8 macroevolution: the big picture

What is macroevolution?

Large-scale evolutionary patterns above the species level caused by speciation and extinction.

What is microevolution?

Changes in allele frequencies within populations over time.

Key difference between microevolution and macroevolution

Microevolution is directly observable and testable experimentally; macroevolution examines large historical patterns over long time scales.

What is the fossil record?

The preserved remains or traces of organisms that lived in the past.

Three types of fossils

Intact fossils (can contain DNA), cast fossils (show morphology), and trace fossils (show ecological activity such as footprints or burrows).

Why is the fossil record incomplete?

Most organisms do not fossilize and many fossils have not yet been discovered.

How many fossil species have been discovered?

About 250,000 fossil species.

Estimated total number of species that have ever existed

Approximately 50 billion species.

Approximate fraction of species represented in the fossil record

About 0.0005% (less than 1 in 1000).

Three major biases in the fossil record

What gets preserved, where fossils form, and how fossils are discovered.

Example of preservation bias

Hard-bodied organisms like crabs fossilize more easily than soft-bodied organisms like jellyfish.

Example of location bias

Fossils are more likely to form in shallow marine environments than tropical forests.

What information can trace fossils provide?

Ecological information about organism behavior.

What was Darwin’s dilemma?

The absence of Precambrian fossils in Darwin’s time, making early evolution difficult to explain.

What was the Cambrian Explosion?

A rapid increase in diversity of animal life around 540 million years ago.

What major extinction event is discussed in the slides?

The Permian extinction.

What is standing diversity?

The total number of species present at a given time.

Formula for evolutionary diversity change

Diversity(t+1) = Diversity(t) + α − Ω

What does alpha (α) represent in macroevolution?

Origination rate (number of new species forming).

What does omega (Ω) represent in macroevolution?

Extinction rate (number of species going extinct).

What is turnover?

The sum of origination and extinction rates (α + Ω).

What is background extinction?

The normal ongoing rate of extinction over time.

What is a mass extinction?

A sudden event where at least 75% of marine animal species go extinct.

How many major mass extinctions are recognized?

Five major mass extinctions (“The Big Five”).

What does the macroevolutionary Red Queen hypothesis suggest?

Species must constantly evolve just to survive while extinction risk stays roughly constant.

What conditions can cause high origination rates?

Vacant ecological niches, evolution of novel traits, and evolution of new mutualisms.

What is adaptive radiation?

Rapid diversification of a lineage when new ecological opportunities become available.

Two hypotheses explaining high tropical diversity

Nursery hypothesis (high speciation rate) and Museum hypothesis (low extinction rate).

What is biogeography?

The study of the geographic distribution of species over time.

Who created the first global map of biogeographic regions?

Alfred Russel Wallace.

What is Wallace’s Line?

A boundary separating Asian and Australian species distributions.

Two evolutionary explanations for species distributions

Dispersal and vicariance.

What is dispersal?

Species moving from one location to another.

What is vicariance?

Species distributions split due to the formation of geographic barriers.

Examples of vicariance events

Continental drift, mountain formation, and rising sea levels.

What example organism group is used to explain biogeography in the slides?

Marsupials.

What event allowed species movement between North and South America?

The Great American Interchange about 3 million years ago.

Example fossil used to predict evolutionary transitions

Tiktaalik (a transitional fish–tetrapod fossil).

What does Tiktaalik demonstrate?

How fossils can predict where transitional forms should be found.

What is gradualism?

Evolutionary change occurring slowly and continuously over time.

What is punctuated equilibrium?

Long periods of little change interrupted by rapid bursts of evolutionary change.

Where does rapid evolutionary change often occur?

Around speciation events.

What is standing diversity used for?

Measuring how species diversity changes over geologic time.

What is turnover used for?

Measuring how quickly species originate and go extinct.