This document discusses speciation and the mechanisms that lead to the formation of new species. It defines a species as a population that can interbreed and produce fertile offspring. New species originate through reproductive isolation caused by pre-zygotic barriers like geographic, ecological, temporal, behavioral, mechanical, and gametic isolation that prevent mating or fertilization. Post-zygotic barriers include reduced hybrid viability and fertility. Speciation can occur gradually over long periods through gradualism or in punctuated bursts separated by stasis. The main modes of speciation are allopatric, with a physical barrier; sympatric within the same area; and parapatric along a contact zone between populations.

1. AP Biology
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
Chapter 18
WHAT IS SPECIATION?

2. 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
Western Meadowlark
Sturnella magna
Eastern Meadowlark
Sturnella neglecta
Distinct species:
songs & behaviors are different
enough to prevent interbreeding

3. AP Biology
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

4. AP Biology
 This means there is an obstacle to
mating or to fertilization if mating occurs
PRE-Reproduction (pre-zygotic) Barriers
behavioral isolation
geographic isolation ecological isolation temporal isolation
mechanical isolation gametic isolation

5. AP Biology
1. Geographic isolation
 New species occur in different areas
 physical barrier separated ancestor species
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 white-
tailed antelope
squirrel
Ammospermophilus spp

6. AP Biology
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

7. AP Biology
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

8. AP Biology
4. Behavioral isolation
 Unique behavioral patterns & rituals isolate species
 identifies members of species
 attract mates of same species
 courtship rituals, mating calls
Blue footed boobies mate
only after a courtship display
unique to their species
That’s hot…

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10. AP Biology
firefly courtship displays
courtship display of
Gray-Crowned Cranes, Kenya
courtship songs of
species of lacewings
(that live in the same area)
Recognizing your
own species

11. AP Biology
5. Mechanical isolation
 Morphological differences can prevent
successful mating
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.
Plants

12. AP Biology
Mechanical isolation
 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!
Damsel fly penises
Animals
Get that thing
away from me!

13. 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.

16. AP Biology
Mules are vigorous,
but sterile
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
Donkeys have 62
chromosomes
(31 pairs)
Horses have 64
chromosomes
(32 pairs) Mules have 63 chromosomes!

17. AP Biology
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.

18. AP Biology
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
 assume that big
changes occur as
the accumulation
of many small ones

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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

21. 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

22. 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

23. AP Biology
Modes of Speciation: Parapatric Speciation
Daughter species form a
small proportion of
individuals along a
common border between
2 populations.
Hybrid Zone is formed
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
HYBRID ZONE
Figure 18.10

24. AP Biology
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

25. AP Biology