Download SF Ev L4 Speciation in animals

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BY2204.Lecture 11
SF Evolution Module
What is a species and how do they
form?
What is a species?
When does it stop being a subspecies, or a distinct population?
“There is probably no other concept in
biology that has remained so
consistently controversial as the
species concept”
(Mayr, 1982)
“What are species? Perhaps no other
issue in comparative or evolutionary
biology has produced quite so much
disparate opinion as this simple
question”
(Eldredge 1995)
We consider blue tits from here and France to be
the same species.
There are 5 main ways scientists have
considered defining the species:
We consider humans and chimps to be clearly
different species.
1)  Biological species concept
Yet the blue tits are genetically more different than
the man and the chimp.
2) Evolutionary species concept
In Indonesia we found a black-backed
version of the red-backed thrush.
3) Recognition species concept
4)  Morphological species concept
Is it a new species, a sub-species or just a
local colour variant?
5) Genetic difference
JMS: “if it will save a rainforest by being a
species, then it’s a species”
Each has strengths and weaknesses, and different ones are
useful for different groups.
How DO we decide?
We will look at the uses and drawbacks of each.
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1) Biological Species Concept
(or Isolation Species Concept)
“Species are groups of actually or potentially
interbreeding natural populations which are
reproductively isolated from other such
groups.”
Mayr 1970
If populations do not breed together, they have
separate gene pools, so remain separate, and will
become more separated with time.
Definition should specify the young need to be
fertile.
Zebroid is offspring of zebra and horse, but cannot
breed with either parent species.
Liger is lion and tiger and the males are infertile.
A Mule finch is an infertile cross between a canary
and another finch species, bred for their song.
Other problems with the Biological
species concept.
1) Inappropriate for asexual organisms.
Do we count all amoeba as the same
species?
2) Too difficult to apply to fossils, as can’t
know who they would have bred with.
3) Species isolated on islands are problematic
as they never get the chance to show
whether they would breed.
Trying in the lab is unsatisfactory as some
populations who breed in the wild don’t in
the lab and v.v.
4) Organisms which just release gametes are
problematic.
2) Evolutionary species concept
If mating is never observed, hard to tell who is
mating with whom or how often. Rare enough
to see it in large animals, but impossible in
wind pollinated plants, or plankton.
A group of organisms that shares an
ancestor; a lineage that maintains its
integrity with respect to other lineages
through both time and space
5) Some species which do occasionally
interbreed, are clearly good species.
May or may not recognise sub-species
and recognises sister-species only when
they have become distinct enough to be
clearly different.
E.g. wolf and coyote, have remained separate
for 500,000 years, despite cross-breeding
occasionally and producing perfectly fertile
offspring.
(Surely that’s the debate?!)
Depends on the evolutionary history being known, and the
phylogenetic tree being correct, which isn’t universal.
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3) Recognition species concept
HEH Paterson 1980s
Species are separated by differences in their
fertilization system (complex of behaviours and
morphology, physiology and biochemical
features).
They are different species if they would not
recognise each other as mates, or could not
physically mate.
Supposedly covers fossils, and species on
different islands
But
Does not cater for asexual organisms
How different is too different to allow mating?
5) Use of genetic markers
Genetic analysis can also show whether
populations are interbreeding.
Problem: the degree of morphological
differences and genetic ones don’t
agree.
Frogs are all morphologically very
similar but have huge genetic diversity.
On the other hand humans and chimps
share 98% of genes but are
morphologically clearly good species.
We share 95% with mice!
Arbitrary how much difference needed
to count as a different species.
4) Morphological Species Concept
In practice this is most often used in difficult
cases - based on the visible differences
between species.
Taxonomists look for discontinuities in the
morphological variation between the
populations in question.
If variation is continuous then it’s not a
species e.g. different galapagos finches are
sufficiently different on each island that they
are clearly distinct species, with no gene flow
between them.
The red backed thrush, on the other hand,
was defined as a sub-species.
But what do these genetic differences actually
mean?
Wakatobi (Tomia)
We share 98% of the same genes with a chimp,
but are those genes the same? (No.) They have
different genetic sequences, even if they both
have similar phenotypic effects.
Runduma
To determine genetic differences between
subspecies and sister species, you need to look
at differences in base pairs within homologous
sequences of DNA.
Mainland
Sulawesi
We have been looking at genetic markers in
White eyes of South East Sulawesi
These allow us to investigate the degree of
separation between the different populations of
closely related birds, living on different islands.
WWWE
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Borneo
Runduma
Bali
Australia
Wakatobi
islands
Second (new)
white-eye
species
Presumed
source
population
Different
LBWE
colonisation
event
Closely related group of
populations of LBWE
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So there is a range of methods for defining a
species, and people use whatever method
works best for their group.
Bacteria, which exchange plasmids of DNA at
will, don’t have genetic isolation, so the idea of
species is merged with the term “strain” (see
Mario’s lecture on Thurs).
Virologists aren’t even sure their study
organism is alive, and call them “phage types”,
again avoiding the term species.
So the term “species” is neither well defined,
nor universally agreed upon, and JMS is not
wrong when he chose a practical definition.
“A species is that which is defined as a species
by a competent taxonomist!”
But why would the two populations become
different when one is isolated on an island?
a) Founder effect
b) Genetic drift
c) Less competition for resources on the mainland as
population smaller
d) Different selective pressures on the mainland from
those on the island
e) Co-adaptation of genes
e.g. imagine greenness and hiding only beneficial when present together.
If no predators on new island, greenness dies out and brownness takes
over.
When new species spreads to mainland brown individuals are not
recognised as same species by the green ones
Speciation has occurred using colour as Specific Mate Recognition
System (SMRS).
Routes to Speciation
However speciation occurs, the two
populations need to stop breeding
for long enough for them to become
too different to breed any more.
There are 3 possible ways:
1) Allopatric speciation
Some form of barrier splits the population,
or part of the population gets stranded
on an island.
The different conditions in the two
populations cause slightly different
selection to occur, so they diverge.
When the two populations meet once
more they can no longer interbreed.
Routes to Speciation 2: Parapatric speciation =
speciation at a border which does not prevent gene
flow entirely.
e.g. 2 species of crickets in the Pyrenees interbreed only at the tops of
certain mountains, and are clearly different species further down the
mountains
Tends to form a hybrid zone at the border as hybrids
between the two species may occur here.
(Hybrid zones may also arise where two originally allopatric
species coming together again.)
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In hybrid zones you often get Character Displacement =
exaggeration of signals, especially those related to breeding, to
make the signal stronger and prevent hybridisation.
Hybrid Zone narrows if hybrid offspring are less fit than
the pure breeds of each type,
Hybrid Zone
Call frequency
It widens if they are equally fit with pure bred offspring.
Normal
frequeny for
species A
Character
displacement
Normal
frequency for
species B
It may even widen so far that it wipes out one or both of the
original species.
Another example of a hybrid zone is between hooded and
carrion crows, which interbreed only at the border between the
two species.
During the last Ice Age, crows were forced
into “refugia” in Spain and the Balkans
Hooded
Crow
In the British Isles this is the
distribution of the Carrion
Crow…
…and this is the hooded
crow’s distribution.
Carrion Crow
By the time they spread back into Europe, the two populations
had drifted apart, and now there were two forms – but are they
now different species?
In Scotland, where the two forms meet, they form a hybrid
zone. Hybrids of different species are often less fit than pure
breeds.
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The crows were classed as one
species until work on the hybrids
made the experts decide that they
are actually two separate species.
The hybrids were found to be less fit
than either pure bred carrion or
hooded crows.
This will make a block to gene flow,
and over time they should hybridise
less and less. Thus they are
increasingly separate species.
(Interestingly, the hybrid zone in
Scotland appears to be moving
north with global warming.)
Routes to speciation:
3) Sympatric Speciation
More controversial than the other routes.
Requires isolation of a subset of the
population within the original population.
Interbreeding must stop if two separate
species are to form though.
If hybrids are less fit, then assortative mating
may be favoured i.e. preferentially mating
with your own morph.
The two forms may become increasingly
separate populations until they become
species.
On the way, you may see stable
polymorphisms i.e. more than one morph of
the same species e.g. 2spot ladybirds
How could it happen? A thought experiment:
Hybrid forms if lettuce eater
mates with cabbage eater
X
But hybrids do badly on both
food plants, so fitness low for
parents.
Mutant with SMRS (blue colour)
differentiating new species is at
advantage as can avoid
hybridising with lettuce eater.
Perhaps through change in food types:
Caterpillars all eat lettuce, and avoid cabbage.
One evolves to cope with cabbage, but now can’t eat lettuce.
What’
s he??
Ugly
cow!
The End (of interbreeding)
Character displacement
accentuates this difference, and
further SMRSs arise (antenna
shape and colour) in both
species.
Now don’t recognise each other
at all, and never hybridise
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Sympatric competitive speciation
Speciation Summary
e.g. Ichneumon flies
Lay eggs in beetle larvae buried in wood of tree
trunks.
If larva already parasitised she doesn’t lay another
egg in it, as other larva would eat 2nd egg.
Big Mums have long ovipositors so lay in deeply
buried larvae, small Mums in shallower larvae.
Resulted in disruptive selection for long and short
but not middle sized ovipositors, with middle sized
least favoured as suffers competition from both.
But to get fixed it needed assortative mating.
Allopatric
Parapatric
Original
population
First step
Barrier formed
New niche entered
Got stable polymorphism for two extremes of size…. Leading to 2 species.
Keywords to revise:
Founder Effect - a change in allele frequency due to the foundation of a subpopulation by an unrepresentative sample of the parent
population.
Genetic Drift
- A change in allele frequency due to chance events
Selectively neutral - A gene which has no effect on fitness.
Fixation - when only one allele survives at a locus
Effective population size - Those individuals of a population which can
breed
Population bottlenecks - When a population is reduced to a very few
individuals.
Polymorphic - having more than one form. In this context, a gene with more
than one possible allele.
Monomorphic - having only one form.
Inbreeding depression - reductions in fitness due to breeding with close relatives
due to increased expression of rare deleterious recessives.
Heterozygote advantage - extra vigour due to the genotype being heterozygotic.
Assortative mating - mate selection on the grounds of sharing the same allele
Disassortative mating - mate selection on the grounds of having a different
allele
Stable polymorphism – A species with more than one form in the population.
Polymorphism occurs
Reproductive
isolation
evolves:
In isolation
Big Dads could reach further into the wood to mate with emerging females
from deep in wood, so big Dads ended up mating more with big Mums =
assortative mating.
Sympatric
In new niche
Within the population
New genetically
distinct species
spreads and
competes with
original species
Real life examples of sympatric speciation:
1) In Madeiran petrels
Ibis Volume 149 Issue 2, Pages 255 - 263 Published Online: 21 Dec 2006
Review:
http://www.sciencedaily.com/releases/2007/11/071113160351.htm
2) In bats
Abstract:
http://www.nature.com/nature/journal/v429/n6992/full/nature02487.html
3) In Darwin’s finches
http://www.journals.uchicago.edu/doi/abs/10.1086/339987
Background Reading:
Peter Skelton “Evolution: A Palaeontological Approach” (1998) Addison Wesley
Chap. 3.5 “Measuring Variation”
Chap. 3.6 “Variation and Population structure”
Chap. 9 “Species, Speciation and Extinction”
Other interesting books relating to speciation:
Song of the Dodo by David Quammen
The Third Chimpanzee by Jared Diamond
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