Chapter 24 The Origin of Species

Question? What is a species? Comment - Evolution theory must also explain how species originate. Darwin’s “Mystery of mysteries”

Question? How many species of African Violets are here?

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 two species, but sterile. The parents are probably separate species.

African Violets 1950-60’s –20 species described 70,000 cultivars or human created hybrids

African Violets 2007 – lumping occurred to 6 species and 10 subspecies 2008 – AVSA recognizes 9 species and 8 subspecies 2012 – Proposed change for all plants lumped under a different genus with 6 species and 10 subspecies. pt – species definitions can be fluid and change.

Speciation Requires: 1. Variation in the population – different genetic forms must exist. 2. Selection – nature must favor some genetic forms over others. 3. Isolation – populations of genetic forms are prevented from interbreeding with each other.

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. Temporal Isolation 3. Behavioral Isolation 4. Mechanical Isolation 5. Gametic Isolation

Habitat Isolation Populations live in different habitats or ecological niches. Ex – mountains vs lowlands.

Temporal Isolation Breeding seasons or time of day different. Ex – flowers open in morning or evening.

Behavioral Isolation Mating or courtship behaviors different. Different sexual attractions operating. Ex – songs and dances in birds.

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 when corals and other organisms shed gametes into water at the same time.

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 or salamanders hybridize the eggs fail to develop.

Reduced Hybrid Fertility Hybrids are viable, but can't reproduce sexually. Chromosome count often “odd” so meiosis won’t work. Ex – donkeys and horses produce mules

Hybrid Breakdown Offspring are fertile, but can't compete as well as the “pure breeds”. Ex – many plant hybrids; lack the specific adaptations that made the parents successful.

Question Consider - actively evolving and interbreeding species in genera like Quercus (Oaks) and Saintpaulia (African violets) Good isolation mechanisms or poor ones? Isolation mechanisms may not have fully developed yet.

How do species originate? 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. (environment is different on the edges)

Result Gene pool of isolate changes from the parent population and new species can form.

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.

2. Allopolyploid - formed as a polyploid hybrid between two species. 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

Assignments Read Chapter 24, 25 or 17 in Hillis Lab – HW modeling and Bio-informatics Lizard species – video for discussion board – due Thursday Chapter 24 - Friday

Adaptive Radiation Rapid emergence of several species from a common ancestor (often 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

Question What if isolation fails and populations come in contact with each other again? Hybrid zones – a region where members of different species meet and mate and produce hybrid offspring.

Possibile Outcomes 1. Reinforcement – barriers hold and hybrids disappear. 2. Fusion – barriers give way and species fuse. 3. Stability – barriers constant, continued production of hybrids

Speed of speciation? How long does it take a new species to form? How many genes are involved?

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). Too fast for fossils to record. 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. Uses more Evo-Devo information of the role of developmental gene control and gene regulation.

How many genes? Can be as little as 1 gene which prevents interbreeding. Ex – snails Probably a larger number of genes in many cases.

1 Gene = 2 species

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. Know adaptive radiation. Be able to discuss gradualism and punctuated equilibrium theories.