Chapter 24:The Origin of Species

Question? What is a species? Comment - Evolution theory must also

explain how species originate.

Question How many species of African Violets are

here?

Two Concepts of Species1. Morphospecies

2. Biological Species

Morphospecies Organisms with very similar morphology or

physical form.

Problem Where does extensive phenotype variation

fit?

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.

Key Points Could interbreed. Fertile offspring.

Morphospecies & Biological Species

Often overlap. Serve different purposes.

African Violets 15+ species 5+ varieties (natural hybrids) 50,000 cultivars

Example Problem S. magungensis Cluster:

S. Magungensis S. magungensis var. minima S. magungensis var. occidentalis

Current Placement One species and two varieties. However :

Not all live in the same habitat. Not all flower under the same temperatures.

Possible New Placement

Two species and one variety. S. magungensis var. occidentalis may be a new

species.

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 BarriersPrezygotic - 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 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 Types1. 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

Introgression Transfer of alleles between two species, but

only a partial gene flow. Result – Some intermixing of genes between

two species.

Importance A possible mechanism for gene flow between

similar species.

Question Actively evolving species like Quercus. Good isolation mechanisms or poor ones? Isolation mechanisms may not have fully

developed yet.

Other Concepts of Species

1. Recognition Species Concept

2. Cohesion Species Concept

3. Ecological Species Concept

4. Evolutionary Species Concept

Recognition Species Concept Species are defined by the ability of the

individuals in the population to recognize certain characteristics in each other.

Ex – mate recognition

Cohesion Species Concept

Emphasizes cohesion of phenotypes (complex of genes and set of adaptations).

Ecological Species Concept Emphasizes a species role or function in the

environment.

Evolutionary Species Concept Emphasizes evolutionary lineages and

ecological roles.

Comment AP exam will focus on morphological and

biological species concepts.

Modes of Speciation1. 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.

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.

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. Common in island and mountain top populations or

other “empty” environments. Ex – Galapagos Finches

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.

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

Animals Don't form polyploids and will use other

mechanisms.

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 New theory on 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.

Exaptation Heterochrony Homeosis

Exaptation

When a structure that was adapted for one context is co-opted for another function.

Ex. – feathers and flying

Heterochrony Changes in the timing or rate of development.

Allometric Growth Paedomorphsis

1. Allometric Growth – changes in the relative rates of growth of various parts of the body.

Ex. – skull growth in primates

2. Paedomorphosis – when an adult retains features that are present in the juvenile form.

Ex. – gills in adult salamanders

HomeosisChanges in the basic body design or arrangement

of body parts.

Ex. – Hox gene clusters that gave rise to vertebrates from invertebrates.

Evolutionary Trends Evolution is not goal oriented. It does not

produce “perfect” species.

Future of Evolution ? Look for new theories and ideas to be

developed, especially from new fossil finds and from molecular (DNA) evidence.

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.