Chapter 18
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
WHAT IS SPECIATION?
AP Biology

So…what
is(Evolutionary
a species?
defined by Ernst Mayr
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
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Eastern Meadowlark Western Meadowlark
Sturnella neglecta
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

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PRE-Reproduction (pre-zygotic) Barriers
 This means there is an obstacle to
mating or to fertilization if mating occurs
geographic isolation
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behavioral isolation
ecological isolation
temporal isolation
mechanical isolation
gametic isolation
Ammospermophilus spp
1. Geographic isolation
 New species occur in different areas

physical barrier separated ancestor species
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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
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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.
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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
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Blue footed boobies mate
only after a courtship display
unique to their species
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Recognizing your
own species
courtship songs of
species of lacewings
(that live in the same area)
courtship display of
Gray-Crowned Cranes, Kenya
firefly
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courtship displays
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.
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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.
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POST-Reproduction (Post-Zygotic) Barriers
 Prevent hybrid offspring from
developing into a viable, fertile adult
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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.
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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
chromosomes
(32 pairs)
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Mules have 63 chromosomes!
Donkeys have 62
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.
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Rate of Speciation
 Current debate:
Does speciation happen gradually or
rapidly? Or both?

Gradualism

Punctuated equilibrium
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Gradualism
 Gradual divergence
over long spans of
time

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assume that big
changes occur as
the accumulation
of many small ones
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

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Time
MODELS OF SPECIATION
18.2 AND 18.3
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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
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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

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Parapatric Speciation
Daughter species form a
small proportion of
individuals along a
common border between
2 populations.
HYBRID ZONE
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
Hybrid Zone is formed
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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
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