Transcript how species evolve

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:
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"
Species concepts
Today: 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 often
gradual (but not in
polyploidy)
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
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} 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-mating
isolation
e.g.
Fireflies
in 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-mating
isolation
e.g.
Haldane’s
Rule in
Drosophila
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.
These two species , the biggest
on the planet (ever) hybridize,
& backcross
– fin whale and blue whale
European mammals:
Archibold
Thorburn 1920-1921
6% of spp.
hybridize
Plate 43 from British Mammals
3) Ecological species concept
Leigh Van Valen (1970s) species concept based on
ecological niche.
e.g. adaptive radiations
Darwin’s finches
character 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 teeth
pelagic piscivore: sharp teeth
algae/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.
Species 1
= large tooth cusps
Species 2
Outgroup
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 via
hybridization
e.g.
Clarkia
“reticulate”
phylogeny
due to
polyploidy
problems for
both
phylogenetic
and biological
concepts
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 1
head
width
Species 2
body size
Species are detectable clusters of genotypes with
discontinuities or gaps separating them.
Species 1
(mate recognition)
gene flow,
hybridization
pre-mating
isolation
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).
Biological species concept – BSC
Phylogeny,
Species 1
diagnostic
characters
Species 2
Outgroup
Phylogenetic species concept – PSC
Species 1
genetic
axis 1
Species 2
genetic axis 2
Most fundamentally, species are clusters of
individuals that can maintain genetic
differences in sympatry
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 genera
of ecosystems
of 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 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.
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)
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?
Hawaiian
honeycreepers
(Drepanididae)
Intraracial
Interracial
F1
Interracial
backcrosses
any
CPxMP
Interspecific
backcrosses
MGxCP
209
144
106
MG
23 (2 families)
CP/MG 0 (10 families)
CPxMG
CP or MP
CPxMG
CPxMP
MP
MPxMG
CP
MG
MPxMG
CPxMP
MP
MGxMP
221
275
943
200
316
881
820
594
501
257
15
451
347
250
516
Mel/cyd steril
MP
MG
MPxMG
MP or MG
MGxMP
CP
MG
MPxMG
MG
CP
MP or MG
MP
Interspecific
F1
CP
MP
MG
MG
MG
MG
MP
MP
MP
CP
CP
Post-mating isolation e.g. Haldane’s Rule
Or hybridization may cause introgression
e.g. Heliconius gene genealogies
Adaptive radiations: silverswords on Hawaii