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Speciation. Proposing Mechanisms for Species Formation and Identifying Ways to Maintain Reproductive Isolation. Two Types of Evolution . Microevolution . Change within a population or species . Change to a new species . Macroevolution . Species Formation.

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Proposing Mechanisms for Species Formation and Identifying Ways to Maintain Reproductive Isolation

two types of evolution
Two Types of Evolution


Change within a population or species

Change to a new species


species formation
Species Formation
  • Species = Group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups
  • Speciation depends on
    • isolation (lack of gene flow)
    • genetic divergence
mechanisms for speciation
Mechanisms for Speciation
  • Allopatric Speciation
    • Occurs as a result of geographical isolation
    • Most common mechanism
  • Sympatric Speciation
    • Occurs in the same location
    • Can be due to Ecological isolation
    • Can be due to Polyploidy
      • Occurs for plants that have a sudden change in numbers of chromosome sets
allopatric speciation
Allopatric Speciation

Single species(white mice);homogeneous habitat


Geographical barrier(impassable river);isolated populations




Genetic drift;genetic divergence;tan vs. white mice

Barrier removed(river dries up);Mice mix but don’t interbreed.

summary of allopatric speciation
Summary of Allopatric Speciation
  • One group separates from the population.
  • Separate evolutionary pressures cause different genetic changes in both groups.

(Is this (1) microevolution or (2) macroevolution?)

  • Sufficient genetic changes accumulate so that interbreeding cannot occur if groups are rejoined.

(Is this (1) microevolution or (2) macroevolution?)

genetic divergence after geographical isolation
Genetic Divergence After Geographical Isolation
  • Four races of mole rats show different numbers of chromosomes
  • Races live in areas with differing moisture content and soil type
  • Hybrids rarely form
adaptive radiation
Adaptive Radiation
  • One species gives rise to many new species in a short period of time
    • populations invade a variety of habitats and evolve in response to differing environmental pressures in those habitats
    • results from
      • availability of unoccupied habitats
      • lack of competition
sympatric speciation
Sympatric Speciation

Single species(white mice);homogeneous habitat


Climate change;two habitats;isolated because don’t mix




Environmental pressure to adapt;genetic divergence;tan vs. white mice

Sufficient divergence;now different species

speciation by polyploidy

Diploid with chromosome set A andchromosome set B.

Chromosomes duplicatebut do not separateTetraploid with two sets of A and B.

Cross between diploid and tetraploid species Triploid with one each of

chromosome sets A, B and D.

Chromosomes duplicate but do not separate 

Hexaploid with three sets of A, B and D.

Speciation by Polyploidy

Modern Wheat

fertility of polyploids





Fertility of Polyploids







Viable TetraploidZygote

Viable TriploidZygote



Meiosis yields unbalanced productsno viable zygotes.


applying your knowledge
Applying Your Knowledge
  • Sympatric Speciation
  • Speciation by Polyploidy
  • Allopatric Speciation
  • Which process involves a sudden, large change in chromosome number?
  • Which process requires geographical separation?
  • Which process can occur as a result of small differences within the same local environment?
maintaining reproductive isolation
Maintaining Reproductive Isolation
  • Premating Isolating Mechanisms
    • Prevent mating between individuals of different species
  • Postmating Isolating Mechanisms
    • Interfere with development, viability or fertility of offspring from matings between species
premating isolating mechanism geographical isolation
Premating Isolating Mechanism:Geographical Isolation
  • The Grand Canyon is a geographical barrier that can promote speciation.
  • The Kaibab squirrel (left) lives on the North Rim while the Avert squirrel (right) lives on the South Rim.
premating isolating mechanism ecological isolation16
Premating Isolating Mechanism:Ecological Isolation

Each of the 750 species of fig wasps pollinates a different species of fig plant.

premating isolating mechanism temporal isolation19
Premating Isolating Mechanism:Temporal Isolation

Periodic Cicada: Males emergeon a 13 or 17-year cycle.

premating isolating mechanism behavioral isolation
Premating Isolating Mechanism: Behavioral Isolation

Fruit fly species have different wing beat frequencies for the mating ritual.

Firefly signaling patterns differ between species

premating isolating mechanism behavioral isolation21
Premating Isolating Mechanism: Behavioral Isolation

White-crowned sparrows from different areas

have distinct song patterns for territorial displays.

postmating isolating mechanism hybrid inviability
Postmating Isolating Mechanism: Hybrid Inviability

Hybrids between Sheep and Goats do not survive

postmating isolating mechanism hybrid breakdown
Postmating Isolating Mechanism: Hybrid Breakdown



Hybrids of Lions and Tigers have been obtainedthrough zoo breeding programs. While the first generation may be healthy, hybrids become weak or sterile in future generations.

applying your knowledge27
Applying Your Knowledge
  • Premating Isolating Mechanism
  • Postmating Isolating Mechanism
  • Hybrids of dogs and wolves can occur. Some have more dog traits and others favor wolves. After two or more generations, the hybrids are feeble and cannot reproduce.
  • The male fruit fly flaps his wing to attract a nearby female. Wing beat frequencies differ between species so a female will only accept the male that has the correct wing beat.
applying your knowledge28
Applying Your Knowledge
  • Temporal Isolation
  • Ecological Isolation
  • Behavioral Isolation
  • Geographical Isolation
  • Mechanical Isolation
  • English Oak is found in Europe and Scrub Oak is found in California.
  • Lions live in groups called prides on open grassland. Tigers are solitary and live in forests. These species do not form hybrids even though their ranges overlap.
  • The male Reggiana bird of paradise displays his feathers to a potential mate. The female trumpet manucode, a related species, is not attracted by this display.
  • Death of all members of a species
  • Factors that contribute to extinction
    • localized distribution of a species
    • overspecialization
    • competition
    • habitat change and destruction
localized species distribution
Localized Species Distribution

The Devil’s Hole Pupfish lives in a single pool at the mouth of a water-filled

cave system in Nevada.


The Everglades Kite depends on a single species of snail as a food source.