Download Speciation

Speciation and evolution
 Mechanisms
Selection and genetic isolation;
founder effects; genetic drift; adaptive
radiation; polyploidy; rates of
speciation;
centres-of-origin
 Tools
cladistic analysis
4 Ga of
evolution =
high organic
diversity
(1.4M species known)
[5-100M overall?]
Number of
known
animal
species
“God, if he exists, shows an
inordinate fondness for beetles”
J.B.S. Haldane (1951)
Number of
known
higher
plant
species
The Darwinian revolution
Charles Darwin
(1809-82)
The species alive today
evolved from a single original
life form as a result of
progressive specialization;
Plants and animals evolve by
natural selection (preservation
of useful variation in a
population);
Evolutionary change has
been gradual, requiring
thousands to millions of years
Selective pressures can reduce or increase
the natural variation in a population
e.g. body size
Disruptive selection may give rise
to new species as a result of:
• Geographic segregation (e.g. islands), or
• Habitat-niche segregation (e.g. savanna vs.
forest), or
• Behavioral segregation (e.g. seasonal or diurnal
mating isolation), or
• Sexual segregation (e.g. lack of female response
to male colouration, smell, etc.)
of the divergent populations; or new species
may be a product of
• instantaneous mutation (polyploidy)
Genetic isolation
Allopatric
Peripatric
Speciation
Parapatric
Sympatric
Original
Population
Initial
Step
Evolution
of genetic
isolation
Barrier
Formation
In isolation
New niche
entered
In isolated
niche
New niche
entered
In adjacent
niche
Increased genetic
variation
Within the
population
Based on Wikipedia graphic
Allopatric speciation
Speciation may arise following the
crossing or creation of barriers
Barrier-crossing events
Barrier-creation events
e.g. dispersal by storms
e.g. continental drift
Vocabulary
allo = “other”
peri = “nearby”
para = “around about”
sym = “same”
patri = “of the country”
Allopatric speciation:
barrier types
Water
island1
island2
Topographic
grass
land
Habitat
forest
Competition?
grass
land
Allopatric speciation
Spotted owl subspecies
living in different
geographic locations show
some genetic and
morphological
differences. Will new
species form through
geographic isolation?
http://evolution.berkeley.edu/evosite/evo101/
Paleo-allopatric speciation?
More
widely
separated
at
15ka BP?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
http://birds.cornell.edu/programs/AllAboutBirds/BirdGuide
Peripatric speciation
(founder effects)
Q uic kTim e™ and a
TI FF ( Unco m pr es sed) d ecom p r essor
ar e needed t o se e t his pict ur e .
Scarlet robin
(Petroica boodang)
Australia
Pacific robin
(Petroica multicolor)
Norfolk Island
Founder
effects
Colonizing populations are
often very small, and
therefore contain only a
small sample of the genetic
diversity of the parent
population.
e.g. if a small flock of blueback grassquits was blown
from the coast of Ecuador
across >1000 km of ocean to
the Galápagos, how much of
the ancestral gene pool would
it contain?
Copyright of
Genetic drift
Small populations are also subject
to changes in their genetic
constitution as a result of chance;
they can therefore diverge quickly
from their ancestors. Changes in
the species gene pool resulting
from these stochastic effects are
referred to as “genetic drift”.
“Darwin’s
finches”
14 species.
Primarily in the genus
Geospiza, they are likely
descended from the blueback grassquit (Volatinia
jacarina) of the South
American mainland.
Source: Lack, D. 1966.
Darwin’s Finches. Harper,
N.Y.
Darwin’s only comment on
“his” finches
“The most curious fact is the perfect gradation in
the size of the beaks of the different species of
Geospiza - seeing this gradation and diversity of
structure in one small, intimately related group of
birds, one might really fancy that, from an original
paucity of birds in this archipelago, one species
had been taken and modified for different ends”
my italics!
Journal of Researches, 1842.
The Galápagos archipelago
1000 km
Daphne major
100 km
Galápagos Islands began to emerge over the hotspot beneath the Nazca plate ~5 Ma a
Evolutionary
relationships:
Galápagos
finches
Adaptive radiation
The evolution of a single species into
many species that adopt diverse
habits, usually as a result of the
invasion of a new area, (=allopatric
speciation) followed by parapatric or
sympatric speciation.
Examples: Galápagos finches; Hawaiian honeycreepers and
honeyswords
Adaptive radiation* in
Galápagos finches
• Seed eaters (like their S. American ancestors feed on ground or in canopy)
• Insect eaters (woodpecker-like and tool-using or
warbler-like)
• Nectar feeding (cactus finch)
• Vampire-like (some finches suck blood of sleeping
seabirds)
* a.k.a. ecological release or niche
segregation
Speciation by
character
displacement:
beak size in
three finches
on four island
groups
G. magnirostris
colonized in 1982
Beak size
and seedeating
efficiency
Large-beaked G. fortis (A) and G. magnirostris (B) can crack or
tear the woody tissues of T. cistoides mericarps (D), whereas
small-beaked G. fortis (C) cannot. Five mericarps constitute a
single fruit. In (D), the left-hand mericarp is intact. The right-hand
mericarp, viewed from the other (mesial) side, has been exploited
by a finch, exposing five locules from which seeds have been
extracted.
Source: Grant, P.R. and Grant, B. R. 2006. Science 313, 224 - 226.
Squawk!
Beak shape
influences song,
which controls
mating
behaviour,
reinforcing
differentiation
Source: Podos, J.
2001.
Nature 409, 185-
Trill!
Competition and character
displacement in drought years
• Mean beak size of G. fortis
on Daphne Major (19732005). G. magnirostris
arrived on the island in 1982.
• NB: Daphne Major is 0.34
km2 in area.
• Population of G. fortis and G.
magnirostris on Daphne
Major (1996-2005). There
was no breeding in the
drought years of 2003 and
2004.
Source: Grant, P.R. and Grant, B.R. 2006. Science 313. 224 - 226
Many interisland
transfers?
e.g. Geospiza
magnirostris on
Daphne major
on previous
slide
The
salamander
Ensatina:
divergence
resulting from
repetitive
colonization
(founder effects)
and genetic drift
“ring
speciation”
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Ensatina semi-species
X
E. e. eschscholtzii
=
E. e. klauberi
hybrid
The salamanders in the Ensatina eschschlotzii complex in
California interbreed with their immediate neighbors, with
one exception. In Southern California, E. e. klauberi and E.
e. eschscholtzii can interbreed (see above), but rarely do so.
Images courtesy of Chuck
Brown, Santa Rosa Community
College
Speciation may be slow, to moderate, ….
e.g. Galápagos finches or
Hawaiian silverswords:
28 silversword species in 3
genera derived from a tarweed
ancestor in the last 5 million years
Baldwin, B.G., and Sanderson, M.J. 1998,
PNAS, 95, 9402-9406,
…..or rapid:
Cichlidae: ~700 species of freshwater fishes
of tropics and subtropics
• Common in lakes of East Africa
• >300 species of cichlid fish in Lake Victoria
• Varied feeding habits: predators [fish, molluscs]
and grazers
• Lake Victoria is shallow, and dried up in the late
Pleistocene, so all cichlid species are a product
of colonization and radiation in about the last 12
000 years
• Is this an example of sympatric speciation?
Lake Victoria cichlids: genetic
segregation by feeding habitat and
colour signals
Pundamilia pundamilia
Neochromis gigas
http://home.t-online.de/home/schraml.e/mbipi2.htm
Lake Victoria:
agricultural pollution -> loss of
clarity -> hybridization
80% loss
80% loss
… or instantaneous: polyploidy
• Polyploid organisms have a chromosome
count that is a multiple (2x, 4x, 6x, 8x, …)
of that of some related form
• All of these represent the spontaneous
addition of extra (whole) sets of
chromosomes in progeny
• Polyploids are “instantaneous” species,
only even-ploid hybrids are likely in nature
~50% of all plant species have a
polyploid origin, e.g. Anemone
Parent species
Daughter
or Chrysanthemum
Chrysanthemum species: many polyploid events and hybridization
between members of polyploid complexes, e.g.
(x[ base chromosome number] = 9)
C. mankinoi (2x, n=9)
C. indicum
C. mankinoi x
C. japonese?
(4x, n=18)
C. japonese (6x,
(hybrid of C.
n=27)indicum and
C. ornatum?)
C. ornatum (8x, n=36)
C. yezoensis and C. pacificum (10x, n= 45)
Species diversity: pheasants
centre-of-origin
(area of maximal diversity)
Species diversity: crocodiles
centre-of-origin?
Biodiversity: palms
(number of genera)
centre-of-origin?
Cladistics I
• Attempts to answer the following question:
“Given any group of taxa, which ones
are most closely related to each other?”
• Methods based on objective techniques based on the
presence/absence of morphological characters
(a.k.a. ‘numerical taxonomy’);
or DNA sequences
Characters used to distinguish
between species of Douglas-fir
(Pseudotsuga)
1. Young cones purplish (not green).
2. Cones >10 cm long.
3. Leaves <1.5 mm wide.
4. Bract scales >2.5 cm long.
5. Bract scales reflexed (curved; not exserted)
6. Vegetative (leaf) buds > 7 mm long.
7. Seed scales have length-width ratio <1.
8. Mature trees have very thick bark with resin
blisters.
Comparing characters
Species
menziesii
macrocarpa
sinensis
japonica
Range
N.America
N.America
Asia
Asia
Characters
1
0
0
+
+
2
0
+
0
0
3
+
0
0
0
4
+
+
+
0
5
0
0
+
+
6
+
+
0
0
7
+
0
0
0
8
+
0
0
0
e.g. P. menziesii shares 4 of 8 characters
(#1,4,5,6) with its North American relative P.
macrocarpa; their relative similarity therefore =
4/8 = 0.5
Similarity matrix
(Douglas-firs)
menziesii macrocarpa
menziesii
macrocarpa
sinensis
japonica
-
sinensis
japonica
0.50
0.25
0.13
-
0.50
0.38
-
0.88
-
A similarity linkage diagram for
Douglas-fir
japonica
menziesii
sinensis
macrocarpa
Cladistics II
• Cladistic analysis is based on the identification of
distinctive morphological characters that can be
placed in an evolutionary sequence.
• Characters may be apomorphous (derived
characters) or plesiomorphous (ancestral
characters).
• Once a taxon divides, an apomorphic character
becomes plesiomorphic.
Constructing a cladogram
• We will use some of the characters
listed in the similarity matrix for
Douglas-fir to construct a
hypothetical cladogram.
• NB: this is for illustrative purposes only (the
characters are not necessarily
plesiomorphic/apomorphic).
The parsimony rule*
Using characters 3,4,6,7,8 only;
3 steps to cladogram
Using characters 3,4,6,7,8 only;
4 steps to cladogram
menziesii macrocarpa japonica sinensis
menziesii macrocarpa sinensis japonica
4
6
6
6
3,7,8
A
4
3,7,8
B
By the parsimony rule cladogram ‘A’ is preferred [3 steps vs 4 steps].
(see Fig 8.1 in text for a more complex example)
*Alternatively known as “economy of hypothesis”; “Occam’s
razor”, or the “KISS principle” ( as in, “Keep it simple, stupid!”).
Alternative cladograms
menziesii macrocarpa japonica sinensis
menziesii macrocarpa sinensis
japonica
4
6
6
3,7,8
A
3,7,8
B
Note that the information is in the branching sequence, and
NOT in the proximity of the branches.
A and B are identical in terms of information.
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