Chapter 24 The Origin of Species

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Chapter 24The Origin of Species
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Question?

• What is a species?
• Comment - Evolution theory must also explain how
  species originate.

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Question

• How many species of African Violets are here?

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Two Concepts of Species

• 1. Morphospecies
• 2. Biological Species

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Morphospecies

• Organisms with very similar morphology or
  physical form.

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Problem

• Where does extensive phenotype variation fit?

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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.

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Biological Species

• A group of organisms that could interbreed in
  nature and produce fertile offspring.

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Key Points

• Could interbreed.
• Fertile offspring.

Heaven Scent an F1 hybrid between 2 species,
but sterile.
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Morphospecies Biological Species

• Often overlap.
• Serve different purposes.

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African Violets

• Originally 20 species
• 70,000 cultivars

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Problem

• What is a species?
• Some plants didnt fit placement.
• Plants freely interbreed.
• Answer coming up later

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Speciation Requires

• 1. Variation in the population.
• 2. Selection.
• 3. Isolation.

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Reproductive Barriers

• Serve to isolate a populations from other gene
  pools.
• Create and maintain species.

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Main Types of Barriers

• Prezygotic - Prevent mating or fertilization.
• Postzygotic - Prevent viable, fertile offspring.

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Prezygotic - Types

• 1. Habitat Isolation
• 2. Behavioral Isolation
• 3. Temporal Isolation
• 4. Mechanical Isolation
• 5. Gametic Isolation

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Habitat Isolation

• Populations live in different habitats or
  ecological niches.
• Ex mountains vs lowlands.

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Behavioral Isolation

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

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Temporal Isolation

• Breeding seasons or time of day different.
• Ex flowers open in morning or evening.

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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.

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Gametic Isolation

• Gametes fail to attract each other and fuse.
• Ex chemical markers on egg and sperm fail to
  match.

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Postzygotic Types

• 1. Reduced Hybrid Viability
• 2. Reduced Hybrid Fertility
• 3. Hybrid Breakdown

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Reduced Hybrid Viability

• Zygote fails to develop or mature.
• Ex when different species of frogs hybridize.

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Reduced Hybrid Fertility

• Hybrids are viable, but can't reproduce sexually.
• Chromosome count often odd so meiosis wont
  work.
• Ex - mules

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Hybrid Breakdown

• Offspring are fertile, but can't compete
  successfully with the pure breeds.
• Ex many plant hybrids

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Question

• Actively evolving species like Quercus and
  Saintpaulia.
• Good isolation mechanisms or poor ones?
• Isolation mechanisms may not have fully developed
  yet.

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Modes of Speciation

• 1. Allopatric Speciation
• 2. Sympatric Speciation
• Both work through a block of gene flow between
  two populations.

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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.

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Example

• Pupfish populations in Death Valley.
• Generally happens when a species range shrinks
  for some reason.

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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.

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Conditions Favoring Allopatric Speciation

• 3. Selection pressure on the isolate differs from
  the parent population.

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Result

• Gene pool of isolate changes from the parent
  population.
• New Species can form.

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Comment

• Populations separated by geographical barriers
  may not evolve much.
• Ex - Pacific and Atlantic Ocean populations
  separated by the Panama Isthmus.

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Examples

• Fish - 72 identical kinds.
• Crabs - 25 identical kinds.
• Echinoderms - 25 identical kinds.

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Adaptive Radiation

• Rapid emergence of several species from a common
  ancestor (Allopatric speciation)
• Common in island and mountain top populations or
  other empty environments.

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Mechanism

• Resources are temporarily infinite.
• Most offspring survive.
• Result - little Natural Selection and the gene
  pool can become very diverse.

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When the Environment Saturates

• Natural Selection resumes.
• New species form rapidly if isolation mechanisms
  work.

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Examples

• Galapagos Finches
• Usambaras Mountains African violets

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Sympatric Speciation

• Sympatric same homeland
• New species arise within the range of parent
  populations.
• Can occur In a single generation.

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Plants

• Polyploids may cause new species because the
  change in chromosome number creates postzygotic
  barriers.

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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

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Autopolyploid
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Allopolyploid
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Animals

• Don't form polyploids and will use other
  mechanisms.

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Gradualism Evolution

• Darwinian style evolution.
• Small gradual changes over long periods time.

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Gradualism Predicts

• Long periods of time are needed for evolution.
• Fossils should show continuous links.

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Problem

• Gradualism doesnt fit the fossil record very
  well. (too many gaps).

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Punctuated Evolution

• theory that deals with the pacing of evolution.
• Elridge and Gould 1972.

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Punctuated Equilibrium

• Evolution has two speeds of change
• Gradualism or slow change
• Rapid bursts of speciation

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Predictions

• Speciation can occur over a very short period of
  time (1 to 1000 generations).
• Fossil record will have gaps or missing links.

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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.

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Possible Mechanism

• Adaptive Radiation, especially after mass
  extinction events allow new species to originate.
• Saturated environments favor gradual changes in
  the current species.

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Comment

• Punctuated Equilibrium is the newest Evolution
  Theory.
• Best explanation of fossil record evidence to
  date.

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Origin of Evolutionary Novelty

• How do macroevolution changes originate?
• Several ideas discussed in textbook (read them)
• Exaptation
• Heterochrony
• Homeosis

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Another idea

• Mutations in developmental or control genes
  (Chapter 21)
• Looking very promising as a source of
  macroevolution

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Exaptation

• When a structure that was adapted for one context
  is co-opted for another function.
• Ex. feathers and flying

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Heterochrony

• Changes in the timing or rate of development.
• Allometric Growth
• Paedomorphsis

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• 1. Allometric Growth changes in the relative
  rates of growth of various parts of the body.
• Ex. skull growth in primates

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• 2. Paedomorphosis when an adult retains
  features that are present in the juvenile form.
• Ex. gills in adult salamanders

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Homework

• Read Chapter 25, 26
• Lab this week - TBA
• Chapter 24 today, but with 48 hr extension if
  needed
• Chapter 26 Fri. 2/29
• Chapter 25 Mon. 3/3 (XC)
• Exam 2 next week

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Ex - Homeosis

• Changes in the basic body design or arrangement
  of body parts.
• Ex. Hox gene clusters that gave rise to
  vertebrates from invertebrates.

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Gene Duplications

• Allow genes to be used for other functions such
  as in the previous slide.
• Many other examples are known.

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Future of Evolution ?

• Look for new theories and ideas to be developed,
  especially from new fossil finds and from
  molecular (DNA) evidence.

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Evolutionary Trends

• Evolution is not goal oriented. It does not
  produce perfect species.
• Remember species survive because of their
  adaptations. They dont adapt to survive.

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Summary

• Be able to discuss the main theories of what is a
  species.
• Know various reproductive barriers and examples.

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Summary

• Know allopatric and sympatric speciation.
• Be able to discuss gradualism and punctuated
  equilibrium theories.

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Summary

• Recognize various ideas about the origin of
  evolutionary novelties.