BIOL2007 (B242) - BIODIVERSITY AND SPECIES

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• BIOL2007 (B242) - BIODIVERSITY AND SPECIES
• Next lecture how species evolve.
• Today
• (1) nature of species
• (2) whether speciation differs from
  microevolution.
• What are species? Species "concepts."
• How do species differ from each other?
• How many species are there?
• Species-level biodiversity.

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Species "concepts" What do we mean by species?
Darwin proved species evolved But a difficulty
Species werent created kinds, with an essence.
They gradually evolved from each other. So where
is the dividing line? A pragmatic solution
Species 1
head width
Species 2
body size
Darwins view -- species differ from races and
morphs via gaps. This was later called by Mayr
Darwin's "morphological species concept"
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• Species conceptsToday much debate, many
  "species concepts".
• Morphological species concept. Species delimited
  by gaps in morphology.
• e.g. according to Darwin
• (i) Primula vulgaris (primrose), Primula elatior
  (oxslip), and Primula veris (cowslip) different
  species, but many intermediates (but all rare,
  sterility) , so evidence of continuity of species
  and "varieties".
• (ii) Similarly, races of humans same species. 
  In this case, cannot find good dividing lines.
• 1960s-1970s, phenetic species concept.
• A multivariate statistical restatement of
  Darwins ideas.

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Primula vulgaris (primrose, top left), Primula
elatior (oxslip, top right), and Primula veris
(cowslip, left) hybridize produce
intermediates. Darwin different species, OK,
but many intermediates!
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Phenetic differences apply to genetic variation,
as well as morphological variation. e.g. enzyme
differences in Drosophila chromosome
number Differences often gradual (but not
in polyploidy)
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However some problems with phenetic concept a)
Morphological gaps within species. e.g. Peppered
moth or Papilio memnon morphs. b) Lack of
differences between species There are often
sibling species which are (i) morphologically
similar, though differ genetically.(ii) evolve
more or less separately(iii) little or no
hybridisation/gene flow e.g. willow warbler
chiff-chaff song, nesting Drosophila
pseudoobscura vs. D. persimilis chrom., steril.
Anopheles mosquitoes habitat, biting, malaria
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2) The biological species concept Species
defined by interbreeding (Poulton 1903,
Dobzhansky1937, Mayr 1942). Gene flow within
each species No hybridization or gene flow
between species Lack of gene flow due to
isolating mechanisms
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• Types of reproductive isolation
• A) Pre-mating isolation or pre-zygotic
  isolation
• a) Ecological/seasonal isolation - mates do not
  meet b) Behavioural isolation - meet but do not
  attempt mating c) Mechanical isolation -
  attempts at mating do not work!
• B) Post-mating or post-zygotic isolation
• d) Gametic incompatibility - gametes die before
  fertilization (note post-mating but
  pre-zygotic) e) Hybrid inviability hybrid
  zygotes have reduced fitness
• genomic factors
• hybrids are not suited ecologically
• reduced mating propensity of hybrids
• f) Hybrid sterility (even though may survive and
  mate as normal). g) Sexual selection against
  hybrids - disfavoured during mating.

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Pre-mating isolation e.g. Fireflies in North
America
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Post-mating isolation Hybrid inviability and
sterility - genomic incompatibility. e.g. Mules,
chromosomal heterozygotes. Other species no
hybrid problems e.g. Darwins finches,
ducks   Special case of hybrid inviability/sterili
ty Haldane's Rule When one sex of F1 hybrid
between species is inviabile or sterile, that sex
is usually the heterogametic sex, rather than
the homogametic sex.   Mammals, Drosophila (XY
males , XX females) Birds, butterflies (ZZ males,
WZ females)
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Post-mating isolation e.g. Haldanes Rule
in Drosophila Recessive effects of epistatic
genes cause incompatibility on the X chromosome. 
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Problems with the biological species concept a)
Does not apply in allopatry or fossil record.
Species become less clear over large spans of
space (in geography) or time (in the fossil
record). b) Natural hybridisation/introgression
occurs. 10 of bird and butterfly species, 6 of
mammal spp. hybridise naturally. (Hybridization
is rare lt 1/1000 in populations). But species
that do so are common. Introgression potentially
common.  Examples ducks 75 of UK species
mammals 6 of European species plants 25 of
British species. Hybridization and
introgression are important topics in
conservation and agriculture.
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These two species , the biggest on the planet
(ever) hybridize, backcross fin whale and
blue whale European mammals 6 of spp.
hybridize

• Archibold Thorburn 1920-1921
• Plate 43 from British Mammals

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3) Ecological species concept Leigh Van Valen
(1970s) species concept based on ecological
niche. e.g. adaptive radiations
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Darwins finches character displacement
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• Problems with the Ecological Species Concept
• a) Sibling species may have same niches.
  (Eventually loss of one species via
  competition?)
• b) Ecological morphs within species. Adaptations
  often differ in different parts of a species
  range (see Evolution in space and time).
• The cichlid fish Cichlasoma sp. from Cuatro
  Cienagas, Mexico three morphs within the
  species
• bottom living mollusc-feeder grinding teeth
• pelagic piscivore sharp teeth
• algae/detritivore rounded teeth
• So hard to say ecology is the definition of
  species.

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4) Cladistic and phylogenetic species concepts
Cladistic movement founded by Willi Hennig in
the 1950s. If higher taxa are defined by means
of phylogeny, then so should species, reasoned
cladists.
large tooth cusps
phylogenetic (based on monophyly) and diagnostic
species concepts (based on diagnostic characters,
such as morphology or mtDNA bases).
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But a) In reality, phylogenies are hypotheses,
not facts unstable. b) Many isolated
populations may be monophyletic. Phylogenetic
concepts ? many spp., only just recognizable.
c) Hybridization between branches of a
phylogeny. A phylogeny is really a mass of
"genealogies" at different loci. So is average
phylogeny ("consensus" phylogeny) the "true"
species phylogeny? Not exactly! Many
alternative evolutionary and phylogenetic species
concepts which attempt to answer these problems.
None are (yet) clearly accepted.
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Species may evolve via hybridiz- ation e.g. Clar
kia reticulate phylogeny due
to polyploidy problems for both
phylogenetic and biological concepts
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5) Rank-free taxonomy, and giving up on species
altogether! Do away with species altogether?
But then how would we communicate about groups of
organisms? Mallets view (but what is yours?)
species are convenient naming devices to classify
animals and plants. There must be a certain
validity to species, or bird or plant guides
wouldn't be very useful. But we shouldnt take
the "reality" of species too seriously.
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Genetics and the definition of species Updating
Darwins view of species with Mendelian genetics
(which Darwin did not know about). Darwin's view
Species 1
head width
Species 2
body size
Species are detectable clusters of genotypes with
discontinuities or gaps separating them. 
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Biological species concept BSC
Species 1
pre-mating isolation
(mate recognition)
gene flow, hybridization
Species 2
disruptive or divergent selection
post-mating isolation
Gene flow, if it exists, can be balanced by
disruptive selection -- intrinsic (post-mating
isolating mechanisms) or extrinsic (as in
ecological concept).
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Phylogeny,
diagnostic characters
Species 1
Species 2
Outgroup
Phylogenetic species concept PSC
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Most fundamentally, species are clusters of
individuals that can maintain genetic differences
in sympatry
Species 1
genetic axis 1
Species 2
genetic axis 2
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Genetic differences between species   
multiple loci    normally more differences than
between geographic races    but not
qualitatively different than between races Most
parsimonious (simplest) explanation The usual
microevolutionary forces such as selection,
drift, mutation ... explain speciation, as
well as within species diversity.
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• Biodiversity
• The sum total of diversity at all levels of the
  evolutionary hierarchy
•    genetic diversity within populations
•    genetic diversity between populations races
•    diversity of species
• of genera
• of ecosystems
• of biomes
• However traditionally,
• the species viewed as most important,
• and most biodiversity studies are concerned with
  this level

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From EO Wilson 1992. The diversity of life
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Species diversity Most of diversity is not
mammalian, or even vertebrate or chordate.
Beetles make up 20 of all described
species! Insects in general 53 of described
species.
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Bias of using described species 1) Entomologists
(Terry Erwin and others)    Fogged canopies of
1 S. American tree species    Counted
unidentified, host-specific beetles   
Calculated may be as much as a 30x more species
than currently described.    30,000,000 species
is their estimate. 2) Bacteriologists   
Prokaryotic world is far more diverse in DNA   
Maybe more diverse in species Mostly not
discovered (e.g. recent discoveries of
extremophiles in deep sea vents, in granite)
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Genetic diversity in the Tree of Life
You are somewhere here!
n.b. phylogenetic tree is based mainly on slowly
evolving ribosomal DNA sequences
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FURTHER READING FUTUYMA, DJ 1998.  Evolutionary
Biology.  Chapter 15 (pp. 447-479).  Species.
WILSON, EO 1992.  The diversity of life.
Science Lbrary View B242 Teaching Collection by
going to eUCLid use Keyword, Basic Search, All
Fields B242 Next time how does all this
diversity evolve?
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Hawaiian honeycreepers (Drepanididae)
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Mel/cyd steril
Post-mating isolation e.g. Haldanes Rule
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Or hybridization may cause introgression e.g.
Heliconius gene genealogies
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Adaptive radiations silverswords on Hawaii