Isolating Mechanisms - PowerPoint PPT Presentation

About This Presentation
Title:

Isolating Mechanisms

Description:

Describe the mechanisms which isolate individuals of a population and allow speciation to occur. – PowerPoint PPT presentation

Number of Views:1524
Avg rating:3.0/5.0
Slides: 19
Provided by: jwi141
Category:

less

Transcript and Presenter's Notes

Title: Isolating Mechanisms


1
Isolating Mechanisms
  • Describe the mechanisms which isolate individuals
    of a population and allow speciation to occur.

2
Geographical Barriers
  • Shifts in the continental plates NZ separated
    from the supercontinent Gondwanaland at a very
    early stage before the evolution of mammals.
  • Our birds, reptiles, insects and plants evolved
    without the pressure of grazing, browsing or
    predator mammals.

3
Geographical Barriers
  • As a result they did not evolve protective
    adaptations to counter the attacks of mammals
    later introduced by settlers.
  • The separation from Gondwanaland prevented gene
    exchange between NZ and the vast continent.

4
Geographical Barriers
  • The only native NZ mammal is a bat thought to
    have blown over to NZ from Australia.
  • Barriers include
  • water oceans, rivers changing course.
  • Desserts and canyons.
  • Mountain ranges.

5
(No Transcript)
6
Ecological Barriers
  • Populations may have developed genetic
    differences to cope with different ecological
    niches or habitats.
  • E.g. different temperatures.

7
Reproductive Barriers
  • Under natural conditions, sympatric populations
    of different species are prevented from mating by
    isolating mechanisms. These can be divided into 2
    categories
  • Prezygotic
  • Postzygotic

8
Reproductive Barriers
  • Premating / Prezygotic
  • These take effect before fertilisation.
  • Habitat Differences the 2 species may never
    meet, e.g. one living on the top of a tree and
    the other on the ground. One may be nocturnal and
    the other diurnal.

9
Reproductive Barriers
  • Differences in Breeding Times the species may
    be sexually receptive at different times of the
    year.
  • Mechanical Differences e.g. the genitalia of
    different insect species will not fit together so
    breeding is prevented. Some plants have flowers
    that can only be pollinated by certain insects.

10
Reproductive Barriers
  • Behaviour Patterns there are many examples of
    courtship behaviour which takes place in a
    stepwise ritual, only members of the same species
    have the behaviour patterns to complete the
    ritual.
  • e.g. in birds, the songs and displays will seem
    wrong to a different species.

11
(No Transcript)
12
Postmating Isolation
  • Postzygotic Isolation
  • These isolations prevent successful reproduction
    after fertilisation.
  • Hybrid Inviability a zygote is formed but does
    not develop properly.
  • Hybrid sterility a hybrid forms but it is
    sterile

13
Postmating Isolation
  • Hybrid Breakdown the hybrid offspring are
    fertile but produce many infertile or non-viable
    offspring.

14
Polyploidy
  • This is the abrupt formation of a new species. It
    is an important sympatric speciation as it does
    not involve geographical isolation.
  • In one generation the parent and the offspring
    can belong to a different species.
  • More common in plants.

15
Isolation by Time
  • A species that disappeared one million years ago
    obviously can not interbreed with a species
    living today.

16
(No Transcript)
17
(No Transcript)
18
  • Although individuals living close together within
    each species can interbreed, as distance
    increases, fertility decreases, indicating
    increasing genetic divergence as the habitat and
    climate change. These frogs are now considered to
    be four different species 1) Rana pipiens, 2) R.
    blairi, 3) R. utricularia, and 4) R. berlandieri.
    The mating calls of each species differ, and
    defective embryos occur between certain
    combinations, indicating that both pre-mating and
    post-mating isolating mechanisms exist.
Write a Comment
User Comments (0)
About PowerShow.com