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Levels of Selection Gametic Selection: meiotic drive or segregation distortion is selection among gametes. Cell Selection: selection among cell lineages.

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Presentation on theme: "Levels of Selection Gametic Selection: meiotic drive or segregation distortion is selection among gametes. Cell Selection: selection among cell lineages."— Presentation transcript:

1 Levels of Selection Gametic Selection: meiotic drive or segregation distortion is selection among gametes. Cell Selection: selection among cell lineages in tissue formation. Individual Selection: Darwinian selection. *Group Selection*:Selection among groups. –Kin Selection –Interdemic or Inter-population Selection Species Selection: selection among taxonomic lineages.

2 Necessary Properties for Evolution by Natural Selection Phenotypic Variation: There must be differences among the units in phenotype. Fitness Variation: The phenotypic differences must result in differences among the units in birth and death rates. Genetic Variation: The differences in phenotype that cause the differences in fitness must be heritable:

3 Levels of Selection Selection Among Individuals within Groups is Standard Darwinian Selection Selection Among Groups of Individuals is Controversial.

4 I confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. Darwin 1859 Problem: Sterile female workers have a fitness = 0 and cannot propagate their own kind.

5 This difficulty, … as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Darwin 1859 Solution to Problem: Selection applied at a higher level of biological organization: Selection among families or among colonies.

6 (Problem=) Breeders of cattle wish the flesh and fat to be well marbled together: an animal thus characterised has been slaughtered, but (Solution=) the breeder has gone with confidence to the same stock and has succeeded.. Darwin 1859 Typical Darwinian style of argument: Uses example from artificial selection of domesticated plants and animals.

7 Here is a better and real illustration: according to M. Verlot, some varieties of the double annual Stock from having been long and carefully selected to the right degree, always produce a large proportion of (Sterile Offspring = ) seedlings bearing double and quite sterile flowers; but they likewise yield some single [flowers] and fertile plants (= Reproductive Offspring). These latter, by which alone the variety can be propagated. Darwin 1859

8 These latter, by which alone the variety can be propagated, may be compared with the fertile male and female ants, and the double sterile plants with the neuters of the same community. As with the varieties of the stock, so with social insects, selection has been applied to the family, and not to the individual, for the sake of gaining a serviceable end. Darwin 1859

9 Benefactors Recipients Bear fitness cost of altruism and bestow fitness benefit on group members No fitness cost because they are not altruistic, but a fitness benefit, reap fruits of the Benefactors in the group W benefactor < W recipient

10 Group 1: P benefactors = 0.80 Group 2: P benefactors = 0.20 Two kinds of Phenotypic Variation: (1) individuals differ from one another; (2) groups differ from one another in frequency of benefactors and recipients.

11 Individual Selection Within Groups Opposes Benefactors Before: After: Individual Selection Within Group 1: p benefactors = 0.78 – 0.80 = Individual Selection Within Group 2 p benefactors = 0.17 – 0.20 = Average p individual = (- 0.02)(0.6) + (- 0.03)(0.4) = < 0

12 Benefactors 0 1 Directional Individual Selection within groups: Benefactors reduce their own relative fitness and increase fitness of others. Low Fitness High Fitness Fitness of Individual Recipients Behavior Phenotype

13 Group 1: Many BenefactorsGroup 2: Few Benefactors Group Fitness Variation: group growth rate increases with the frequency of benefactors. Before: After:

14 After Replication: p group = {(0.8)*(1.2) + (0.2)*(0.8)}/2 – 0.5 = > 0 Group Selection Favors Benefactors: Groups with more benefactors grow faster than Groups with few benefactors.

15 More Benefactors 0 1 Directional Group Selection among groups: Groups with a high frequency of benefactors Grow faster and to larger size. Low Fitness High Fitness Fitness of Group Few Benefactors Group Behavior Phenotype

16 Directional Group Selection Fitness of Group Group Behavior Phenotype 0 1 Directional Individual Selection Fitness of Individual 0 1 Individual Behavior Phenotype TWO Opposing Levels of Selection

17 Selection Among Groups: p group = {[(0.8)*(1.2) + (0.2)*(0.8)]/2} – 0.5 = > 0 Selection Within Groups: p individual = (- 0.02)(0.6) + (- 0.03)(0.4) = < 0 p total = p individual + p group = = When Group Selection is stronger than Individual Selectionthe Benefactor Type INCREASES. p total = /Total } { After /Total } { Before - Total Selection: Sum of Within and Among Group Selection

18 Opposing Evolutionary Forces Natural Selection versus Deleterious Mutation Reach a balance where every copy of a deleterious allele removed from a population by selection is replaced by mutation. Natural Selection versus Sexual Selection Traits that increase male mate numbers at the expense of male viability spread through a population. Group Selection versus Individual Selection Traits that increase the fitness of the group can spread at the expense of the individual when group selection > individual selection. Traits that increase the fitness of the individual can spread at the expense of the group when individual selection > group selection = Tragedy of the Commons

19 Selection at any higher level than that of an individual is essentially "impotent and is "not an appreciable factor in evolution" (1966:8; cf., Williams 1992). Many, perhaps most, evolutionary biologists believe that it [group selection] is only rarely an important force of evolution. (Futuyma 1998, p. 352) Group vs. Individual Selection Controversy

20 It is sometimes said, Group selection was vanquished in the 1960s by William Hamilton and his genes eye view of selection, which is the basis of Richard Dawkins notion of the selfish gene. The fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity. (Dawkins, 1976)

21 Group vs. Individual Selection PropertyIndividualGroup Phenotypic Variation LargeSmall FitnessVariation LargeSmall Association between phenotype and fitness CommonRare

22 The Biology of Group Phenotypes Artificial Individual and Group Selection on Migration in the flour beetle, Tribolium confusum (Craig 1982)

23 Individuals differ in tendency to emigrate and the differences are genetic Phenotypic variation V migration > 0 And it is heritable V Genetic > 0 Stay at home beetle Traveling beetle

24 Variation in Group Density leads to Variation in Group Migration Low density = every beetle is more likely to stay home High density = every beetle is more likely to be traveling

25 Variation in Group Density leads to Variation in Group Migration Low density = every beetle is more likely to stay home High density = every beetle is more likely to be traveling Group Density Variation V Density > 0 And Density is heritable V Genetic > 0

26 High Density Low High High Density Causes Increased Migration Low Migration High Migration Migration Phenotype of Group Low Density Social Environment

27 0 1 Directional Individual Selection imposed by Dr. Craig: Experimentor creates variation in fitness: V W > 0 and the association between migration and fitness Low Fitness High Fitness Fitness of Individual Stay at home beetleTraveling beetle

28 High Migration = High Density 0 1 Directional Group Selection imposed by Dr. Craig: Group (High) Experimentor creates variation in group fitness: V Group W > 0 and association between Migration and Group Fitness Low Fitness High Fitness Fitness of Group Low Migration = Low Density

29 0 1 Low Fitness High Fitness Fitness of Group High Migration = High Density Low Migration = Low Density Directional Group Selection imposed by Dr. Craig: Group (Low) Experimentor creates variation in group fitness: V Group W > 0 and association between Migration and Group Fitness

30 Stay at home beetle High Fitness Individual SelectionGroup Selection None Control None Ind Low Group Low Low Migration High Fitness None Ind & Group Low Stay at home beetle High Fitness Low Migration High Fitness Ind Low Vs Group High Stay at home beetle High Fitness High Migration High Fitness High Migration High Fitness Group High None

31 Experimental Test for measuring Migration 160 beetles Empty

32 Experimental Test for measuring Migration 160 beetles Empty Pipe Cleaner and string

33 Experimental Test for measuring Migration Less than 160 beetlesMigrators Pipe Cleaner and string

34 Results of 14 Generations of Artificial Selection Treatment Migrators Out of 160 % of Control Only Individual Selection (Low) Only Group Selection (Low) Only Group Selection (High) Ind versus Group (Low vs High) Ind & Group (Low) No Selection Fewer than Control Much Fewer Than Control Much More Than Control Much More Than Control Surprizing!

35 Experimental Test for measuring Offspring Density 30 Adult beetles Wait 40 Days

36 Experimental Test for measuring Offspring Density 30 Adult beetles 40 Days Hundreds of Offspring

37 Results of 14 Generations of Artificial Selection Treatment Offspring Density % of Control Only Individual Selection (Low) Only Group Selection (Low) Only Group Selection (High) I vs G (Low vs High) I and G (Low) No Selection Equal to Control Much Fewer Than Control More Than Control More Than Control Surprizing! 1% ~ 10 offspring


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