Other Mechanisms of Isolation

1. Other Mechanisms of Isolation • Changes in Chromosome Number • Polyploidization can create barriers to gene flow that are not geographic • Very important mechanism of isolation in plants • 43-70% of extant plant species may have arisen by polyploidization • Much less common in animals but occasionally important in barrier creation • Sometimes smaller scale chromosomal changes can cause speciation as well

2. What Can Cause Divergence? Mutation? - maybe, but we know it’s incredibly slow Drift? - SURE, especially when populations are small Inbreeding? - Yes for genotypes, but only causes gene frequency change when coupled with drift or selection Selection? - Of course, especially if environment differs between two populations Migration? - NO WAY! Gene flow causes populations to converge, not diverge. It will negate other differentiating forces (drift and selection). Populations must be isolated from one another to undergo independent evolution.

3. What causes divergence? NOTE: isolation by itself is not enough to cause differentiation, other factors must be acting to cause populations to diverge

4. Mechanisms of Divergence • After a barrier has been formed, genetic drift and natural selection act on mutations to cause divergence in isolated populations • Drift is most important in small populations • Most species are formed from few individuals • Drift may rapidly cause divergence because of bottleneck effects

5. The Role of Drift • This is very difficult to determine because it is tough to tease apart the magnitude of drift’s impact • However, it is definitely a strong force in founder effect speciation and other situations where Ne is small

6. The Role of Selection • Widely held view that vicariant speciation is caused by Natural Selection resulting in pre- & post zygotic incompatibility • This occurs while the taxa are allopatric • Thus, both pre- and post zygotic isolating mechanisms are by-products of divergent selection that occurs during allopatry

7. Categories of Selection • Ecological Selection – side effect of adaptive divergence • Sexual Selection – variation in reproductive success due to variation in the ability to acquire mates

8. Ecological Selection • Almost no evidence but probably is significant • Requires the genes underlying the adaptive differences either have pleiotropic effects resulting in reproductive isolation or be closely linked to those genes responsible for isolation

9. Ecological Selection • Little known about genes that control pre & post zygotic isolation • Sperm lysin genes of marine inverts.: prezygotic isolation results from constant change in female egg proteins which is ‘chased’ by evolution of male lysin proteins • Ground finch (Geospiza) – different beak sizes will court different species with wrong head

10. Evidence for Ecological Selection  “Parallel Speciation” • Independent evolution of the “same” reproductively isolated forms in different localities suggests that ecological selection can result in speciation • Sticklebacks

11. Parallel Speciation Sticklebacks

12. Sexual Selection • Arises from competition among individuals of one sex (usually males) for access to the other sex, or from the preferences of one sex • The males secondary sex characteristics frequently play a role in both contexts • Females choose among conspecific males on the basis of the state of one or more features in the courtship display • Generally the female prefers the male with more elaborate stimuli or traits

13. Sexual Selection – WHY? • Females have a fixed maximum fecundity • Males fecundity essentially based on number of mates only • So, variance in male fitness is always > female fitness

14. Sexual Selection • Why females have preference is controversial • Runaway selection: self reinforcing • Preference is adaptive: Male traits are correlated with fitness • Females prefer traits that will produce more competitive sons • Perceive louder, brighter, etc. against background noise • Females have sensory biases towards certain specific stimuli

15. Sexual Selection • The weight of evidence is that sexual selection is a major source of pre zygotic isolation • Sexually selected characters act as a barrier to interbreeding (e.g. Physalaemus call preferred only intraspecifically) • High species richness of many taxa in which sexual selection appears to be diverse and intense

16. Sexual Selection • Many species that are genetically similar and have diversified rapidly • These show females which look very similar yet males which look VERY different!! • Suggests strong female mate choice powering speciation (e.g. sea lions!) • Study by Barroclaugh – found the clade with sexual dimorphism contains more species

17. Sexual Selection • Hawaiian Drosophila have elaborate courtship behaviors and secondary sexual characteristics • Males meet in leks and fight to attract females • Males have bizarre head shapes to help them win fights

18. Sexual Selection D. heteroneurahave hammerhead and butt heads during fights D. silvestris have heads just like females and grapple during fights Evolutionary scenario • Ancestor of both species had normal heads • Mutation occurred to cause head-butting behavior • Mutants won more contests • Mutation increased to fixation in a sub-population • Additional mutations caused change in head shape for better fighting

20. Peripatric Speciation • The process by which drift effects initial changes in allele frequencies • Then selection acts to further diverge colony from parent population

21. Peripatric Speciation How does this differ from Vicariant Allopatry? • Typically, most allopatric events are thought of as a major split leaving two large populations on either side, whereas a Peripatric division typically results in the nearly intact parent pop. and a small founder population (thus, drift is very powerful in the founder)

22. Peripatric Speciation: Remember... • Genetic change can be very rapid in localized populations founded by few individuals cut off from gene flow • Differences in ecological selection can play a role because the environment of a small area is often more homogenous than the larger area • So... conflicting pressures that act on a widespread species may be less numerous and more directed at certain adaptations

23. Peripatric Speciation: Remember... • Allele frequencies at certain loci will be different from the parent population due to drift and founder effects • Changes in allele freq. could create genotypes that have higher fitness that Natural Selection may select for or against • Many species have arisen by peripatric speciation • eg. Peromysus maniculatus

24. Peripatric Example: Peromysus maniculatus

25. ParapatricSpeciation Evolution of reproductive isolation between populations that are continuously distributed in space, so that there is substantial movement of individuals and genes between them

26. Parapatric Speciation • NOTE: The observation that two sister species have parapatric (or sympatric) ranges does not mean that they have speciated parapatrically or sympatrically!  this may instead be allopatry followed by secondary contact of ranges once distinct

27. Parapatric Speciation • Divergent selection GREATER than gene flow over time • Generally a hybrid zone may be formed • The degree of selection and gene flow depend on the size of the character clines

28. Secondary Contact • If populations come back into contact after steps 1 and 2, have opportunity to interbreed • Hybridization is common in plants and birds • Will hybrids be viable and fertile? • Will hybrids have characteristics of parent species or new characteristics? • Depends on outcome of speciation event

29. Secondary Contact • Theodosius Dobzhansky formulated the reinforcement hypothesis about the third stage of speciation • If populations sufficiently diverged in allopatry, hybrids should have reduced fitness • Should be strong selection for assortive mating • Selection that reduces fitness of hybrids is Reinforcement • Completes reproductive isolation

30. Secondary Contact • Reinforcement Hypothesis • Predicts that pre-mating isolation will evolve in species in secondary contact • Prezygotic isolating mechanisms prevent fertilization • Mate choice, time of breeding, genetic incompatibility • Postzygotic isolation • Offspring are sterile • Reinforcement not necessary

31. Secondary Contact • Study by Coyne and Orr showed that prezygotic isolation occurs more often in sympatric than allopatric species

32. Secondary Contact • Hybridization • Hybrids should have reduced fitness by reinforcement hypothesis • Some have normal or increased fitness • Sorghum is an important crop • Johnsongrass is closely related weed • Agriculturalists worried that if they genetically engineered herbicide resistance into sorghum it might be transferred to johnsongrass

33. Secondary Contact • Hybridization • Significant gene flow occurred and herbicide resistance introgressed into johnson grass • If hybrids have increased fitness, will they form their own species? • Biologists attempted to recreate the hybridization event that lead to the formation of Helianthus anomalous • Found that certain crosses had higher fitness and certain had low • High fitness hybrids became new species

34. Secondary Contact • Hybridization • Hybrid zones occur where recently diverged populations overlap • May occur by secondary contact or during parapatric (or peripatric) speciation • Hybrid zones are often present where hybrids have equal fitness to parental species • Size of hybrid zone depends on fitness of hybrids

35. Secondary Contact • Hybridization • Study on sagebrush in western US • Basin sagebrush found at low elevations • Mountain sagebrush found at high elevations • Species make contact at middle elevations and hybridize • Graham compared several fitness measures of hybrids and parentals • Found that hybrids have superior fitness in transitional habitats

38. SympatricSpeciation • Among the most controversial subjects in evolutionary biology • Complete panmictic mating and a reproductive isolating mechanism evolves within the population

39. Sympatric Speciation – Possible Mechanisms • Polyploidy • Disruptive selection, whereby certain homozygous genotypes have high fitness on one or the other of two resources & intermediate (heterozygotes) has low fitness

40. Sympatric Speciation & Polyploidism

41. Hybridization in Helianthus

42. Sympatric Speciation – Disruptive Selection • Selection may favor alleles in homozygous condition and you get assortive mating • But, antagonism likely to arise from alternative selection which promotes association between alleles for adaptation and alleles for assortive mating, and recombination which destroy this association • This is the greatest opponent to the process of Sympatric Speciation • In such a model strong selection and tight linkage are required... BUT recombination will break this down!

43. Sympatric Speciation – Disruptive Selection • Another similar model was proposed by Bush based on his work on the apple maggot flies • Rhagoletis – economically important pest which are parasites on the fruits of trees in the hawthorn clade • Rhagoletis ID host by sight, tough and smell • Courtship and mating occur on the fruit • Females lay eggs in fruit on the tree • eggs hatch in 2 days and develop after fruits fall to ground • insects burrow into ground, overwinter, and emurge next spring

45. Mechanisms of Divergence • Apple and hawthorn maggot flies • Are they different populations? • Live in sympatry on adjacent trees • Recently diverged because apples are not native to US • A mark-recapture allozyme study revealed they do form distinct populations • How have they diverged without initial isolation? • Did they skip Step 1?

46. Mechanisms of Divergence • Apple and hawthorn maggot flies • There are other mechanisms for speciation besides allopatric • Through assortive mating there is only 6% gene flow among populations • Separated in time by pupating at different times of year • They are able to maintain distinct populations even with gene flow because of strong natural selection

47. Genetics of Differentiation and Isolation • What changes in the genome are necessary for speciation? • Most F1 hybrids are sterile • Postzygotic isolation is pronounced • Which genes are responsible for sterility? • If one sex is sterile, it is usually the heterogametic sex • Human males have Y • Bird females have W

48. Genetics of Differentiation and Isolation • Haldane’s Rule • Pattern of sterility is in heterogametic sex, regardless of which is male or female

49. Genetics of Differentiation and Isolation • Why does Haldane’s Rule work? • Consider an autosomal locus A and an X-linked locus B • Individuals from one species are fixed for A1 and B1 • Sister species fixed for A2 and B2 • A2and B1 interact to cause inviability • If females from first species mate with second: A1A2B1 males and A1A2B1B2females • Males are inviable, females are viable

50. Genetics of Differentiation and Isolation • Quantitative Trait Loci (QTL) Mapping • Most traits involved in reproductive isolation are quantitative • QTL mapping attempts to locate genes with small effects on quantitative traits • Find genetic markers that are unique to each parental species and the value of that trait in hybrids