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SPECIES AND SPECIATION

SPECIES AND SPECIATION

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SPECIES AND SPECIATION

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  1. 0 SPECIES AND SPECIATION Species Concepts

  2. 0 This idea goes all the way back to Darwin where he used visible “gaps” in morphology to delimit species. “I believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links.”(1859: pg 177) “We shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species…”(1859: pg 282)

  3. Species Concepts 0 • Based on judgments about the similarities among organisms • challenge is to make it mechanistic and testable • want to accurately reflect evolutionary history of organisms. • we don’t really know whether such a thing as a species actually exists in reality

  4. There are many proposed species concepts in the literature of evolution, ecology and conservation biology 0

  5. Three species definitions are most widely accepted 0 • All three assume two things in common 1) no gene flow-species form a boundary for the spread of alleles 2) species have their own evolutionary pathway

  6. The BSC Biological Species Concept 0 • Proposed by Dobzhansky and Mayr, elucidated by Mayr as … • “ Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” • Definition implies: no hybridization or hybrids fail to form fertile offspring lack of gene flow

  7. BSC (cont) 0 PROBLEMS: • Can not always tell if two groups of individuals are reproductively isolated If two groups are separated by geographical barriersthere is no way to know if they are reproductively isolated • Many plantshybridize freely; we will discuss hybridization later in some detail • Cannot test it in fossil forms • Irrelevant to asexual populations

  8. The Phylogenetic Species Concept (PSC) 0 • Also called Evolutionary Species Concept • This is the systematists contribution to the idea of a species • “A species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historicalfate” • Focuses on the idea of monophly (a set of species are all descended from one common ancestor)

  9. PSC (cont) 0 • A monophyletic group contains all of the known descendants of a single common ancestor. There are no parallel branches or interconnecting branches (such as in hybridization) • Fig 16.1 species are on the tips of the trees. Circles represent the monophyletic groups Notice that …(1) Common ancestor does not continue on as a species and (2) every “species” divides to form only two new “sister species” You do not see branching such as this. or this

  10. PSC (cont) 0 • RATIONALE • can only form separate species if the populations have diverged from one another in isolation • The original species will always form two new species and cease to exist itself. • Appeal is that it is testable • Species are identified (named) on the basis of statistically significant differences in the traits used to estimate the phylogeny (ancestry)

  11. PROBLEMS with PSC(cont) 0 • Populations must have been independent long enough for diagnostic traits to emerge • Phylogenies are only available for a handful of groups • Very tiny differences, even a single DNA substitution may be used as a trait that separates groups • Could end up doubling the number of species • Very difficult to interpret when new species actually becomes a new species

  12. Morphospecies 0 • Define species based on the morphological differences. Commonly used with fossils. • This definition does not demand proof of reproductive isolation or phylogenetic relationships • Used when we do not have tests for reproductive isolation or well-estimated phylogenies • Assignment to species is often arbitrary and cannot distinguish cryptic species

  13. Cryptic species 0 • ones which are strongly divergent based on non-morphological characters. • Things such as song, temperature or drought tolerance, habitat use, or courtship displays

  14. Morphospecies (cont) 0 • Today used mostly by paleontologists • For at least some instances there is good evidence that fossil Morphospecies may indicate real species differences

  15. Application of the 3 species definitions to red wolf 0 • In 1930 the Red wolf appeared to be a Morphospecies being intermediate in appearance between the gray wolf and the coyote, all 3 appearing to be distinct. • Studies have shown that the red wolf is actually a hybrid between gray wolves and coyotes. Therefore its intermediate characteristics are the result of hybridization and not independent evolution. This makes the Red wolf not a distinct species for most biologists because… • Neither the BSC or the PSC allow for hybridization • However, it is still considered a separate species and the morphospecies is the only one of these 3 definitions that works.

  16. Speciation 0

  17. Possible conditions for speciationwe will consider 0 • Allopatric model- speciation occurs in populations that have been physically isolated from one another • Sympatric model - Populations can diverge without geographical separation, with low to moderate gene flow between them if… 1. Selectionfor divergence is strong 2. Mate choice is correlated with the factor that is promoting divergence • Parapatric model– Strong selection for divergence causes the gene frequencies to diverge along a gradient • Peripatric model a subset of the allopatric model involving colonization

  18. 0 Parapatric speciation

  19. Allopatric model of speciation 0 Involves 3 steps • Isolation of members of a population from one another • Genetic divergence of the separated populations • Renewed sympatry of the populations with reinforcement of the genetic differences which have arisen

  20. 0 FIRST STAGE OF ALLOPATRIC SPECIATION Physical Isolation

  21. Physical Isolating Mechanisms 0 • Necessary to prevent gene flow which would keep populations homogenized • may occur when small populations become isolated at the periphery of a species’ range. • If selection is strong and gene flow is low divergence could then occur rather rapidly

  22. Ways in which physical isolation may occur 0

  23. Geographic isolation 0 • By dispersal and colonization Dispersal to novel environment such as rafting a portion of a population to an island • By Vicariance events

  24. Dispersal and Colonization 0

  25. Hawaiian island Drosophilids 0 • Involves Founder effect= Peripatric Speciation (Mayr) • small group of individuals cut off from the original population colonizes a new habitat • drift and selection on genes involved in mating and habitat use leads to divergence

  26. Evidence show this is a valid interpretation 0 • closely related species should be found on adjacent islands • some of the phylogenetic branching sequence should follow island formation • using mitochondrial DNA it was shown that four closely related species were found in the expected pattern • Figure 16.7 page 613

  27. 0 Figure 15.7 page 593

  28. Vicariance events 0 • Events which split a species into two or more isolated ranges and prevents gene flow between them (or at least greatly reduces it) • Can be slow processes like rising of a mountain range, long term drying trend etc • or rapid like a lava flow that splits non-flying insect populations

  29. Isthmus of Panama 0 • A land bridge opened as the isthmus closed about 3 million years ago • Found 7 pairs of closely related morphospecies of snapping shrimp. One member of each pair on each side of the land bridge • The pairs from either side of the bridge are shown to be sister species (each other’s closest relative) believed to share the same common ancestors which split to form each pair

  30. 5 0 Also, interestingly, shrimp populations would have been isolated in a staggered fashion as the land bridge gradually formed in stages Species 6 and 7 live in the deepest water and were cut off first 1-5 were in shallower water and diverged later Figure 16.8 pg. 614

  31. 0 SECOND STAGE OF ALLOPATRIC SPECIATION Genetic Divergence

  32. Mechanisms of divergence 0 • Vicariance events and dispersal events only provide conditions for speciation • Usually you also need to have genetic drift and/or selection work on mutations in these isolated populations in order to get genetic divergence. • Sexual selection may also lead to genetic divergence

  33. 0 3rd and Final Stage Of Allopatric Speciation Secondary Contact(return to sympatry)

  34. Examining potential outcomes of secondary contact (return to sympatry) 0

  35. Four possible outcomes After return to sympatry

  36. No Speciation Possible Outcome #1 after secondary contact

  37. Fully fertile hybrids form – no speciation has actually occurred while in allopatry. • Hybrids thrive and interbreed with both parental populations, any divergence is erased.

  38. Reinforcement Possible Outcome #2 after secondary contact

  39. Reinforcement of parental forms as two recently diverged species. • The two groups are considered now to be two species.

  40. Outcome #2 Reinforcement 0 • If populations have sufficiently diverged while in allopatry, their hybrid offspring should have markedly reduced fitness when compared to individuals in both parental populations. • Parental populations will reduce their fitness if they produce hybrid offspring, therefore this should favor assortative mating within each new species. • Selection that reduces the frequency of hybrids is called reinforcement

  41. Reinforcement finalizes the speciation process by completing reproductive isolation 0 • The final stage of speciation, that of establishing reproductive isolation by reinforcement can occur in any number of ways. • These are called pre-zygotic isolation mechanisms

  42. Reinforcement hypothesis predicts any number of possible Pre-zygotic isolation mechanisms which will prevent fertilization from occurring 0 • Temporal isolation – individuals of different species do not mate because they are active at different times of day or seasons of the year • Ecological isolation- Individuals mate in their preferred habitat, and therefore do not meet individuals of other species. • Behavioral isolation- potential mates from incipient species meet but choose members of their own species • Mechanical isolation – copulation is attempted but transfer of sperm does not take place • Gametic incompatibility – sperm transfer takes place but egg is not fertilized

  43. Some facts from experimental and observational research 0 • Plot of genetic similarity versus the degree of interbreeding for various sister species of Drosophila. A value of 0 on the Y axis indicates free interbreeding, 1 indicates no interbreeding. Figure 16.12 pg 625

  44. Reinforcement, (total lack of interbreeding) is not absolutely necessary for populations to remain genetically isolated when brought back into contact 0 • Post-zygotic mechanisms may lead to hybrid offspring which are sterile or infertile

  45. Post-zygotic mechanisms 0 • Zygotic mortality- egg is fertilized but zygote does not develop • Hybrid inviability– Hybrid embryo forms but of reduced viability • Hybrid sterility– hybrid is viable but adult is sterile • Hybrid breakdown– F1 hybrids are viable and fertile but F2 and backcrosses to parents are inviable or sterile

  46. End Day 1 Chapter 16

  47. Hybridization Possible Outcome #3 after secondary contact

  48. 0 Creation of a new speciesthrough hybridization. Formation of a new third speciesfrom the hybrid formed. Hybrid is fertilebut cannot back cross to either parent.

  49. 0 The role of Hybridization • Hybridization is a common occurrence in plants • At least in some cases the outcome of these hybridization events determines the outcome of the speciation event