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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?

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BIOL2007 (B242) - BIODIVERSITY AND SPECIES Next lecture: how species evolve . Today:

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  1. 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.

  2. 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: Species 1 head width Species 2 body size Darwin’s view -- species differ from races and morphs via gaps. This was later called by Mayr: Darwin's "morphological species concept"

  3. 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 Darwin’s ideas.

  4. 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!

  5. 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)

  6. 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

  7. 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”

  8. Types of reproductive isolation • A) Pre-mating isolation {or pre-zygotic isolation} • a) Ecological/seasonal isolation - mates do not meetb) Behavioural isolation - meet but do not attempt matingc) 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.

  9. Pre-mating isolation e.g. Fireflies in North America

  10. 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)

  11. Post-mating isolation e.g. Haldane’s Rule in Drosophila Recessive effects of epistatic genes cause incompatibility on the X chromosome. 

  12. 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.

  13. Archibold Thorburn 1920-1921 Plate 43 from British Mammals These two species , the biggest on the planet (ever) hybridize, & backcross – fin whale and blue whale European mammals: 6% of spp. hybridize

  14. 3) Ecological species concept Leigh Van Valen (1970s) species concept based on ecological niche. e.g. adaptive radiations

  15. Darwin’s finches character displacement

  16. 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 thedefinition of species.

  17. 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).

  18. 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.

  19. Species may evolve via hybridiz- ation e.g. Clarkia “reticulate” phylogeny due to polyploidy problems for both phylogenetic and biological concepts

  20. 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.

  21. 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 1 head width Species 2 body size Species are detectable clusters of genotypes with discontinuities or gaps separating them. 

  22. Biological species concept – BSC Species 1 (mate recognition) pre-mating isolation 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).

  23. Phylogeny, diagnostic characters Species 1 Species 2 Outgroup Phylogenetic species concept – PSC

  24. Most fundamentally, species are clusters of individuals that can maintain genetic differences in sympatry Species 1 genetic axis 1 Species 2 genetic axis 2

  25. 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.

  26. 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

  27. From: EO Wilson 1992. The diversity of life

  28. 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.

  29. 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)

  30. Genetic diversity in the “Tree of Life” You are somewhere here! n.b. phylogenetic tree is based mainly on slowly evolving ribosomal DNA sequences

  31. 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?

  32. Hawaiian honeycreepers (Drepanididae)

  33. Mel/cyd steril Post-mating isolation e.g. Haldane’s Rule 15 451 516 347 943 200 250 316 881 257 501 594 820 275 221 23 (2 families) 0 (10 families) 144 106 209 MP MP MG MG MP MG MG MP CP MP CP MG MG CP CP or MP CPxMG CPxMG CP/MG CP CP MP or MG MPxMG MP or MG MPxMG MGxMP MG MGxMP MP MPxMG MG MPxMG MP CPxMP any MGxCP MG MP CPxMP CP CPxMP Inter- racial F1 Inter- specific F1 Intra- racial Interracial backcrosses Interspecific backcrosses

  34. Or hybridization may cause introgression e.g. Heliconius gene genealogies

  35. Adaptive radiations: silverswords on Hawaii

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