Transcript Lecture 6: Units of Selection cont`d

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
relative
focal
mother daughter sister father son brother
individual:
female 0.5
0.5
0.75 0.5 0.5 0.25
male 1
1
0.5
0.5
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
Royal Jelly
Worker
Queen
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
A
50%
a
50%
Aa
• Meiotic Drive: non-random segregation of
alleles into gametes
Segregation Distorters
• Drosophila spp.
sd
90%
+
10%
sd/+
• sd prevents dev’t of sperm with + allele
• sd/+ have low fertility
• sd has advantage: genes at other loci suffer
• selection for suppressors & recombination
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
some alleles gain fitness at the expense of
genes at other loci
SOMA
Mutations die out
GERM LINES
Mutations inherited
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
e.g. Eye colour
HIGH
e.g. Number of eyes
LOW
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”