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  1. BIOL102

  2. BIOL102 Origin of Species Part 1 – A few reminders from lecture 2 • Modern Synthesis of Genetics and Evolution • Hardy-Weinberg Principle • Factors Changing Allele Frequencies Source of cover picture: Reece et al. (2010) , Campbell Biology, 9th edition, Pearson Benjamin Cummings, San Francisco (CA), Figure 24.4c

  3. BIOL102 Origin of Species Part 2 – Species Concepts • Species • Biological Species Concept • Morphological Species Concept • Ecological Species Concept • Phylogenetic Species Concept

  4. BIOL102 Origin of Species Part 3 – Speciation • Allopatric Speciation • Sympatric Speciation • Rates of Speciation • Dynamics

  5. • A population is the smallest biological unit that can evolve and is defined as a group of individuals of the same species that live, interbreed and produce fertile offspring in a particular geographic area • A gene pool consists of all alleles (forms of genes) for all loci in a population and is the source of genetic variation that produces the phenotypes and their traits on which natural selection acts • A population evolves when individuals with different genotypes survive or reproduce at different rates Part 1 – A few reminders from lecture 2 Modern Synthesis of Genetics and Evolution

  6. Part 1 – A few reminders from lecture 2 Hardy-Weinberg Principle • • states that frequencies of alleles and genotypes in a • population remain constant from generation to generation • if certain conditions are met (Hardy-Weinberg equilibrium) •  no mutations •  random mating •  no natural selection •  extremely large population size (no effect of • genetic drift) •  no gene flow (migration into or out of a population)

  7. • Hardy-Weinberg equilibrium is a null hypothesis, which assumes that allele frequencies are not changed • However, there are at least four mechanisms of evolution, which cause changes in allele frequencies of populations:  mutations gene flow genetic drift natural selection Part 1 – A few reminders from lecture 2 Factors Changing Allele Frequencies

  8. • is defined as an evolutionarily independent population or group of populations • Biologists commonly use the following four approaches to identify species:  the biological species concept  the morphological species concept  the ecological species concept  the phylogenetic species concept Part 2 – Species Concepts A. Species

  9. Species

  10. • defines a species as a population or group of populations whose members have the potential to interbreed and produce fertile offspring • considers populations to be evolutionarily independent if they are reproductively isolated from each other and no gene flow occurs between them Part 2 – Species Concepts B. Biological Species Concept

  11. • Biologists categorize the mechanisms that stop gene flow between populations into prezygotic barriers (before fertilization) and postzygotic barriers (after fertilization)  prezygotic barriers: individuals of different species are prevented from mating postzygotic barriers: individuals from different populations do mate, but the hybrid offspring produced have low fitness and do not survive or produce offspring Biological Species Concept Prezygotic and Postzygotic Barriers

  12. Prezygotic and Postzygotic Barriers Individuals of different species Prezygotic barriers Mating attempt Fertilization (zygote forms) Postzygotic barriers Viable, fertile offspring

  13. • block fertilization from occurring by:  impeding different species from attempting to mate  preventing the successful completion of mating  hindering fertilization if mating is successful Biological Species Concept Prezygotic Barriers

  14. • Habitat isolation: two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers • Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes • Behavioral isolation: courtship rituals and other behaviors unique to a species are effective barriers Prezygotic Barriers

  15. • Floral traits of plants can influence the behavior of pollinators, and thus whether plants can hybridize  two species of columbines (Aquilegia) in California can produce fertile hybrids, but flower structure determines that one species is pollinated by hummingbirds, the other by hawkmoths, so hybridization is rare Behavioral Isolation

  16. Behavioral Isolation

  17. • Mechanical isolation: morphological differences (e. g., size and shape of reproductive organs) can prevent successful mating • Gametic isolation: sperm of one species may not be able to fertilize eggs of another species Prezygotic Barriers

  18. • In plants, mechanical isolation may involve pollinators  many orchid flowers look and smell like the females of particular pollinator species  male insects attempt to mate, thereby transferring pollen Mechanical Isolation

  19. Prezygotic Barriers Prezygotic barriers Gametic Isolation Mechanical Isolation Habitat Isolation Temporal Isolation Behavioral Isolation Individuals of different species MATING ATTEMPT FERTILIZATION (g) (c) (a) (e) (f) (d) (b)

  20. • prevent the hybrid zygote from developing into a viable, fertile adult due to:  reduced hybrid viability  reduced hybrid fertility  hybrid breakdown • Hybrids are the offspring of crosses between different species Biological Species Concept Postzygotic Barriers

  21. • Reduced hybrid viability: genes of the different parent species may interact and impair the hybrid’s development • Reduced hybrid fertility: even if hybrids are vigorous, they may be sterile • Hybrid breakdown: some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile Postzygotic Barriers

  22. Reduced Hybrid Fertility

  23. Postzygotic barriers Postzygotic Barriers Hybrid Breakdown Reduced Hybrid Fertility Reduced Hybrid Viability VIABLE, FERTILE OFFSPRING FERTILIZATION (i) (h) (l) (j) (k)

  24. • If two formerly isolated populations are reunited before complete reproductive isolation has developed, interbreeding can occur with three possible outcomes:  if hybrid offspring are as fit as those resulting from matings within each population, hybrids will mate with individuals of both parental species. The gene pools will gradually become completely mixed (no speciation) Biological Species Concept Hybrid Zones

  25.  if hybrid offspring are less fit, reinforcement may result in more prezygotic barriers and complete reproductive isolation may evolve (speciation)  ahybrid zone may develop in the absence of reinforcement, or before reinforcement is complete, and may contain recombinant individuals resulting from many generations of hybridization Hybrid Zones

  26. • Example: two species of European toads have a long narrow hybrid zone  the toad hybrids have many defects, some of which are lethal  on average, a hybrid toad is significantly less fit as a purebred individual  the hybrid zone is narrow, because there is strong selection against hybrids. But it persists because individuals of both species continue to move into it and mate Hybrid Zones

  27. Hybrid Zones

  28. Biological Species Concept Limitations • • The criterion of reproductive isolation cannot be evaluated • in fossils or in species that reproduce asexually •  for example, prokaryotic and viral species must be • defined differently • • this concept can only be applied to populations that • overlap geographically • • it also emphasizes absence of gene flow, which can occur • between distinct species •  for example, grizzly bears and polar bears can mate • to produce “grolar bears”

  29. Grizzly bear (U. arctos) Limitations of the Biological Species Concept Polar bear (U. maritimus) Hybrid “grolar bear”

  30. • defines a species by differences in morphological or structural features  is based on the idea that distinguishing features are most likely to arise if populations are independent and isolated from gene flow  applies to sexual and asexual species but relies on subjective criteria  also cannot identify cryptic species that differ in non-morphological traits Part 2 – Species Concepts C. Morphological Species Concept

  31. • views a species in terms of its ecological niche  applies to sexual and asexual species and emphasizes the role of disruptive selection  is widely used for viral species (in addition to genetic homologies) Part 2 – Species Concepts D. Ecological Species Concept

  32. • defines a species as the smallest group of individuals on a phylogenetic tree (monophyletic group)  applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species  on phylogenetic trees, an ancestral population plus all of its descendants is called a monophyletic group or clade, which is identified by synapomorphies, homologous traits inherited from a common ancestor that are unique to certain populations or lineages Part 2 – Species Concepts E. Phylogenetic Species Concept

  33. Phylogenetic Species Concept

  34. • This concept can be applied to any population, but there are disadvantages:  phylogenies are currently available for only a tiny (though growing) subset of populations on the tree of life  would probably lead to recognition of many more species than either of the other species concepts Phylogenetic Species Concept

  35. • A key event in the potential origin of a species occurs when a population is somehow severed from other populations of the parent species. With its gene pool isolated, the splinter population can follow its own evolutionary course and become reproductively incompatible • Two modes leading to reproductive barriers can be distinguished  allopatric speciation sympatric speciation Part 3 – Speciation

  36. Part 3 – Speciation • Allopatric speciation occurs when geographic isolationcreates a reproductive barrier (extrinsic mechanisms) • Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms)

  37. • Genetic isolation happens routinely when populations become physically separated. Physical isolation, in turn, occurs in one of two ways: dispersal or vicariance.  dispersal occurs when a population moves to a new habitat, colonizes it, and forms a new population  vicariance occurs when a physical barrier splits a widespread population into subgroups that are physically isolated from each other • Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation Part 3 – Speciation A. Allopatric Speciation

  38. Allopatric Speciation by Dispersal or Vicariance

  39. Allopatric Speciation • Geographic separation prevents species from mating • Speciation occurs only with the evolution of reproductive barriers between the isolated population and its parent population

  40. Allopatric Speciation • • The definition of barrier depends on the ability of a • population to disperse •  for example, a canyon may create a barrier for • small rodents, but not birds, coyotes, or pollen • • Separate populations may evolve independently • through mutation, natural selection, and genetic drift •  for example, speciation of snapping shrimp (Alpheus) • populations due to separation by the Isthmus of • Panama

  41. Physical Isolation and Reproductive Barriers A. harrisii A. leucurus

  42. Physical Isolation and Reproductive Barriers

  43. Physical Isolation and Reproductive Barriers

  44. Allopatric Speciation • Regions with many geographic barriers typically have more species than do regions with fewer barriers • Reproductive isolation between populations generally increases as the distance between them increases  however, barriers to reproduction are intrinsic; separation itself is not a biological barrier

  45. Allopatric Populations and Reproductive Isolation EXPERIMENT Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on maltose medium Some flies raised on starch medium Mating experiments after 40 generations

  46. Allopatric Populations and Reproductive Isolation RESULTS Female Female Starch population 2 Starch population 1 Starch Maltose Starch population 1 Starch 9 18 15 22 Male Male Starch population 2 Maltose 12 15 20 8 Number of matings in experimental group Number of matings in control group

  47. Part 3 – Speciation B. Sympatric Speciation • • In sympatric speciation, speciation takes place in • geographically overlapping populations • • can occur if a genetic change produces a reproductive • barrier between mutants and the parent population • • may be the result of: •  polyploidy •  extreme habitat differentiation •  sexual selection

  48. Sympatric Speciation

  49. Sympatric Speciation Polyploidy • is the presence of extra sets of chromosomes due to accidents during cell division  an autopolyploid is an individual with more than two chromosome sets, derived from one species  an allopolyploid is a species with multiple sets of chromosomes derived from different species • is much more common in plants than in animals  many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

  50. Autopolyploidy