Download Biological Species Concept

Speciation
Darwin explained evolution, but didn’t do so well with
speciation
Natural selection causes change in a pop., but what causes
pop. to split?
- had idea of
‘diversification’,
but not of gene pool
What is a species?
Most popular idea (still?) is Biological Species Concept
(Ernst Mayr):
- pop. or group of pops. whose members have potential
to interbreed & produce fertile offspring
- largest interbreeding unit, genetically isolated from
other pops.
1904-2005
Species usually look different, but may be very hard
to distinguish (or a species may be very variable)
Eastern
Meadowlark
Western
Meadowlark
Polymita
In practice, many species hybridize – if only rare,
generally considered distinct spp.
Blue-winged Warbler
Golden-winged Warbler
‘Lawrence’s Warbler’
‘Liger’
Hard to classify if:
- disjunct range
- asexual reproduction
- fossils
Can have ‘ring species’ – don’t interbreed in 1 zone
of contact, but connected by interbreeding pops
Ensatina
Deer Mouse
Difficulties expected if species arise by gradual
divergence of pops.
- other species concepts proposed, but they have
difficulties too…
Morphological Species Concept – based on similar
structure & appearance
- scientists may not agree on what differences define a sp.
Phylogenetic Species Concept – species is smallest group
of related organisms that share a defining feature
- could greatly increase # of species
Another possibility: Genetic Species Concept - a group of
genetically compatible interbreeding natural populations
that is genetically isolated from other such groups.
- allows use of extensive genetic data, but not clear how
much difference is required to qualify as a species
Little Penguin – Australia?
Little Penguin – New Zealand
For sexual species, most popular concept is still
Biological Species Concept
- relies on idea that reproductive isolation (lack of
interbreeding between populations) maintains
separate gene pools (total set of genes in a population)
Northern (‘Yellow-shafted’) Flicker
Northern (‘Red-shafted’) Flicker
Reproductive isolation important to speciation – can be
achieved in various ways:
A – Prezygotic barriers
1) Habitat isolation (flycatchers)
Dusky
Flycatcher
Hammond’s
Flycatcher
2) Behavioral isolation (fireflies)
3) Temporal isolation (spotted skunks)
Eastern Spotted Skunk
Western Spotted Skunk
4) Mechanical isolation (flowers, insects)
5) Gametic isolation (marine animals, flowers)
B May have 2 individuals of different spp. mate, & yet
maintain distinctiveness through postzygotic isolating
mechanisms or…
– Postzygotic barriers
1) Reduced hybrid viability (sunfish)
Longear Sunfish
Green Sunfish
abnormal hybrid larva
2) Reduced hybrid fertility (mules)
Horse, Donkey & Mule
3) Hybrid breakdown (rice & cotton)
How does speciation work?
Most popular model of speciation: allopatric speciation
- allopatry: inhabiting different ranges
- suggests original pop. becomes divided by new
geographic barrier, or colonization of new
isolated area
White-tailed Antelope Squirrel
Harris’Antelope Squirrel
- isolated pops. gradually diverge; genetic diffs.
develop due to selection in diff. environments,
drift, or founder effect
- when & if pops. make contact isolating mechanisms
may be reinforced (hybrids selected against)
An alternative model: sympatric speciation
- sympatric: living in same area; two new species
form while their geographic ranges still overlap
- controversial – some systematists doubt it occurs in
animals, most think probably rare (& hard to prove)
allopatric
sympatric
- might originate through host specialization
(e.g. Apple Maggot Fly) or strong assortative mating
Sympatric speciation does occur commonly in plants
via polyploidy
Autopolyploidy: chromosomes fail to separate
(nondisjunction), diploid gametes, may
self-fertilize & form tetraploid offspring
- new species, cannot interbreed with diploids
Allopolyploidy: more frequent
- hybridization leads to unusual chromosome #, either
non-disjunction or second hybridization event
increases chromosome #
- new polyploid species, can
breed with other polyploids
Vegetative reproduction &
self-fertilization keep
hybrids alive until plant
can reproduce sexually
- 25-50% of plant species
may be formed this way
(e.g. wheat & other crops)
Systematics
Repeated speciation leads to tremendous diversity of life
– needs to be organized
Systematics – study of relationships between organisms
& their classification & naming
Modern system begins with Carl Linnaeus – 1758
- first to consistently use binomials (e. g. Homo sapiens)
e.g. Bald Eagle: Haliaeetus leucocephalus
First part – genus name, second part – specific epithet
Bald Eagle, Haliaeetus leucocephalus
Steller’s Sea Eagle, Haliaeetus pelagicus
Specific epithet is label that (almost) always stays with
sp. once named (Law of Priority)
Genus name indicates closest relatives (plural = genera)
Species & genus only lowest levels in classification:
Kingdom: Animalia
Phylum:
Chordata
Class:
Aves
Order:
Falconiformes
Family:
Accipitridae
Genus:
Haliaeetus
Species:
leucocephalus
Any ‘unit’ in the classification is a taxon (pl. taxa)