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SPECIATION
the formation of new species
Speciation follows the results of
DIVERGENT (branching)
EVOLUTION
Phylogenetic Tree for all living things
Time ---------------------------------------->present
Nodes indicate most recent common ancestor of two branches
What is a SPECIES?
Biological Species Concept:
A group of naturally or potentially
interbreeding organisms that can produce
fertile offspring.
How Do New Species Form?
Take 2 minutes to think about how it might
happen…
Steps of Speciation:
1) Reproductive isolation of two populations
•
Allopatric speciation – due to a geographic barrier
that prevents gene flow
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Squirrels and the Grand Canyon
Birds and the Rocky Mountains
Galapagos Finches separated from the mainland
Sympatric speciation – two populations in same
place, but behavior or niche prevents interbreeding
•
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Cichlid Fish
Polyploidy in plants
2) Genetic divergence
•
The gene pools become more and more
different due to selection and/or drift.
3) Genetic barriers to reproduction arise.
•
The two populations are no longer able to
interbreed, due to genetic differences in habitat
preference, mating behavior, or physical
compatibility.
Now there are two separate species!
What can make a population
reproductively isolated?
A) Pre-zygotic isolating mechanisms
(before the zygote is formed)
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Geographic isolation
Habitat isolation
Behavioral isolation
Temporal isolation
Mechanical isolation
Chemical isolation
B) Post-zygotic isolating mechanisms
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Hybrid inviability (zygote or offspring dies)
Hybrid inferiority (hybrid is weak and sickly)
Hybrid sterility (offspring can’t reproduce)
Ring
Species
Ligers and tigons and geeps (oh my!)
Speciation results in
PATTERNS in EVOLUTION:
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Adaptive radiation
Stasis
Gradualism
Punctuated equilibrium
How can we describe the
RATE OF EVOLUTIONARY
CHANGE?
Two opposing theories:
• Gradualism – slow and steady change.
• Punctuated equilibrium – change is
relatively rapid, interspersed with periods
of stasis
Gradualism
vs. Punctuated
equilibrium
Punctuated equilibrium happens
when…
• The environment changes quickly
• A barrier arises quickly
• One mutation enables a large shift in the
population
Evidence: fossils, experiments
Evolutionary STASIS
A long period of little or no
evolutionary change
ADAPTIVE RADIATION
The development of many species
from a common ancestor
Adaptive
radiation may
result from:
1. New
Environments
with diverse
habitats
…especially on islands
Fruit fly evolution in Hawaii
Galapagos finches
2. New traits that then diversify
Example: Flowering plants – once
flowers emerged as a strategy, it worked
so well in so many different niches that
flowering plants diversified greatly.
3. New Communities of Organisms
Example 1: One new organism allows other to
diversify – e.g. flowering plants and insects
Example 2: Extinction of one group allows
diversification of another – e.g. dinosaurs and
mammals
Phylogenetic Tree for all living things
Time ---------------------------------------->present
Nodes indicate most recent common ancestor of two branches