Download SPECIATION Why do species exist? •Altitude symbolises fitness

SPECIATION
•Estimates of the number of species alive today range from 3-100
million
•Very few of these have actually been described (approx 1.5 million).
Lots of them are insects!
•From the fossil record we know that
1.
Species can give rise to other species (speciation)
2.
Species can go extinct
•Why are there different species?
•What is a species?
Why do species exist?
•Altitude symbolises
fitness … level of
adaptation to
environment
•Why are there many
species?
•Because there are
many adaptive peaks.
•In other words, species
exist because there are
many possible ways to
adapt and survive.
•How does speciation happen?
What is a species?
1
Western spotted frogs (America)
Eastern Blue Jay
Cyanocitta cristata
Steller’s Jay
Cyanocitta stelleri
Look different
Live mostly in different areas
In Colorado overlap and interbreed
One species or two?
Dog and wolf
Once considered a single species
Look very similar
Don’t interbreed
Now classified as two species
WHAT IS A SPECIES?
•One theoretical definition is that a species is the smallest
evolutionarily independent unit – mutation, selection,
gene flow and drift operate on different species separately.
•However, this is hard to put into practice.
•What criteria can we use to identify species?
Canis lupus familiaris
Canis lupus lupus
2
BIOLOGICAL SPECIES CONCEPT
Ernst Mayer, 1942
“A species is a group of actually or potentially interbreeding natural
populations that are reproductively isolated from other groups”
Main criterion is reproductive isolation
- impossible for gene flow to take place between reproductively isolated
populations
Phylogenetic Species Concept
Members of a species must include all genetically closely
related organisms
Morphospecies Concept
Define species on the basis of morphological differences
If not done carefully, may make arbitrary species definitions
Problems:
1.
If the populations do not overlap, it is not easy to tell if
they would be able to reproduce together if they did meet
2.
It is not possible to apply this definition to the fossil
record (extinct species)
3.
It is not possible to apply this definition to asexual
populations
4.
In some plants see hybridisation between strongly
divergent populations – what does the biological species
concept mean in that case?
How does speciation happen? – Mechanisms of
Genetic Isolation
Classic model: Three steps
1.
Geographic isolation of populations
2.
Divergence of traits (e.g., mating system or habitat)
3.
Reproductive isolation (inability to successfully
mate)
Recent information indicates that steps 1 and 2 may occur
together, and that step 3 may not occur in some cases
Physical isolation as a barrier to gene flow
When a population is physically isolated from another, it is
then free to differentiate through mutation, natural selection
and drift. Migration between the populations will be blocked
by the physical barrier, and so there will be no
homogenising effect of gene flow.
This kind of speciation is called allopatric speciation
(Allopatric = other country)
Describes speciation that happens through geographic
isolation
Once gene flow has been dramatically reduced or halted,
the populations are independent evolutionarily and
speciation is under way.
3
Geographic isolation may occur through dispersal to a
new remote habitat, or through vicariance
(fragmentation of the environment)
Examples of vicariance could be the formation of a mountain range,
the movement (splitting) of tectonic plates, river formation, formation of
isthmus (e.g., Panama Isthmus separates Pacfic and Atlantic Oceans)
… etc. etc.
1. Geographic isolation through dispersal and colonisation
•Hawai’ian drosopholids (flies)
Over 500 named species
Huge ecological diversification (habitat, food)
Many of the species are found on only one island (endemic)
•How did all this speciation come about?
Founder hypothesis: small populations of flies, or one pregnant female,
disperse to new habitat or island and found a new population. Divergence
begins after the founding event.
How can we test this hypothesis?
It predicts that (1) closely related species should be found on closely
positioned islands; and (2) the order of some speciation events should
correspond to the order in which the islands were formed. Examine both
of these questions by looking at DNA sequence similarity.
The order of speciation events is seen in the order of
branches in the tree.
4
Observe:
1.
The newest species are on the newest
islands.
2.
Some of the species branching events
correspond to island formation events
3.
Hawai’I (Big Island) flies are more closely
related to Maui flies than to flies from other islands
(closer genetic relationships of flies from
geographically closer islands)
These observations are in line with the predictions of the
founder hypothesis.
What about speciation in the absence of geographic
isolation? Can this happen?
Called sympatric speciation
Sympatric = same country
Examples of sympatric speciation come from polyploidy
(more about this in your plant evolution lectures of this
module).
Polyploidy is a type of mutation event and is the
instantaneous doubling (or tripling, … etc.) of the genome of
an organism. For example, humans have 23x2
chromosomes, two of each kind. A polyploid human (if he
arose and could survive) would have more than two of each
kind of chromosome, e.g., a tetraploid would have 23x4
chromosomes.
1.
Geographic isolation through vicariance
•A species’ geographic distribution is split between two
isolated regions.
•Gene flow between the groups is impeded or prevented by
geographic separation. => populations start to evolve
independently.
•Example: rise of the land bridge between North and South
America (Panama Isthmus) isolated the marine populations
in the Atlantic from those in the Pacific.
•Did this lead to speciation?
•Yes – in an experiment where male and female shrimp
from either side of the isthmus were placed together in a
tank, those with a greater genetic distance (and therefore a
longer isolation time) were less interested in courting each
other.
•A polyploidy individual is genetically isolated from nonpolyploid relations – even if they are crossed, the offspring
will not survive (or will have a very low survival rate) or will be
infertile because of the unbalanced number of chromosomes.
•It also has been observed in plants that plants with different
ploidy levels (different numbers of copies of the
chromosomes) have different flowering times. This further
decreases the likelihood of gene flow.
•Polyploidy produces instant genetic isolation in the absence
of geographic separation.
5