Extinctions

A. Prokaryotes took 1 billion years to evolve

1. Prebiotic atmosphere contained: CH4, NH3, CO, CO2, H20
2. Amino acids and nucleotides can be created from these in the presence of electricity
3. Self-replicating RNA w/ catalytic ability is likely ancestor of DNA (ribozyme)
4. Needed coacervates or clay particles to enable molecules to work together

B. Eucaryotes appeared 2 billion years later

1. Once photosynthesis evolved, O2 was created, and life moved out of water

C. Multicellular animals appeared in last 550 milion years

D. Current phylogeny of life

1. Eucaryotes are sister taxa to Archae, and Bacteria are sister to this clade
2. Root the phylogeny of life using duplicated genes as outgroups

II. Diversity represents a balance between speciation and extinction

A Many of the geological periods represent major gaps in the fossil record. These are due to major extinction events - now known to be true

B. Mass extinctions recur, and have been a regular pattern in earth's history

C. Five mass extinctions have been identified

1. Probably all mediated by climatic changes, but only one caused by a meteor
2. All were followed by adaptive radiations

III Mass extinctions

A. Five major extinctions

1. 440 MYA, late Ordovician - global cooling, plants colonized land
2. 365 MYA, late Devonian - global cooling and anoxia of seas, followed by insect diversification
3. 245 MYA, End of Permian - 80-96% of marine species went extinct, sustained cooling, coincided with formation of Pangea, followed by gymnosperm radiation
4. 210 MYA, End of Triassic - possibly related to increased rainfall - followed by dinosaur radiation
5. 65 MYA - KT (Cretaceous-Tertiary) - 65-70% of marine species went extinct, end of the dinosaurs, followed by mammal radiation, likely due to large impact event

B. Causes

1. Climatic change - some evidence for all

a. sudden cooling, increases polar ice caps
b. Fall of sea level and increased salinity of oceans when ice caps increased

2. Continental drift

a. Continental coalescence (Pangea) at the end of the Permian would have reduced perimeter and caused extinctions - only seems relevant for Permian

3. Meteor impact - only known to be true for KT extinction

a. 1980 Large concentrations of iridium found at Gubbio, Italy. Iridium is a rare element normally found associated with iron, but is common in meteorites. KT border is marked by iridium around the world. Proposed a 10 km diameter meteorite collided with earth

b. 120 known impact craters on earth, but most likely site is Chicxulub, 180 diameter crater, off coast of Yucatan. Crater is the right size, evidence of surrounding tidal waves from KT deposits

c. Shocked quartz and tektites have been found in KT deposits and around Chicxulub

d. Extinctions coincide with the iridium layer, are synchronous for a wide range of taxa

e. Explosion would have been 1000 times Krakatao, which put ash into the atmosphere for 2.5 years. Would have caused

i. nuclear winter from loss of sunlight
ii. tidal waves
iii. acid rain
iv. global fires due to enhanced vulcanism

C. Effects - Mass extinctions are followed by new communities

1. Fungal spikes occur after mass extinctions - indicate massive decaying plants

2. After the Permian, shallow-water communities of brachiopods and bryozoans were replaced by bivalves and gastropods. On land pteridophytes were replaced by advanced gymnosperms. Paleopteran insects were replaced by neopteran insects (fold wings over back - now 98% of all insects).

3. After Triassic extinction, mammal-like reptiles were replaced by turtles, crocodilians, dinosaurs and mammals

4. Mammals and birds radiated after the KT extinction of dinosaurs

5. Extinctions were more prone for species with narrow ranges, nonplanktonic larvae

III Human caused extinctions

A. Pleistocene mammal disappearance 10 KYA - overkill or climate change?

1. 70% of North American mammals disappeared
2. 80% of South American genera
3. 86% of giant marsupials in Australia
4. Only 29% of northern Eurasian genera went extinct
5. Only 14% of African mammals disappeared
6. No loss of marine invertebrates, insects, plants and birds!
7. Coincides with glaciation and spread of humans into N and S America

B. Polynesian birds

1. 60 Hawaiian spp have gone extinct in last 1500 years. Disappearance coincides with fire pits, middens and tools
2. New Zealand - 44 spp of birds went extinct after human colonization, including 8 spp of flightless moas
3. Estimate that at least 10 spp have been lost from each of 800 islands. High levels of island endemism suggests that 2000 spp have been lost in last 2000 years. Bird loss coincides with human colonization of Polynesian islands
4. On the Galapagos, only 3 populations have been lost over 8000 years preceding arrival of humans. Since 1535, 20 taxa have been lost.

C. Evidence for a current mass extinction

1. From 1986-1900, 15 vertebrates went extinct. If this rate continues, in 7000 years half of the described 45,000 vertebrate species will be gone
2. Estimates from rainforest loss suggest extinctions will occur at 100-1000 times background levels.

D. Will another radiation occur after this extinction? Will it take 5-10 MY?