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