Transcript Speciation
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)