Classification

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Classification

• Organisms are classified into a hierarchical
  classification that groups closely related
  organisms and progressively includes more and
  more organisms.

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Species

• The species is the basic biological unit around
  which classifications are based.
• However, what constitutes a species can be
  difficult to define and there are multiple
  definitions of species in use today.

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What is a species?

• The species is a basic biological unit and humans
  seem to intuitively recognize species.
• However, why do species exist?
• Why dont we see a smooth continuous blending of
  one species into another?

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Why do we see discrete species?

• Because intermediate forms between closely
  related organisms are usually selected against.
• If they were not selected against, then the two
  forms would merge into one as their gene pools
  mixed.

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Why do we see discrete species?

• Organisms are very well adapted to their
  environments having evolved over millions of
  years.
• Each organism has specialized characteristics
  such as camouflage, feeding structures, behavior,
  and genitalia that equip it to survive well in
  its environment.

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Why do we see discrete species?

• An offspring that results from a cross between
  members of two different species or between
  members of different populations that have been
  evolving in isolation from each other, will
  probably have traits intermediate between its
  parents.
• As a result, it likely will be less well adapted
  to its environment than either parental form and
  be selected against.
• Thus, we see distinctively different species.

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What is a species?

• John Ray (1627-1705) gave first general
  definition of a species.
• A species consists of all individuals that can
  breed together and produce fertile offspring.

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A female donkey mated to a male horse produces
what?
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A mule (which is sterile) Hence, donkeys and
horses are separate species.
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Biological Species Concept

• Rays idea was updated into the Biological
  Species Concept. Two definitions of the BSC are
  given below
• Species are groups of actually or potentially
  interbreeding natural populations, which are
  reproductively isolated from other such groups.
  Ernst Mayr.
• A species is a reproductive community of
  populations (reproductively isolated from others)
  that occupies a specific niche in nature. Ernst
  Mayr.

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Biological Species Concept

• The biological species concept emphasizes that a
  species is an interbreeding population of
  individuals sharing common descent and that
  members of that community because they share a
  niche constitute an ecological entity in nature.
• Members of a species we expect to be similar to
  each other but different from other organisms,

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Criticisms of the Biological Species Concept

• The BSC has been criticized for several reasons
• 1. It applies only to sexually reproducing
  species.
• 2. Distinguishing between species on the basis of
  reproductive separation is problematic because it
  can be difficult to determine how much
  reproductive separation is needed to distinguish
  between species.
• 3. The definition refers only to current
  populations and ignores the species status of
  ancestral populations.

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Evolutionary Species Concept

• George Gaylord Simpson proposed the Evolutionary
  Species Concept in the 1940s to add an
  evolutionary time dimension to the Biological
  Species Concept.

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Evolutionary Species Concept

• Evolutionary species concept A single lineage of
  ancestor-descendant populations that maintains
  its identity from other such lineages and that
  has its own evolutionary tendencies and
  historical fate.

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Evolutionary Species Concept

• Definition applies to both sexually and asexually
  reproducing species and emphasizes common
  descent. As long as diagnostic features are
  maintained a lineage will be recognized as a
  single species.

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Phylogenetic species concept

• A third species concept is the phylogenetic
  species concept.
• an irreducible (basal) grouping of organisms
  diagnosably distinct from other such groupings
  and within which there is a parental pattern of
  ancestry and descent.

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Phylogenetic species concept

• The phylogenetic species concept also emphasizes
  common descent and covers both sexually and
  asexually reproducing organisms.
• Under the PSC any population that has become
  separated and has undergone character evolution
  will be recognized as a species.

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Phylogenetic species concept

• Criterion of irrreducibility requires that no
  more than one diagnosibly distinct population can
  be included in a single species. Thus, the
  emphasis is placed on monophyly lineages that
  contain all the descendents of a single common
  ancestor.
• Main difference in practice between ESC and PSC
  is that PSC recognizes as species the smallest
  groupings of organisms that have undergone
  independent evolutionary change.

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Phylogenetic species concept

• The ESC would group into one species a series of
  geographically disjunct populations that show
  some genetic divergence, but the PSC would treat
  them as discrete species.
• Thus, subspecies under the ESC would be species
  under the PSC and in general more species would
  be recognized under the PSC than either the BSC
  or ESC.

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Typological Species concept

• For historical interest this is the pre-Darwinian
  idea that species are defined by fixed and
  unchanging features and do not change over time
  (i.e., evolve).
• Biologists discarded the idea after Darwins
  theory of evolution by natural selection became
  established.
• Creationists still cling to the typological
  species concept and youll often see types
  referred to in creationist writings.

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Applications of species concepts

• Diversification in marine copepods.
• Copepods are small abundant crustaceans. Numerous
  populations of Eurytemora affinis have been
  described from estuaries in the northern
  hemisphere and traditionally grouped into one
  species on the basis of similarity of appearance.

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Diversification in marine copepods

• A study by Lee (2000) in which she compared gene
  sequences of populations and also carried out
  breeding trials showed that at least 8
  phylogenetic species exist, which are
  reproductively isolated.
• Clearly, assuming species identity on the basis
  of morphology alone will underestimate species
  diversity.

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How many species of African elephants are there?

• Traditionally one species of elephant Loxodonta
  africana has been recognized in Africa (a second
  species Elephas maximus occurs in Asia).
• However, recent morphological studies have
  pointed out that forest dwelling elephants in
  West Africa appear to differ from elephants found
  in Savannah habitats elsewhere on the continent.

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How many species of African elephants are there?

• A comparison of DNA from 21 populations suggests
  that two phylogenetic species exist and it has
  been suggested by Roca et al. (2001) that forest
  elephants be named Loxodonta cyclotis.
• Whether the two populations are capable of
  interbreeding is unclear, but the clear genetic
  differences between populations suggest that
  conservation biologists should be attempting to
  conserve members of both populations.

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How species form

• Classically, speciation has been viewed as a
  three stage process
• Isolation of populations.
• Divergence in traits of separated populations
  (e.g. mating system or habitat use).
• Reproductive isolation of populations that
  maintains isolation when populations come into
  contact again (secondary contact).

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How species form

• Recent research shows that steps one and two may
  take place simultaneously in the same place and
  often the third step does not occur.

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Genetic Isolation physical isolation

• Physical separation reduces or stops gene flow
  between populations and as a result there may be
  a balance between gene flow and natural selection
  (recall the Lake Erie water snake example from
  chapter 6).
• On the islands selection favors elimination of
  alleles for banding, but migration constantly
  introduces them. If the islands were to be
  completely separated so no snakes migrated
  natural selection would result in the island
  populations becoming different from the mainland
  ones.

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Allopatric speciation

• This is the essence of Ernst Mayrs allopatric
  model of speciation.
• A physical barrier isolates a population or
  populations from the rest of the species and
  selection favors genetic divergence of that
  population.

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Allopatric speciation

• Separation of populations can occur by two major
  means
• Dispersal of some individuals across a barrier.
• Development of a new barrier that separates
  populations Vicariance (the vicariance event
  could be e.g. change in flow of a river, lava
  flow, development of a mountain range, habitat
  destruction)

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Geographic isolation through dispersal

• We have already encountered example sof
  speciation after individuals crossed a barrier.
• The ancestors of Darwins finches colonized the
  Galapagos Islands after dispersing from South
  America and speciated into the current range of
  species.
• Similarly, the Hawaiian Islands were colonized by
  ancestral Drosophila fruit flies that appear to
  have speciated to produce more than 500 endemic
  species of Drosophila on the islands.

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Evidence for founder hypothesis of speciation in
Hawaiian Islands

• The main hypothesis for how the Hawaiian Islands
  became populated with a diverse variety of
  endemic species most of which occur on only a
  single island is the founder hypothesis.
• According to the founder hypothesis new species
  are formed when a small population of
  individuals disperses to a new island and after
  being separated diverges from the ancestral form.

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Evidence for founder hypothesis of speciation in
Hawaiian Islands

• The Hawaiian Islands were formed by a stationary
  geological hot spot over which the continental
  plate drifts northwest.
• Periodically, the hot spot produces magma flows,
  which form islands that are then carried away on
  the plate and ultimately erode away. Thus the
  newest islands are close to the hot spot and the
  oldest further northwest.

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Evidence for founder hypothesis of speciation in
Hawaiian Islands

• Based on the geological information te founder
  hypothesis makes two predictions about the
  pattern of speciation that should be observed.
• Closely related species should be found on
  adjacent islands and
• Some speciation sequences should match the
  sequence in which islands formed.

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Evidence for founder hypothesis of speciation in
Hawaiian Islands

• A study of mitochondrial DNA of four species of
  closely related Drosophila by DeSalle and
  Giddings (1986) found the predicted patterns.
• The most recent species occur on the youngest
  islands and several of the branching events match
  the order of island formation.

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Geographic isolation through vicariance events

• There are many ways in which a species
  distribution may be split into two by a physical
  event. Some such as mountain formation are slow,
  others such as a lava flow are rapid.
• The Isthmus of Panama closed about 3 million
  years ago separating marine populations on either
  side. Did these populations speciate?

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Geographic isolation through vicariance events

• A DNA sequence study by Knowlton et al. 1993 of
  snapping shrimp populations from both sides of
  the isthmus suggests they did.
• Seven pairs of morphologically closely related
  species pairs occur, one of each pair on each
  side of the isthmus and the DNA sequence results
  confirm that these are each others closest
  relatives, which is consistent with the
  vicariance hypothesis.

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Phylogenetic tree of numbered species of snapping
shrimp. P and C refer to Pacific and Caribbean
species respectively.
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Geographic isolation through vicariance events

• Mating experiments with the snapping shrimp fund
  that males and females with the greatest genetic
  divergence were least interested in each other
  and almost none the pairs produced clutches that
  yielded fertile young.

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Polyploidization as a mechanism of speciation

• Polyploidy (production of multiple sets of
  chromosomes) appears to have played a major role
  in the speciation patterns of plants.
• An estimated 70 of flowering plants appear to
  have had polyploid events in their evolutionary
  history as have 95 of fern species.

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Mechansims of divergence

• Dispersal, vicariance and polyploidization create
  opportunity for speciation to take place.
• For speciation to occur populations must diverge
  genetically from each other.

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Genetic drift

• Genetic drift is a sampling phenomenon in which
  only some alleles occur in a population as a
  result of its small size because of founder
  effect and bottlenecking.
• If a population remains small for a period of
  time many alleles may be lost from the gene pool.

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Genetic drift

• The length of time the population is bottlenecked
  has a strong influence on how great allele
  frequency changes will be. Theoretical studies
  show that if populations remain very small for
  only a short time then only rare alleles are
  likely to be lost and little effect on speciation
  is likely.
• Thus, scientists are increasingly focusing on
  natural selection as a more important force
  driving speciation than drift.

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Natural selection and speciation in apple and
hawthorn maggot flies

• The apple maggot fly (Rhagolestis pomonella) is a
  major pest of apples that occurs throughout the
  northeastern U.S. It also parasitizes hawthorn
  trees a close relative of apples.
• Maggot flies recognize trees on the basis of
  visual, tactile and olfactory cues and mate on or
  near the fruit.

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Natural selection and speciation in apple and
hawthorn maggot flies

• Eggs are laid on fruits and larvae develop in
  them. When the fruit falls the larvae burrow
  into the ground and pupate emerging as adults the
  next year.
• Apple trees are a novel food source for these
  native flies, which exploited apples after they
  were introduced about 300 years ago.

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Natural selection and speciation in apple and
hawthorn maggot flies

• The question is does the new food source
  represent an island and are the populations that
  breed on apples genetically distinct form those
  that breed on hawthorn trees?
• Do apple and hawthorn populations interbreed or
  not and are they diverging?

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Natural selection and speciation in apple and
hawthorn maggot flies

• Hawthorn and apple trees are often in very close
  proximity so it would seem hard for the
  populations to diverge.
• However, a protein electrophoresis study by Feder
  et al. (1988,1990) showed that the populations
  are genetically distinct.

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Natural selection and speciation in apple and
hawthorn maggot flies

• Each population shows a strong preference for its
  own fruit type, which because mating takes place
  on fruit results in strong nonrandom mating.
• There is gene flow between populations because
  about 6 of matings are cross-population matings,
  but despite this gene flow natural selection
  appears to driving the populations apart.

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Natural selection and speciation in apple and
hawthorn maggot flies

• Natural selection favors divergence because
  hawthorn fruits ripen 3-4 weeks after apples. As
  a result hawthorn fly larvae experience cool
  temperatures before pupating whereas apple fly
  larvae experience warmer temperatures.
• Hawthorn flies and apple flies thus depend on
  different temperature signals to time their
  pupation and emergence the next spring and have
  different developmental timetables.

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Natural selection and speciation in apple and
hawthorn maggot flies

• Experimental tests show that these developmental
  schedules have a genetic basis and individuals
  need the correct alleles to develop under each
  temperature regime.
• Individuals that are the result of crosses
  between apple and hawthorn flies are thus
  selected against and the populations have
  diverged and continue to do so.

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Secondary contact

• Theodosius Dobzhansky (1937) the famous
  geneticist reasoned that for populations that had
  diverged and come back into contact hybrid
  offspring between them would have reduced
  fitness.
• As a result there should be strong selection
  favoring assortative mating (individuals mating
  within their own population) and as a result a
  variety of isolating mechanisms should evolve to
  reduce the likelihood of interbreeding.

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Isolating mechanisms

• Isolating mechanisms fall into two categories
• prezygotic (those that reduce chances of mating
  and fertilization taking place) and
• postzygotic (those that reduce the viability or
  hybrid offspring).

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Isolating mechanisms

• Examples of prezygotic isolating mechanisms
• Different habitat choice
• Activity at different times of day
• Differences in sexual advertisements calls,
  displays, pheromones.

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Isolating mechanisms

• Examples of postzygotic isolating mechanisms
• Failure of zygote to develop
• Reduced viability of zygote
• Sterility

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Hybridization

• In many cases hybrid offspring have reduced
  fitness and this maintains two distinct gene
  pools and incipient species.
• However, in some instances, hybridization appears
  to promote speciation, especially in plants, as
  some hybrids may obtain combinations of genes
  from parental species that enable them to occupy
  habitat that neither parental strain can.

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Hybridization

• For example, Helianthus anomalous a southwestern
  species of sunflower, possesses a unique
  combination of genes from H. annuus and H.
  petiolaris and is clearly the result of a
  hybridization event.

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Genetics of speciation

• How much genetic differentiation is needed to
  separate populations enough that two new species
  are produced?
• Historically, it was considered that large
  differences would be necessary, but more recent
  thinking is that large differences are not
  necessary

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Genetics of speciation pea aphids

• Pea aphids are small sap-sucking insects.
• Via et al. have studied two populations one that
  lives on red clover and the other on alfalfa.
• They have shown that members of each population
  actively chooses its preferred plant and each
  does poorly if reared on the other plant.

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Genetics of speciation pea aphids

• Crosses between the two populations produce F1
  hybrids that do poorer than either parental
  population on both plants.
• Via et al. have identified alleles at several
  locations in the genome that increase fecundity
  on one plant, but decrease it on the other.

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Genetics of speciation pea aphids

• These data suggest that there is a genetic
  trade-off and that alleles that lead to high
  fitness on one plant lead to low fitness on the
  other.
• In addition, alleles for plant preference and
  success on that particular plant appear closely
  related, which suggests the same allele may have
  multiple effects or that alleles for success and
  preference are closely linked.

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Genetics of speciation pea aphids

• If it is common for the same genes or closely
  linked sets of genes to simultaneously alter
  preference and increase success on host plants
  then mutations of these genes should lead to
  speciation on the basis of host plant use.
• Because there are millions of plant-feeding
  insects, this may be an important mechanism of
  speciation.