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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BIOL2007 (B242) - BIODIVERSITY AND SPECIES Next lecture: how species evolve . Today:
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? - PowerPoint PPT Presentation
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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.
Species "concepts" – What do we mean by species? 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? A pragmatic solution:
Darwin’s view -- species differ from races and morphs via gaps. This was later called by Mayr: Darwin's "morphological species concept"
body size
head width
Species 1Species 1
Species 2Species 2
Species conceptsToday: much debate, many "species concepts".
1) 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 Darwin’s
ideas.
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!
Phenetic differences apply to genetic variation, as well as morphological variation.
e.g. enzyme differences in Drosophila
chromosome number
Differences oftengradual (but not inpolyploidy)
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, nestingDrosophila pseudoobscura vs. D. persimilis: chrom., steril. Anopheles mosquitoes: habitat, biting, malaria
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”
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 factorshybrids are not suited ecologicallyreduced mating propensity of hybrids
f) Hybrid sterility (even though may survive and mate as normal). g) Sexual selection against hybrids - disfavoured during mating.
Pre-matingisolation
e.g.Firefliesin North America
Post-mating isolation
Hybrid inviability and sterility - genomic incompatibility. e.g. Mules, chromosomal heterozygotes.
Other species no hybrid problems: e.g. Darwin’s finches, ducks Special case of hybrid inviability/sterility: 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)
Post-matingisolation
e.g.Haldane’s Rule inDrosophila
Recessive effects of epistatic genes cause incompatibility on the X chromosome.
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: < 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.
Archibold Thorburn 1920-1921Plate 43 from British Mammals
These two species , the These two species , the biggest on the planet biggest on the planet (ever) hybridize, & (ever) hybridize, & backcross backcross – – fin whale and blue fin whale and blue whalewhale
European mammals: European mammals: 6% of spp. hybridize6% of spp. hybridize
3) Ecological species concept Leigh Van Valen (1970s) species concept based on ecological niche.
e.g. adaptive radiations
Darwin’s finchescharacter displacement
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 teethpelagic piscivore: sharp teethalgae/detritivore: rounded teeth
So hard to say ecology is the definition of species.
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
OutgroupOutgroup
Species 1Species 1
Species 2Species 2
phylogenetic (based on monophyly) and diagnostic species concepts (based on diagnostic characters, such as morphology or mtDNA bases).
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.
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?
Mallet’s 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 shouldn’t take the "reality" of species too seriously.
Genetics and the definition of species Updating Darwin’s view of species with Mendelian genetics (which Darwin did not know about). Darwin's view:
Species are detectable clusters of genotypes with discontinuities or gaps separating them.
body size
head width
Species 1Species 1
Species 2Species 2
Biological species concept – Biological species concept – BSCBSC
gene flow, hybridizati
on
disruptive or divergent selection
pre-mating isolation
= post-mating
isolation
(mate recognition)
Species 1Species 1
Species 2Species 2
Gene flow, if it exists, can be balanced by disruptive selection -- intrinsic (post-mating isolating mechanisms) or extrinsic (as in ecological concept).
PhylogenyPhylogeny,,
OutgroupOutgroup
diagnostic diagnostic characterscharacters
Species 1Species 1
Species 2Species 2
Phylogenetic species concept – PSCPhylogenetic species concept – PSC
Most fundamentally, Most fundamentally, species are species are clusters of individuals that can clusters of individuals that can maintain genetic differences in maintain genetic differences in
sympatrysympatry
Species 1Species 1
Species 2Species 2
genetic axis 2
genetic axis
1
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.
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 generaof ecosystemsof biomes
However: traditionally, the species viewed as most important, and most biodiversity studies are concerned with this level
From: EO Wilson 1992. The diversity of life
Species diversityMost 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.
Bias of using described species1) 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)
Genetic diversity in the “Tree of Life”
You are somewhere here!
n.b. phylogenetic tree is based mainly on slowly evolving ribosomal DNA sequences
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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Or hybridization may cause introgressione.g. Heliconius gene genealogies