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Chapter 24: The Origin of Species
•Speciation:
(the appearance of new species is the source of biological
diversity)
•Microevolution: the change in the genetic makeup of a population from generation to generation
•Macroevolution: evolutionary change
•Niche:
(what an organism eats,
does, & where it lives)
Biological Species Concept
•Definition: a population or group of populations whose members have the potential to interbreed in nature
and produce viable, fertile offspring, but

Members of a biological species are united by being ____________________________
Reproductive Isolation
Definition: barriers that prevent members of two species from producing __________________________
Two types:
1)Prezygotic barriers:
2) Postzygotic barriers:
Prezygotic Barriers
•Habitat isolation: different
•Temporal isolation: different
•Behavioral isolation: different
•Mechanical isolation:
differences
•Gametic isolation: sperm can’t fertilize eggs of another species
Postzygotic barriers
Example of _______________
type of prezygotic barrier
•Reduced hybrid viability:
•Reduced hybrid fertility:
•Hybrid breakdown: 1st generation hybrids are viable and fertile, the 2nd generation is feeble/sterile
Speciation
•Speciation is the beginning of ________________________________________
•Cumulative change (during many speciation episodes) accounts for sweeping evolutionary change
Modes of speciation: Two ways speciation can occur:
1) Allopatric speciation: a population forms a new species while
2) Sympatric speciation:
Allopatric Speciation
•Gene flow is interrupted when a population is divided into two geographically isolated subpopulations
–Examples:
Once the geographic separation has occurred, the populations may be
acted on by genetic drift, sexual selection, etc.
Sympatric Speciation
•
•Common in plants: polyploidy
–Weeds, oats, cotton, potatoes, tobacco, wheat
Sympatric speciation: example in animals
•200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthorns
–today, these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to
America by immigrants and bred).
•Females generally choose to lay their eggs on the type of fruit they grew up in
–males tend to look for mates on the type of fruit they grew up in.
•Hawthorn flies generally end up mating with other hawthorn flies
–apple flies generally end up mating with other apple flies.
•This means that gene flow between parts of the population that mate on different types of fruit is reduced.
•This host shift from hawthorns to apples may be the first step toward sympatric speciation!
Adaptive Radiation
•
•EX: Hawaiian Islands: “Each island was born ‘naked’ and was gradually populated by stray organisms that rode the ocean
currents and winds”
Hawaiian honeycreepers: Best example of the evolutionary phenomenon known as adaptive radiation (changing to survive new habitats).
•From a single finch like ancestor who colonized these islands, over 40 species have evolved.
 Variation in color and bill shape in selected honeycreepers is related to their habitat and diet
Tempo of Speciation
•Gradualism:
•Punctuated equilibrium:
Evolution: NOT goal oriented
•Evolution= branching tree; NOT ladder of progress
•EX: evolution of horses: today’s horse is the only surviving
“twig” of an evolutionary tree where many other species died out