11. Patterns of speciation and extinction

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Mass extinctions

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Mass extinctions

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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

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 corax Corvus corone Corvus frugilegus

Does any species concept fit?

Meteorus pulchricornis from New Zealand

Salmonella typhiHeliconius butterflies

Dog races

Thelytokous waps

Presexual species

Morphologically divergent races

Genetical fingerprint „species”

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

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 isolationbehavioural isolationtemporal isolation (separated generations)host switch in parasites and herbivoresselective 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 incompatibilitymorphological incompatibilityearly death of hybridssterility

Basal population

Lineage A Lineage B

Genetic distance

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 speciation Peripatric speciation

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 speciation Parapatric speciation

How fast is speciation?

Lineage length

Time to genetic

isolation

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Angiosperms

Mammals

Hawaiian Drosophila

Cichlids

Horses

Frogs

Lineage length

Genetic isolation

N.A. songbirds

Drosophila

Mya

Bivalvia

Trees

It seems that evolutionary speed is not correlated with generation length

and body size

Are species reproductively independent lineages?

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Amphibia

Anthozoa

Birds

Fishes

Ferns

Insects

Mammals

Seed plants

Taxon

Proportion of taxonomic species that represent reproductively independent

lineages

Many ‘species’ do not represent genetically isolated lineages. However ecological, morphological or spatial mating barriers

exist

Time to ecological

isolation

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

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

rangeThis 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

Natura non facit saltus?

Species A

Species B

Species A

Species B

Species C

Time

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Speciation event

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

Stasis

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-

Adaptation or species selection?

Morphological divergence

Tim

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Morphological divergence

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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.

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Speciation

Speciation

Speciation

Subspeciation

Stasis

Stasis

Stasis

Stasis

Evolution is assumed to proceed via fast genetic transitions within an peripatric

speciation framework.

Punctuated equilibrium

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Stasis

Stasis

Stasis

Cladogenesis

Cladogenesis

Cladogenesis

Mean thorax width of Trilobite species

The evolution of man is a good example of punctuated equilibrium.

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Cranial capacity

Australopithecus

Homo habilis/ergaster

Homo erectus

Homo sapiens

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.

Ambulocetans natans

Dorudon atroxEoc

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40 mya

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Rhodocetus kasrani

The history of whales: Gradualism or saltationism?

46 mya

The history of birds: Gradualism or saltationism?

Protarchaeo-pteryx robusta Ju

ra

150mya

135 mya

Caudipteryx zoui

Sinosauro-pteryx prima

First feathers

The rise of major lineages

Cryogenian

850-630

Ediacaran

630-540

Cambrian

540-490

Mass extinction

Ordovician490-440

Silurian440-410

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.

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.

EvoDevo and the constraints

Genes for cell division and

adhesion

HOX genesGenes 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

mutationsfor 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

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.

Extinctions

R2 = 0.21p(t) < 0.0001

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Mass extinctions

Trade off between extinction and speciation

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The background extinction rate e(t) of marine taxa decreased!

Marine taxa

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Mass extinctions might be followed by high origination rates

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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.

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Mass extinction

Brachiopoda

Bivalvia

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Mass extinction

Unbuffered species

Buffered species

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Mass extinction

Sessile species

Motile species

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Mass extinction

Predators

Prey

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

Data from Mc Peek, Brown (2007)

Species richness increases with taxon age.

Speciation rates l are independent of species richness but decrease with taxon age.

Younger taxa have higher speciation rates l.

Total species richness is also determined by species survival rates.

Chordata

Arthropoda

Mollusca

Species richness and taxon age

Insecta and Vertebrata

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

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Survival time [mya]

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Survival times for extinct genera of Echinoidea (sea urchins).

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Negative age dependent rate of extinction

Positive age dependent rate of extinction

Constant rate of extinction

Today’s reading:

Speciation: http://en.wikipedia.org/wiki/Speciation

Observed instances of speciation: http://www.talkorigins.org/faqs/faq-speciation.html

The origin of species: http://bill.srnr.arizona.edu/classes/182/Lecture%202007-03.htm

Punctuated equilibrium: http://en.wikipedia.org/wiki/Punctuated_equilibrium

Punctuated equilibrium: http://www.mun.ca/biology/scarr/2900_Fossils.htm