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.

Species may evolve viahybridiz-ation

e.g.Clarkia “reticulate” phylogenydue topolyploidy

problems forboth phylogeneticand biologicalconcepts

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. 

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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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Post-mating isolation e.g. Haldane’s Rule

Or hybridization may cause introgressione.g. Heliconius gene genealogies

Adaptive radiations: silverswords on Hawaii