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BIOL2007
SPECIES AND BIODIVERSITY
Kanchon Dasmahapatra
What are species?
How do species differ from each other?
Biodiversity: How many species are there?
Darwin proved species evolved
But a difficulty:
Species weren’t created kinds, with an essence. They
gradually evolved from each other.
So where is the dividing line between species?
Darwin’s view -- species differ from races and morphs via gaps.
This was later called by Mayr: Darwin's "morphological species concept"
What do we mean by a species?
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).
(ii) Races of humans: same species.
1960s-1970s, phenetic species concept.
A multivariate statistical restatement of Darwin’s ideas.
Morphological species concept Morphological species concept
Peppered mothPapilio memnon
Species delimited by gaps in morphology.
Problems:
a) Morphological gaps present within species
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Species delimited by gaps in morphology.
Problems:
a) Morphological gaps present within species
b) Lack of morphological gaps between species
closely related, but genetically distinct
2-956 Hel mel ama
2-1882 Hel mel ama
2-1839 Hel mel ama
2-2060 Hel mel ama
mel 2224
mel 1309
mel 2217
mel 1315
mel 1520
mel 2214
2-1850 Hel mel ama
mel 1303
mel 1929
mel 1921
mel 922
2-1598 Hel mel
mel 2173
2-2146 Hel mel
mel 1927
mel 1517
mel 1937
mel 1934
mel 1933
0.005
Heliconius
melpomene
Heliconius
“timareta”
numerous
chromosome
differences
Morphological species concept
mtDNA
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/
reproductive isolation
Biological species concept
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} isolationd) 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
Pre- and post-mating isolation in Heliconius
Heliconius melpomene
Heliconius cydno
Closely related
&
find occasional hybrids
Predators do not recognise hybrid pattern
Unpalatable.
Warningly coloured.
3
Pre- and post-mating isolation in Heliconius
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)
F1 males fertile
F1 females sterile
Pre- and post-mating isolation in Heliconius
PROBLEMS
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: < 1/1000 in
populations). Introgression potentially common.
ducks ~75% of UK species
mammals ~6% of European species
plants ~25% of British species.
Biological species concept
2-956 Hel mel ama
2-1882 Hel mel ama
2-1839 Hel mel ama
2-2060 Hel mel ama
mel 2224
mel 1309
mel 2217
mel 1315
mel 1520
mel 2214
2-1850 Hel mel ama
mel 1303
mel 1929
mel 1921
mel 922
2-1598 Hel mel
mel 2173
2-2146 Hel mel
mel 1927
mel 1517
mel 1937
mel 1934
mel 1933
Heliconius
melpomene
Heliconius
“timareta”
Phylogenetic species concept 2-956 Hel mel ama
mel 2224
2-944 Hel mel ama
2-1839 Hel mel ama
2-1882 Hel mel ama
mel 1309
2-2060 Hel mel ama
mel 2217
mel 1315
mel 1520
mel 2214
2-1850 Hel mel ama
mel 1303
2-1894 Hel mel agl
4-288 Hel mel agl
4-286 Hel mel agl
2-366 Hel mel agl
2-1598 Hel mel
mel 922
mel 1921
mel 1929
mel 2173
mel 1517
mel 1937
mel 1934
mel 1933
2-2146 Hel mel
mel 1927
Heliconius
melpomene
Heliconius
“timareta”
PROBLEMS
Species defined by monophyly:based on diagnostic characters: morphology and/or DNA
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PROBLEMS
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!
Phylogenetic species concept
Updating Darwin’s view of species with Mendelian genetics
(which Darwin did not know about).
Species are clusters of genotypes
with gaps separating them
body size
head
width
Species 1Species 1
Species 2Species 2
Species delimited by
gaps in morphology
genetic axis 1
genetic
axis 2
Species 1Species 1
Species 2Species 2
Genetics and the definition of species
Species 1Species 1
Species 2Species 2
genetic axis 2
genetic
axis 1
Species are clusters of individuals that can maintain
genetic differences in sympatry
Species as genotypic clusters
gene flow?
Clusters can be maintained
in the presence of gene flowSpecies 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.
Don’t take the "reality" of species too seriously.
Give up on species?
5
Species, subspecies and populations
Populations subspecies/races species
Species, subspecies and populations
Allozyme differences
in Drosophila
Total of diversity at all levels of the evolutionary hierarchy
Traditionally, the species viewed as most important,
and most biodiversity studies are concerned with this level.
BiodiversityFrom: EO Wilson 1992. The diversity of life
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Beetles make up ~20% of all described species!
Insects in general: ~55% of described species.
Known species diversity
1,589,000Total
29,000Others (incl. lichen, fungi)
297,000Plants
flowering plants: 259,000
1,203,000Invertebrates
insects: 950,000
molluscs: 81,000
crustaceans: 40,000
60,000Vertebrates
mammals: 5,500
birds: 10,000
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) Invertebrate diversity in deep-ocean sediments
- annelids, molluscs
3) Bacteriologists:
- Prokaryotic world is far more diverse in DNA
- Maybe more diverse in “species”
UNknown species diversity
Deep sea biodiversity
Grassle & Maciolek (1992) Am. Nat
~1 new species per km
Mainly polychaetes + molluscs
~10 million bottom dwelling species
Genetic diversity in the “Tree of Life”
You are somewhere here!
n.b. phylogenetic tree is based mainly
on slowly evolving ribosomal DNA sequences
7
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) Invertebrate diversity in deep-ocean sediments
- annelids, crustacea, cnidaria
3) Bacteriologists:
- Prokaryotic world is far more diverse in DNA
- Maybe more diverse in “species”
UNknown species diversity Importance of biodiversity
Distribution of biodiversity
25 biodiversity hotspots: 12% of land
20% world pop. in these areas
High pop. growth of 1.8%/year
Cincotta et al. (2000) Nature
FUTUYMA, DJ 1998. Evolutionary Biology. Chapter 15
(pp. 353-378). Species.
May RM (1988) How many species are there on earth?
Science 241: 1441-1449.
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: Speciation: how does all this diversity evolve?
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