What do we mean by a species? Morphological species concept. Morphological species concept BIOL2007 SPECIES AND BIODIVERSITY. Kanchon Dasmahapatra

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1 BIOL2007 SPECIES AND BIODIVERSITY Kanchon Dasmahapatra What are species? How do species differ from each other? Biodiversity: How many species are there? What do we mean by a 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 between species? Darwin s view -- species differ from races and morphs via gaps. This was later called by Mayr: Darwin's "morphological species concept" 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). Morphological species concept Species delimited by gaps in morphology. Problems: a) Morphological gaps present within species (ii) Races of humans: same species. 1960s-1970s, phenetic species concept. A multivariate statistical restatement of Darwin s ideas. Papilio memnon Peppered moth 1

2 Morphological species concept 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 numerous chromosome differences melpomene timareta Hel mel ama Hel mel ama Hel mel ama Hel mel ama mel 2224 mel 1309 mel 2217 mel 1315 mel 1520 mel Hel mel ama mel 1303 mel 1929 mel 1921 mel Hel mel mel Hel mel mel 1927 mel 1517 mel 1937 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/ reproductive isolation mel mel 1933 mtdna 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 Pre- and post-mating isolation in melpomene cydno Closely related & find occasional hybrids Unpalatable. Warningly coloured. Predators do not recognise hybrid pattern 2

3 Pre- and post-mating isolation in Pre- and post-mating isolation in F1 males fertile F1 females sterile 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) Biological species concept 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. Phylogenetic species concept Hel mel ama Species defined by monophyly: based on diagnostic characters: morphology and/or DNA melpomene timareta Hel mel ama Hel mel ama Hel mel ama Hel mel ama mel 2224 mel 1309 mel 2217 mel 1315 mel 1520 mel Hel mel ama mel 1303 mel 1929 mel 1921 mel Hel mel mel Hel mel mel 1927 mel 1517 mel 1937 mel 1934 mel 1933 PROBLEMS melpomene timareta mel Hel mel ama Hel mel ama Hel mel ama mel Hel mel ama mel 2217 mel 1315 mel 1520 mel Hel mel ama mel Hel mel agl Hel mel agl Hel mel agl Hel mel agl Hel mel mel 922 mel 1921 mel 1929 mel 2173 mel 1517 mel 1937 mel 1934 mel Hel mel mel

4 Phylogenetic species concept PROBLEMS a) In reality, phylogenies are hypotheses, not facts: unstable. b) Many isolated populations may be monophyletic. Phylogenetic concepts many spp., only just recognizable. Genetics and the definition of species head width Updating Darwin s view of species with Mendelian genetics (which Darwin did not know about). Species 1 Species 2 genetic axis 2 Species 1 Species 2 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! body size Species delimited by gaps in morphology genetic axis 1 Species are clusters of genotypes with gaps separating them Species as genotypic clusters Give up on species? genetic axis 1 Species 1 gene flow? Clusters can be maintained in the presence of gene flow Species 2 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. Don t take the "reality" of species too seriously. genetic axis 2 Species are clusters of individuals that can maintain genetic differences in sympatry 4

5 Species, subspecies and populations Species, subspecies and populations Populations subspecies/races species Allozyme differences in Drosophila Biodiversity From: EO Wilson The diversity of life 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. 5

6 Known species diversity Vertebrates mammals: 5,500 birds: 10,000 Invertebrates insects: 950,000 molluscs: 81,000 crustaceans: 40,000 Plants flowering plants: 259,000 Others (incl. lichen, fungi) Total 60,000 1,203, ,000 29,000 1,589,000 Beetles make up ~20% of all described species! Insects in general: ~55% of described species. UNknown species diversity 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 Deep sea biodiversity Genetic diversity in the Tree of Life You are somewhere here! ~1 new species per km Mainly polychaetes + molluscs ~10 million bottom dwelling species Grassle & Maciolek (1992) Am. Nat n.b. phylogenetic tree is based mainly on slowly evolving ribosomal DNA sequences 6

7 UNknown species diversity Importance of biodiversity 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 Distribution of biodiversity Reading Cincotta et al. (2000) Nature 25 biodiversity hotspots: 12% of land 20% world pop. in these areas High pop. growth of 1.8%/year FUTUYMA, DJ Evolutionary Biology. Chapter 15 (pp ). Species. May RM (1988) How many species are there on earth? Science 241: WILSON, EO 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? 7