Chapter 23The 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
Galapagos – Finches Usambaras Mountains –
African violets
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.