Chapter 19
Changes in Species
Speciation
Q: When are two populations new species?
A: When populations no longer interbreed they
are thought to be separate species.
•As natural selection proceeds, populations
occupying different environments will diverge
into races, subspecies, and finally separate
species.
•Barriers to gene flow between populations
isolate those populations, ultimately leading
to the formation of new and separate species
Reproductive Isolating Mechanisms
•Pre-zygotic Isolating Mechanisms-
these act before fertilization can happen
-Ecological:
-Different habitats
»Example Lions and Tigers
–Geographical:
–Mountains, islands, or rivers separate
»Chipmunks in the Grand Canyon
–Seasonal/Temporal:
–Different breeding season
»Blue whales in northern and southern hemisphere
–Mechanical:
–Parts don’t fit
»bushbabies
–Behavioral:
–Different mating behavior
»Crickets & Birds of Paradise
Post-zygotic Isolating Mechanisms•these happen, even if fertilization happens
Gametic incompatibility
Sperm transfer takes place,
but egg is not fertilized.
Zygotic mortality
Egg is fertilized, but zygote does not develop.
Hybrid inviability
Hybrid embryo forms, but of reduced viability.
Hybrid sterility
Hybrid is viable, but resulting adult is sterile.
Hybrid breakdown
First generation (F1) hybrids are viable and fertile, but further hybrid
generations (F2 and backcrosses) may be inviable or sterile.
Paths of Speciation:
•Phylentic Speciation –
a single population is
transformed enough to be
considered a new species
• Allopatric Speciation
– Populations begin to
diverge when gene flow
between them is
restricted.
– Geographic isolation
is often the first step in
allopatric speciation.
Parapatric Speciation• The splitting of a population into 2 species under
conditions where members of each population reside
in adjacent areas
Hybridization
• Two species mate and make a hybrid
that cannot mate with either parent.
Patterns of Evolution:
• Divergent Evolution- two species evolve from a
common ancestor and become different over time.
• Adaptive Radiation- many species evolve from a single ancestral line
• Convergent Evolution- species with different ancestors colonize
similar habitats with a superficial resemblance
Comparing them:
Parallel
evolution occurs
when two species
When people
evolve
hear the word
independently of
"evolution," they
each other, but
usually think of
maintain the
divergent
same level of
evolution, the
similarity. Parallel
pattern where
evolution usually
two species
occurs between
gradually
unrelated species
become
that do not
increasingly
occupy the same
different.
or similar niches
in a given habitat.
Convergent evolution takes place when species of different
ancestry begin to share analogous traits because of a shared
environment or other selection pressure
Extinction
Extinction occurs when the
population cannot adapt to
changing environmental
conditions.
The golden toad of Costa Rica’s
Monteverde cloud forest became
extinct because of climate
change.
video
Pace of Evolution
• Gradualism– Very gradually, over a long time, a population changes.
– Over a short period of time it is hard to notice.
– Change is slow, constant, and consistent.
– The steady uninterrupted process has small adaptive steps
– Darwin’s view
• Punctuated Equilibrium• change comes in spurts. There is a period of
very little change, and then one or a few huge
changes occur
• Often due to major changes in the
environment.
• Gaps in the fossil record