Evolution and Systematics

Evolutionary trends; extinctions (Chapt. 15)

Meganosed fly carrying pollinariato Pelargonium suburbanum

SUDDEN CHANGES?• Richard Goldschmidt (1940) suggested that

large (“systemic”) mutations could produce rare “hopeful monsters” that could endure

• Casarea, Bolyeria (Serpentes: Boidae) on small island near Mauritius

2-part maxilla: unique among amniotes

ADAPTIVE RADIATIONSDiversification of large number of descendant

species from ancestral species:• Galapagos finches, African cichlid fishes,

Hawaiian swilverswords, etc.

(Text Fig. 15.14)

KEY INNOVATIONS

• “Novel morphological or behavioural traits that open up new ‘adaptive zone’ or facilitate rapid speciation” (e.g., after environmental or colonization event)

• Traits involved in reproductive isolation may increased speciation rates

• Invoked to explain large radiations in some clades

• Key innovation in jaw morphology: pharyngeal jaws in cichlid fishes (Liem, Syst. Zool. 22:425; 1974)

AFRICAN RIFT-LAKE CICHLIDS

• Cichlids advanced teleost; include angelfish• ~2,000/~30,000 species

• Lake Malawi: 500 species in last 2 my• Lake Tanganyika: 150 species in last 2 my• Lake Victoria: 250 species in last 750,000 yr• Much rapid evolution: e.g., 300 species in Lake

Victoria derived from single founding population with last 12,000 years

L. Malawi

L. Victoria

L. Tanganyika

Cichlid fishes of East Africa’s rift lakes:>1500 species in last 10 million years

Corematodus shiranus (upper):scale-eating mimic

Caprichromis orthognathus:Head-ramming paedophage

Melanochromis labrosus:Seals openings w/lips, sucks

Tangle-veined fly (Prosoeca ganglbaueri) visits mountain drumstick (Zaluzianskya microsiphon)

FLORAL SPURS

• Floral spur: long outgrowths of

floral parts, increase distance

between floral reward and

reproductive parts of flower• Associated with reproduction so

may diverge rapidly and

rapid reproductive isolation

Between closely related lineages

Malagasy orchid (Angraecum sesquipedale):• Nectar pool of nectar ~30 cm from flower opening• Darwin received flower from Kew Gardens,

predicted existence of moth with long proboscis: discovered, described 1903!

• Geographic variation: in Aquilegia caerulea, spur length and colour vary in association with different types of pollinators

• Spur differences prevent hybridization between Aquilegia species that can hybridize

• Spur morphology varies with morphological variation in bee pollinators of Diascia

• Aquilegia: little DNA sequence divergence, many species: suggest recent radiation

• Floral spurs arose independently several times; other groups like Aquilegia

Taxon with

floral spurs

No. species:

TFS Sister groups

Aquilegia 70 1, 14

Delphinium, Aconitum 350 37

Fumariaceae 450 15

Tropaeolaceae 88 2

Lentibulariaceae 245 2

Pelargonium 280 399

STASIS

Major pattern: New morphospecies appear in fossil record, then change little over long periods of time

Amborella trichopoda:sister group to all extant angiosperms

(LCA ~140-210 Ma)

“LIVING FOSSILS”

• Gingko (Permian; maximal diversity and geographic range in Cretaceous; 1 species left in Paleocene): phylogenetic and geographic (distributional) relict

• Horseshoe crabs (Early Triassic, ~230Ma)• Snapping turtles (Late Paleocene, ~57 Ma)

THE THEORY OFPUNCTUATED EQUILIBRIUM

Eldredge & Gould (1972):• Stasis real, not artifact• Stasis normal: relatively rapid change (saltation)

occurs periodically• Patterns viewed over different temporal scales

lead to different interpretations (What does “gradual” mean?)

(Text Fig. 15.16)

STASIS vs. GRADUALISM

Darwin:• Evolution and extinctions gradual• Sudden extinctions are artifacts• Extinctions result from competition between species• Extinctions tied to natural selection and trend to “progressive” evolution

George Gaylord Simpson:• Extinctions rarely result from

competition• Dominant groups die out and

are then replaced• Successor group often present

during decline of other group

DEMONSTRATING STASIS

• Distinguish stasis from gradual change: fine temporal scale best

• Is change independent of or coincident with speciation events?

• Is rapid change followed by stasis or by gradual change?

Test for stasis:• Phylogeny of clade must be known• Ancestral species must overlap with descendants

CHEILOSTOME BRYOZOANS:Morphological and genetic concordance

(text p. 407)

BuskeaBracebridgia

DEMONSTRATING STASIS:BRYOZOANS OVER ~16 x 106 YEARS

• Stylopoma: 12 fossil, 7 extant species• Morphologically established phylogeny• Phylogeny scaled: branch points at first appearance

in fossil record, tips at last appearance

• Similar analysis for Metrarabdotos

(Text Fig. 15.17a)

1) No intermediates2) Species stable through time

(Text Fig. 15.17b)

1) No intermediates2) Species stable through time

PUNCTUATED EQULIBRIA NOT UNIVERSAL

• Varied patterns across taxa• Gradualism common in microscopic marine forms

• Lack of genetic variation?• Compare horseshoe

crabs with other arthropod clade

CAUSES OF STASIS

(Text Fig. 15.19)

DYNAMIC STASISAND ZIGZAG EVOLUTION

• Averaging over long periods can blur trends

(Text Fig. 15.20)

QUANTIFYING RATE OF EVOLUTION

J.B.S. Haldane (1949) defined darwin:• “change by factor of e per 106 years”:• Simpson’s Tertiary horse data: 0.04 d• Domestic animals: thousands of d• Kurtén: post-glacial Holocene mammals, 13 d;

Pleistocene mammals, 0.5 d; Tertiary mammals, 0.02 d

RECORD OF EXTINCTION

• Known fossils ~ 250 000 species (~ 1% of species that have ever lived)

• Bias: marine animals with hard skeletons• Incomplete sampling biases extinction rates (few

short-lived localized fossils)• Distribution of recorded lifespans of genera

highly skewed

BACKGROUND EXTINCTION• Within radiation, likelihood of extinction is

constant, not affected by how long clades have been in existence

• Select random samples within clade• Plot proportion of surviving clades vs. time• Analogy: survivorship curves in ecology

EPISODES OF EVOLUTION• “Mass extinction” often defined as extinctions of

75-95% in brief period• Dramatic but combined effect ~ 4%• Implies 2 extinction modes: but this is not clear cut

CLUSTERING OF EXTINCTIONS

• Due to removal of key taxa? Ecosystem loss?• Major extinction events cannot be explained by

species interactions• Long aftermath following mass extinctions• Post-mass-extinction biota very different

SELECTIVITY OF EXTINCTIONS• Sometimes taxonomic selectivity• Species with planktonic larvae survive background

extinction better than species with direct development• Widespread species survive mass extinctions better

MASS EXTINCTIONS

Five commonly recognized:• End-Cambrian (~ 500 Ma)First vertebrates in CambrianTrilobites and conodonts reduced; many invertebrate

groups disappeared

• Terminal-Ordovician (~ 440 Ma)

• Late-Devonian (~ 365 Ma)~75% of fish families died out: most agnathans,

placoderms & several families of acanthodians, bony fishes

• End-Permian (~250 Ma)Amniotes diversified, large terrestrial nonamniotes

declinedLarge “therapsids” became dominantPangaea formed; reduced epicontinental shallow seasLarge land area, fewer geographic barriersWidespread marine extinctions (88-96% of all species);

also many large land animals

• End-Triassic (~215 Ma)

• Cretaceous-Tertiary (K-T) boundary (65 Ma) (text pp. 403-405)

EVIDENCE OF K-T IMPACT

Iridium layer at K-T boundary; rare in earth’s crust, common in meteorites

Shocked-quartz crystals

Microtektites

Magnetic and gravitational anomalies; crater confirmed 1990s

EFFECTS OF IMPACT

• Huge influx of [SO2 + water vapour] H2SO4 (therefore acid rain)

• SO2 also scatters solar radiation• Cooling enhanced by dust• Extensive forest fires soot• Thus Earth became cold and dark

• Massive earthquakes triggered volcanoes• Huge magma deposits (Indian Decca Traps)

contemporaneous; connected?• Vulcanism release of SO2, CO2, ash• Tsunami (4 km high?)• Oceanic primary productivity reduced• Disruption of ecological processes

Stack of ~ 20 Columbia River Basalt lava flows(Grande Ronde River, Washington State)

EFFECTS OF MAGMA DEPOSITS

• Environmental impact from released gases• Climatic cooling from H2SO4 aerosols?• Greenhouse warming from

CO2 and SO2 gases, acid rain?• Indirect effects: changes in ocean chemistry,

circulation, and oxygenation?

K-T VERTEBRATE EXTINCTIONS

• 100% of dinosaurs, pterosaurs, plesiosaurs• 75% of marsupial families• 75% of bird families• 36% of squamate families• 27% of turtle families• 23% of mammal families• 18% of chondrichthyian families• 12% of bony fish families• Overall (families): 30% of vertebrates, 48% of

amniotes, 43% of tetrapods, 15% of fishes