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Species and Speciation
Chapter 15
1
What is a species?
• The “species” is a fundamental concept in biology –
paradoxically, there is no definition of species that is
universally applicable
• Freeman and Herron suggest that “most biologists agree …
[that a species] is the smallest evolutionarily independent
unit” (p. 584)
– Evolutionary independence occurs when gene flow between
populations ceases (or becomes very low); and when drift,
mutation, and selection operate on populations separately
• The “essence of speciation is lack of gene flow,” and
speciation involves at least two stages:
– Genetic isolation
– Differentiation
2
Species concepts
•
•
•
•
Biological species concept
Phylogenetic species concept
Morphological species concept
Ecological species concept
3
Biological species concept:
(Ernst Mayr 1942)
• A biological species is a group of interbreeding (or
potentially interbreeding) individuals, that is
reproductively isolated from other groups of
interbreeding individuals
– A species is a gene pool that is not exchanging genes
other gene pools
– Reproductive isolation means that individuals of
different species will not mate and produce offspring,
or offspring will be sterile
4
Biological species concept – 2
• The BSC is the “textbook” standard definition of
species
• It is applicable to sexually reproducing organisms
• It can be difficult to apply in practice because we
generally do not know if populations that have
separate ranges can interbreed, unless we do the
experiment
• Doesn’t apply to asexual organisms
• Can’t be applied to fossils
• How to apply to many plant and some animal
species that hybridize freely?
5
Phylogenetic species concept
= genealogical species concept
• A species is the smallest monophyletic group
• Suppose we sample 5 individuals from each of two
populations, A and B, and construct a tree – based
perhaps on genetic information
6
Phylogenetic species concept – 2
• If our tree looks like this, then populations A and B
represent different species:
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
7
Phylogenetic species concept – 3
• If our tree looks like this, then populations A and B
are NOT different species:
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
8
Phylogenetic species concept – 4
• What happens if our tree looks like this?
A1
B4
A3
A4
B2
B1
A5
B3
A2
B5
9
Phylogenetic species concept – 5
• The logic of the phylogenetic species concept is that traits
can only distinguish populations on a phylogeny if the
populations have been isolated in terms of gene flow and
have diverged genetically and/or morphologically
• To be considered separate phylogenetic species,
populations must have been separated long enough to have
evolved unique derived diagnostic characters
• Unlike the BSC, the PSC can be applied to asexual
organisms
• Like the BSC, the PSC may be difficult to apply in practice
because it requires a lot of information to construct good
trees that will identify monophyletic groups
10
BSC and PSC overlap
• A “good” biological species is a monophyletic group of
individuals – individuals of the same species are more
closely related to one another than they are to individuals of
other species
• Therefore, the BSC and PSC may agree on species
designations some of the time
• However, the PSC could greatly increase the number of
species because separate populations of a biological species
might be monophyletic but still able to interbreed – if
individuals from population A can interbreed with
individuals from population B, we have one biological
species but two phylogenetic species
11
BSC and PSC overlap – 2
• If individuals from population A can interbreed with
individuals from population B, then we have one biological
species but two phylogenetic species
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
12
Morphological Species Concept – 1
• A species is a group of phenotypically
similar individuals
• This is, in fact, how most species are
actually defined in the absence of detailed
information about reproductive
compatibility and/or phylogenetic
relationship
• Only species concept applicable to fossils
13
Morphological Species Concept – 2
• Disadvantages
– Not “evolutionary” (genetic or phylogenetic)
– Arbitrary and idiosyncratic: two people may
disagree about where to draw species
boundaries, or statistical phenetic methods may
disagree about how to create groups of
morphologically similar individuals
– Cryptic species cannot be distinguished
14
Ecological Species Concept
• A species is a group of phenetically similar organisms that
occupy a given ecological niche (or set of niches)
• Species integrity is maintained not so much by
reproductive isolation, but by selection to adapt each
species to its niche
• Hybridization is not a problem if hybrids are less fit than
parental species or have very restricted ranges
• Works for asexual species
• Problem:
– Hard to define niches independently of the species that occupy
them
15
Applying Species Concepts
The marine copepod, Eurytemora affinis
• Small estuarine copepod (1 – 2 mm), world
-wide distribution, important part of
zooplankton and marine food chains.
• Lee (2000) sequenced 2 genes in
individuals from 38 populations and
performed matings between individuals
from different populations in the laboratory
16
Eurytemora affinis
http://life.bio.sunysb.edu/marinebio/pl_23.jpg
17
A phylogeny of E. affinis populations
(Lee 2000) (Fig. 15.3 b)
•
•
This
phylogeny
supports “at
least 8”
phylogenetic
species
Breeding tests
indicate that
individuals
from two
different
phylogenetic
species are
reproductively
isolated
18
How many species of elephants?
(Roca et al. 2001) (Fig. 15.4 b)
•
•
•
This
phylogeny
supports two
African
elephant
species
195 elephants
from 21
African
populations
4 genes
sequenced
19
Mechanisms of genetic isolation
• Geographic (physical) isolation – allopatric
speciation
• Isolation based on differential resource use
without geographic isolation – sympatric
speciation
• Isolation by selection and limited gene flow in
continuously distributed populations – parapatric
speciation
• Isolation based on changes in chromosome
number or chromosomal rearrangements –
chromosomal speciation
20
Allopatric speciation
(Ernst Mayr 1942, 1963)
• Two or more populations of a species become
geographically isolated from one another (either
by dispersal or vicariance)
• Separated populations will evolve independently
of one another provided there is no (or very little)
gene flow between them
• In a “pure” allopatric model, speciation is an
“accidental” by-product of separate evolutionary
trajectories that eventually result in genetic
(reproductive) incompatibility between individuals
from different populations
21
Isolation by dispersal and vicariance (Fig. 15.5)
22
Geographic isolation by dispersal:
Hawaiian drosophilids
•
•
•
•
•
Over 500 endemic species in 2 genera
Occupy a wide variety of ecological niches
Many species have ranges restricted to single islands
One of the most famous adaptive radiations
“Founder hypothesis” – one gravid female (or a few
individuals) disperse to another island and start a new
isolated population
• Predictions:
– Closely related species should tend to be on neighboring islands
– At least some phylogenetic branching sequences should
correspond to the sequence in which islands were formed
23
Evidence for
speciation by
dispersal and
colonization
events –
Hawaiian
Drosophila
(DeSalle and
Giddings
1986) (Fig.
15.7)
Based on
mtDNA
24
Geographic isolation by vicariance
Snapping shrimp and the Isthmus of Panama
(Knowlton et al. 1993)
• Before the formation of the Panamanian land bridge about
3 million years ago, the Pacific Ocean and Carribean Sea
were connected. The formation of the land bridge was a
vicariant event that divided populations of marine
organisms
• Seven species pairs identified on the basis of morphology
– one member of each pair in the Carribean and the other
member in the Pacific. This is what would be expected if
there were seven species originally, and each was
subsequently split by the formation of the land bridge.
• Data on mtDNA sequences confirms this hypothesis
25
mtDNA phylogeny of 7 morphological species pairs of
snapping shrimp (Knowlton et al. 1993)(Fig. 15.8 b)
(P = Pacific Ocean; C = Carribean Sea; numbers designate morphospecies)
Alpheus armillatus
http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/Alpheus%20armillatus.jpg
26
Mechanisms of divergence
• Given that populations become
geographically isolated, what then causes
them to diverge genetically?
– Founder effect speciation – maybe, if number
of founders is very small and new population
stays small for a long time
– Genetic drift – likely to require a long time
unless populations are very small
– Natural selection, sexual selection, mutation
operating differently in isolated populations
27
Sympatric Speciation
• Genetic isolation without geographic isolation
• A single population becomes subdivided by
alternative habitat preferences or alternative
resource use
• Sympatric speciation commonly invoked for
herbivorous insects – alternative host plants give
rise to host races that ultimately become separate
species
28
Apple and hawthorn maggot flies (Rhagoletis
pomonella) – 1
• Fly is native to northeastern and north-central US
• Native host plant is hawthorn (Crategus)
• Flies first recorded as a pest of apples in the mid
1800’s
• Female flies lay eggs on the fruit, larvae hatch and
burrow into the fruit: after about a month the fruit
falls to the ground, the larvae leave and pupate in
the soil, and emerge as adults the following
summer
29
Rhagoletis pomonella
(Photo: Guy Bush)
(http://www.ento.psu.edu/mcpheronlab/schwarz/Copy%20of%20Rhagoletis1.jpg
30
Apple and hawthorn maggot flies (Rhagoletis
pomonella) – 2
• Are flies that parasitize apple fruits and hawthorn fruits
distinct populations?
– host race hypothesis
– implies selection for exploiting different hosts
• Or, do flies that parasitize apple fruits and hawthorn fruits
interbreed freely and are they a single population?
– this hypothesis seems more likely because the two host trees occur
together (sympatrically) throughout their ranges, and flies are
known to travel distances of more than a mile
31
Apple and hawthorn maggot flies (Rhagoletis
pomonella) – 3
• Flies that parasitize apple and hawthorn fruits are distinct
host races
– genetically differentiated
– show preference for their own fruit type in choice tests
• Field observations indicate that only 6% of matings take
place between apple and hawthorn flies
• This is still a fair amount of gene flow
– it would prevent genetic divergence of host races unless selection
was acting differently on the two hosts
• Natural selection for divergence appears to be based on
different times of fruit ripening
– apple fruits ripen 3 – 4 wks before hawthorn
– natural selection for apple flies to complete development well
before hawthorn flies
32
Allele frequency changes caused by differences in
temperature experienced by hawthorn maggot flies
(Feder et al. 1997) (Fig. 15.10)
33
How common is sympatric speciation?
• Historically, a controversial idea
• Simple population models suggest that genetic
isolation in sympatry is likely to be difficult
• However, host race formation is much more likely
to occur if individuals tend to mate and lay eggs
on the same kind of fruit that they grew up on –
assortative mating reduces gene flow
• There is more and more evidence that sympatric
speciation may be relatively common – most
examples involve herbivorous insects
34