speciation n.
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  1. Speciation Chapter 25

  2. Speciation • Speciation: the creation of new species. • May occur in a population due to: • reduced gene flow • Mutation • genetic drift • natural selection • Distinct species usually derive from one ancestral group

  3. Speciation • Species - evolutionarily independent population or group of populations • Evolutionary independence starts with lack of gene flow. • Once gene flow stops, mutation, selection, and drift can act on populations and genetic divergence can occur • Genetic divergence, in turn, may lead to speciation

  4. When is a population evolutionarily independent?

  5. Biological Species Concept • Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring • Speciation is due to reproductive isolation • They cannot breed whether they are in the same area or not, biological incompatibility • Two types of reproductive isolation, prezygotic and postzygotic

  6. Reproductive Isolation • Reproductive isolation is the existence of biological factors that impede members of two species from producing viable, fertile hybrids • Prezygotic barriers • Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate • Postzygotic barriers • Often prevent the hybrid zygote from developing into a viable, fertile adult

  7. Prezygotic barriers impede mating or hinder fertilization if mating does occur Behavioral isolation Habitat isolation Temporal isolation Mechanical isolation Individualsof differentspecies Matingattempt HABITAT ISOLATION MECHANICAL ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION (g) (b) (d) (e) (f) (a) (c) Prezygotic Barriers

  8. Gameticisolation Reducehybridfertility Reducehybridviability Hybridbreakdown Viablefertileoffspring Fertilization REDUCED HYBRID VIABILITY GAMETIC ISOLATION HYBRID BREAKDOWN REDUCED HYBRID FERTILITY (k) (j) (m) (l) (i) (h) Postzygotic Barriers

  9. Limitations to the Biological Species Concept • The biological species concept cannot be applied to • Asexual organisms • Fossils • Organisms about which little is known regarding their reproduction

  10. Other Definitions of Species • The morphological species concept • Characterizes a species in terms of its body shape, size, and other structural features • The paleontological species concept • Focuses on morphologically discrete species known only from the fossil record • The ecological species concept • Views a species in terms of its ecological niche • The phylogenetic species concept • Defines a species as a set of organisms with a unique genetic history

  11. Morphospecies Concept • Morphological species concept can be applied easily but is rather subjective

  12. Phylogenetic Species Concept • an ancestral population plus all its descendants is called a monophyletic group • A phylogenetic species is the smallest monophyletic group on a tree • By definition, it is isolated from gene flow with other groups • Precise and is applicable to all populations

  13. Phylogenetic Species Concept

  14. Species Definitions in Action: The Case of the DuskySeaside Sparrow

  15. Seaside Sparrows • Seaside sparrow subspecies were believed to be genetically isolated because: • populations are geographically isolated • young birds breed near their hatching ground • Biologists compared gene sequences • found that seaside sparrows belong to two monophyletic groups • Atlantic Coast and, Gulf Coast

  16. Speciation

  17. Speciation • Much speciation takes place due to geographic separation, but some speciation can occur without separation • Speciation can occur in two ways • Allopatric speciation- when speciation occurs due to geographic isolation • Sympatric speciation- when speciation occurs in a small population without geographic isolation

  18. Allopatric Speciation • Physical separation occurs in two ways: • A group colonizes a new habitat (dispersal) • New physical barrier divides a population (vicariance). • Once geographic separation has occurred • One or both populations may undergo evolutionary change during the period of separation

  19. A. harrisi A. leucurus Allopatric Speciation • Example: The antelope squirrel • Ammospermophilus harrisi on the south rim • Ammospermophilus leucurus on the north rim

  20. Causes of Allopatric Speciation • Cause speciation because the physical separation reduces gene flow • Colonists are likely to experience genetic drift (founder effect) • If the new habitat differs, natural selection may also cause divergence • When an existing population is split by a new physical barrier, it is called vicariance

  21. Vicariance • Last ice age caused vicariance as a result of glaciation • Continental drift separated species physically

  22. Sympatric Speciation • Takes place in geographically overlapping populations • Often this happens when populations become isolated by habitat preference • Can be due to chromosomal changes and non-random mating that reduces gene flow

  23. Habitat Differentiation and Sexual Selection • Sympatric speciation can also result from different niches or sexual changes • Temporal separation in mating • Different pollen production times • Sexual preference or morphology • Prettiest peacock • Mating strategy- • Mating calls in frogs

  24. Polyploidy • Polyploidy • Is the presence of extra sets of chromosomes in cells due to accidents during cell division • Has caused the evolution of some plant species • Gene flow is reduced between polyploid and wild-type plants • Two types: autotetraploid and allotetraploid

  25. Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue. Offspring with tetraploid karyotypes may be viable and fertile—a new biological species. Gametes produced by flowers on this branch will be diploid. 2n 2n = 6 4n 4n = 12 Polyploidy • An autopolyploid is an individual that has more than two chromosome sets, all derived from a single species

  26. Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Viable fertile hybrid (allopolyploid) Hybrid with 7 chromosomes Meiotic error; chromosome number not reduced from 2n to n Species A 2n = 4 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species B 2n = 6 Polyploidy • An allopolyploid is a species with multiple sets of chromosomes derived from different species

  27. Polyploid Speciation • Creates species because a mating between a normal diploid (2n) individual and a tetraploid (4n) individual results in an infertile triploid (3n) zygote. • Tetraploid and diploid individuals rarely mate and produce fertile offspring, so these populations are reproductively isolated

  28. Polyploid Speciation

  29. Hybrid Infertility • Many hybrid offspring are sterile because their chromosomes do not pair normally during meiosis • Including triploid • If mutation doubles the chromosome number, then homologs synapse normally • Gametes may be able to self-fertilize to produce a tetraploid offspring

  30. Hybrid Zones • Sometimes if two species that are geographically isolated are able and willing to breed there is a hybrid zone between them • A geographic area where interbreeding is common and there are lots of hybrid offspring is called a hybrid zone • Sometimes the hybrid itself is a new species