Extinction
This is the fate of all species.

"Stochastic Extinction"
Extinction resulting from random fluctuations in very small populations.

In a study of local populations of hawks and crows, populations comprising fewer than 4 breeding pairs (N_e < 8) nearly always went extinct within 10 years, even in stable environments.

Modifications of environments by humans have provided unintentional "experiments" in extinction.

Based on human induced extinctions, it appears that reduction in habitat and introduction of new species (as competitors or predators) are particularly effective at causing extinctions.

Introduced species

Most of the species that have gone extinct in historic times are island species, often driven extinct by introduced predators.

According to the IUCN, 77 species of Mammals have gone extinct since the year 1500, with 47 of these disappearing since 1800.
Since 1500, 58.4% were island species.

When causes are known or suspected:
Predation (usually by introduced species) or Hunting is implicated in 86% of cases.
Competition from introduced species is implicated in 34% of cases.
Habitat loss is implicated in 25% of cases.

Loss of habitat
The long term persistence of many species requires more usable habitat than they are actually using at any one time.

Consider a population that exists in a number of semi-isolated habitat patches such that at any given time, some of the patches are vacant due to local extinction.
The species can still persist, though, if individuals colonize empty habitats at a higher rate than the rate of extinction within a habitat.
If the number of habitat patches is reduced, the species may be doomed to extinction, though this might not happen for many generations.

Loss of genetic variation.

In very small populations, drift and inbreeding cause a loss of genetic variation that causes many deleterious recessive alleles to be expressed, often increasing juvenile mortality.

If this causes the population to further decline in size, then increasing drift can cause other deleterious mutations to go to fixation despite selection - further reducing the population size.
This positive feedback, called a mutational meltdown (or an "extinction vortex"), can lead to extinction.

Example: Scandinavian wolves
A small population of wolves was discovered in the early 1980s on the Scandinavian peninsula. This was a single pack with fewer than 10 individuals.

This small population persisted but was unable to increase, even in good years when resources were plentiful. Infant mortality was high.

In 1990, a single male wolf from a faraway population migrated in, introducing new genetic variation.

The population promptly started to increase in size after this.

Mass Extinctions
5 since the base of the Cambrian.

End of:
Ordovician (~444 mya)
Devonian (~359 mya)
Permian (~252 mya)
Triassic (~201 mya)
Cretaceous (~66 mya)

Largest was at the end of the Permian
>90% of marine species went extinct.
~70% of terrestrial species.

Properties:

• Stand out statistically from background extinctions
• Are global and cut across habitats
• Hit many, unrelated, groups at the same time.

Selectivity
Mass extinctions are selective, but in different ways from background extinctions.

Between mass extinctions (in background conditions), having planktonic larvae and broad geographic range correlate with low extinction rate.
    These do not help during mass extinctions.

However, some groups are hit harder during mass extinctions than would be expected by chance.

Thus, it appears that the rules change during mass extinctions. What was advantageous during background conditions may suddenly not be.

This can have long term consequences:
Brachiopods resemble Bivalve Mollusks (clams) and are ecologically similar, but are distantly related (they belong to another phylum).
Before the end of the Permian, Brachiopods were much more diverse than Bivalves
However, Brachiopods become rare and Bivalves common after the Permian extinction.

The graph below shows the actual data for number of genera of Brachiopods and Bivalve Mollusks (clams) over the Phanerozoic.

(Note that the second big drop in Bivalve diversity is the end Cretaceous mass extinction.)

Possible causes of mass extinction:

Convergence of continents
    Permian

Recall that loss of habitat, and introduced competitors and predators are the principle drivers of extinction. Convergence of continents reduces the amount of shallow nearshore marine habitat (where most marine species live), and combines previously separated terrestrial ecosystems, bringing many terrestrial organisms into contact with new competitors and predators.

Volcanism
    Siberian Traps at the end Permian extinction.
    Deccan Traps in India at Cretaceous Extinction

Impacts
    Good evidence from Cretaceous-Paleogene extinction. Some recent evidence from P-Tr extinction.
K-P_G: Iridium spike, "Fern spike", Identified crater.

Iridium
Very rare on earth, but rather common in meteors.
A distinct layer with high Iridium content is found all over the world. It dates from the end of the Cretaceous and suggests a very large impact at that time.

Large meteor impacts produce shocked quartz and glass spherules as a result of the impact and melting of rock. Both of these are found in high concentrations in a layer just below the Iridium spike in North America, but not elsewhere.
This suggests that the impact was somewhere near North America.

The best candidate site is just north of Yucatan (Chicxulub), and occurred near the end of the cretaceous. Jul 8, 2021