Download Chapter 24 The Origin of Species

Chapter 24
The Origin of Species
Question?
• What is a species?
• Comment - Evolution theory must
also explain how species originate.
• Darwin’s “Mystery of mysteries”
Question?
• How many species of African Violets are
here?
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
two species, but sterile.
The parents are probably
separate species.
African Violets
• 1950-60’s –20 species described
• 70,000 cultivars or human created hybrids
African Violets
• 2007 – lumping occurred to 6
species and 10 subspecies
• 2008 – AVSA now recognizes 9
species and 8 subspecies
• 2012 – all plants lumped under a
different genus with 6 species and
10 subspecies.
• pt – species definitions can be
fluid and change.
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. Temporal Isolation
3. Behavioral Isolation
4. Mechanical Isolation
5. Gametic Isolation
Habitat Isolation
• Populations live in different habitats
or ecological niches.
• Ex – mountains vs lowlands.
Temporal Isolation
• Breeding
seasons or time
of day different.
• Ex – flowers
open in morning
or evening.
Behavioral Isolation
• Mating or courtship
behaviors different.
• Different sexual
attractions operating.
• Ex – songs and
dances in birds.
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 or salamanders hybridize.
Reduced Hybrid Fertility
• Hybrids are viable, but can't
reproduce sexually.
• Chromosome count often “odd” so
meiosis won’t work.
• Ex – donkeys and horses produce
mules
Hybrid Breakdown
• Offspring are fertile, but can't
compete as well as the “pure
breeds”.
• Ex – many plant hybrids
Question
• Actively evolving species like
Quercus and Saintpaulia.
• Good isolation mechanisms or poor
ones?
• Isolation mechanisms may not
have fully developed yet.
How do species occur?
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.
(environment is different on the
edges)
Result
• Gene pool of isolate changes from
the parent population and new
species can form.
Sympatric Speciation
• Sympatric = same homeland
• New species arise within the range
of parent populations.
• Can occur In a single generation.
Plants
• Polyploids may cause new species
because the change in
chromosome number creates
postzygotic barriers.
Polyploid Types
1. Autopolyploid - when a species
doubles its chromosome number
from 2N to 4N.
2. Allopolyploid - formed as a
polyploid hybrid between two
species.
• Ex: wheat
Autopolyploid
Allopolyploid
Adaptive Radiation
• Rapid emergence of several
species from a common ancestor
(often 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
• Galapagos – Finches
• Usambaras Mountains – African
violets
Question
• What if isolation fails and populations
come in contact with each other again?
• Hybrid zones – a region where
members of different species meet and
mate and produce hybrid offspring.
Possibile Outcomes
• 1. Reinforcement – barriers hold
and hybrids disappear.
• 2. Fusion – barriers give way and
species fuse.
• 3. Stability – barriers constant,
continued production of hybrids
Speed of speciation?
• How long does it take a new
species to form?
• How many genes are involved?
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).
• Too fast for fossils to record.
• 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.
• Uses more Evo-Devo information
How many genes?
• Can be as little as 1 gene which
prevents interbreeding. Ex – snails
• Probably a larger number of genes
in many cases.
1 Gene = 2 species
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.
• Know adaptive radiation.
• Be able to discuss gradualism and
punctuated equilibrium theories.