Group Selection, Individual Selection and Gene Level Selection

Problem of the cheat

• alleles can invade when initially rare

Why Group Selection predicts female-biased sex ratios

• females do all the investing (eggs are costly, provision of nutrients\\\\\0

• almost all investment into the next generation comes from females

• if the goal is to maximise total number of offspring, you want as many females as possible

• group selection wants to produce the minimum number of males necessary

Why would group selection make sense?

• groups with more females grow faster

• faster growing groups outcompete others

Why is group selection incorrect?

• natural selection does not act for the good of the group

• selection acts on:

  • individuals

  • genes

Why does Kin Selection produce female-biased ratios?

• an allele spreads if it helps copies of itself

• helping kin can increase an allele success

If it helps non relatives

• alleles do not spread as it is helping competitors genes

If it helps relatives

• the alleles spread only under certain conditions

Why sex ratios tend to be 50;50 instead? (Fisher’s Insight)

If males are rare

• each male will have high reproductive success

• mothers who produce sons have high advantage

If males are common

• each male has low reproductive success

• producing daughters is favoured

this frequency dependent selection drives the system toward: equal investment in males and females

Hamiltonian Sex Ratios

• females make less female biased sex ratios depending on the number of females who have laid their eggs in the same host

• Fisher assumed that two sons can mate with anyone

• e.g wasps injects eggs into a catepillar

  • those eggs grow up in the catepillar and mature into adults in the catepillar

  • mate inside the catepillar

  • catepillar is eaten from the inside

• produce less males

Where does Fisher’s case not hold?

SOCIAL INSECTS  

• Queen, dad, and the progeny 

• Haplodiploid: 2 copies of chromosome of female, 1 copy of chromosome of males 

• Sons don't have a father in this species  

• Because the father only has one copy of chromosomes, daughters become identical on the paternal side  

  • Father is the same for two females  

  • Daughters related by r=1 on the paternal chromosomes  

  • On the maternal di-chromosomes they are normal  

  • Average relatedness between sisters is 75%  

  • Only related to their brothers 25% since brothers don't have paternal chromosome

• From allele invade argument: as sisters are more related to each other they would want to continue the production of mothers 

 

The Sperm Sprint

• heterozygote male

• under mendelian inheritance says all sperm have ann equal chance

• what really happens in some sperm of killer alleles that kill of competitors

Meiotic Drive

• if you have drive gene on sperm X, it will kill Y variant sperm

  • male produces half the sperm it should

• X linked drive in flies

• Viable sperm of Xdy males are almost all Xd

• this reducles male fertility, female fertility

• however because of the sperm level advantage in males this mutation speeds

Meiotic drive is an example of a selfish genetic element

• hese are genes that spread in populations even though they are costly, because they don't play the inheritance game fairly  

• NOT to be confused with Darwins' term "selfish gene"  

• Examples include cytoplasmic alleles that kill sons (e.g male killers in ladybirds, butterflies), cytoplasmic alleles that turn males into females (feminiser gene in woodlice)  

• Those that distort the sex ratio can send a population extinct (and in the lab do)  

  • Evolution has no foresight 

• Cytoplasm only from mother