Lecture 8 - Extinctions, Radiations, & Evolutionary Trends

01/31/25

punctuated model

something dramatic occurs → significant phenotypic shift in descendants → long period of stasis

essentially a rapid change followed by equilibrium

gradual model

evolutionary change happens in small increments over time

most species that ever lived are now ______

extinct

how many extinctions have happened leading to mass species loss

5

mass extinctions 1, 2, and 4

• 70-85% species lost

• loss of trilobites, echinoderms, precursor to vertebrates (conodonts)

• these were driven by global cooling (moving into ice age) because it caused changes in sea levels and oxygen availability (especially in marine ecosystems)

the great dying

• 90-96% of marine species due to greenhouse gases and anoxic oceans

5th extinction

• massive cooling caused the dinosaur death along with squishing of dinosaurs

6th extinction?!?!

• current rates of extinctions are super duper high

• modern extinctions are due to humans

  • we are supposed to reach 6th extinction soon

species radiation

follows a mass extinction because we physically leave room for new species to arise

mechanisms that drive species radiation

• changes in environment (cooler, wetter, dryer)

• availability of new habitat (e.g. as result of earthquake)

• a new adaptation or mutation developing

• new species interacting together

cambrian explosion

• most important species radiation

• occurred after multicellularity

• extensive radiation occurred due to…

  • change in glaciation

  • emergence of new species interactions like predation (not only herbivores now)

• most extant phyla appeared during this period

what can lead to species radiation

the presence of coevolution

coevolution

the reciprocal of 2 interacting species where both respond to selection from the other

this diversifies the group

e.g. different flowers are pollinated by different bees

red queen hypothesis

interacting species are required to maintain their relationship (e.g. if flower gets longer then toad’s tongue becomes longer)

it’s like an arms race

• species are continually adapting in response to evolutionary changes in other organisms to avoid extinctions