The Origin of Species 2 December, 2005 Text Chapter 24

1
The Origin of Species2 December, 2005Text
Chapter 24
2
(No Transcript)
3
(No Transcript)
4

• Historically, definitions of species have been
  based on appearance.
• Modern definitions focus on reproductive
  isolation.
• A species is a population or group of populations
  whose members have the potential to interbreed to
  produce viable, fertile offspring.
• Different species, then, cannot interbreed under
  natural conditions.
• Breeding involves two individuals meeting at the
  same place and at the same time.
• The behavior of mating individuals must generate
  sexual attraction.
• The genitalia of the individuals must be
  mechanically compatible.
• Gametes must recognize and fuse to produce a
  zygote.
• The zygote must develop into a fertile adult.

5
Each of the steps in the production of viable
fertile offspring can be a potential barrier to
the interbreeding of different species. Habitat
isolation and temporal isolation prevent
individuals from different species from meeting.
(examples parasites on different host species,
periodic cicadas) Behavioral isolation prevents
individuals from attracting a mate from another
species. (examples bird song, courtship
rituals, firefly flash codes)
6
(No Transcript)
7
Mechanical isolation occurs when the reproductive
anatomy of different species is incompatible.
8
Gametic isolation prevents gametes form different
species from fusing to form a zygote, even if
they do happen to meet. (examples aquatic and
terrestrial spawning organisms)
9

• Post zygotic reproductive isolation can arise
  when hybrid zygotes (arising from interspecific
  mating)
• Fail to develop (example Rana spp.).
• Have reduced fertility (example mules).
• Produce a non-viable second generation (hybrid
  breakdown)

10
Summary and review of reproductive barriers.
11

• The classification of species by reproductive
  isolation does not work for all organisms.
• Extinct species are classified on the basis of
  morphology alone.
• Reproductive isolation is not meaningful for
  species that reproduce completely asexually
  prokaryotes, some plants animals, and fungi.
• Speciation in progress is difficult to classify.

12
Investigating the genotype of individuals can
answer questions about phylogeny (relatedness).
PCR
Liquify mite Purify DNA
Mt COI gene
13
Species that do interbreed will (by definition)
exchange DNA. Gene flow will keep even variable
sequences similar within a species. Populations
that do not exchange genetic information will
have divergent sequence, especially for variable
regions.
14
Sequence analysis allows detailed phylogenic
relationships to be determined.
15
(No Transcript)
16
(No Transcript)