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Chapter 24:The 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 Species1. 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 Schools1. 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.
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Morphospecies & Biological Species
Often overlap. Serve different purposes.
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African Violets 15+ species 5+ varieties (natural hybrids) 50,000 cultivars
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Example Problem S. magungensis Cluster:
S. Magungensis S. magungensis var. minima S. magungensis var. occidentalis
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Current Placement One species and two varieties. However :
Not all live in the same habitat. Not all flower under the same temperatures.
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Possible New Placement
Two species and one variety. S. magungensis var. occidentalis may be a new
species.
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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 BarriersPrezygotic - Prevent mating or fertilization.
Postzygotic - Prevent viable, fertile offspring.
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Prezygotic - Types1. 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 Types1. 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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Introgression Transfer of alleles between two species, but
only a partial gene flow. Result – Some intermixing of genes between
two species.
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Importance A possible mechanism for gene flow between
similar species.
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Question Actively evolving species like Quercus. Good isolation mechanisms or poor ones? Isolation mechanisms may not have fully
developed yet.
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Other Concepts of Species
1. Recognition Species Concept
2. Cohesion Species Concept
3. Ecological Species Concept
4. Evolutionary Species Concept
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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
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Cohesion Species Concept
Emphasizes cohesion of phenotypes (complex of genes and set of adaptations).
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Ecological Species Concept Emphasizes a species role or function in the
environment.
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Evolutionary Species Concept Emphasizes evolutionary lineages and
ecological roles.
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Comment AP exam will focus on morphological and
biological species concepts.
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Modes of Speciation1. 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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Another Example
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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. Common in island and mountain top populations or
other “empty” environments. Ex – Galapagos Finches
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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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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 doesn’t fit the fossil record very
well. (too many “gaps”).
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Punctuated Evolution New theory on 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.
Exaptation Heterochrony Homeosis
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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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HomeosisChanges 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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Evolutionary Trends Evolution is not goal oriented. It does not
produce “perfect” species.
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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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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.
![Page 81: Chapter 24: The Origin of Species. Question? What is a species? Comment - Evolution theory must also explain how species originate](https://reader035.vdocument.in/reader035/viewer/2022062801/56649e745503460f94b7456f/html5/thumbnails/81.jpg)
Summary Recognize various ideas about the origin of
evolutionary novelties.