What affects the Rate of Evolution?

What can The Cambrian Explosion and Mass Extinction

Events teach us about the rate of the Evolution of life?

Two Factors affect the rate of Evolution: Variation and Selection

• Variation: The differences among

members of the same species. Due to

differences in genetics caused by

mutations

• Selection: How strongly the

environment is selecting for of against

a specific characteristic

What was the Cambrian Explosion and what caused it?

The Cambrian Explosion is used to define the boundary

between the Precambrian and the Cambrian (544 MYA),

Before this time life on earth was mostly single cellular

Prokaryotic and Eukaryotic, some simple colonial species

present.

• During the Cambrian we see a sudden increase in

the number, types, and complexity of life on earth

• What caused the sudden increase?– A type of

mutation called Gene duplication occurred to a group

of genes called homeotic genes.

• What do Homeotic genes do?– They are active during

embryonic development and determine body segment

identity.

Tree of Multicellular life according to Number and type of Homeotic Genes

The embryos highlighted show the region

where that Homeotic gene is turned on to

regulate development of segment identity

In Summary, Gene duplication of the

Homeotic Genes that regulate embryonic

development and body segment identity

dramatically increased the variety of

forms that life could produce. This

occurred shortly before and during the

Cambrian explosion, producing all the

major body plans that are present on earth

today.

How do Mass Extinctions lead to new diversity oon earth?

• Mass Extinctions are events that cause a

massive die-off of a large percentage of the

species on earth

• Most often caused by cataclysmic volcanic

activity, asteroid impacts, and climate

change.

• Mass Extinctions increase the rate of

selection

• They open up previously occupied niches

• Organisms that are able to survive diversify

to occupy the niches left available in a

process called adaptive radiation

Example: Cretaceous-Tertiary Mass Extinction65 mya

• This was most likely caused by an asteroid

impact followed by climate change

• The dinosaurs, except for the ancestors of

birds, became extinct.

• Mammals were small and able to survive this

period of upheaval.

• After Climate normalized, mammals

expanded and diversified to take the niches

once occupied by dinosaurs