Macroevolution (Speciation)

1. On the Road to Speciation
   1. What is a Species?
      0. The morphological species concept - what about subtlety?
      1. The biological species concept - what about asexual individuals?
      2. The genetic species concept - what about isolation?
      3. The ecological species concept - what about history?
2. Reproductive Isolation Mechanisms
   1. Reproductive isolating mechanisms are any heritable features of anatomy, physiology, or behavior that prevent interbreeding between populations.
   2. Prezygotic Isolation (mechanisms take effect before or during fertilization)
      0. Temporal Isolation: different groups may not be reproductively mature at the same season, or month, or year (ie- periodical cicadas, coral spawning).
      1. Behavioral Isolation: patterns of courtship may be altered to the extent that sexual union is not achieved (for example: albatross courtship rituals).
      2. Mechanical Isolation: two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding (ie- sage plants and bee pollinators, Great Dane & Chihuahua).
      3. Ecological Isolation: potential mates may be in the same general area by not in the same habitat where they are likely to meet (for example: different species of Manzanita shrubs live at different altitudes and habitats).
      4. Gametic Mortality: incompatibilities between egg and sperm prevent fertilization (for example: chemoreception between coral larvae).
      5. Embryonic (larval) Mortality: gametes combine but the result is an incompetent larvae or embryo. (Corals…)
   3. Postzygotic Isolation (mechanisms take effect after fertilization)
      0. Sometimes fertilization does occur between different species, but the hybrid embryo is weak and dies.
      1. Hybrid sterility - In some instances the hybrids are vigorous but sterile (example: mule produced by a male donkey and a female horse).
      2. F2 breakdown - The hybrid is fertile, but the "grandchildren" are sterile.
3. Speciation in Geographically Isolated Populations

   

   1. Geographic Isolation
      0. Isolation may be sudden as in an earthquake that results in separation of species.
      1. Glaciers, on the other hand, may produce conditions that yield separate species over vast periods of time.
      2. Studies of enzymes from fishes on the Atlantic and Pacific sides of the Isthmus of Panama reveal molecular differences.
   2. Allopatric Speciation
      0. In this model some physical barrier arises and prevents gene flow between populations.
      1. Allopatric refers to the "different lands" the two species occupy.
      2. Reproductive isolating mechanisms evolve in the genetically diverging populations and will result in complete speciation when the two species can no longer interbreed.
   3. Allopatric Speciation
      0. An archipelago is an island chain some distance away from a continent.
      1. The finches of the Galápagos Islands are evidently ancestors of mainland finches that invaded first a few islands and then spread to others, diverging as time progressed.
4. Models for Other Speciation Routes
   1. Sympatric Speciation
      0. In this model, species may form within the home range of an existing species, in the absence of a physical barrier.
      1. In two crater lakes of East Africa exist small fish called cichlids.
         1. The species in each lake are alike in their mitochondrial DNA and unlike the species in neighboring lakes and streams.
         2. The lakes are small so the fish must live in sympatry.
      2. Polyploidy - speciation is instantaneous for plants that are polyploid.
   2. Parapatric Speciation
      0. Daughter species form from a small proportion of individuals along a common border between two populations.
      1. Interbreeding individuals produce hybrid offspring in this region called a hybrid zone.
5. Patterns of Speciation
   1. Branching and Unbranched Evolution
      0. Cladogenesis applies to populations that become isolated from one another and subsequently diverge in different directions.
      1. Anagenesis is a pattern of descent in which species form within a single, unbranched line.
   2. Evolutionary Trees and Rates of Change

      

      0. A dendrogram summarizes information about the continuity of relationship among species.
      1. The gradual model of speciation is represented by tree diagrams with branches at slight angles to each other to show slow change over time.
      2. The punctuation model of speciation is drawn with short, horizontal branches that represent abrupt periods of speciation followed by stable periods.
   3. Adaptive Radiations
      0. An adaptive radiation is a burst of microevolutionary activity that results in the formation of new species in a wide range of habitats.
      1. The presence of adaptive zones presents new ways of life by physical, evolutionary, or ecological access.

      

   4. Extinctions–End of the Line
      0. Background extinction is the rather inevitable loss of species as local conditions change over periods (usually long) of time.
      1. Mass extinctions are abrupt disappearances due to catastrophic, global events.
      2. Inbreeding refers to nonrandom mating among closely related individuals.
      3. It tends to increase the homozygous condition, thus leading to lower fitness and survival rates.
   5. Biodiversity on Oceanic Islands: its Origin and Extinction; American Zoology, by Gustav Paulay
   6. Human migration through time…the Neanderthal puzzle…