Species and Speciation

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Species and Speciation

• Chapter 15

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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

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Species concepts

• Biological species concept
• Phylogenetic species concept
• Morphological species concept
• Ecological species concept

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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

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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
• Doesnt apply to asexual organisms
• Cant be applied to fossils
• How to apply to many plant and some animal
  species that hybridize freely?

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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

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Phylogenetic species concept 2

• If our tree looks like this, then populations A
  and B represent different species

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Phylogenetic species concept 3

• If our tree looks like this, then populations A
  and B are NOT different species

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Phylogenetic species concept 4

• What happens if our tree looks like this?

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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

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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

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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

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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

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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

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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

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Applying Species ConceptsThe 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

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Eurytemora affinishttp//life.bio.sunysb.edu/mari
nebio/pl_23.jpg
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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

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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

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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

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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

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Isolation by dispersal and vicariance (Fig. 15.5)
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Geographic isolation by dispersalHawaiian
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

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Evidence for speciation by dispersal and
colonization events Hawaiian Drosophila
(DeSalle and Giddings 1986) (Fig. 15.7)Based on
mtDNA
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Geographic isolation by vicarianceSnapping
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

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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/photo20
gallery/Alpheus20armillatus.jpg
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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

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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

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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 1800s
• 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

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Rhagoletis pomonella(Photo Guy Bush)
(http//www.ento.psu.edu/mcpheronlab/schwarz/Copy
20of20Rhagoletis1.jpg
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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

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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

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Allele frequency changes caused by differences in
temperature experienced by hawthorn maggot
flies(Feder et al. 1997) (Fig. 15.10)
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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