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Secondary Contact

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?

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Secondary Contact

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  1. 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

  2. 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

  3. 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

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

  5. 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

  6. Secondary Contact • Hybridization • Significant gene flow occurred and herbicide resistance introgressed into johnsongrass • 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

  7. 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

  8. 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

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

  10. 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

  11. Sympatric Speciation & Polyploidism

  12. Hybridization in Helianthus

  13. 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 assortative 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!

  14. 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

  15. 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?

  16. 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

  17. 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

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

  19. 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

  20. 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

  21. Genetics of Differentiation and Isolation • Quantitative Trait Loci (QTL) Mapping • If statistically significant associations are found between traits and markers that have been mapped, implies that at QTL near the marker contributes to that trait • Try to identify “speciation genes” in a diverse array of organisms to understand the genetics behind speciation

  22. CHAPTER 17 Character Evolution:Form and Function

  23. Character Evolution • Why do species vary in temperature tolerance? • Why variance in reproductive rate or reproductive mode? • Why differences in size?

  24. Character Evolution • The next four chapters discuss evolutionary theory and how it relates to: • 17: Form and Function • 18: Species Interactions • 19: Life History Evolution • 20: Behavior

  25. Character Evolution • We need to understand Morphology & Physiology to understand biological diversity • We can then use this understanding to gain insight into the mechanisms by which organisms function • This approach will allow us to develop hypotheses about How and Why traits have evolved

  26. Morphological & Physiological Adaptations Morphology Performance Fitness rm,prp,f rm,p = correlation between morphology and performance rp,f = correlation between performance and fitness rm,p is easy to find rp,f is difficult!

  27. Morphological & Physiological Adaptations • We need to understand the benefits that an adaptation has, along with its costs and constraints •  Trade-offs!

  28. Flight in Birds • Costs  work required to keep a bird aloft increases with “wing-loading” (=weight of the body relative to wing area) • Large birds have low wing loading by having long wings that are broad at the tip

  29. Flight in Birds • Terrific innovation!!! • Primary flight feathers separated so each feather acts as an individual pointed wing (reducing drag and adding loft)

  30. Flight in Birds

  31. Desert Animals & Heat • Counter current heat exchangers • Panting cools blood in nasal area (in capillaries) • This cooled blood then runs into the arteries just before the brain and cool blood cools the brain • This keeps the brain from overheating (denaturing proteins necessary for fxn)

  32. Desert Animals & Heat

  33. Body Size • Spans 1021 Magnitude as measured by mass!!! • Small organisms take less time to grow to maturity (generation time is shorter) • Therefore, fitness is greater, all else being equal... • Small organisms require less food than large ones and can inhabit smaller microhabitats

  34. Body Size • Large plants set more seeds • Large animals carry more eggs • Large males can often win more contests with smaller males • Thus, Large size also has reproductive advantages that may outweigh the reproductive advantages of rapid development and fewer necessary resources...

  35. Body Size • Although they require more food, Large animals can often overpower, handle, or swallow a greater range of potential prey or food items • Predators select for small or large size in prey species depending on predators size and mode of feeding

  36. Allometry and Isometry • y changes as a function of x • Allometric equation  y = bxa • (where y= one char, x=another, a=coeff. of allometry, and b=constant proportion relating y and x) • if “a” = 1 then b = y/x which means that y changes in direct proportion to x • a<1  y increases less rapidly than x • a>1  y increases more rapidly than x

  37. Allometry and Isometry • Many times it is easiest to express this equation like this: • log y = log b + a log x • This gives a straight line with slope = a and an intercept = log y • Most morphological evolution can be described in terms of Allometric relationships. • Allometric relationships with body mass are often the consequence of adaptation

  38. Structures that support an organism must change disproportionately in shape as weight increases. • Tree trunk mass to cross sectional are is 3/2 power of height

  39. Evolution of Tolerance • In animals, a series of responses occur sequentially in response to stress • Lets examine these steps...

  40. Allometry and Isometry • Example: • In cold environments, large size is advantageous in birds and mammals because they lose heat more slowly, Thus requiring less food to maintain constant body temp. • Bergmann’s Rule “birds and mammals larger in colder climates than same/related species in warmer climates

  41. Evolution of Tolerance • Changes in behavior • Hormone-modulated biochemical and physiological functions • Slower, longer lasting changes in physiology (“acclimation”) • In some instances, developmental changes in morphology • At population level, genetic changes due to differences among genotypes in survival and reproduction rates caused by the stress

  42. Evolution of Tolerance • If the responses of individual organisms cannot fully compensate for the stress, fitness is reduced • This may lead to genetic changes • Some changes entail developmental responses and these are reversible • e.g., Seasonal Responses

  43. What Limits Geographical Ranges of Species? • Some ranges are set by biotic factors, interspecific competition & predation, or by abiotic factors such as temperature and water availability • This question is thus complex and difficult to answer • The simplest hypothesis is the lack of genetic variation for tolerance of physiological stress • However, in general this is not likely...

  44. What Limits Geographical Ranges of Species? • Successful colonization of sites may require numerous coincident adaptive changes • This suite of adaptations may be an improbable concatenation of genetic variants for many characteristics • e.g. Seasonal timing of reproduction & Growth...

  45. What Limits Geographical Ranges of Species? • Trade-offs exist between adaptation to conditions within and beyond the margin of the range • Trade-offs limit adaptation to a new environment due to gene flow from old  new (center of range  periphery)

  46. What Limits Geographical Ranges of Species? • The explanation put fourth by Mayr • Gene flow from the main range of a species into the marginal populations prevents them from further adapting by breaking down adaptive combinations of interacting genes • So, a marginal population may be better if able to adapt & expand range if it could not exchange genes with interior populations • Perhaps species have evolved broader ranges then we give them credit  because the adapted extralimital population we call different species

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