Species Concepts

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SPECIES AND SPECIATION
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• Species Concepts

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(No Transcript)
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• This idea goes all the way back to Darwin where
  he used visible gaps in morphology to delimit
  species.
• I believe that species come to be
  tolerably well-defined objects, and do not at any
  one period present an inextricable chaos of
  varying and intermediate links.(1859 pg 177)
• We shall have to treat species in the same
  manner as those naturalists treat genera, who
  admit that genera are merely artificial
  combinations made for convenience. This may not
  be a cheering prospect but we shall at least be
  freed from the vain search for the undiscovered
  and undiscoverable essence of the term
  species(1859 pg 282)

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Species Concepts
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• Based on judgments about the similarities among
  organisms
• challenge is to make it mechanistic and testable
• want to accurately reflect evolutionary history
  of organisms.
• we dont really know whether such a thing as a
  species actually exists in reality

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There are many proposed species concepts in the
literature of evolution, ecology and conservation
biology
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Three species definitions are most widely
accepted
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• All three assume two things in common
• 1) no gene flow- species form a boundary for
  the spread of alleles
• 2) species have their own evolutionary pathway

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The BSC Biological Species Concept
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• Proposed by Dobzhansky and Mayr, elucidated by
  Mayr as
• Species are groups of interbreeding natural
  populations that are reproductively isolated from
  other such groups.
• Definition implies
• no hybridization or hybrids fail to form fertile
  offspring
• lack of gene flow

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BSC (cont)
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• PROBLEMS
• Can not always tell if two groups of individuals
  are reproductively isolated
• If two groups are separated by geographical
  barriersthere is no way to know if they are
  reproductively isolated
• Many plants hybridize freely we will discuss
  hybridization later in some detail
• Cannot test it in fossil forms
• Irrelevant to asexual populations

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The Phylogenetic Species Concept (PSC)
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• Also called Evolutionary Species Concept
• This is the systematists contribution to the idea
  of a species
• A species is a single lineage of ancestral
  descendant populations of organisms which
  maintains its identity from other such lineages
  and which has its own evolutionary tendencies and
  historical fate
• Focuses on the idea of monophly (a set of species
  are all descended from one common ancestor)

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PSC (cont)
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• A monophyletic group contains all of the known
  descendants of a single common ancestor. There
  are no parallel branches or interconnecting
  branches (such as in hybridization)
• Fig 16.1 species are on the tips of the trees.
  Circles represent the monophyletic groups

Notice that (1) Common ancestor does not
continue on as a species and (2) every species
divides to form only two new sister species
You do not see branching such as this.
or this
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PSC (cont)
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• RATIONALE
• can only form separate species if the populations
  have diverged from one another in isolation
• The original species will always form two new
  species and cease to exist itself.
• Appeal is that it is testable
• Species are identified (named) on the basis of
  statistically significant differences in the
  traits used to estimate the phylogeny (ancestry)

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PROBLEMS with PSC(cont)
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• 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
• Very difficult to interpret when new species
  actually becomes a new species

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Morphospecies
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• Define species based on the morphological
  differences. Commonly used with fossils.
• This definition does not demand proof of
  reproductive isolation or phylogenetic
  relationships
• 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

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Cryptic species
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• ones which are strongly divergent based on
  non-morphological characters.
• Things such as song, temperature or drought
  tolerance, habitat use, or courtship displays

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Morphospecies (cont)
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• Today used mostly by paleontologists
• For at least some instances there is good
  evidence that fossil Morphospecies may indicate
  real species differences

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Application of the 3 species definitions to red
wolf
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• In 1930 the Red wolf appeared 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.

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Speciation
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Possible conditions for speciation we will
consider
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• Allopatric model- speciation occurs in
  populations that have been physically isolated
  from one another
• Sympatric model - Populations can diverge without
  geographical separation, with low to moderate
  gene flow between them if
• 1. Selection for divergence is strong
• 2. Mate choice is correlated with the factor
  that is promoting divergence
• Parapatric model Strong selection for divergence
  causes the gene frequencies to diverge along a
  gradient
• Peripatric model a subset of the allopatric model
  involving colonization

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Parapatric speciation
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Allopatric model of speciation
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• Involves 3 steps
• Isolation of members of a population from one
  another
• Genetic divergence of the separated populations
• Renewed sympatry of the populations with
  reinforcement of the genetic differences which
  have arisen

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FIRST STAGE OF ALLOPATRIC SPECIATION
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• Physical Isolation

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Physical Isolating Mechanisms
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• Necessary to prevent gene flow which would keep
  populations homogenized
• may occur when small populations become isolated
  at the periphery of a species range.
• If selection is strong and gene flow is low
  divergence could then occur rather rapidly

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Ways in which physical isolation may occur
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Geographic isolation
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• By dispersal and colonization
• Dispersal to novel environment such as rafting a
  portion of a population to an island
• By Vicariance events

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Dispersal and Colonization
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Hawaiian island Drosophilids
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• Involves Founder effect Peripatric Speciation
  (Mayr)
• small group of individuals cut off from the
  original population colonizes a new habitat
• drift and selection on genes involved in mating
  and habitat use leads to divergence

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Evidence show this is a valid interpretation
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• closely related species should be found on
  adjacent islands
• some of the phylogenetic branching sequence
  should follow island formation
• using mitochondrial DNA it was shown that four
  closely related species were found in the
  expected pattern
• Figure 16.7 page 613

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Figure 15.7 page 593
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Vicariance events
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• 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 etc
• or rapid like a lava flow that splits non-flying
  insect populations

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Isthmus of Panama
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• A 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 others closest
  relative) believed to share the same common
  ancestors which split to form each pair

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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
Figure 16.8 pg. 614
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SECOND STAGE OF ALLOPATRIC SPECIATION
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• Genetic Divergence

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Mechanisms of divergence
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• Vicariance events and dispersal events only
  provide conditions for speciation
• Usually you also need to have genetic drift
  and/or selection work on mutations in these
  isolated populations in order to get genetic
  divergence.
• Sexual selection may also lead to genetic
  divergence

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3rd and Final Stage Of Allopatric Speciation
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• Secondary Contact (return to sympatry)

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Examining potential outcomes of secondary contact
(return to sympatry)
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Four possible outcomes

• After return to sympatry

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No Speciation

• Possible Outcome 1 after secondary contact

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• Fully fertile hybrids form no speciation has
  actually occurred while in allopatry.
• Hybrids thrive and interbreed with both parental
  populations, any divergence is erased.

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Reinforcement

• Possible Outcome 2 after secondary contact

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• Reinforcement of parental forms as two recently
  diverged species.
• The two groups are considered now to be two
  species.

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Outcome 2 Reinforcement
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• 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 within each new
  species.
• Selection that reduces the frequency of hybrids
  is called reinforcement

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Reinforcement finalizes the speciation process by
completing reproductive isolation
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• The final stage of speciation, that of
  establishing reproductive isolation by
  reinforcement can occur in any number of ways.
• These are called pre-zygotic isolation mechanisms
  

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Reinforcement hypothesis predicts any number of
possible Pre-zygotic isolation mechanisms which
will prevent fertilization from occurring
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• Temporal isolation individuals of different
  species do not mate because they are active at
  different times of day or seasons of the year
• Ecological isolation- Individuals mate in their
  preferred habitat, and therefore do not meet
  individuals of other species.
• Behavioral isolation- potential mates from
  incipient species meet but choose members of
  their own species
• Mechanical isolation copulation is attempted
  but transfer of sperm does not take place
• Gametic incompatibility sperm transfer takes
  place but egg is not fertilized

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Some facts from experimental and observational
research
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• Plot of genetic similarity versus the degree of
  interbreeding for various sister species of
  Drosophila. A value of 0 on the Y axis indicates
  free interbreeding, 1 indicates no interbreeding.
  

Figure 16.12 pg 625
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Reinforcement, (total lack of interbreeding) is
not absolutely necessary for populations to
remain genetically isolated when brought back
into contact
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• Post-zygotic mechanisms may lead to hybrid
  offspring which are sterile or infertile

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Post-zygotic mechanisms
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• Zygotic mortality- egg is fertilized but zygote
  does not develop
• Hybrid inviability Hybrid embryo forms but of
  reduced viability
• Hybrid sterility hybrid is viable but adult is
  sterile
• Hybrid breakdown F1 hybrids are viable and
  fertile but F2 and backcrosses to parents are
  inviable or sterile

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End Day 1

• Chapter 16

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Hybridization

• Possible Outcome 3 after secondary contact

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• Creation of a new species through hybridization.
• Formation of a new third species from the hybrid
  formed.
• Hybrid is fertile but cannot back cross to either
  parent.

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The role of Hybridization

• Hybridization is a common occurrence in plants
• At least in some cases the outcome of these
  hybridization events determines the outcome of
  the speciation event

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Interspecific hybridization is an important
source of evolutionary change in plants
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• In newly colonized areas or in new habitats,
  hybrids may have higher fitness than the parents
• These hybrids can mate with siblings and
  backcross to their parents the result is a
  variety of hybrid gene combinations. This is
  called introgression.
• If certain of these combinations is best suited
  to a new habitat, a third species may arise that
  is somewhere intermediate between the parental
  species.
• However, it is also possible that, if the hybrids
  have equal or greater fitness than either
  parental population, complete introgression may
  occur. The result is one species somewhat like
  the one that existed prior to geographic
  separation

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Hybrid Zone Formation

• Possible Outcome 4 after secondary contact

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Size, shape and longevity of hybrid zones are
determined by 3 possible outcomes....
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• When parents and hybrid are equally fit
• The zone is wide.
• Hybrid traits are found with highest frequency
  at the center of the zone.
• Gene frequency changes are dominated by drift.
• Width of the zone is determined by
• a) distance of dispersal in each generation
• b) How long zone has existed

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Factors (cont)
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• 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.

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Factors (cont)
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• 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 (Parapatric speciation)
• Often found in areas called ecotones where
  markedly different plants and animals meet

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Example from Big sagebrush in Utah
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• 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

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Hybridization Issues
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• 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

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Genetic differentiation and isolation
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• How much genetic variation is necessary to
  produce a new species?
• The BSC requires that no hybridization whatsoever
  occur, however.....
• 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 in an attempt to uncover past
  speciation through hybridization

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Sympatric Speciation no physical isolation
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• Requires development of reproductive isolation
  while individuals are still in contact and gene
  flow is still possible

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Possible mechanisms for sympatric speciation

• Polyploidy
• Genetic divergence (drift, natural selection
  etc.)
• Sexual selection

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Polyploidy Isolation by chromosome
incompatibility
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• Polyploidy, or the condition of having extra sets
  of chromosomes can lead to genetic isolation of
  populations.
• Polyploids have 3n, 4n, 5n, 6n etc. numbers of
  chromosomes rather than the normal diploid
  number.
• This condition is detected by looking for two
  factors
• In plants at least, chromosome numbers greater
  than n14 are considered to be of polyploid
  origin
• Related individuals will have chromosome numbers
  which are multiples of some basic number, for
  example, in Chrysanthemum different species have
  2n numbers of 18,36,54,72, and 90

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Two Types of Polyploidy

• Autopolyploids
• Allopolyploids

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Autopolyploidy

• The union of unreduced gametes from genetically
  and chromosomally compatible individuals, that
  may be thought of as being from the same species.
• During meiosis In the autoploids the chromosomes
  pair up into quadrivalents and mostly get uneven
  segregation of chromosomes. Called aneuploid
  gametes (have either too many or too few
  chromosomes).
• This leads to reduced fertility or sterility

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Unreduced gametes
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Allopolyploidy

• Polyploids are derived from a hybrid between
  unreduced gametes of two different diploid
  species.
• Genetic or chromosomal incompatibility arises.
• In meiosis, there is a natural formation of
  homologous pairs with twice as many pairs as
  either parent.
• Get balanced segregation and normal gametes
• Typically these hybrid polyploids have near
  normal fertility.

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Unreduced gametesTwo different species
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Alloploidy can lead to isolation due to
chromosomal incompatibility

• Because cannot back cross to either parent the
  resulting gametes are sterile
• Limits the individuals they can cross with to
  other allopolyploids
• If species cannot self fertilize then...
• May have difficulty finding each other and
  cannot out-compete the original parent
  populations. But
• If they do interbreed with each other, or are
  self-compatible, they are chromosomally isolated
  and can begin to diverge from parent populations
  immediately.

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2. Sympatric speciation due to Genetic divergence
occurs through the linkage natural selection
forces acting on behavioral differences
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3. Sympatric speciation though Sexual
Selection
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• Sexual selection promotes divergence efficiently
  because it affects gene flow directly
• There is an example in the book of Drosophila
  flies that may have diverged due to sexual
  selection
• In the Beak of the Finch we read about cases of
  sexual selection which can help prevent
  hybridization in the finches.
• Recent studies are suggesting that sexual
  selection is an important and necessary factor
  for sympatric speciation

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End here for winter 2008