Lecture 11: evolution of complexity Flashcards
How does greater complexity arise in evolution
- A small number of events led to major changes in how inheritance worked
- Previously independently evolving units merged, leading to higher-level complexity & specialization through division of labour
What Is the ‘Unit of Selection’?
- Most phenotypic traits we study in organisms arose due to selection that increases the fitness of individuals
- Traits that are “good for the species” but that reduce fitness of the individual cannot be favored by individual selection
When is cooperation adaptive?
1) High relatedness
* Genes that lead to helping relatives can spread via natural selection
2) Reciprocal altruism
* In cases where organisms repeatedly encounter each other
* Mutual cooperation can lead to the highest fitness
3) BUT: Cooperation sometimes breaks down eg. selection for ‘cheaters’
what is the ultimate target for selection
genes
How do genes stay cooperative ?
1) Mitosis and meiosis
* Ensures that alleles don’t compete within an individual
* Fair representation of gene variants among daughter cells
2) Development and multicellularity
* Starting from a single cell prevents initial competition
among cell lineages
3) Uniparental inheritance of organelles
* Chloroplasts and mitochondria replicate asexually
* Prevents competition within cells of different organelle
genomes
How can meiotic drive contribute to cheating a fair meiosis
*If an allele can bias its own transmission
* Then it can spread to higher frequency
* Even while reducing individual fitness
*Meiotic drive can RAPIDLY ELIMINATE alleles that have higher individual fitness
what mechanisms contribute to genomes staying cooperative
1) Fair meiosis
2) When cheating alleles spread, strong selection on rest of genome for suppression of cheating
How does over-replication contribute to cheating a fair meiosis
Transposable elements (TE)
Self-replicating segments of DNA
TE replication separated from cellular replication
Ensure their own over-representation in offspring
Approximately what percentage of DNA do transposable elements make up in the genome
Transposable elements can make up >50% of DNA in genomes of some species
How Do Genomes Not ‘Explode’ From Transposition?
1) Alleles arising elsewhere in genome that silence TEs will be favoured by individual selection
* Mechanisms controlling DNA & histone methylation
* piRNAs and RNA interference may have evolved as silencing mechanisms
2) Transposition-selection balance
* Transposition is a form of mutation that can disrupt a gene
How Do Genomes Not ‘Explode’ From Transposition?
1) Alleles arising elsewhere in the genome that silence TEs will be favoured by individual selection
* Mechanisms controlling DNA & histone methylation
* piRNAs and RNA interference may have evolved as silencing mechanisms
2) Transposition-selection balance
* Transposition is a form of mutation that can disrupt a gene
Explain transposition -selection balance
- Transposition increases TE abundance
- Natural selection against harmful effects on the organism reduces abundance of chromosome copies with most TEs
– Overall abundance results from a balance between these opposing forces
how do mitochondria stay cooperative?
- Uniparental plastid inheritance strongly reduces competition within individuals
- Consistent with hypothesis that it evolved to maintain cooperation
– e.g. active exclusion of sperm mitochondria at fertilization
How Do Collections of Cells Stay Cooperative?
- Starting from a single cell reduces competition within individuals
- Separation of germline with limited numbers of cell divisions inhibits transmission of selfish cell lineages
- Tumor suppressors, other features inhibit unregulated cell division
- BUT:somatic mutation is inevitable in long-lived multicellular organisms
*Some of those somatic mutations might be selectively favoured within an individual
How Do Individual Genomes Stay So Cooperative?
- Many features ensure that the variance in fitness WITHIN an individual is minimized
- Ensures that many genes succeed by enhancing the fitness of the organism (‘group’)
- BUT: countless ways to evade cooperation
- Presence of strong selection on rest of genome (‘policing’) seems essential to maintain higher-level cohesion!