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
