Chapter 24: The Origin of Species, Lecture Notes
I.Introduction
Microevolution – Changes in a population
Speciation –The origin of new species
Macroevolution – The origin of new taxonomic groups
II.What is a Species
A. The Biological Species Concept - emphasizes reproductive barriers!
Developed by Ernst Mayr in 1942
Key components:
Interbreeding group or population
Producing viable fertile offspring
Have a protected gene pool – genetic isolation
B. Isolation of Gene pools begins speciation
1. Prezygotic barriers – impedes mating or fertilization between different species
Habitat isolation
Behavioral isolation
Temporal isolation
Mechanical isolation – anatomical incompatibility
Gametic isolation – gametes will not fuse
2. Postzygotic barriers - prevents hybrid zygote from becoming a viable fertile adult
Reduced Hybrid Viability – development fails at some point
Reduced Hybrid Fertility – Hybrids are sterile
Hybrid Breakdown – produce inviable or infertile offspring
C. Limitations of the Biological Species Concept
Using reproductive isolation to define a species doesn’t always work
We often lack information on interbreeding results and have to rely on morphology
What about asexual organisms?
D. Alternative Species Concepts
1. The Ecological Species Concept – defines in terms of its ecological niche
2. The Pluralistic Species Concept – Uses multiple concepts (Biological and Ecological)
3. The Morphological Species Concept – uses structure or morphology
4. The Genealogical Species Concept – organisms with a unique genetic history
Often utilizes gene sequences
III.Modes of Speciation
A. Allopatric Speciation – Geographic barriers isolate sub-groups
1. Conditions for Allopatric Speciation
Geographic isolation of population sub-groups
Barriers that may arise – Mt. Ranges, loss of water or land bridges, flooding.
Inability of organisms to migrate out ….. loss of gene flow between sub-groups
Isolation is more likely in smaller populations
Isolation must be long enough for divided populations to diverge by
Genetic drift …. Or
Differing selective pressures
Divergence must be long enough for reproductive barriers to develop
Origin of prezygotic barriers (Campbell example 471)
Origin of postzygotic barriers (Campbell example 472)
2. Ring Species (Campbell 470)
3. Adaptive Radiation on Island chains
Adaptive Radiation – when 2 or more species diverge from a common ancestor
Classic examples – Galapagos Islands
The Hawaiian Archipeligo
B. Sympatric Speciation – New species emerge within the geographic midst of the parent population.
A reproductive barrier emerges to isolate a sub-population
In plants – hybridization coupled with division errors leading to polyploid individuals
that are still fertile.
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In Animals – isolation may result from a new habitat preference or female mating
preference in a subset.
C. Closely Related species
Very Often allopatric, isolated somehow from each other – suggests allopatric speciation
Example – Eastern – Black-capped Chickadees and Western Chickadees
When closely related species are sympatric – a hybrid zone often exists between
Examples – Warblers
Or , at the borders of ranges, reproductive or behavioral barriers exist
Example – Carolina Chickadee ---- Black-capped Chickadee ---- Boreal Chickadee
D. Punctuated Equilibrium
1. The fossil record seems to suggest that new forms often arise suddenly, then remain relatively
unchanged for a long period of time.
This seems at odds with Darwin’s gradualism.
The many transition fossils that should be found due to gradualism, have not been.
2. Allopatric speciation may answer some of this ….. new forms arise as a splinter population, separate
from the parent group. When the new species re-enters parental range, fossils would indicate a
sudden appearance.
3. New View to explain absence of transition fossils - Punctuated Equilibrium
Eldridge and Gould, 1970s.
Evolutionary history is often
Spurts of rapid change ……… punctuated
Interspersed by periods of stability …………..Equilibrium
Understand “Rapid” change – thousands of generations
50,000 years to emerge, followed by 5,000,000 years of stasis
emergence is only 1%, will appear abrupt.
4. Cause of abrupt emergence?
Fast mutation due to environmental stress
Long term accumulated mutations are suddenly (over 1,000s of generations) expressed
IV. From Speciation to Macroevolution
A. Evolutionary Novelties modify previous structures
See Campbell’s example of the eye (477)
B. Development genes play a role in evolution
Allometric growth in embryos– relative growth rate of differing body sections
develops correct body proportions.
Evolutionary changes from modification of allometric growth - Heterochrony
Sexual maturity can change to arrive earlier, while juvenile characteristics remain
Called – Paedomorphosis
Slight mutations in homeotic genes can cause radical changes in body structure
C. Evolutionary trend does not imply teleology (goal oriented evolution)
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