Chapter 24:The Origin of
Species
Essential Knowledge 1.c.1 – Speciation and extinction have
occurred throughout the Earth’s history (24.3 & 24.4).
1.c.2 – Speciation may occur when two populations become reproductively isolated from each other (24.1).
1.c.3 – Populations of organisms continue to evolve (24.2).
2.e.2 – Timing and coordination of physiological events are regulated by multiple mechanisms (24.1).
Two Concepts of Species
1. Morphospecies
2. Biological Species
Morphospecies Organisms with very similar
morphology
Two Schools1. 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
Don’t identify species based upon similar likeness ALONE!
Key Points Could interbreed Fertile offspring May or may not
be similar in phenotype
Morphospecies & Biological Species Often overlap Serve different purposes
Other Concepts of Species
1. Recognition Species Concept
2. Cohesion Species Concept
3. Ecological Species Concept
4. Evolutionary Species Concept
Ecological Species Concept Emphasizes a specie’s role or
function in the environment.Called an organism’s niche
(Added Slide ) Lions and tigers are ecologically isolated
Live in open
grassland
Live in forest
Hybridization has been successful in captivity But it does not occur in the wild
Tiglon
Evolutionary Species Concept
Emphasizes evolutionary lineages and ecological roles
Used extensively in comparative biology
Popular among paleontologists
Speciation Speciation = formation of a NEW
speciesCaused by disruptive selection (see
Ch 23) 3 Requirements for speciation:
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 Reproductive Barriers
Prezygotic – Prevent mating or fertilization
Postzygotic – Prevent viable, fertile offspring
Prezygotic - Types1. 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 of flower 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 mature or develop Ex:
When two different species of frogs hybridize○ Egg is fertilized (zygote IS formed)○ Zygote never develops properly (if at
all)
Reduced Hybrid Fertility
Hybrids/offspring are living, viable Offspring cannot reproduce sexually
(nonfertile) Chromosome count often “odd”
Meiosis doesn’t work out Ex: mules, ligers
Hybrid Breakdown Hybrid/offspring are living, viable AND
fertile Offspring cannot compete
successfully with the “pure breeds” Ex:
Most plant hybridsPlants are “born” and can produce seedsBlooms/plants may lack certain color or
size of petal
• Hybrids between them produced defective embryos in the lab
• Their mating calls also differ substantially
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 species
range shrinks for some reason
Another Example
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
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 IsthmusFish - 72 identical kindsCrabs - 25 identical kindsEchinoderms - 25 identical kinds
Adaptive Radiation
Rapid emergence of several species from a common ancestor
Common in island and mountain top populations or other “empty” environments
Ex – Galapagos Finches
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
Animals: Don't form polyploids and will use other mechanisms
Polyploid Types1. Autopolyploid - when a species
doubles its chromosome number from 2N to 4N
2. Allopolyploid - formed as a polyploid hybrid between two speciesEx: wheat
Autopolyploid
Allopolyploid
Rate of Evolution??
Gradualism Evolution
Darwinian style of evolution Small gradual changes over long
periods of time
Gradualism Predicts:
Long periods of time are needed for evolution
Fossils should show continuous links
Problem: Gaps in fossil record
Punctuated Evolution
New theory on rate/pace of evolution
Elridge and Gould – 1972
Punctuated Equilibrium
Evolution has two speeds of change:1. Gradualism or slow change
2. Rapid bursts of change/speciation
Predictions Speciation can occur over a very
short period of time Fossil record will have gaps or
missing links New species will appear in the
fossil record Established species will show
Comment Punctuated Equilibrium is the
newest, most widely accepted view
Best explanation of timing
Summary Identify several concepts of "species". Recognize reproductive barriers that keep
two species from reproducing with each other.
Recognize various mechanisms of speciation and hybrid zones.
Identify the Gradual and Punctuated interpretations of speciation.
Identify genetic mechanisms of speciation.