Species:
• One or more populations of individuals.
• Can interbreed under natural conditions.
• And produce fertile offspring.
• That are reproductively isolated from other such
populations.
ONLY WITH
CAN produce
fertile offspring.
When does Speciation happen?
• When changes in allele frequencies occur that are
significant enough
• To mark the formation of a new species
• Distinct from the parental species.
• How did divergence/change in allele frequency happen?
• Last lesson: mutation, selection, genetic drift.
Barrier to reproduction
Preventing inter-breeding
between 2 closely related
species.
What’s the main POINT?!
Interbreeding happens
Interbreeding prevented
BARRIER in place
New species CANNOT
form.
Speciation begins.
Different ways speciation
occurs – will be mentioned
later on
Speciation process began……
I’m the Strong evolutionary
pressure (Natural Selection) and
this is my COMMAND
Command! DO THIS OR ELSE!
• Members of NEW species are to
AVOID MATING with members of the
old.
What happens if interbreeding (i.e. mating between different
species) happened?
Because of genetics, the hybrid offspring produced are
often less successful. They die early or produce few or no
offspring.
PUNISHMENT (OR ELSE) – Natural
Selection punished individuals that
accidentally mate outside their own
species
How? They will leave fewer successful
offspring.
You DISOBEYED ME… prepare to
be PUNISHED!
SO that’s a waste..
• Producing unsuccessful offspring – a waste.
• Strong selection pressues – ensuring species mate with
members of their own only.
• Preventing interbreeding – Reproductive barriers.
Biological barrier
(genomes too different)
Geographical barrier
(physical separation)
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Reproductive Isolating Mechanisms
• Anything that PREVENTS successful reproduction from
happening.
• Important in speciation… why?
• ALLOWS one species to diverge (i.e. separate) into two separate species.
TWO TYPES
Pre-zygotic
Post-zygotic
1) Pre-zygotic Isolating Mechanisms:
• What’s a ZYGOTE? FERTILIZED EGG.
• Pre-zygotic isolating mechanisms?
• BARRIERS to reproduction.
• Occur BEFORE a zygote can form.
• Usually before mating.
• Think about it:
2 populations can’t get
together to mate.
DIVERGE GENETICALLY from
each other more over time
Genes WON’T MIX
Pre-Zygotic isolating mechanisms:
• Favored in evolution – because they are more efficient
• Resources are NOT wasted producing unsuccessful offspring.
• Several – can occur through differences in space, time,
behavior, or breeding mechanisms.
• Ways pre-zygotic isolation can occur:
1) Geographic Isolation.
2) Habitat Isolation.
3) Temporal Isolation.
4) Behavioral Isolation.
5) Mechanical Isolation.
6) Gametic Isolation.
1) Geographic Isolation
• Populations become isolated
because of physical barrier.
• Occurs very gradually.
• Once speciation occurred, the
two populations won’t be able
to interbreed anymore EVEN
if BARRIER is REMOVED.
2) Habitat Isolation
• Two populations occupying different habitats
• So they are less likely to meet and attempt to reproduce.
• Commonly experienced by non-moving organisms (ex:
Plants).
Common ancestor
Closely related
tree species
Scarlet Oak
Black Oak
Prefer moist-low
lying areas
Prefer higher drier
areas
• Occur in same ecosystem BUT occupy DIFFERENT HABITATS within it.
• Speciation happened? YES
• WHY? Because the two populations of the ancestral species began occupying
slightly different habitats.
• SO? Over time each became adapted to its own environment (wet or dry).
• Because of the physical separation, less chances their gametes would meet.
• With time, genetic changes built up within each population – became separate
species!
3) Temporal Isolation
• 2 populations may share an
ecosystem.
• BUT reproduce at a different time of
year.
• Ex:
• Plants – blooming different times of the
day.
• Animals – different mating seasons
(spring and fall).
Prefer Spring
Two species of termites
are REPRODUCTIVELY
ISOLATED because they
are not mating at the
same time of the year.
Prefer Fall
4) Behavioral Isolation
• Even if 2 populations/species occur in the same place and
same time, they may still be isolated by BAHEVIOR.
• Animals have characteristic behaviors that allow males
and females to recognize each other and trigger mating.
• Enough difference between populations – NO mating.
• Ex: firefly.
No mating
Emitting specific patterns of
flashes to attract females.
Female not recognizing pattern of
flashes (from different species).
Another example: crickets
Mating happens
Female recognizing it
Emit characteristic chirps to
attract females
No Mating
Female NOT recognizing
chirp pattern
Pattern of chirps is UNIQUE to each species.
• This helps prevent mating between different members of 2 different species (i.e.
PREVENTS INTERBREEDING).
• Other examples of behavioral barriers to reproduction: bird songs, chemicals
given off by insects.
5) Mechanical Isolation
• When animal or plant reproductive structures are NOT
compatible.
• Result: Mating CAN’T occur.
• Ex: Dragonfly
• Male dragonfly has claspers for holding on to female
during mating.
• If male grabs female of wrong species, he will be in wrong position,
and sperm transfer won’t occur.
• Genitals – like lock and key (one key for the lock).
• So those dragonflies can only SUCCESSFULLY mate
with member of their own species.
6) Gametic Isolation
• Even if gametes (sperm and egg) meet, they may not form a
viable zygote.
• Ex:
• Sperm of one animal species can’t survive in the
reproductive tract of another species.
• Egg may have chemical barriers that prevent sperm of
another species from entering it.
Post-zygotic Isolating Mechanisms
Sometimes individuals mistakenly mate with a member of the
wrong species and transfer gametes.
BUT there are STILL mechanisms to prevent interbreeding
Post-zygotic isolating mechanisms.
• When? happen at or after a zygote is being formed.
• How? they prevent hybrid offspring from developing OR
being able to breed later.
interbreeding
Sterile (can’t reproduce when
mating)
Hybrid produced.
DIED
Worst case:
F2 generation
Even if Zygote forms and develops into an adult, barriers still
occur to prevent interbreeding.
• Hybrid could be sterile (can’t reproduce) or
• Hybrid’s offspring (F2 generation) can have high mortality (death) rate.
Post-zygotic mechanisms:
1) Zygote mortality
• Several species interbreed but when they mate, the zygote fails to
develop (ex several species of frog).
2) Hybrid offspring are sterile
• Donkey interbreeds with a horse, offspring is a mule.
• Mule can’t reproduce.
• Genetics of the two species then don’t PERMANATENTLY mix.
3) If hybrid offspring (F2 generation) are not sterile,
• They still may be less fit than their purebred competitors.
Reproductive Isolating Mechanisms - Summary
Support Questions
21, 22, 23
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Examples of Speciation
• It occurs when enough genetic differences build up
between two populations
• That they can no longer interbreed successfully.
• Easiest way – two populations physically seperated.
Question:
• What about if two populations were in the same place at
the same time. Would speciation occur? How?
Ex 1 - Grasses
Stage 1 – Geographic Isolation
Stage 2: Reproductive
Isolation (barrier to gene flow)
Can interbreed but diverged
genetically so hybrid
offspring are not successful
Oil Spill
Evolving higher frequency of
genes tolerant to oil
OIL
TOLERANT
PART
Darwin’s Finches
• Speciation happens faster in small than large populations.
• Galapagos finches became geographically isolated n their
separate islands.
• Developed different genetic makeups.
• Evolved to different selection pressures on their islands.
Long beak
Eating insects
in rocks
Genetic differences became so
great
Result: when brought together,
unable to reproduce
successfully.
Short beak
Eating seeds and fruit.
Polyploidy in Plant
• Random mutations in some plants – chromosome number
doubles in gametes.
Fertilization - NO
Polyploid gametes
Polyploid gametes
normal gametes
Fertilization - Yes
Polyploid gametes
Fertile offspring with
double chromosomes.
• Polyploidy individuals are reproductively isolated from the rest. If enough is
produced, they can create their own breeding alongside the normal plants.
• First step to creating a new species (accomplished even by genetic barrier
rather than physical one).
Hybrids
Cloning
• Speciation can happen through hybrids.
HOW?
If two species interbreed and create a sterile hybrid, the hybrid
can still reproduce ASEXUALLY (ex: sending up shoots or
cloning/copying).
If produce enough copies, they create a small breeding
population.
Reproductively isolated from parents but can breed amongst
themselves.
Gradually, sterile hybrids can change into fertile polyploids
(remember mutation, drift, and selection as ways of changing
allele frequency) and a new species can form.
Support Questions
24
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilitizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Types of Selection
• Genetic differences between populations build up to the
point where they become different species.
• How does genetic divergence happen?
• Natural selection creates either a divergence (i.e.
separating) or convergence (i.e coming together) of traits,
leading to a change in the population and perhaps
eventually speciation.
• In natural population, traits are usually distributed in a normal bell-shaped
curve.
• Most individuals are close to the average for the trait (peak).
• Few are either very low or high (lower & upper tails of curve).
• Ex: Height (most are average, few shorter, few, very few extremely short
and very few extremely tall).
• Height shows some variation which natural selection can act upon.
How does selection relate?
• Selection changes the shape of the bell curve for traits
over time.
• 3 kinds of selection: stabilizing, disruptive, and directional.
1) Stabilizing Selection
• When selection favors middle values of a trait + is against
the two extreme values.
• Ex: height in plants and birth weight of babies.
Tall plants (Affected by wind)
Medium size plants
Short plants (Sunlight can’t
reach)
What do selection pressures do in this
case?
• Lower the fitness of short or tall plants.
• Reward plants of medium height.
• Result:
• Number of medium height plants increase
• Number of tall and short plants decrease
• Result on bell curve: narrowing it.
2) Disruptive Selection
• Selective pressures act againts individuals possessing the
average trait.
• Result: bimodal/two-peaked curve
• The two extremes of the curve create their own smaller curves.
Ex: plants with variation in height + pollinated by three
different bee species.
Attracted to
Tall plants (Affected by wind)
Attracted to
Medium size plants
Attracted to
Short plants (Sunlight can’t
reach)
• Population would tend toward both short and tall, but not medium-height
plants.
• Divergence in the population & reproductive isolating mechanism (short and tall can’t
exchange pollen).
• Eventually the two populations could diverge enough on other traits – leading to
speciation.
This bee species
disappeared
So won’t be able
to reproduce
Another example – Darwin’s finches
• Plants produce either large or small seeds.
Bird population start with medium-sized beak would not be
as efficient at gathering seeds.
• Birds with slightly smaller beaks – better eating smaller seeds.
• Birds with larger than average beaks – better eating larger seeds.
• Birds with medium beak – nor efficient at gathering either.
• Over many generations, two beak sizes are selected
(small and large), and birds with medium-sized beaks die
out.
Small beaks
Large beaks
Medium beaks
Directional Selection
• Individuals that are either above or below the average do
better, depending on where natural selection was pushing
them.
• Result: over time, population’s trait distribution shifts
towards the favored type.
• Mean/average of the population graph shifts to the
left/right.
• Ex: Giraffe necks.
Selected against
(couldn’t reach as many
leaves to feed)
So: distribution of neck lengths shifted to favor those with long necks.
• Leading to higher percentage of giraffes having long necks,
eventually dominating (taking over) the gene pool
• Today – only long-necked giraffes are present.
Another example - horses
• Early horses – small and slow runners
(adapted to forest environment).
• Modern horse – large and fast (adapted to
life on grasslands).
• WHY?
• Environment changed - forests replaced by
grasslands.
• SO larger individuals did better because they had
strength and speed to outrun predators.
• With generations, average size of horses in the
population increased (so shifting the bell curve
average).
Support Questions
25
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Human Impact on Evolution – Polar Bears
• Considered at risk of being endangered due to global warming.
• Eat seals and small whales as they hunt out on the frozen
Arctic Ocean.
• If ice melts, bears can’t hunt for seals and they will starve.
• Arctic ice pack has remained stable but that is rapidly changng.
Impact of Global Climate Change
• Earth’s climate – rapidly changing (humans burning fossil fuels, releasing carbon
•
•
•
•
•
•
dioxide and greenhouse gases into atmosphere).
Gases trap heat against Earth’s surface – gradual warming.
Earth slowly warms up – effects dramatically felt in polar areas.
Arctic Sea forms later every fall and melt earlier every spring.
Amount of Ice shrinks + longer ice-free periods every year, polar bears are forced
onto shore earlier. So they slowly starve until ice forms again.
Bears are dying younger + fewer offspring produced as ice gradually disappears.
Grizzly beards are brought into polar bear territory since the Arctic, as it warms
up, is becoming a more favorable habitat for grizzly bears.
Hybridization
• Grizzly bears and polar bears – close relatives.
• Two species kept separated by isolation in different
habitats.
• Polar bear – lives on ice and feeds on seals.
• Grizzly bear – lives on land and feeds on vegetation and prey.
• Example of Reproductive isolation (georgraphic &
behavioral mechanisms).
• Today, climate change changed bear habitats and
movements throughout the Arctic.
• Taking grizzly beards into habitats brining them in direct contact
with polar bears.
Hybridization two concerns:
1) Aggressive Grizzly bear outcompeting or attacking the
polar bears.
2) Grizzly and polar bears interbreed, forming hybrids.
Could lead to less fit individuals not well adapted to life on the
land or ice.
Why know this? Because human-caused climate change can
be leading to new directions in evolution for the polar bear.
Support Questions
26 & 27
CONTENT SUMMRY:
1. Species & Speciation.
2. Reproductive isolating mechanisms
• Pre-zygotic.
• Post-zygotic.
3. Examples of Speciation.
• Grasses.
• Darwin’s Finches.
• Polyploidy in Plants.
• Hybrids.
4. Types of Selection
1. Stabilizing.
2. Disruptive.
3. Directional
5. Human Impact on Evolution – Polar Bears, Climate
Change, & Hybrids.
Key Questions
• 12 to 15.
• Unit End.