Download Speciation.2004

The Process of Speciation
• Every individual alive today, the highest as well as
the lowest, is derived in an unbroken line from the
first and lowest forms
August Frederick Lopold Weismann
(1834-1914)
• From the remotest past which Science can fathom,
up to the novelties of yesterday, that in which
Progress essentially consists, is the transformation
of the homogeneous to the heterogeneous
Hebert Spencer (1820-1903)
The Process of Speciation
We wish to ask:
• What is biological evolution?
• How are the theories of microevolution and
macroevolution related?
• What is a species, and what are the different ways is
can be defined?
• How is reproductive isolation important to speciation,
and what forms can it take?
• Why should natural selection reinforce reproductive
isolation?
• Can species be formed in ways other than geographic
isolation?
Biological Evolution Defined
• Changes in the genetic composition of a
population with the passage of each generation
– Micro-evolution
– Small, gradual changes
• The gradual change of living things from one form
into another over the course of time, the origin of
species and lineages by descent of living forms
from ancestral forms, and the generation of
diversity
• Macro-evolution and taxonomic hierarchy
• May not be so gradual
Species formation
• To ask, “how are species formed”, it helps
first to ask, “What is a species?”
• Two main definitions
– Morphological species
– Biological species
Morphological Species Concept
• “members of a species are individuals that look
similar to one another”
-based on appearance and professional
judgment
-good enough for Linneaus
• Criticisms
-arbitrary
-may fail to discriminate (e.g. mimicry
complexes)
Biological Species Concept
• “a species is a group of actually or potentially
interbreeding individuals who are reproductively
isolated from other such groups”
-critical test of ‘species-hood’ –ability to
produce successful, fertile offspring
-species is evolutionary unit
• Criticisms
-useless with fossils, museum spec
-can not use with spatially disjunct populations
- hybridization
How does Reproductive Isolation
Arise?
• First, what are reproductive barriers ?
• Pre-zygotic isolating mechanisms
- ecological
- temporal
- behavioral
- mechanical
• Post-zygotic isolating mechanisms
- hybrid invariability
- hybrid sterility
- hybrid less fit
Courtship
displays are a
behavioral, prezygotic isolating
mechanism
By adapting to breed at different times (temporal
isolation), these four frog species achieve at least
partial pre-zygotic isolation.
Post-zygotic reproductive isolation
A sheep-goat chimera produced by injecting a goat inner cell
mass into a sheep blastocyst. The chimera was carried to term
in a sheep recipient female. Dept Animal Science, UC-Davis
Speciation by Geographic
Isolation
• Intact population becomes separated by
geographic barrier (note link to earth processes)
• Different geographic regions have different
environments, competitors, predators
• Separated populations are genetically independent
(different mutations)
• Given enough time, the two populations may
diverge enough to become distinct species
• If reproductively isolated,  two species
How geographic isolation may lead to the splitting of one
species into two, following the allopatric model of speciation
How might geographic
isolation occur?
Dispersal
Continental drift
Climate fluctuations,
including glaciation
Drifting Continents and
Biogeography
100-millionyear old
Rugops primus
Freshwater and land dinosaurs
became isolated on different
continents as plates drifted apart.
Five species of toucans occur in the rain forest
of South America. How did they arise?
Contemporary rainfall. Areas
receiving > 2500 mm annually are
suggested Pleistocene forest refugia
Proposed pleistocene
refugia for forest-dwelling
plans and animals
Savannahs occur where rainfall is < 1500 mm annually
Species Formation and the
Hierarchy of Life
• Speciation results in the splitting of one
ancestral species into two (or more)
descendent species
• Extended over great expanses of time, this
same process gave rise to the entire tree of
life
• The link from micro-evolution to macroevolution is time – lots and lots of time
Species Formation and the Tree
of Life
• Time + speciation => diversity
• Outcome depends on:
- strength of selection pressure
- length of time
• Explains why species is not an
unambiguous entity
- continuum of differentiation
Species formation (Revisited)
• Cladogenesis – the splitting of one lineage into
two
• Geographic (allopatric) isolation is the
prerequisite condition for reproductive isolation to
evolve (animals)
• Except in plants, where hybridization and/or
chromosome doubling can instantly produce
reproductively isolated, new species (see notes)
• How can we re-construct phylogenetic trees?
B
C
D
E
F
G Living species
Fossil species
Time
A
Ancestral (common) species
A phylogenetic tree
Clocks in Molecules
Figure: the rate of
evolution of hemoglobin.
Each point on the graph
is for a pair of species, or
groups of species. From
Kimura (1983).
"One of the most startling things that has come
to light is that molecular evolution seems to
happen at a surprisingly constant rate. If you
plot molecular changes in a particular lineage of
organisms against time they seem to accumulate
these changes at a remarkably constant rate, and
that has come to be known as the molecular
clock.“ Linda Partridge
This suggests that molecular evolution
is constant enough to provide a
molecular clock of evolution.
This means that the amount of
molecular change between two species
measures how long ago they shared a
common ancestor.
A phylogenetic tree
constructed from
similarities between
cytochrome c
molecules in various
organisms.
Summary
• Biological evolution
- genetic change (microevolution)
- change from one form into another over time (macroevolution)
• Species definitions
- morphological, biological
• Reproductive isolation
- central to biological species concept
• Allopatric speciation
- main mechanism in animals
- long-term geographic isolation is prerequisite
• Diversity and the tree of life
- repeated splittings over great expanse of time