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.