Download SPECIES AND SPECIATION

Speciation
Diversity
Question
It is estimated that
of all species that
have ever lived on earth are now extinct.
A. 20 %
B. 45 %
C. 80 %
D. 90 %
E. 99 %
Mass Extinctions Are a Fact of Life
It is estimated that 99% of all species that have ever lived on earth are now extinct.
Did the creation end in the Garden?
• Before Charles Darwin,
many naturalist believed
the creation of new
species ended in the
Garden of Eden.
• By the 18th century
scientist began to
understand that extinctions
were a common
phenomena.
• Was the Diversity of life
winding down?
Charles Darwin
There is grandeur in this view of life,
with its several powers, having been
originally breathed by the creator into
a few forms or into one; and that,
whilst this planet has gone cycling on
according to the fixed law of gravity,
from so simple a beginning endless
forms most beautiful and most
wonderful have been, and are being
evolved.
From the Origin of Species by Charles
Darwin
Darwin’s Tree of Life
• The ascent of life through
time.
• Each new branch or
species arise from the
modification of an
ancestral species.
• The process by which new
species arise is called
Speciation.
Evolution creates (and destroys) new species, but …
What is a species?
Its not as straightforward a question as most believe.
These are members of different species - eastern (left) and western (right) meadowlark.
What is a Species?
There is only one extant (existing) human species.
What is a Species?
And these are all members of a single species.
Are these the Same Species?
What is a Species?
• It is the smallest evolutionarily independent unit
– Different species follow different evolutionary trajectories.
– The essence of speciation is lack of gene flow
It is difficult
to determine
when
populations
are actually
evolving
independently
.
species form a boundary
for the spread of alleles
Proposed species concepts
•
There are three different species concepts
currently in use.
1. The Biological species concept
2. The Phylogenetic species concept
3. Morphospecies concept
The Biological Species Concept (BSC)
“ Species are groups of interbreeding natural populations
that are reproductively isolated from other such groups.”
Ernst Mayr 1942
• Definition implies:
– A population is reproductively isolated if it possesses genetic
differences that prevent it from inbreeding with other biological
species.
– There is no hybridization or hybrids fail to form fertile offspring.
– Lack of gene flow ensures independent evolutionary trajectories
between populations.
The BSC does not include geographical
isolation.
Two populations that
cannot interbreed because
they are separated by some
physical barrier such as a
river or a mountain range
are not defined as separate
species. Why?
This makes sense, because geographic isolation may be
temporary (rivers change their course mountains erode),
whereas reproductive isolation caused by genetic differences is
usually irreversible.
Problems associated with the BCS
1. Many plants hybridize freely
2. Cannot test fossil forms
3. Irrelevant to asexual populations
The Phylogenetic Species Concept
(PSC)
Species are identified by estimating the
phylogeny of closely related populations
and finding the smallest monophyletic
groups.
PSC (cont)
• Taxa labeled A-G represent
populations.
• They re the smallest
monophletic groups on the tree
and represent distinct species.
• If populations cannot be clearly
distinguished in a phylogeny by
unique, derived characters, then
they will form clusters like the
populations designated B, E,
and G.
• Clusters B, E and G would be
considered part of the same
species
Explain the Figure below
In this example, salamander lineages A and B are separate species. Each has a
common ancestor that individuals of other species do not. Even though it has
diversified a lot, Lineage C is a single species, according to the phylogenetic
species concept. None of the subspecies of Lineage C has a single common
ancestor separate from the other subspecies.
PSC Rational
• Species are formed when a single interbreeding
population splits into two lineages that do not
exchange genetic material.
– Form Sister species (Lineages that diverged from
the same ancestral node on a phylogenetic tree)
– Identified from diagnostic traits called
synapomorphies (shared derived characteristics)
• Species are identified on the basis of
statistically significant differences in the traits
used to estimate the phylogeny (ancestry)
PROBLEMS with PSC(cont)
• Populations must have been independent long
enough for diagnostic traits to emerge
• Phylogenies are only available for a handful of
groups
• Very tiny differences, even a single DNA
substitution may be used as a trait that separates
groups
• Could end up doubling the number of species
Morphospecies
• Define species based on the morphological
differences. Commonly used with fossils.
• Used when we do not have tests for reproductive
isolation or well-estimated phylogenies
• Assignment to species is often arbitrary and
cannot distinguish cryptic species
cryptic species= ones which are strongly
divergent based on non-morphological
characters.
Things such as song, temperature or drought tolerance,
habitat use, or courtship displays
Application of the 3 species
definitions to red wolf
• Red wolf appeared in 1930 to be a Morphospecies
being intermediate in appearance between the gray
wolf and the coyote, all 3 appearing to be distinct.
• Studies have shown that the red wolf is actually a
hybrid between gray wolves and coyotes. Therefore
its intermediate characteristics are the result of
hybridization and not independent evolution. This
makes the Red wolf not a distinct species for most
biologists because…
• Neither the BSC or the PSC allow for hybridization
• However, it is still considered a separate species and
the morphospecies is the only one of these 3
definitions that works.
http://news
How do you miss a whole Elephant
Species?
• Up until recently,
scientists believed there
were two species of
elephant: the African
elephant and the Asian
elephant.
• Geneticists conducting a
comprehensive DNA
sampling of elephants
from across Africa
recently found that there
are in fact two species of
African elephants.
Speciation
Speciation
• Classically, speciation has been
hypothesized to be a three-stage process
– 1 Population becomes reproductively isolated.
– 2 divergence in traits
– 3 Development of isolating mechanisms
(Secondary contact)
Allopatric Speciation
• Allopatric (Allo= different, Patric=homeland)
• Population are separated due to geographical
barriers
Geographic isolation
1. By dispersal and colonization
Dispersal to novel environment such as rafting a
portion of a population to an island
2. By Vicariance events
Events that physically divide a population into
two or more isolated ranges like rising of a
mountain range, long term drying trend, or lava
flow.
Example of Dispersal and
Colonization
• Question Why do each of the islands of
Hawaii have their own drosophilid species?
• Hypothesis: It is due to the Founder effect.
– small group of individuals cut off from the
original population colonizes a new habitat
– Drift, mutations and selection leads to
divergence
Predictions
• Closely related species should be found on
adjacent islands
• The phylogenetic branching sequence should
follow island formation
• Figure 15.7b
The analysis of mitochondrial DNA showed that closely related
species were found in the predicted pattern .
Vicariance events
• Events which split a species into two or
more isolated ranges and prevents gene
flow between them (or at least greatly
reduces it)
• Can be slow processes like rising of a
mountain range, long term drying trend or
rapid like a lava flow.
Case study of Mexican Cavefish
• In Mexico, researchers
identify a river running
through rocks to a cave
system below.
• In the cave they find fish
that are like the population
on the outside, however
with some important
differences
Case study continued
• The eyes of the cavefish
have become vestigial
(no longer functional)
• The fish have also lost
their pigmentation.
• The two populations of
fish are no longer able to
mate
• Speciation has occurred.
Example from isthmus of
Panama
• land bridge opened as the isthmus closed about 3
million years ago
• Found 7 pairs of closely related morphospecies of
snapping shrimp. One member of each pair on
each side of the land bridge
• The pairs from either side of the bridge are shown
to be sister species (each other’s closest relative)
believed to share the same common ancestors
which split to form each pair
5
Also, interestingly, shrimp populations
would have been isolated in a staggered
fashion as the land bridge gradually
formed in stages
Species 6 and 7 live in the deepest water
and were cut off first
1-5 were in shallower water and
diverged later
SECOND STAGE OF
SPECIATION
Genetic Divergence
Mechanisms of Genetic Divergence.
• If the environments are different between
isolated populations then natural selection,
mutations, and genetic drift will work
independently on each group. This will
speed up genetic divergence and speciation.
Genetic Drift
• This sampling process leads to the random
fixation of alleles with the concurrent loss of other
alleles
• Effect most predominant in small populations and
founder effect populations. Especially with
bottlenecking
• Because peripheral populations in colonization
events are almost always small, genetic drift is
hypothesized to be the key to the second stage of
speciation
The role of genetic drift is controversial
• Hundreds of small populations have been
introduced to new habitats around the world
in the last 150 years, but few dramatic
changes in genotypes have occurred.
• Now a more balanced approach is taken and
the role of natural selection is also brought
into the equation.
Natural selection
If one of the split populations occupies a
novel environment, Natural selection
can quickly lead to divergence
Secondary Contact
Reinforcement
• If populations have sufficiently diverged while
in allopatry, their hybrid offspring should have
markedly reduced fitness when compared to
individuals in both parental populations.
• Parental populations will reduce their fitness if
they produce hybrid offspring, therefore this
should favor assortative mating
• Selection that reduces the frequency of hybrids
is called reinforcement
Reinforcement finalizes the speciation process
by completing reproductive isolation
• The final stage of speciation, that of establishing
reproductive isolation occurs as Isolating
mechanisms develop between groups.
• These are mechanisms that prevent interbreeding
between two groups and assure continued genetic
divergence.
Isolating mechanisms
• Mechanisms that lead to, or cause genetic
divergence by preventing interbreeding
between two groups.
– As long as two groups do not interbreed their
gene pools will continue to drift further and
further apart.
– The longer two species are genetically isolated
from each other the more different they become
from each other.
Isolating mechanisms
• Isolating mechanisms can be divide into
two different types.
– Isolating mechanisms that occur before
fertilization are called Prezygotic mechanisms
– Isolating mechanisms that occur after
fertilization are called postzygotic mechanisms
Types of Prezygotic Isolating
mechanisms
• Behavioral isolation: Potential mates meet but cannot
figure out what to do about it because patterns of courtship
may be altered to the extent that sexual union is not
achieved
• Temporal isolation: (Time) Different groups overlap in
range but may not be reproductively mature in the same
season.
• Mechanical isolation: Potential mates attempt engagement
but sperm cannot be successfully transferred . This may be
due to differences in reproductive organs.
Types of Prezygotic Isolating
mechanisms cont.
• Gametic isolation: Sperm is transferred but sperm
and egg are incompatible.
• Ecological isolation: potential mates never meet
because they live in different habitats
Types of Postzygotic isolating
mechanisms
• Zygotic mortality: Egg is fertilized but zygote does
not develop properly dies before birth because
parents are genetically incompatible.
• Hybrid inviability: Hybrid very weak and can’t
live outside the uterus.
• Hybrid offspring: Hybrid is sterile.
Sympatric Speciation
A new species can arise in a single generation if a
genetic change produces a reproductive barrier
between mutants and the parents.
Sympatric speciation
• Sympatric (Sym = same, Patric=homeland) speciation
occurs in the same geographical region without physical
isolation.
• A new species can arise in a single generation if a genetic
change produces a reproductive barrier between mutants
and the parent population.
• Example: Accidents during cell division that result in extra
sets of chromosomes (Polyploidy). Self-fertilization can
give rise to new individuals that are unable to mate and
form fertile off-spring with the parent species.
Sympatric Speciation
• Accidents during cell division may results in extra sets of
chromosomes in the progeny, a condition called polyploidy.
• The differences in chromosome number act as a reproductive
barrier between parent and offspring.
• This is type of speciation is quite common in plants that are able
to self-fertilize.
Sympatric Speciation
• Botanist estimate that up to 50% of all plant
species are polyploids.
The evolution of Wheat
Parapatric Speciation
Parapatric Speciation
• Parapatric (Para = near, Patric=homeland)
neighboring populations become distinct
species while maintaining contact through
Hybrid zones
Parapatric
speciation
Hybridization
Creation of new species
through hybridization
There are 3 possible outcomes
when hybridization occurs
1. Reinforcement of parental forms as two
recently diverged species
2. Formation of a new third species
3. Formation of secondary hybrid zones
New Species through Hybridization
• Three species of Sunflower
– Helianthus annuus, H. petiolaris and H. anomalous.
• Rieseberg crossed individuals of H. annuus and H.
petiolaris to produce three hybrid populations.
– The three experimental hybrids were genetically almost
identical to the natural H. anomalous.
• Only certain alleles work in combination and that
other hybrids are inviable and have reduced fitness.
Speciation through Hybridization
• Hybrids can mate with siblings and backcross to their
parents the result is a variety of hybrid gene
combinations.
• If certain of these combinations is best suited to a
new habitat, a third species may arise that is
genetically somewhere intermediate between the
parents.
• Secondary contact and gene flow between recently
diverged species can result in the formation of a new
third species.
Size, shape and longevity of hybrid zones
are determined by 3 possible outcomes of
hybridization
1. When parents and hybrid are equally fit…
The zone is wide.
Hybrid traits are found with highest frequency at
the center of the zone.
Width of the zone is determined by
a) distance of dispersal in each generation
b) How long zone has existed
Factors (cont)
2. When hybrids are less fit than purebred
individuals (parents)…
Fate of the hybrid zone depends on the strength of
selection against them.
(a) Strong selection leads to reinforcement, with a
very narrow and short-lived hybrid zone.
(b) If selection is weak, the hybrid zone is wider and
longer lived
A balance develops between formation of the
hybrids and the selection pressure against the
hybrids. (similar to the banded and non-banded
snakes we used as an example of selectionmigration balance)
Factors (cont)
3. When Hybrids are more fit than purebreds
Depends on the extent of the environment in
which the hybrids are at an advantage
New species results if hybrids are more fit in
areas outside the range of the parental
species
If the advantage is at the boundaries between
the two parental species then will form a
stable hybrid zone
Often found in areas called ecotones where
markedly different plants and animals meet
Example from Big sagebrush in Utah
• Between the basin and mountain subspecies
• Hybrids shown to be more fit than parents in
transitional zones
• Showed that the hybrid zone is maintained
because the hybrids have superior fitness in the
transitional zone
Hybridization Issues
• Potential practical problems
• Some crop species are closely related to
weed species
• Herbicide resistance genetically engineered
into a crop plant could be transferred to
weed plants through hybridization
Genetic differentiation and isolation
• How much genetic variation is necessary to produce a
new species?
• The BSC requires that no hybridization whatsoever
occur, so it predicts that a virtual “ genetic revolution”
or radical reorganization of the genome would be
required in order for hybrids to survive.
• Large scale changes in the genome are unlikely
• They have also been shown to be unnecessary.
• Fertile hybrid offspring can be found even when the
parental populations are markedly different from one
another.
• Current research focuses on the number, location or
nature of the genes which distinguish closely related
species
Do we see speciation occurring today?
Sockeye Salmon
Sea run vs. lake dwelling
Skipper butterflies
Different host plant
Blueberry and apple
maggot flies
Warbler finches
Different host plant
Heliconius butterflies
Lake White fish
Habitat selection (high vs
low elevation)
Different habitats
Dwarf versus normal
sized individuals
THE END