5 Origin of Sex and Multicellular Organisms Flashcards
What happens to Acritarchs at 950-850 Mya and from 850 Mya?
Reach their zenith in terms of diversity and size
Suffer a major wane during various extinction events
What is the idea of snowball Earth?
Suggests that at one point the Earth was completely frozen
- Found glacial deposits
- Some life was able to survive
What caused snowball Earth scenarios?
Two extensive glaciations extending into equatorial latitudes
- Cryogenian (720-635Mya)
- Marinoan Ice Age (650-635Mya), Sturtian Ice Age (720-660 Mya)
What is the composition of the boulder clays that represent the glaciations?
Bounded below by carbonates
BIFs occur within the clays
Bounded above by cap carbonates
What caused the glaciations?
Carbonates below the boulder clays
- Have very high C-isotope values
- Due to continental break up = narrow seaways with enhanced C burial = reduced atmospheric CO2 leading to the glaciations
What ended the glaciations?
Cap carbonates with very low isotope values
- Due to rapid accumulation and/or absence of organisms and/or burst of methane that ended the glaciations
What is meiosis?
Sexual reproduction in unicellular or multicellular organisms
What causes variation in sexual and asexual organisms?
Asexually reproducing:
Only variation is due to mutation
Sexually reproducing:
Variation due to mutation, independent assortment and crossing over
What are the problems of sex?
Expensive
- uses lots of resources
50% of pop is not reproducing
- Inefficient
- Time also spent waiting for birth or looking after young
Offspring not immediately available for reproduction
What are the advantages of sex?
Can pass beneficial mutations around many lineages so they build in the population
Can combine advantageous mutations
Makes populations more diverse
Mutations can be shed to remove harmful ones
How do mutations spread in asexual reproduction?
Spread only through direct descendants
All offspring get the mutations
What are the main theories of why sex evolved?
1) Method of adapting to change by evolving faster
- More variation, get good mutations, remove bad ones
2) Method of fending off disease/parasites
3) Method of repairing genes
- During meiosis, chromosomes have had to align so proofreads
4) Historical accident
When did sex evolve?
Probably by 1200 Mya
How and when did multicellular life evolve?
By 1000 Mya
- Evolved several times independently in different biological groups
- Probably evolved via experimentation with colony formation
- Has the advantage of division of labour
