Chapter 23 The Origin of Species. Question? u What is a species? u Comment - Evolution theory must...
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Transcript of Chapter 23 The Origin of Species. Question? u What is a species? u Comment - Evolution theory must...
![Page 1: Chapter 23 The Origin of Species. Question? u What is a species? u Comment - Evolution theory must also explain how species originate.](https://reader036.fdocuments.in/reader036/viewer/2022081504/56649ea95503460f94bae05a/html5/thumbnails/1.jpg)
Chapter 23The 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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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 didn’t 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 won’t 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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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 specie’s 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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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 doesn’t 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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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 don’t 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.