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 Species Morphological Species Concept – characterizes species by body shape and other structural features; can be applied to sexual and asexual organisms Ecological Species Concept – views a species in terms of how members of the species interact with the biotic and abiotic factors of their environment 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 Habitat isolation – populations cannot get together Behavior isolation – different mating behavior Temporal isolation – breed or flower at different times Mechanical isolation – different anatomical structures 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 Reduced hybrid viability – zygote either fails to develop or to reach sexual maturity Reduced hybrid fertility – hybrid is sterile 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: Small population at fringe of larger one Better change is gene pool already somewhat different 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 Polyploidy – accident during cell division that results in extra sets of chromosomes (~80% of plants today are the result of this) Habitat differentiation – subpopulations exploit a habitat or resource not utilized by the entire population Sexual selection – mate choice based on different appearance of mates of same population

Genetic Change of Subpopulations Adaptive Divergence 2 parts of population adapt to different environments Each accumulates different gene pools Reproductive Barriers Develop Usually secondary to change in gene pool May be the side effect of sexual selection

TEMPO OF SPECIATION Gradualism One species gradually evolves into a new species Represents microevolution Big changes occur through the accumulation of small changes

TEMPO OF SPECIATION Punctuated Equilibrium Long periods of stasis (equilibrium) punctuated by episodes of speciation Species undergo most change when they branch from parent species; then change very little after Species develop in spurts of rapid change Neither slow nor gradual; end of age of dinosaurs, beginning of age of mammals