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Chapter 24

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  1. Chapter 24 Species & Speciation

  2. Chapter 24 in a nutshell • Microevolution v. Macroevolution • Speciation • Prezygotic • Postzygotic • Gradualism v. Punctuated equilibrium

  3. Species • Morphological species concept • Divisions of organisms into discrete units called species • If two organisms look different enough, then they are considered different species • Biological species concept • Population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring • This population is unable to produce viable, fertile offspring with members of another population

  4. Micro- vs. Macro- Evolutions • Microevolution • Change in the genetic makeup of a population over generations • Changes (adaptations) confined to a particular gene pool or population • Macroevolution • Changes (evolution) above the species level • Changes used to define higher taxa

  5. Speciation • The process that produces a new species • May be the result of microevolution OR macroevolution • How could microevolution lead to speciation? • How could a new species arise from macroevolution?

  6. Reproductive Isolation • Biological barriers that impede members of 2 species from producing viable, fertile offspring • There are 2 types of barriers that lead to reproductive isolation • Prezygotic • Pre-mating • Post-mating • Postzygotic

  7. Prezygotic Barriers • Pre-Mating • Habitat isolation • Temporal isolation • Behavioral isolation • Post-Mating • Mechanical isolation • Gametic isolation

  8. Prezygotic Barriers (Page 1) • Pre-Mating • Habitat isolation • 2 species occupy different habitats in the geographic area • Tree-dwelling species, Bush-dwelling species • Temporal isolation • Temporal = time • Species breed at different times • Different time of day, different seasons, or different years • Behavioral isolation • Certain signals or types of behavior are integral to courtship, and these signals/behaviors are unique to the species • Other species do not respond to these signals or behaviors

  9. Boobies!! (blue-footed) Different mating season – late winter or late summer Different habitats – water or terrestrial

  10. Prezygotic Barriers (Post-Mating) • Mechanical isolation • Anatomical incompatibility • Insect copulatory organs are elaborated for a particular species • Gametic isolation • Even if gametes meet, they are unable to fuse • Sperm may not be able to survive in the reproductive tract of females from another species • Purple + Red sea urchins cannot mate b/c gametes cannot fuse

  11. Postzygotic Barriers • Reduced hybrid vitality • Reduced hybrid fertility • Hybrid Breakdown

  12. Postzygotic Barriers • Reduced Hybrid vitality • Although zygote forms, genetic incompatibility causes developmental cessation • In some salamanders, hybrids form but do not complete development • Reduced Hybrid fertility • Sterile hybrids • Donkey + Horse = mule (Sterile) • Hybrid breakdown • Although first-generation hybrids survive & reproduce, their offspring are feeble or sterile

  13.  No Geographic Barrier  Geographic Barrier

  14. Allopatric Speciation • Causes • Geologic events or processes that fragment the population • Emergence of a mountain range • Formation of a land bridge • Evaporation of a large lake into multiple smaller lakes • Leads to a significant alteration of a gene pool

  15. Sympatric Speciation • Sympatric = sym (same) + patr (fatherland) • Same country • Geographically overlapping populations • Major mechanism of sympatric speciation is Polyploidy • Autopolyploidy • Allopolyploidy • Preferential Habitation

  16. Polyploidy • Extra set(s) of chromosomes • Autopolyploidy • > 2n • Typically, nondisjunction (Meiosis) in Plants • All chromosomes from the same species • Example • Tetraploid (4n) + diploid (2n) • Produce triploid (3n) offspring which are sterile • But plant tetraploids can self fertilize or mate with other tetraploids = viable, fertile polyploid offspring • Tetraploids are reproductively isolated from the diploid population

  17. Polyploidy (Page 2) • Allopolyploidy • Plants • 2 different species interbreed = hybrid • Hybrids cannot breed with either original species • But hybrids can successfully mate with each other • Self-pollination successfully occurs as well

  18. Polyploidy (Page 3) • Can happen in animals but not common • In animals, sympatric speciation is usually due to part of the population switching to a new habitat, food source, or other resource

  19. Adaptive Radiation • Many new species arise from a common ancestor given new environmental opportunities or challenges • Think Darwin’s finches & Galapagos islands • As the finches moved from South America to the Galapagos, different environments and food sources were encountered • Hawaiian Plants • As the plants spread to Hawaiian islands, different environments on different islands = evolution into several distinct forms

  20. Hawaiian Plants that had a common ancestor  Arose from Adaptive Radiation

  21. Gradualism vs. Punctuated Equilibrium • Gradualism – species descended from a common ancestor and gradually diverge in morphology (acquiring unique adaptations) until they are reproductively isolated • Punctuated Equilibrium – long periods of apparent stasis punctuated by brief periods of sudden and rapid change • Fossil record confirms the punctuated equilibrium model • Also eliminates the necessity of finding “missing links”