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 Extinction 65 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