1) Allopatric Speciation - populations become reproductively isolated during periods of geographic separation (e.g. by mountains, land bridge disappearance, sea strait). - this mode is recognized as important for speciation of many plants and animals; - the best evidence comes from birds (Darwin's finches, Hawaiian honeycreepers), mammals, butterflies and Drosophila (e.g. Hawaii)
One of the best examples is the accepted history behind Darwin’s Galapagos finches. The initial colonist is believed to be a finch from Cocos Island, off the mainland of Costa Rica. The Cocos finch is the sole finch species on Cocos Island, whereas many species occupy the Galapagos islands owing to emigration followed by allopatric speciation on ‘new’ islands in the Galapagos. Cocos Island Habitat and food specialization and geographic isolation facilitated species
Hawaiian Honeycreepers same process as per Darwin's finches except the scale of radiation and morphological variation in beak size was far more extensive. Process occurred in <10 million years S. Olson (2004) Evolution in Hawaii
Kevin Edwards, Illinois State Univ. Hawaiian Drosophila: ~1000 diverse species from a single introduction of an Asian Drosophila species that spread before the current Hawaiian Islands were formed, and colonized the chain by 'island hopping' about 26 million years ago. New islands were colonized as they became suitable ecologically. Lava flows and geographic separation fragmented species' ranges, causing bottlenecks that facilitated evolution of species Hawaiian Drosophila
Wrasse Speciation A classic example of allopatric speciation as a result of separation of populations by the uplifting of the Isthmus of Panama (<3 million years ago) separating some species into Atlantic and Pacific populations
2) Sympatric Speciation - more controversial and in most cases less common than allopatric speciation - best examples are from plants with polyploidy (multiplication of chromosome number resulting in instantaneous speciation) - this mode is successful only if new individuals can interbreed through assortative mating, inbreed (self-mating), or become polyploid - believed responsible for ~40% of plant species diversity, far less in animals, but is found in parasitic Chalcidoidea, and may occur in parasitic and hyperparasitic species through shifts in host selection.
Hawaiian Silverswords Hawaiian silversword alliance consists of about 30 species in 3 genera (Wilkesia, Argyroxiphium, Dubautia). The species exhibit an extraordinary range of anatomical, morphological, and ecological adaptations genetically very close evolved from a single ancestor that colonized Hawaii by way of long-distance dispersal from North America
Limnetic Male Benthic Male 3-Spine Sticklebacks in British Columbia Lakes The similarity of their mtDNA (<0.5% sequence divergence) suggests that each pair arose independently by sympatric speciation, following a single invasion of each lake by ancestral marine sticklebacks Other possibility is an initial invasion from the sea, with individuals developing benthic feeding mode, followed by a later 2nd invasions, with individuals focusing on planktonic foods Incipient speciation is driven by positive assortative mating Vamosi, & Schluter. 1999. Evolution
Cichlid Fishes in African Great Lakes hundreds of species in each of Lakes Victoria, Malawi and Tanganyika, that have both habitat and feeding specialization, the latter driven largely by mouth morphology
3) Parapatric model - speciation occurs among adjacent populations of the same species as a result of strong and differing selective pressures in the local environments - there is only limited evidence, perhaps best exemplified by local 'races' of plants like Deschampsia caespitosa that establish on mine wastes rich in heavy metals like copper and nickel; these populations are different ecologically and genetically from adjacent individuals on non-toxic soils.
Deschampsia caespitosa is an extremely variable grass with a wide distribution, though mainly in the northern hemisphere 40 subspecies or varieties in North America there exist 5 morphologically different forms taxonomy of the group, particularly in Central Europe, is still puzzling, because of the extreme morphological variability and ambiguous boundaries between the taxa Adaptation to particular environments (metal vs. nonmetal) may result in the formation of these ecotypes and represent the beginning of speciation Chiapella (2000) Biol. J. Linn. Soc.
Parapatric speciation in Orioles? May occur in zones where distinct species have range overlap and hybridize Sibley & Short (1964) Condor
How does speciation occur? 1.Gradual divergence in the face of differing selective pressures for different, isolated populations; and/or 2.genetic revolution in which more than 1 genetic constitution is favoured by selection for populations with the same environmental conditions. Speciation in the latter model occurs if a population switches from one adaptive peak to another, most likely occurring because of genetic drift or by polyploidization, which causes instantaneous speciation.
Diversification has occurred both continuously and sporadically, with dramatic increases in diversity following each of the mass extinctions. Without repeating the history, we can review the times of major diversification of particular phyla… Cambrian: sea invertebrates: archaeocyathans, inarticulate branchiopods, trilobites
Rest of Paleozoic: corals, articulate branchiopods, cephalopods (e.g. squids), ostracods, crinoids, starfish, graptolites (extinct colonial group), first land plants; insects in late Paleozoic
Mesozoic: bivalves, gastropods, malacostracans, echinoids, bony fishes, marine reptiles, first gymnosperms and (later) angiosperms; dinosaurs appear. Cenozoic: social insects; modern and placental mammals; hoofed mammals and apes; man. Most geologic records are based on marine taxa, specifically invertebrates.
Family-level Diversification: Benton (1995) used data collated from 90 experts for taxa including microbes, algae, fungi, protists, plants and animals for 7186 different families. Benton selected family level distinctions for a number of reasons, the main one being that he could avoid uncertainty regarding species designations by working at a higher level of resolution. Family level diversification shows the following history:
Family Diversification through time based on fossil record Benton (1995) Science
log (diversity) thru time new families in period Speciation through Time
Extinction through Time Benton (1995) Science # families lost thru time % families lost in period % families lost thru time
Loss of families as a function of the number that started the period and the length of the period
1) family-level diversity increased in a burst during the Vendian (Precambrian); 2) diversity fell to 120 families during late Cambrian; 3) diversity increased to 450 during Ordovician; 4) diversity rose to ~600 during remaining Paleozoic; 5) fell to 420 at beginning of Triassic; 6) rose to 2600 during Cretaceous (144 myr) and 2159 during Pleistocene and Holocene (<2 myr); 7) most continental families arose following Silurian – land plants, insects and vertebrates; Benton (1995), Science
8) by contrast, many marine families arose during the Vendian and early Cambrian period followed by massive losses during the late Cambrian and late-Permian; then exploded in diversity until the late Cretaceous; further expansion occurred at the beginning of the Tertiary (65 myr); 9) family diversity increases were clearly logarithmic for all taxa and for continental taxa, though a more complex pattern of radiation and extinction occurred with marine families.
Overall Patterns: many of the radiations occurred following massive extinctions; most of the early radiations were marine, most of the later ones terrestrial; huge increases during the Eocene (55 myr) correspond with radiation of the insects insect and flowering plants appeared to undergo radiation together suggesting coevolutionary radiations
- mass extinctions occurred during the early Cambrian, late Ordovician, middle and late Devonian, late Carboniferous, late Permian, end Triassic, end-Jurassic, mid-Cretaceous, late Eocene - 4 of extinction periods were protracted (not fast) - some extinctions (and radiations) were not well recorded in the fossil record (referred to as slippage) -different measures of extinction (per family, total rate etc.) each have their own weakness (e.g. dividing by total extant diversity where the small number of families early on skews that statistic)
For the 5 accepted mass extinctions: Time Period % of families Extinct Late Ordovician 20-22 Late Devonian 20-24 Late Permian 31-52 Late Triassic 15-24 end-Cretaceous 11-16 Remember, the clock ticks faster for genera or for species than for families, so large family losses would correspond with huge species losses
General patterns in extinctions have been identified: terrestrial vertebrate taxa that went extinct were large-bodied (e.g. dinosaurs; end-Pleistocene megafauna). That would be consistent with the expected population biology of these species: low population numbers, long-lived, slow maturing, low reproductive rates, large home ranges and high food demands. Widespread genera fared extinction periods much better than those with narrow geographic limits.
Factors (e.g. species richness) that protected genera from background extinctions were ineffective protection during the mass extinction that occurred at the end of the Cretaceous. Tropical marine taxa (corals) were harder hit during mass extinctions than taxa from higher latitudes. Species diversity losses of marine bivalves took, on average, 10 million years to recoup through new radiations. Species responses to changing environments (particularly following the Pliocene-onset [<5.2 myr]) were highly individualistic.
Thus, current reserve designs should reflect the broad requirements and environmental responses of different taxa if large-scale biodiversity is to be successfully preserved. In many cases, a rich diversity of protected microhabitats may prove more important in reserve design than maximizing reserve area.
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