L7. Evolution and Early Diversification Flashcards
Lines of evidence
- selection experiments
- present biodiversity
- fossil record
- molecular phylogeny
Assessing descent through comparisons involving phenotypic traits
- Uses observation and measurement
- evaluate degrees of similarity and account for patterns of change in time to reconstruct relationships and lines of descent
- fossil sequences test inferred patterns of change
Issues:
- environmental influence on phenotype
- ontogenetic changes
- identifying “derived” traits
- Insufficient data from living and fossil organisms to unambiguously trace changes in time
- lack of universal traits
What is chance of finding a fossil
extremely small
need:
- dying in one piece
- being fossilized
- remaining undisturbed
- being exposed
- being found
- being recognized
Pros and Cons of assessing descent through comparisons involving molecular traits
pros:
- nucleotide sequence provide a direct record of all information stored in the genome
- no environmental or ontogenetic effects on the observed traits
- potential for universal traits
cons:
- only works with sophisticated tech
- inferring patterns of change is hard
- back mutation at a site in the sequence is possible and complicates analysis
- assuming constant molecular clock
How to reconstructing phylogeny (molecule systematics)
- Acquire nucleotide sequence data. The amount sampled depends on how far back in time the relationships of interest lie
- Align sequences from different organisms. Allow for mismatches due to point mutation, insertions, and deletions
- Reconstruct most likely lines of descent. Assess the minimal number of steps required to change from one sequence to another and use this as a measure and use this as a measure of relatedness across all the organisms in the analysis
Why did diversity increase?
- increase in size of genome: more proteins and enzymes coded for
- sexual reproduction: meiosis increases possible variation/mixing
- increase in structural complexity: locomotion, protection
- ecological changes: oxygen, energy sources, physical landscape, photosynthesis in shallow water
Origin of multicellular life
colonial hypothesis evidence:
- dividing cells do not separate after division
- mutation in cell membranes or walls
- colonial protists and algae illustrate intermediate stage of complexity
- allows evolution of separate functions for individual cells
Ediacaran diversity
- variety of forms: leaflike fronds, round pads, worms, jellyfish-like, sponge-like
- most are unique and unlike know animals
- ancestors of many modern groups: arthropods, worms, sponges, jellyfish
- on earth around 570 Ma
- evidence: small shelly fossils in ocean sediments,
Burgess Shale Diversity
- a fossil marine community circa 515 Ma
- found in Yoho National Park, BC in 1909
- fossil of Pikaia, a ancestor of chordates, vertebrates, mammals, and humans
Cambrian Explosion
- occurred between 541Ma - 485 Ma
- All modern animal body plans established in less than 25 million years
- changes since are just variations on those established plans
Why did the Cambrian explosion occur?
unsure
- biological factors
- increase in genetic complexity: enough DNA to code for a much greater range of structural and metabolic processes
- increase in structural complexity: more body parts for selection to act on, more structures that were adapted to multiple functions
- change in the environment
- change in ecological relationships
Is there a purpose in evolution?
NO
- variations arise through change mutations
- some mutations are positive, some negative, many neutral
- selection, not the organisms, decides which variations will survive or succeed
- always interpret evolution in terms of change variations, selection, probabilities of survival, not purpose and progress towards an objective
Predator-Prey arms race
- when organisms are trying to exploit the same niche and evolve in a race like manner (co-evolution)
Pikaia
The first chordate (animal with a long central nerve). The lineage that would go on to give rise to all vertebrates!
