Lecture 23 Flashcards
Complexity evolution
- Traits start off simple but still adaptive, some species experience selection for improvements, which add greater complexity over time
Greater complexity arises from greater cooperation amongst previously independent units
- A small number of events led to major changes in how inheritance worked
- Previously independently evolving units merged, leading to higher-level complexity and specialization through division of labor
Unit of Selection
- Most phenotypic traits we study in organisms arose due to selection that increases the fitness of individuals
- May or may be good for species
- Individual selection usually stronger than group selection
Traits that are “good for the species” but that reduce fitness of the individual cannot be favoured 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 highest fitness
3) BUT : cooperation sometimes breaks down
- Selection for cheaters
- Worker bees are sterile, but selection acts to favour non-reproductive helpers for the closely related queen
( they are closely related to queen)
Genes as the ultimate target of selection
- Because genes are the unit of inheritance, ultimately the target of selection is the gene
Selection on individual organisms is a form cooperation
- Genomes are composed of unrelated genes and alleles that have been inherited from different places
- Segregation, recombination, and random mating ensures that they are mostly passed on independently
Yet genes typically persist by improving fitness of the group
- Segregation, recombination, and random mating ensures that they are mostly passed on independently
How do genomes stay so cooperative
- Many features of individual organisms prevent competition within an individual
- Prevents evolution within individuals
- Align fitness interests
- Ensures that many genes succeed by enhancing the fitness of the organism
How do individual genomes stay so cooperative
1) Mitosis and meiosis
- Ensures that alleles don’t complete within an individual
- Fair representation of gene variants among daughter cells
2) Development and multi cellularity
- 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 to different organelle genomes
Positive natural selection on Alleles
- Alleles spread through a population by increasing individual fitness
Cheating a fair meiosis
MEIOTIC DRIVE
if an allele can bias its own transmission
- Then it can spread to higher frequency
- Even while reducing individual fitness
- Selfing genetic element relative to organisms fitness interests
By enhancing its own transmission, meiotic drive allele (a) can spread
- Increased frequency of a even while reducing individual fitness
- Meiotic drive can rapidly eliminate alleles that have higher individual fitness
Meiotic drive = cheating Mendel’s law of segregation
Ss x ss
Almost all offspring are Ss, except 50% offspring with fair meiosis
- S allele prevents proper ‘s’ sperm formation
- Counteracting ‘restorer’ alleles are favoured at other genes in the genome to silence the S allele
- When cheating alleles spread, strong selection on rest of genome for suppression of cheating
Cheating a fair meiosis
OVER REPLICATION
Transposable elements (TEs)
Cheating mendels law through over replication
- Self replicating segments of DNA
- TE replication separated from cellular replication
- Ensures their own over - representation in offspring
- The most common DNA in Genomes often is selfish, 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 and histone methylation
- piRNAs and RNA interference may have evolved as silencing mechanisms
Mutations in genes for DNA methylation leads to rampant activation of transposable elements
2) Transposition-selection balance
- Transposition is a form of mutation that can disrupt a gene
- 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
Mitochondrial transmission cooperation
Lack of mitosis and meiosis by plastids sets up potential for spread of selfish elements
- Uniparental plastid inheritance strongly reduces competition within individuals
- Consistent with hypothesis that it evolved to maintain cooperation
- Example. Active exclusion of sperm mitochondria at fertilization
Uniparental inheritance creates new conflict
Conflict of interest
- Maternal inheritance of cytoplasmic genome
- Biparental inheritance of nuclear genome
Mitochondrial mutations that enhance maternal fitness can spread
- Even if cost is severe to male fitness
Cytoplasmic male sterility in plants
- New mutations in the mitochondria that make hermaphroditic plants ‘male sterile’ can spread
- Male sterile hermaphrodites = female
- Because they favour mitochondrial transmission
- Can reduce fitness of plant as a whole
- Leads to evolution of nuclear ‘restorer’ alleles that re-enable fertility through pollen
- -arms-race co-evolution of CMS and restorer genes
- Male sterility decreases fitness through pollen, might increase fitness through ovules, re allocation of resources to ovules, but not double that of the hermaphrodite
- Mitochondrial mutations that make a hermaphrodite plant into a “male sterile” female
- Will spread
Even if they reduce the fitness of individuals
How do collections of Cells stay cooperative?
1) Starting from a single cell reduces competition within individuals
2) Separation of germline with limited numbers of cell divisions inhibits transmission of selfish cell lineages
3) Tumor suppressors other features inhibit unregulated cell division
4) But, somatic mutation is inevitable in long-lived multicellular organisms
- Some of these somatic mutations might be selectively favoured within an individual
CANCER = selfish cell lineages evolving within an individual
- Spreads commonly in tissue that is relatively undifferentiated
- Evolves resistance to treatment/immune system
Illustrates the short-sightedness of the evolutionary process
- Evolves resistance to treatment/immune system
