Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
WHAT IS SPECIATION?
AP Biology
So…what is a species?
defined by Ernst Mayr (Evolutionary Biologist)
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
AP Biology Sturnella magna
How and why do new species originate?
New species are formed by a series of
evolutionary processes which cause
reproductive isolation.
Reproductive Isolating Mechanisms
Pre-zygotic barriers are various forms of
isolation
Post-zygotic barriers are generally related to
fertility & viability of offspring
AP Biology
PRE-Reproduction (pre-zygotic) Barriers
This means there is an obstacle to mating
or to fertilization if mating occurs
geographic isolation ecological isolation temporal isolation
AP Biology
behavioral isolation mechanical isolation gametic isolation
Ammo Spermophilus spp
1. Geographic isolation
New species occur in different areas
physical barrier separated ancestor species
Harris’s
Harris’s antelope
antelope
squirrel
squirrel inhabits
inhabits the
the
canyon’s
canyon’s south
south rim
rim
(L).
(L). Just
Just aa few
few miles
miles
away
away onon the
the north
north
rim
rim (R)
(R) lives
lives the
the
closely
closely related
related white-
white-
tailed
tailed antelope
antelope
AP Biology
squirrel
squirrel
2. Ecological isolation
Species occur in same region, but occupy
different habitats so they rarely encounter
each other
22 species
species of
of garter
garter snake,
snake, Thamnophis,
Thamnophis,
occur
occur in
in same
same area,
area, but
but one
one lives
lives in
in water
water &
&
other
other is
is terrestrial
terrestrial
lions
lions & & tigers
tigers could
could
hybridize,
hybridize, butbut they
they
live
live in
in different
different
habitats:
habitats:
lions
lions inin grasslands
grasslands
AP Biology tigers
tigers inin rainforest
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
AP Biology
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
AP Biology unique to their species
AP Biology
Recognizing your
own species
courtship songs of
species of lacewings
(that live in the same area)
courtship display of
Gray-Crowned Cranes, Kenya
firefly courtship displays
AP Biology
5. Mechanical isolation
Morphological 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
cross-pollination does not
happen.
AP Biology
Mechanical isolation Animals
For many insects, male & female sex organs
of closely related species do not fit together,
preventing sperm transfer
lack of “fit” between sexual organs:
hard to imagine for us… but a big issue for insects with
different shaped genitals!
Get that thing
away from me!
Damsel fly penises
AP Biology
6. Gametic isolation
Sperm of one species may not be able to
fertilize eggs of another species
biochemical barrier: sperm cannot penetrate egg
receptor recognition: lock & key between egg & sperm
chemical incompatibility
sperm cannot survive in female reproductive tract
Sea urchins release sperm
& eggs into surrounding
waters where they fuse &
form zygotes.
Gametes of different
species are unable to fuse.
AP Biology
POST-Reproduction (Post-Zygotic) Barriers
Prevent hybrid offspring from
developing into a viable, fertile adult
AP Biology
Post Zygotic: Reduced hybrid viability
Genes of different parent species may
interact & impair the hybrid’s development
Species of salamander
genus, Ensatina, may
interbreed, but most
hybrids do not complete
development & those
that do are frail.
AP Biology
Post Zygotic: Reduced hybrid fertility
Even if hybrids are vigorous they may be sterile
chromosomes of parents may differ in number or
structure & meiosis in hybrids may fail to produce
normal gametes
Mules are vigorous,
but sterile
Horses have 64 Donkeys have 62
chromosomes chromosomes
(32 pairs)
AP Biology Mules have 63 chromosomes! (31 pairs)
Post Zygotic: Hybrid breakdown
Hybrids may be fertile & viable in first
generation, but when they mate offspring
are feeble or sterile
In strains of cultivated rice,
hybrids are vigorous but
plants in next generation are
small & sterile.
AP Biology
Rate of Speciation
Current debate:
Does speciation happen gradually or
rapidly? Or both?
Gradualism
Punctuated equilibrium
AP Biology
Gradualism
Gradual divergence
over long spans of
time
assume that big
changes occur as
the accumulation
of many small ones
AP Biology
Punctuated Equilibrium
Rate of speciation is
not constant
rapid bursts of
change
long periods of little
or no change
species undergo
rapid change when
they 1st bud from
parent population Time
AP Biology
Modes of Speciation: Allopatric Speciation
A physical barrier arises and prevents gene
flow between populations
Reproductive isolating mechanisms evolve in
the genetically diverging populations
Speciation occurs when the members of the
divergent populations no longer interbreed
AP Biology
Modes of Speciation: Sympatric Speciation
A new species may form within the home
range of an existing species
(there is no physical barrier)
May be:
Temporal
Mechanical
Behavioral
AP Biology
Modes of Speciation: Parapatric Speciation
HYBRID ZONE
Daughter species form a
small proportion of
individuals along a BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
common border between
2 populations.
Hybrid Zone is formed
AP Biology Figure 18.10
The two types of plants are close enough that
tolerant and non-tolerant individuals could potentially
fertilize each other—so they seem to meet the first
requirement of parapatric speciation, that of a
continuous population. However, the two types of
plants have evolved different flowering times. This
change could be the first step in cutting off gene flow
entirely between the two groups
AP Biology
AP Biology
