Download Selective Pressures and Speciation

Key points about natural selection
• Affects the population not the individual
• Changes in the gene frequency (how often a gene
shows up in the population) is the driving force of
natural selection
• ‘Survival of the fittest’
• When natural selection is not occurring, we call that
genetic equilibrium
What influences natural selection?
• Mutations
• Recombination of genes
• Environmental Changes
• Selective pressures
Types of Natural Selection:
1. Directional - One phenotype is favored in the
environment.
(ex. "dark" peppered moths have higher survival rates in
soot covered forests)
2. Stabilizing - Organisms with extreme phenotypes
are
eliminated.
(ex. birth weight in humans, low survival rate for
extremes)
3. Disruptive - Organisms with common traits are
eliminated, extremes are favored.
(ex. small female and large male elephant seals)
4.Artificial - A breeder chooses which traits to favor.
(ex. seedless grapes)
Mechanisms of Evolution: Speciation
 Speciation

Process by which new biological species arise.
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
So…what is a species?
 A population whose members can interbreed & produce viable,
fertile offspring.
 Being reproductively compatible is a key component
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
Eastern Meadowlark Western Meadowlark
Sturnella neglecta
Sturnella magna
1. Geographic isolation
 New species occur in different areas
 physical barrier separated ancestor species
Ammospermophilus spp
Harris’s antelope
squirrel inhabits the
canyon’s south rim
(L). Just a few miles
away on the north
rim (R) lives the
closely related whitetailed antelope
squirrel
2. Ecological isolation
 Species occur in same region, but occupy different habitats
so they rarely encounter each other
2 species of garter snake, Thamnophis,
occur in same area, but one lives in water &
other is terrestrial
lions & tigers could
hybridize, but they
live in different
habitats:
 lions in grasslands
 tigers in rainforest
3. Temporal “time” isolation
 Species that breed during different times of day, different
seasons, or different years cannot mix gametes.
Eastern spotted skunk (L) &
Western spotted skunk (R)
overlap in range but eastern
mates in late winter &
western mates in late
summer
4. Behavioral isolation
 Unique behavioral patterns & rituals isolate species
 identifies members of species
 attract mates of same species
That’s hot…

courtship rituals, mating calls
Blue footed boobies mate
only after a courtship display
unique to their species
5. Mechanical isolation
 Morphological (physical) differences can prevent
successful mating
Plants
Even in closely related
species of plants, the
flowers often have distinct
appearances that attract
different pollinators.
These 2 species of monkey
flower differ greatly in shape
& color, therefore crosspollination does not
happen.
Other forms of Speciation
 Remember:
 Genetic Drift: Change to gene pool of a population that
takes place strictly by random chance
 Two major results can stem from Genetic Drift:
 Bottleneck Effect
 Founder Effect
Bottleneck Effect
 Population
bottleneck occur when
a population’s size is
drastically reduced for
at least one generation.
 Results in reduced
genetic variation
Here is what it can look
like:
Example:
 Northern Elephant Seals- due
excessive hunting in the late
1890’s, the population
bottleneck leaving fewer than
20 seals left in the world.
 Today there are more than
30,000 of these seals BUT
still have reduced genetic
variation
Founder Effect
 A founder effect occurs when a new colony is started
by a few members of the original population.
 This small population size means that the colony may
have:
 reduced genetic variation from the original population.
 may lead to increased chances of interbreeding
 Example #2- Birds
 Which colonies would could result from the Founder
Effect?