Download CH 22: DESCENT WITH MODIFICATION

I.
CH 24: The Origin of Species
Macroevolution is the origin of new taxonomic groups
II.
What is a species?
A. Biological species concept emphasizes reproductive isolation- A population or group of populations whose
members have the potential to interbreed with one another in nature to produce viable/fertile offspring, but who
cannot produce the same with other species (note that humans have interbreed different species artificially
(ligers,tigrons,geeps etc but this is not natural environment)
Other species concepts include:
Morphology (forms), paleontological (fossils), ecological, phylogenetic ( lines of ancestry)
B. Prezygotic and Postzygotic barriers isolate gene pools of species
1. prezygotic barriers (before making a zygote)
1. habitat isolation-two species that live in same area but in different habitats may run into each other
infrequently (Ex in book one snake lives primarily in water, and other on land)
2. behavioral isolation-they way species behave to attract/communicate with mates isolates them (Book
Ex bird mating songs are unique to species)
3. temporal isolation- the two species breed during different times of the year
4. mechanical isolation-two species are anatomically incompatible
5. gametic isolation-sperm may not be able to survive the conditions within the female body, or certain
molecules on egg cell will stick to certain molecules on only certain sperm cells
2. postzygotic barriers (after the zygote is formed)
1.
reduced hybrid viability-low survival rate for those gametes that were not suppose to fuse 
abortion
2.
reduced hybrid fertility-the hybrid offspring is infertile
3.
hybrid breakdown-although the zygote will be viable and fertile, its next generation of offspring
will not be
III. Modes of Speciation (origin of a new species)
A.
Allopatric speciation (“different country”) speciation due to a geographic barrier physically isolating
populations to block gene flow
1. geographical barriers-can be anything from a mountain range, to a glacier, to a land bridge
how large must it be? Depends on the organisms ability to disperse (Ex. Grand Canyon can be crossed by
birds but not small rodents)
usually it is the peripheral individuals within a population that are favored for allopatric speciation since
their number may be small (genetic drift) and since their environment may be different from the center
habitat and act on them differently
2. adaptive radiation-the evolution of many diversely adapted species from a common ancestor,
‘explosion’ of new species, occurs a lot on islands
B.
Sympatric speciation – speciation due to changes within the same environment, or other changes within
the group.
1.
polyploidy-an error during mitosis or meiosis which results in gametes with extra sets of
chromosomes. Although the gametes can self fertilize, they cannot fertilize a normal gamete (whose
chromosomes separated normally during mitosis or meiosis)
2.
allopolyploidy-a hybrid produced between two species is usually infertile because it does not have
homologous chromosomes (therefore cannot produce viable gametes)
3.
sexual selection- non random mate selection, usually by the female, to ensure best reproductive
fitness
IV. The tempo of speciation
A. gradualism-species that descended from a common ancestor gradually diverge more and more in
morphology as they acquire unique adaptations/however, transitional fossils are rare
B. punctuated equilibrium model-a new species will change most as it buds from its parent species, and
then changes little for the rest of its life
**argument continues today
V. Evolutionary novelty (new stuff)
A. exaptation-a structure that evolved in one context is used today for another function. Ex. birds’ bones
are light-weight due to air pockets, but light bones pre-dated flight, so they must have served a purpose for
the birds while on ground – possible quick movement
B. heterochrony-evolutionary changes in timing or rate of development (can create new stuff)
1.
allometric growth-difference in the relative rates of growth of various parts of the body to help
shape the organism/subtle changes in this growth will affect the adult’s shape
C. paedomorphosis-when a sexually mature adult retains features that were juvenile structures in its
evolutionary ancestors. The sequence in which different body parts start/stop developing can affect the
traits of an organism (ex human brains are larger than chimpanzees because human brain growth signals are
turned off relatively later)