On the Origin of Species
(Evolution II)
I. What is a Species?
- Organisms capable of having fertile offspring – good start, but incomplete.
- "Species are groups of interbreeding natural populations that are reproductively isolated from other groups." As Mayr suggests, a new species must be reproductively isolated from other species in order to form.
II. Reproductive Isolating Mechanisms
- Prezygotic (before the zygote forms, so these mechanisms prevent breeding)
1. Ecological Isolation
2. Temporal Isolation: Distinct breeding periods.
3. Behavioral Isolation: Breeding dances, songs, calls, etc.
4. Mechanical Isolation: Some plants have flower shapes designed to deposit or extract pollen from insects.
5. Gametic Mortality: Immune system may attack sperm.
B. Postzygotic (after fertilization)
Some species can interbreed, but hybrid offspring are infertile or weak.
III. Geographic Isolation
What would initially cause two species to diverge?
Allopatric Speciation: Think of an ‘alley’ being a physical seperation between two buildings; allopatric speciation requires a physical seperation (for example, an ocean).
IV. Other Modes of Speciation
A. Sympatric Speciation: Each new population begins to occupy a new range or ecological niche.
B. Parapatric Speciation: Populations adapt to new niches, but share a hybrid zone.
V. Patterns of Speciation
A. Gradual Model of Speciation: Allele frequency changes gradually over time.
B. Punctuated Equilibria: Evolution occurs in short, active periods of change.
C. Adaptive Radiation: One line of organisms diverging a following separate evolutionary paths.
D. Convergent Evolution: Different lineages evolving similar structure to deal with common environmental challenge.
Chapter 20: Macroevolution
I. Fossil Record of Changing Earth
A. Fossils : the remnants of ancient life.
1. Steps of fossil formation:
a.) quick burial (so oxygen and bacteria are not able to degrade the carcass),
b.) the preserved organism is covered for years, during which water brings minerals to the remants, ultimately
replacing the organic molecules with minerals.
c.) result is a fossil with little or no organic remnants but (often) remarkable detail. Because of this process,
fossils are often rock-like, very hard and heavy.
B. Reading the fossil record:
1. We don’t have a static earth. Over 99% of all species that ever lived on earth are now extinct.
2. There are dramatic changes in fossil strata (layers).
3. The boundaries of the major eras are denoted with massive extinctions (there have been five, and we are currently in
the 6th).
- Homologous Structures: Same biological tissue used for different functions.
B. Morphological Convergence: Distant lineages converge upon common adaptation for similar environmental conditions.
A. Differential Gene Expression: Similar genes expressed in different organisms.
B. Comparative Bone Structure:
C. DNA/Comparative Biochemistry
Comparisons of DNA, cytochrome C amino acid structure, hemoglobin structure, etc. back up other lines of evolutionary evidence.
