Lecture 11 Flashcards
give a broad description of the tree of life
- has both simple and complex organisms
- bacteria, archaea, and eukaryotes are main 3 groups
- eukaryotes typically more complex in cell number, tissue types, physiology…
major transitions in evolution
- origin of cells
- origin of chromosomes
- origin of genetic code
- origin of eukaryotes
- origin of sexual reproduction
- origin of multicellularity
- origin of colonies (eg non-productive castes)
what is the ultimate target of selection and why?
genes because they are the unit of inheritance
what are the units competing?
- DNA/gene
- cells
- individual organisms
- species
- larger clades
why does multi-level selection pose a problem for complexity
- selection at a given level of organisation means that units compete to maximise fitness
- competition among lower-level units of organisation may reduce fitness at higher levels
what is the solution to the multi-level selection problem?
if lower-level units of organisation cooperate rather than competing, higher-level fitness costs can be avoided
how do biological subunits stay so cooperative?
many features of individual organisms prevent competition within an individual:
- prevents evolution within individuals
- align fitness interests across levels of organisation
- this ensures that many genes succeed by enhancing the fitness of the individual
two ways in which biological subunits stay cooperative
- meiosis and mitosis:
- ensures that alleles don’t compete within an individual
- fair representation of gene variants among daughter cells - development and multicellularity
- starting from a single cell prevents initial competition among cell lineages
what is fair meiosis?
meiosis provides a fair representation of an allele’s fitness effects on individuals
two ways to cheat a fair meiosis
- meiotic drive
- over-replication
define and describe meiotic drive
- if an allele can bias its own transmission then it can spread to higher frequency even while reducing individual fitness
- selfish genetic element relative to organism’s fitness interests
give an example of meiotic drive and cheating Mendel’s law of segregation
drosophila segregation distorter locus (SD)
- almost all (95-99%) of offspring are Ss
- S allele prevents proper ‘s’ sperm formation
- counteracting restorer alleles are favoured at other genes in the genome to silence the S allele
what is the evolutionary response to meiotic drive?
when cheating alleles spread, there is strong selection on rest of genome for suppression of cheating
define and describe over-replication
transposable elements are self-replicating segments of DNA (transposons)
- TE replication is separated from cellular replication
- ensure their own over-representation in offspring
how do genomes not explode from transposition?
- 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 - transposition-selection balance
- transposition is a form of mutation that can disrupt a gene
- natural selection against harmful effects on the organism reduces abundance of chromosome copies with most TES
- abundance of TEs in an organism results from a balance between these opposing forces
what can lead to rampant activation of transposable elements?
mutations in genes for DNA methylation:
- mutation in DDM1 gene reduces methylation
- this reactivates silenced TEs
what experiments were done with C.Elegans and why?
- cell lineage mapped from 1 cell zygote to 959 cells of adult
- how do collections of cells maintain cooperation to make an organism?
what features may inhibit unregulated cell division?
tumour suppression
what makes it harder for collections of cells to stay cooperative?
- somatic mutation is inevitable in long-lived multicellular organisms
- some of those somatic mutations might be selectively favoured within an individual
use cancer as an example of selfish cell lineages evolving within an individual
- spreads commonly in tissue that is relatively undifferentiated
- evolves resistance to treatment and the immune system
- illustrates the short sightedness of the evolutionary process
