Transcript Ch. 23 Origin of Species - Crestwood Local Schools

Chapter 23
The Origin of
Species
Question?
What
is a species?
Comment - Evolution theory
must also explain how
species originate.
Two Concepts of
Species
1. Morphospecies
2. Biological Species
Morphospecies
Organisms
with very similar
morphology or physical form.
Problem
Where
does extensive
phenotype variation fit?
Two Schools
1. Splitters - Break apart
species into new ones on the
basis of small phenotype
changes.
2. Lumpers - Group many
phenotype variants into one
species.
Biological Species
A
group of organisms that
could interbreed in nature
and produce fertile offspring.
Key Points
Could
interbreed.
Fertile offspring.
Heaven Scent
an F1 hybrid between
2 species, but sterile.
Morphospecies &
Biological Species
Often
overlap.
Serve different purposes.
African Violets
Originally
~20 species
70,000 cultivars
Problem
What
is a species?
Some plants didn’t fit placement.
Plants freely interbreed.
Answer – coming up later
Speciation Requires:
1. Variation in the population.
2. Selection.
3. Isolation.
Reproductive Barriers
Serve
to isolate a populations
from other gene pools.
Create and maintain
“species”.
Main Types of Barriers
Prezygotic - Prevent mating or
fertilization.
Postzygotic - Prevent viable,
fertile offspring.
Prezygotic - Types
1. Habitat Isolation
2. Behavioral Isolation
3. Temporal Isolation
4. Mechanical Isolation
5. Gametic Isolation
Habitat Isolation
Populations
live in different
habitats or ecological niches.
Ex – mountains vs lowlands.
Behavioral Isolation
 Mating
or courtship
behaviors different.
 Different sexual
attractions
operating.
 Ex – songs and
dances in birds.
Temporal Isolation
 Breeding
seasons or time
of day different.
 Ex – flowers
open in morning
or evening.
Mechanical Isolation
Structural
differences that
prevent gamete transfer.
Ex – anthers not positioned
to put pollen on a bee, but
will put pollen on a bird.
Gametic Isolation
Gametes
fail to attract each
other and fuse.
Ex – chemical markers on
egg and sperm fail to match.
Postzygotic Types
1. Reduced Hybrid Viability
2. Reduced Hybrid Fertility
3. Hybrid Breakdown
Reduced Hybrid
Viability
Zygote
fails to develop or
mature.
Ex – when different species
of frogs hybridize.
Reduced Hybrid
Fertility
Hybrids
are viable, but can't
reproduce sexually.
Chromosome count often
“odd” so meiosis won’t work.
Ex - mules
Hybrid Breakdown
Offspring
are fertile, but can't
compete successfully with
the “pure breeds”.
Ex – many plant hybrids
Modes of Speciation
1. Allopatric Speciation
2. Sympatric Speciation
Both work through a block of
gene flow between two
populations.
Allopatric Speciation
Allopatric
= other homeland
Ancestral population split by
a geographical feature.
Comment – the size of the
geographical feature may be
very large or small.
Example
Pupfish
populations in Death
Valley.
Generally happens when a
specie’s range shrinks for
some reason.
Conditions Favoring
Allopatric Speciation
1. Founder's Effect - with the
peripheral isolate.
2. Genetic Drift – gives the
isolate population variation
as compared to the original
population.
Conditions Favoring
Allopatric Speciation
3. Selection pressure on the
isolate differs from the parent
population.
Result
Gene
pool of isolate changes
from the parent population.
New Species can form.
Comment
Populations
separated by
geographical barriers may
not evolve much.
Ex - Pacific and Atlantic
Ocean populations separated
by the Panama Isthmus.
Examples
Fish
- 72 identical kinds.
Crabs - 25 identical kinds.
Echinoderms - 25 identical
kinds.
Adaptive Radiation
Rapid
emergence of several
species from a common
ancestor (Allopatric
speciation)
Common in island and
mountain top populations or
other “empty” environments.
Mechanism
Resources
are temporarily
infinite.
Most offspring survive.
Result - little Natural
Selection and the gene pool
can become very diverse.
When the Environment
Saturates
Natural
Selection resumes.
New species form rapidly if
isolation mechanisms work.
Examples
– Finches
Usambaras Mountains –
African violets
Galapagos
Sympatric Speciation
Sympatric
= same homeland
New species arise within the
range of parent populations.
Can occur In a single
generation.
Gradualism Evolution
Darwinian
style evolution.
Small gradual changes over
long periods time.
Gradualism Predicts:
Long
periods of time are
needed for evolution.
Fossils should show
continuous links.
Problem
Gradualism
doesn’t fit the
fossil record very well.
(too many “gaps”).
Punctuated Evolution
theory
that deals with the
“pacing” of evolution.
Elridge and Gould – 1972.
Punctuated Equilibrium
Evolution
has two speeds of
change:
Gradualism
or slow change
Rapid bursts of speciation
Predictions
Speciation
can occur over a
very short period of time
(1 to 1000 generations).
Fossil record will have gaps
or missing links.
Predictions
New
species will appear in
the fossil record without
connecting links or
intermediate forms.
Established species will
show gradual changes over
long periods of time.
Possible Mechanism
Adaptive
Radiation,
especially after mass
extinction events allow new
species to originate.
Saturated environments favor
gradual changes in the
current species.
Comment
Punctuated
Equilibrium is the
newest ”Evolution Theory”.
Best explanation of fossil
record evidence to date.
Origin of Evolutionary
Novelty
How
do macroevolution changes
originate?
Several ideas discussed in
textbook (read them)
Exaptation
Heterochrony
Homeosis
Another idea
Mutations
in developmental
or control genes (Chapter 21)
Looking very promising as a
source of macroevolution
Ex - Homeosis
Changes in the basic body
design or arrangement of
body parts.
Ex. – Hox gene clusters that
gave rise to vertebrates from
invertebrates.
Gene Duplications
Allow
genes to be used for
other functions such as in the
previous slide.
Many other examples are
known.
Future of Evolution ?
Look
for new theories and
ideas to be developed,
especially from new fossil
finds and from molecular
(DNA) evidence.
Evolutionary Trends
Evolution
is not goal
oriented. It does not produce
“perfect” species.
Remember – species survive
because of their adaptations.
They don’t adapt to survive.
Summary
Be
able to discuss the main
theories of what is a
“species”.
Know various reproductive
barriers and examples.
Summary
Know
allopatric and
sympatric speciation.
Be able to discuss
gradualism and punctuated
equilibrium theories.
Summary
Recognize
various ideas
about the origin of
evolutionary novelties.