Transcript Macroevolution and Speciation 2013 for Test 2

Macroevolution :
How do species arise?
How do we go from species A to Species B?
Mom, Dad…
There’s something
you need to know…
I’m a MAMMAL!
Tip! Use a simple flow chart to organize your information….
Speciation
Barriers!
Geographic
Allopatric
Sympatric
Pre-zygotic
Types
A-F
Post-zygotic
Ex.
Hybrids
What IS a Species?
“Biological species
concept”


population whose
members can
interbreed &
produce viable,
fertile offspring
reproductively
compatible
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
Eastern Meadowlark
What is the first step to speciation?
 END Gene flow

between groups!
(reproductive
isolation )
Once gene flow ends..
Populations can:
1. Diverge genetically
via mutation
2. Be subject to natural
selection pressures
3. Experience genetic
drift
1. Obviously…different types of
geographic isolation can occur…
 2 different types of
isolation can be…

allopatric
 geographic
separation between
groups

sympatric
 Group still live in
same area
There are many other ways to keep groups
apart including Reproductive isolating
mechanisms
Such mechanisms
 Arise as populations move apart (diverge)
 They are Heritable aspects of body form
 They can be functions of body parts Or
Behaviors (that prevent interbreeding among
species)
There are two types of reproductive isoolating
mechanisms
 Pre-zygotic
isolating
mechanisms
(prevents
successful
mating/pollination)
 Post-zygotic
isolating
mechanisms
(inability of new
individuals to mate
successfully)
2. PRE- zygotic (reproduction)
 Obstacles to mating or to fertilization if
mating occurs
geographic isolation
behavioral isolation
ecological isolation
temporal isolation
mechanical isolation
gametic isolation
A. Geographic isolation
Ammospermophilus spp
Most common mode of speciation
physical barrier arises and ends gene flow
 We call this: allopatric speciation

(“other country”)
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
B. Ecological isolation
 Species occur in same region, but occupy
different habitats (microenvironments) so rarely
encounter each other
They are…reproductively isolated
2 species of garter snake, Thamnophis,
occur in same area, but one lives in water &
other is terrestrial
Ex. lions & tigers could hybridize, but
they live in different habitats:
lions in grasslands
tigers in rainforest
C. Temporal isolation: timing of reproduction differs
 Species that breed during different times of day,
different seasons, or different years cannot mix
gametes


Another form of reproductive isolation
WE call this: sympatric speciation : new species
form within home range of an existing species and in
the absence of a physical barrier!
 Means: “same country”
Eastern spotted skunk
(L) & western spotted
skunk (R) overlap in
range but eastern mates
in late winter & western
mates in late summer
D. Behavioral isolation
Unique behavioral patterns & rituals isolate species
1.
2.
identifies members of species
attract mates of same species (females of different
species are not impressed!)•
 courtship rituals, mating calls
 reproductive isolation
Blue footed boobies mate
only after a courtship display
unique to their species
Recognizing your
own species
courtship songs of sympatric
species of lacewings
courtship display of
Gray-Crowned Cranes, Kenya
firefly courtship displays
E. Mechanical isolation
sympatric speciation
Morphological differences can prevent
successful mating
Plants
Even in closely related species
of plants, the flowers often
have distinct appearances that
attract different pollinators.
(You have to have the RIGHT
pollinator!)
For example: Black sage (left) v White Sage (right)
Only smaller bees (honeybees)
can pollinate Black Sage; larger
bees cannot be supported by
smaller flowers
Filaments stick out above
petals making it
impossible for little
honeybees to access
anthers: larger bumble
bees OK!
F. Gametic isolation
sympatric speciation
 Sperm of one species (or group) may not be able
to fertilize eggs of another species

mechanisms
 biochemical barrier so sperm cannot penetrate egg
 receptor recognition: lock & key between egg & sperm
 chemical incompatibility
 sperm cannot survive in female reproductive tract
Ex. Sea urchins release
sperm & eggs into
surrounding waters where
they fuse & form zygotes.
Gametes of different
species— red & purple sea
urchins —are unable to
fuse.
3. POST-zygotic barriers (ex. Hybrids)
These will prevent hybrid offspring from
developing into a viable, fertile adult
(3 ways)
1.
2.
3.
reduced hybrid viability
reduced hybrid fertility
hybrid breakdown
zebroid
A. Reduced hybrid viability
sympatric speciation
 Genes of different parent species may
interact & impair the hybrid’s development
 Or recent divergence of genes cause some
missing or some extra genes; incompatible!
Species of salamander
genus, Ensatina, may
interbreed, but most
hybrids do not
completely develop;
those that do are frail.
B. 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)
Donkeys have 62
chromosomes
Mules have 63 chromosomes! (31 pairs)
C. Hybrid breakdown
sympatric speciation
 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.
On path to separate species.
How Fast can we go from
Species A
to
Species B?
Current debate:
Does speciation happen gradually or rapidly

Phyletic Gradualism
 Charles Darwin
 Charles Lyell

Punctuated equilibrium
 Stephen Jay Gould
 Niles Eldredge
Niles Eldredge
Curator
American Museum of Natural History
Stephen Jay Gould (1941-2002)
 Harvard paleontologist &
evolutionary biologist
punctuated equilibrium
 prolific author

 popularized evolutionary thought
Phyletic Gradualism – Darwin and Lyelle
 Gradual divergence
over long spans of
time
Species stay stable
for long periods of
time
 assume that big
changes occur as
the accumulation
of many small ones

Punctuated Equilibrium – Gould/Eldridge
 Rate of speciation is
not constant
rapid bursts of
change
 long periods of little
or no change
between
 Occurs with drastic
change in
environments

Time
Is Evolution goal-oriented? No!
An evolutionary trend does not mean that
evolution is goal-oriented.
Surviving species
do not represent
the peak of
perfection. There
is compromise &
random chance
involved as well
Remember that for
humans as well!
Evolution is not the
survival of the fittest.
Rather it is the
survival of the just
good enough.
View Mr. Anderson’s
“Speciation and Extinction”
Any Questions??
2007-2008