Transcript Chapter 24 Slides

THE ORIGIN OF SPECIES
Chapter 24
Speciation
Speciation
The origin of a new species
Speciation is a focal point of evolutionary theory
Must explain how new species originate and how populations
evolve
Microevolution: Changes in allele frequency in a
population over time
Macroevolution: Broad patterns of evolutionary change
above the species level
Biological Species Concept
• Biological Species Concept (used by Campbell’s Biology)
defines a species as:
•
A group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring
•
Gene flow between populations keeps the populations as one
species
•
Drawbacks:
•
Can not use biological species concept to analyze fossil species
•
Can not apply to species that reproduce asexually (all prokaryotes)
Other Definitions of Species
• Morphological Species Concept:
•
Defines species by structural and anatomical differences and
similarities
• Ecological Species Concept:
•
Defines species by their ecological niche
• Phylogenetic Species Concept:
•
Defines species as the smallest group of individuals on a
phylogenetic tree
Getting Back to Biological Species
Concept
• By defining a species as a group of individuals that can
successfully reproduce, let’s focus on what keeps
organisms from NOT reproducing
• Reproductive isolation: Existence of biological barriers that
prevent two species from producing viable, fertile offspring
• Hybrids
•
Offspring of crosses between species
• Can classify reproductive isolation as before or after
fertilization
Prezygotic Barriers: Before Fertilization
• Prezygotic barriers
• Block fertilization from ever happening
by:
• Impeding different species from attempting
to mate
• Preventing the successful completion of
mating
• Hindering fertilization if mating is successful
Prezygotic barriers
Habitat
Isolation
Temporal
Isolation
Individuals
of
different
species
(a)
MATING
ATTEMPT
(c)
(d)
(b)
Gametic
Isolation
Mechanical
Isolation
Behavioral
Isolation
(e)
(f)
FERTILIZATION
(g)
Postzygotic Barriers: After Fertilization
• Postzygotic barriers
• Prevent hybrid zygote from developing
into a viable, fertile adult by:
• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown
Postzygotic barriers
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(i)
(j)
(k)
(l)
How Does Speciation Occur?
• Speciation can occur with or without
geographic isolation
• Allopatric speciation (with isolation)
• Sympatric speciation (without isolation)
(a) Allopatric speciation.
A population forms a
new species while
geographically isolated
from its parent population.
(b) Sympatric speciation.
A subset of a population
forms a new species
without geographic
separation.
Allopatric Speciation
• Allopatric (“other country”)
• Gene flow is interrupted when population divided
into geographically isolated subpopulations
• Geographic barrier prevents members of
subpopulation from interacting
• Ability of population to disperse defines how much of a
barrier is needed
• Separate populations may evolve independently through
mutation, natural selection and genetic drift
Sympatric Speciation
• Sympatric (“same country”)
• Speciation takes place in geographically
overlapping populations
• Although less common than allopatric, speciation
can occur if gene flow is reduced by factors such
as:
• Polyploidy
• Habitat differentiation
• Sexual selection
Polyploidy
• Polyploidy
• Mistakes in cell division result in extra sets of
chromosomes
• Rare in animals but common in plants
• Many important crops such as oat and wheat
are polyploids
Cell
division
error
Polyploidy
2n = 6
Tetraploid cell
4n = 12
2n
2n
Gametes produced
by tetraploids
New species
(4n)
Habitat Differentiation
• Ecological niche
• Sum of an organism’s use of biotic and
abiotic resources in an environment
• If some organisms can exploit a new niche
from the parental population, they may
eventually evolve into a new species
Hybrid Zones
• Hybrid zone
• Region in which members of different species
mate and produce hybrids (incomplete
reproductive barriers)
• Hybrids often have reduced fitness compared
to either parental species
Hybrid Zones Over Time
• When closely related species meet in a hybrid zone,
3 possible outcomes:
Reinforcement
1.
•
Hybrids less fit than parent species, fewer and fewer hybrids
Fusion
2.
•
Hybrids as fit as parents, forms single species
Stability
3.
•
Continued production of hybrids
Outcomes in Hybrid Zones
Possible
outcomes:
Isolated
population
diverges
Hybrid
zone
Reinforcement
OR
Fusion
OR
Gene flow
Population
Barrier to
gene flow
Hybrid
individual
Stability
How Long Does Speciation Take?
• Hard to say but patterns in fossil records,
morphological data or molecular data can give
some clues
• Fossil record shows some species appearing
suddenly, remaining unchanged and then
disappear
• Punctuated equilibrium
•
Contrasts with model of gradual change
•
Speciation by punctuated equilibrium model appears fast in
geological time (still a long time!)
Speciation Patterns
(a) Punctuated
pattern
Time
(b) Gradual
pattern
How Long Does Speciation Take?
• Lots of variation in speciation time
• Interval can range from 4,000 years in
some species (like cichlids) to 40 million
years (some insects)
• Average is 6.5 million years