Mechanisms of Evolution Organizer
section 15.3
Types of Natural Selection:
1.Stabilizing Selection
A. most common form of natural selection
B. Average expression of trait is selected for and extremes are selected against
C. Ex: average birth weight of infants has higher survival rate
D. Sketch graph:
2.Directional Selection
A. Increases the expression of the extreme trait and decreases the expression of the average trait
B. Ex: Increase in dark moths in England after
Industrial Revolution
C. Sketch graph:
3.Disruptive Selection
A. Both extremes are selected for and the average
Trait is selected against
B. Ex: Northern water snakes having 2 colorations
that are adaptations; one grey, one brown. An intermediate color would be a disadvantage.
C. Sketch graph:
4.Sexual Selection
A. Operates in populations where males and females
differ greatly in appearance
B.Largest, more colorful peacocks are selected by females; most threatening elk or deer mate with the
Most females
C. No graph for this type
Reproductive Isolation
1.Prevents gene flow among populations
2.Prezygotic Isolation
A. Prevent reproduction by making fertilization unlikely
B. Ex: Different mating songs (meadowlarks), different mating times (fireflys, trout) – prevent mating between different species
3.PostzygoticIsolation
A. Operate after fertilization has occurred to ensure that the
Hybrid offspring remains infertile
B. Prevent offspring survival or reproduction
C. Ex: lion + tiger = liger (sterile); horse + donkey = mule (sterile)
Speciation
1. For speciation to occur, a population must diverge and then be reproductively isolated.
2.Definition (from p. 437):Process whereby some members of a
Population change so much that they can no longer reproduce
With the original population
3.Allopatric speciation (Geographic Isolation)
A. a physical barrier divides one population into two or more
populations
B. Results in 2 different species incapable of breeding together
C. Most common form of speciation
D. Geographic barriers include :mountain ranges, channels between islands, wide rivers, lava flows
E. Ex: Grand Canyon – Kaibab squirrel and Abert squirrel – reproductively isolated – may evolve into separate species
4.Sympatric Speciation
A. No physical barrier – ancestor species and new species evolve
Side by side
B. Ex: Insects – diverge based on food they eat
C. Common in Plants due to polyploidy
1. Plants with different chromosome numbers can’t
reproduce with rest of population
Patterns of Evolution
1.Evidence of speciation can be seen in these patterns
2. Adaptive Radiation (Divergent Evolution)
A. One species gives rise to many species due to environmental
changes
B. Often follows large scale extinctions – rise of mammals after
extinction of dinosaurs
C. Ex: Cichlid fish in Africa: 300 species diverged from one
common ancestor in 14, 000 years.
1. Cause:appearance of unique double jaw
3.Coevolution
A. When two species evolve in response to each other
B. Mutualism
1. Two species benefit each other
2. Ex: Comet orchid and moth: foot long tongue matches
Foot long flowers
C. Coevolutionary Arms Race
1.One species evolves a parasitic dependency on another
species
2. Ex:plant and insect that feeds on plant, plant evolves chemical defenses against insect; insects evolve to resist chemical defense
4.Convergent Evolution
A. Unrelated species evolving similar traits even though
they live in different parts of the world
B. Occurs in environments that are geographically far apart, but have similarecology and climate.
C. Ex: list three examples of animals showing convergent evolution:mara and rabbit; mole and marsupial mole; wolf and Tasmanian wolf
5.Rate of Speciation
A. gradualism
1.evolution proceeds in small gradual steps
B. punctuated equilibrium
1. explains abrupt transitions in the fossil record
2.rapid spurts of genetic change cause species to diverge quickly
3. Ex: snail shells changing dramatically in a few thousand years