the origin of species chapter 24. microevolution vs macroevolution microevolution – changes in...
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THE ORIGIN OF SPECIES
CHAPTER 24
Microevolution vs Macroevolution Microevolution – changes in allele frequencies in a
given population Macroevolution – broad pattern of evolution above
species level (origins of new groups of organisms through a series of speciation events
Biological Species Concept Species - A group of populations whose members
have the potential to interbreed in nature and produce viable, fertile offspring – but do not produce viable fertile offspring with members of other such groups
The members of a biological species are united by being potentially reproductively compatible
Speciation – process by which one species splits into two or more species
Biological Species Concept emphasizes the separateness of species from one another due to reproductive barriers
Other Definitions of Species
Definitions Based on Unity of Specieso Morphological Species Concept – characterizes
species by body shape and other structural features; can be applied to sexual and asexual organisms
o Ecological Species Concept – views a species in terms of how members of the species interact with the biotic and abiotic factors of their environment
o Phylogenetic Species Concept – defines species as smallest group of individuals that share a common ancestor
Reproductive Barriers: Pre-zygotic
Pre-zygotic – barriers that prevent fertilization
1. Habitat isolation – populations cannot get together
2. Behavior isolation – different mating behavior
3. Temporal isolation – breed or flower at different times
4. Mechanical isolation – different anatomical structures
5. Gamete isolation – gametes fail to fuse; do not recognize each other
Reproductive Barriers: Post-zygotic
Post-zygotic – barriers that prevent zygote from developing into fertile offspring
1. Reduced hybrid viability – zygote either fails to develop or to reach sexual maturity
2. Reduced hybrid fertility – hybrid is sterile
3. Hybrid breakdown – hybrid produces offspring by offspring are either not viable or not fertile
Modes of Speciation
Allopatric – gene flow is interrupted when a population is segregated by a geographic barrier (ocean, mountain range, etc.)
Favoring conditions:o Small population at fringe of larger one
o Better change is gene pool already somewhat different
o Different selection factors in same population
Adaptive Radiation – emergence of numerous species from a common ancestor introduced into a new environment
Modes of Speciation
Sympatric – new species arise within the parent population
Reproductive isolation without a geographic barrier o Polyploidy – accident during cell division that results in
extra sets of chromosomes (~80% of plants today are the result of this)
o Habitat differentiation – subpopulations exploit a habitat or resource not utilized by the entire population
o Sexual selection – mate choice based on different appearance of mates of same population
Genetic Change of Subpopulations
Adaptive Divergenceo 2 parts of population adapt to different
environmentso Each accumulates different gene pools
Reproductive Barriers Developo Usually secondary to change in gene poolo May be the side effect of sexual selection
TEMPO OF SPECIATION
Gradualismo One species gradually evolves into a new specieso Represents microevolutiono Big changes occur through the accumulation of
small changes
TEMPO OF SPECIATION
Punctuated Equilibriumo Long periods of stasis (equilibrium) punctuated
by episodes of speciationo Species undergo most change when they branch
from parent species; then change very little aftero Species develop in spurts of rapid changeo Neither slow nor gradual; end of age of dinosaurs,
beginning of age of mammals
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