Lecture 6: Units of Selection continued Most Extreme example of Kin Selection: EUSOCIALITY Eusociality: 1)Overlap in generations 2)Co-operative brood care 3)Specialized castes of non-reproductive individuals

Examples Hymenoptera (bees, ants, wasps) Isoptera (termites) Mammalia (naked mole rats) How did this evolve?

Hypothesis 1: Kin Selection Plant Galls: a)stick around & raise your siblings (r =0.5) b)Leave, find mate & raise offspring (r =0.5) Difference is in RISK LEVEL

Haplodiploidy Eusociality is COMMON in haplodiploid hymenoptera (evolved 11 times) UNCOMMON in diploid ( only 1 time)

Haplodiploidy Diploid (2n): produce ova by MEIOSIS females from fertilized eggs Haploid (1n): sperm by MITOSIS males come from unfertilized eggs focal individual: female male relative motherdaughtersisterfathersonbrother Favours reproduction of sisters over all others

Origin of Eusociality Problems with Haplodiploid hypothesis : 1) Isoptera (diploid) –Sterile workers of both sexes –Social transmission of gut symbionts + + 2) Many species queens are promiscuous - females not most closely related to sisters 3) Many species that are haplodiploid are not eusocial So…

Hypothesis 2: Parental Manipulation Queen suppresses reproduction of other workers Chemical cues Physical inhibition Lots of experimental support

Specialized Castes Queens & Workers Fertilized egg Worker Queen Royal Jelly Royal Jelly – hormones that affect development of tissue including ovaries

Stingers Worker: barbed, weak attachment Purpose: defense of colony Queen: unbarbed, long & curved Purpose: Killing sisters

Hypothesis 3: Mutualism Worker manipulation? Cryptic reproductives Greater fitness by helping rather than founding a new colony. Unmated workers lay haploid eggs in many species.

Most likely… Eusociality ONLY occurs in species with: 1.Complex nests 2.Larvae cared for extensively Thus, females are unlikely to be successful if breed on their own. Ecological not genetic? “Best of a bad situation”

Unit of Selection Entity that benefits from adaptation What about adaptations that benefit genes at the expense of other genes in the same individual Segregation Distorters Paternal Sex Ratio Trait

Normal Mendelian Segregation Alleles segregate in a 1:1 ratio in gametes produced by heterozygotes Meiotic Drive: non-random segregation of alleles into gametes Aa A a 50%

Segregation Distorters Drosophila spp. sd prevents dev’t of sperm with + allele sd/+ have low fertility sd has advantage: genes at other loci suffer selection for suppressors & recombination sd/+ sd + 90% 10%

Paternal Sex Ratio Trait Parasitic wasp (Nasonia vitripennis) Father-son transmission Impossible because M are haploid? PSR : on B chromosome B= small, unusual, nonessential chromosomes that don't go through meiosis normally high meiotic drive: most sperm get B chromosome

B Chromosome all other chromosomes in sperm supercondense so lost in mitosis sperm carries only B chromosome sperm empty of all other genes than PSR Ultimate ‘selfish gene’ : copies itself while destroying all other genes !!

Intragenomic Conflicts SOMA Mutations die out GERM LINES Mutations inherited some alleles gain fitness at the expense of genes at other loci If somatic lines give rise to germ cells selection for cell lines

Unit of Selection is unit that benefits from adaptation But adaptation benefits: –Genes –Cells –Individuals –Kin groups –Unrelated groups What unites these? Units that show adaptation are units that show heritability

Heritability Proportion of variation in a phenotype in a population attributable to individual differences in genotype Related to the genetic & phenotypic makeup of a population

Heritability HIGH LOW e.g. Eye colour e.g. Number of eyes

Group Selection Group fitness cannot be inherited Thus, group selection can’t work Unit of selection is entity whose frequency is adjusted by natural selection over generations

Richard Dawkins entity that replicates itself with fidelity & is stable through time is the gene organism is “vehicle” gene is “replicator” organism’s fitness affects frequency of genes over time adaptations exist b/c they  repro of an allele relative to other alleles by  the fitness of the “vehicle”