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11. Patterns of speciation and extinction The rise and fall of biodiversity Four major mass extinctions of marine organisms: End of Silurian Devonian,

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Presentation on theme: "11. Patterns of speciation and extinction The rise and fall of biodiversity Four major mass extinctions of marine organisms: End of Silurian Devonian,"— Presentation transcript:

1 11. Patterns of speciation and extinction The rise and fall of biodiversity Four major mass extinctions of marine organisms: End of Silurian Devonian, Permian, and Cretaceous) Rise in diversity during Cambrian, Silurian, Cretaceous, and Paleogene The rise and fall of biodiversity Eliminating all groups known only from a single stage (5-6 mya): Rise in diversity during Cambrian, and Ordovicium and in the Paleogene Decline of longer lasting taxa from Ordovicium to Triassic

2 What is a species in the evolutionary context? The biological species concept states that species are actually or potentially interbreeding natural populations that are genetically isolated from others The evolutionary species concept states that species are ancestor – descendent lineages of organisms that have their own evolutionary fate. The phylogenetic species concept states that a species is the smallest monophyletic group of organisms of common ancestry (a lineage from one node to another). The genetic species concept states that a species is a genetically sufficiently distinct group of organisms as identified by a genetic fingerprint. The ecological species concept states that a species is a group of organisms (population) that are ecologically distinct from other groups. The heuristic species concept states that a species is a group of organisms that are practically clustered together for the aims of a certain study. Corvus coraxCorvus coroneCorvus frugilegus

3 Does any species concept fit? Meteorus pulchricornis from New Zealand Salmonella typhi Heliconius butterflies Dog races Thelytokous waps Presexual species Morphologically divergent races Genetical fingerprint „species”

4 How do species emerge? A classical example Darwin finches, Geospiza spp.) 1. Large cactus finch (Geospiza conirostris) 2. Large ground finch (Geospiza magnirostris) 3. Medium ground finch (Geospiza fortis) 4. Cactus finch (Geospiza scandens) 5. Sharp-beaked ground finch (Geospiza difficilis) 6. Small ground finch (Geospiza fuliginosa) 7. Woodpecker finch (Cactospiza pallida) 8. Vegetarian tree finch (Platyspiza crassirostris) 9. Medium tree finch (Camarhynchus pauper) 10. Large tree finch (Camarhynchus psittacula) 11. Small tree finch (Camarhynchus parvulus) 12. Warbler finch (Certhidia olivacea) 13. Mangrove finch (Cactospiza heliobates

5 Speciation is the divergence of genetic structure between subpopulations until new separate populations emerge. Any mechanism that promotes the emergence of sublineages is therefore a potential speciation mechanism Premating examples are: spatial isolation behavioural isolation temporal isolation (separated generations) host switch in parasites and herbivores selective habitat choice Divergence can be triggered by premating and postmating mechanisms: Premating mechanisms are those that keep populations isolated before mating occurs. Postmating mechanisms prevent hybrids to develop or breed. Postmating examples are: genetic incompatibility morphological incompatibility early death of hybrids sterility Basal population Lineage ALineage B Genetic distance

6 Speciation due to ecological or spatial or temporal isolation Barriers of gene flow or genetic isolation Allopatric barrier Ancestral population Spatial barrier Lineage A Lineage B Peripatric barrier Ancestral population Lineage A Lineage B Founder effect Allopatric speciationPeripatric speciation

7 Sympatric lineage emergence Ancestral population Genetic differences within the same geographical region result in genetic isolation and lineage divergence. Parapatric lineage emergence Differential selection pressures cause lineage divergence even within narrow spatial ranges. Ancestral population Barriers of gene flow or genetic isolation Sympatric speciationParapatric speciation

8 How fast is speciation? Lineage length Time to genetic isolation It seems that evolutionary speed is not correlated with generation length and body size Are species reproductively independent lineages? Many ‘species’ do not represent genetically isolated lineages. However ecological, morphological or spatial mating barriers exist Time to ecological isolation

9 Examples of fast evolutionary speed Cameraria ohridella Minotetrastichus frontalis (=ecus) The Faroer Island house mouse originated from the Western European House Mouse (Mus domesticus). During 250 years of colonization it has evolved three distinct isolated island populations. The Nólsoy House Mouse is a sub-species called (Mus musculus faeroensis) and the Mykines House Mouse is also a sub-species called (Mus musculus mykinessiensis). Its closest relative was the now extinct St Kilda House Mouse (Mus musculus muralis). The Aesculus miner C. ohridella was first described in 1984 in Albania as a rare new species. Since then it colonized whole Europe and became a dominant mining species on Aesculus hippocastanus. It is unknown what caused the rapid spread. Nevertheless it is a good example how an evolutionary novelty can trigger dispersion. This dispersion initiated host switches and lineage divergence of its major parasite Minotetrastichus frontalis. Mus musculus

10 The classic view of speciation Phyletic gradualism asserts that Species arise by the transformation of an ancestral population into its modified descendants. The transformation is even and slow. The transformation involves large numbers, usually the entire ancestral population. The transformation occurs over all or a large part of the ancestral species' geographic range This implies that Ideally, the fossil record for the origin of a new species should consist of a long sequence of continuous, insensibly graded intermediate forms linking ancestor and descendant. Morphological breaks in a postulated phyletic sequence are due to imperfections in the geological record. Ernst Mayr, 1904-2005 Classical Darwinian selection implies a continuous (graduate) change in species characters. The combination with population genetics gave rise to the neodarwinean synthetic theory of evolution formulated mainly by Ernst Mayr and J.B.S. Haldane. John B. S. Haldane, 1892-1964

11 Natura non facit saltus? Species A Species B Species A Species B Species C Time Genetic divergence Speciation event Gradual speciation Saltatorial speciation Gradualism in Pliocene snails, 10 to 3 Mya. Saltatorial speciation means sudden rapid evolutionary change that is manifest in genetic isolation. Stasis

12 The theory of punctuated equilibrium of Niles Eldredge and Stephen Jay Gould states that The fossil record is relatively complete. Most speciation occurs via peripatric speciation. Widespread species usually change slowly, if at all, during their time of existence. Daughter species usually develop in a geographically limited region. Daughter species usually develop in a stratigraphically limited extent. Sampling of the fossil record will reveal a pattern of most species in stasis, with abrupt appearance of newly derived species being a consequence of ecological succession and dispersion. Adaptive change in lineages occurs mostly during periods of speciation. Trends in adaptation occur mostly through the mechanism of species selection. Tempo and mode of evolution reconsidered Stephen Jay Gould, 1941-2002 Niles Eldredge 1943-

13 Adaptation or species selection? Morphological divergence Time Species selection Morphological divergence Time Adaptive trend Species selection means that evolution proceeds via differential extinction of species with certain characteristic features. Adaptive trends imply differential speciation rates of better adapted lineages.

14 Time Genetic distance Speciation Subspeciation Stasis Evolution is assumed to proceed via fast genetic transitions within an peripatric speciation framework. Punctuated equilibrium Mean thorax width of Trilobite species The evolution of man is a good example of punctuated equilibrium.

15 Does evolution need hopeful monsters? Or evolution above the species level Richard Goldschmidt, 1878-1958 Classical Darwinian theory assumes character evolution to be a gradual process. However higher taxa are of often distinguished without any intermediate fossils (fossil gaps). Did major evolutionary branches evolved very fast or is our fossil record too incomplete? Goldschmidt assumed that major evolutionary transitions are caused by mutations in regulatory genes giving rise to major morphological changes. Most of these highly altered creatures have no chance to survive, but few succeed and are ‘hopeful monsters’ that are ancestors of new higher taxa. Punctuated equilibrium is a modern form of this saltationism.

16 Ambulocetans natans Dorudon atrox Eocene 50 mya 40 mya Paleocene 65 mya Rhodocetus kasrani The history of whales: Gradualism or saltationism? 46 mya The history of birds: Gradualism or saltationism? Protarchaeo- pteryx robusta Jura 150 mya 135 mya Caudipteryx zoui Sinosauro- pteryx prima First feathers

17 The rise of major lineages Cryogenian 850-630 Ediacaran 630-540 Cambrian 540-490 Mass extinction Ordovician 490-440 Silurian 440-410 Sponges Rangeomorpha Erniettomorpha Cnidaria Mollusca Annelida Basal arthropods Basic members of nearly all major phyla Chordata Echinodermata Cephalopoda Pisces Chelicerata Trilobites „Crustacea” „Myriapoda” Insects Very probably all animal phyla (except sponges) appeared during the Ediacarian and Cambrian periods. About 35 of the lineages survived. Later, only new classes appeared. By the end of the carbon all extant classes were already present.

18 Evolution and development (EvoDevo) August Weismann (1834-1914) The soma - germ line distinction makes it impossible to transmit acquired characters to the next generation Ernst Haeckel (1834-1919) Theory of recapitulation The ontogeny of advanced species recapitulates respective stages in ancestral forms. In fact, only basic genetic programs are conserved and modifications at all stages of ontogenesis appear. Haeckel’s rule is only a crude approximation.

19 EvoDevo and the constraints Genes for cell division and adhesion HOX genes Genes for basic body shape and cell types Segment differentiation Supply and neural networks Segment differentiation organ development Common to all extant animals Phylum specific body plans Class specific body plans Steps of gene switching Probability of lethal mutations for higher advanced organisms Seastar Gastrula Zygote New phyla arise from free living gastrula stages New classes arise from free living larval stages, for instance by Neoteny Tunicate larva Vertebrate embryo Adult Tunicate

20 Phagocytic Eukaryotes First filter- feeding Porifera All major types of marine animals First land living Cyanobacteria First land living Eukaryotes First land living arthropod predators Complex terrestrial arthropod based food chains Chains including parasitoid levels The evolution of ecological complexity By the end of the Cambrium marine food chains nearly reached today’s complexity Terrestrial food chains still appear to increase in complexity By the end of the Cambrium all major marine and freshwater ecological niches were occupied, leaving no room for additional aquatic born phyla.

21 Extinctions Trade off between extinction and speciation The background extinction rate e(t) of marine taxa decreased! Marine taxa

22 Trade off between extinctions and speciations Extinction and origination rates are connected. Peaks in speciation of marine taxa occurred often after mass extinctions. Mass extinctions might also change ecological dominance. Bivalvia raised after the mass extinction of the ecologically similar Brachiopoda.

23 Mass extinctions are not equally distributed among taxa. Advanced species that are physiologically more buffered against environmental changes increased in frequency after mass extinctions Motile species were often less affected than sessile species Predator species richness increased after mass extinctions

24 Data from Mc Peek, Brown (2007) Species richness increases with taxon age. Speciation rates  are independent of species richness but decrease with taxon age. Younger taxa have higher speciation rates. Total species richness is also determined by species survival rates. Chordata Arthropoda Mollusca Species richness and taxon age Insecta and Vertebrata

25 The Red Queen hypothesis Extinction rates (probabilities) are roughly constant through time. One explanation for this is the Red Queen hypothesis (after Lewis Carroll’s Through the Looking Glass). Each species has to run as far as possible (to evolve continuously) only to stay in the same place. Its competitors, predators and parasites also evolve continuously. Under these circumstances extinction probabilities will remain roughly constant in time. Leigh M. Van Valen 1935- 2010 Survival times for extinct genera of Echinoidea (sea urchins).

26 Today’s reading: Speciation: Observed instances of speciation: The origin of species: Punctuated equilibrium: Punctuated equilibrium:

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