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
Speciation
Occurs when a group within a species
separates from other members of its
species and develops its own
characteristics.
4. 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
5. 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
6. 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
7. 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
8. 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
9. 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…
11. 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
12. 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
13. 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!
14. 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.
16. 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.
17. 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!
18. 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.
19. AP Biology
Rate of Speciation
Current debate:
Does speciation happen gradually or
rapidly? Or both?
Gradualism
Punctuated equilibrium
20. AP Biology
Gradualism
Gradual divergence
over long spans of
time
assume that big
changes occur as
the accumulation
of many small ones
21. 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
22. 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
23. 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
24. 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
25. 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
Humans are so diverse but considered one species, whereas these Meadowlarks look so similar but are considered different species.
Meadowlarks Similar body & colorations, but are distinct biological species because their songs & other behaviors are different enough to prevent interbreeding
The most comedic species of the Galapagos Islands is the Blue Footed Booby, what a ridiculous outfit and expression! Their name is in fact taken from the Spanish 'bobo' which means clown.
The Blue Footed Boobies above display part of their humorous courtship ritual whereby they raise their feet one at a time and then swivel their heads away from the prospective mate looking to the sky.
Other interesting Booby features are the highly evolved airbag systems in their skulls which allow them to dive bomb into the sea for fish from great height, and the egg and hatchling nesting boundaries they make which are rings of Boobie poop. They aren't the only Booby on the island — there are also Masked and Red Footed Boobies about.
The most comedic species of the Galapagos Islands is the Blue Footed Booby, what a ridiculous outfit and expression! Their name is in fact taken from the Spanish 'bobo' which means clown.
The Blue Footed Boobies above display part of their humorous courtship ritual whereby they raise their feet one at a time and then swivel their heads away from the prospective mate looking to the sky.
Other interesting Booby features are the highly evolved airbag systems in their skulls which allow them to dive bomb into the sea for fish from great height, and the egg and hatchling nesting boundaries they make which are rings of Boobie poop. They aren't the only Booby on the island — there are also Masked and Red Footed Boobies about.
The selection is intense because it directly affects offspring production -- it is affecting sex itself
What’s wrong with having 63 chromosomes?
Odd number! Cannot pair up in meiosis.
A physical barrier such as mountain range or a water way, makes it impossible for them to breed with one another. Each species develops differently based on the demands of their unique habitat or the genetic characteristics of the group that are passed on to offspring.
