CHAPTER 22-24 OUTLINE Part II
SPECIATION AND PHYLOGENY
Reminder:
Population: group of individuals that interbreed and share a common gene
pool.
Deme: small, local population. (ie. deer mice in a certain woodland) Usually
temporary…these example are not absolute
 Members of an individual deme resemble each other more than
other members of other demes of the same species.
 WHY?
Species: largest unit of population within which EFFECTIVE gene flow occurs or
can occur. (reproduction in their natural environment)
Variation within Species:
Demes tend to evolve adaptations to the specific environmental conditions to
which they live.
This “geographic variation is found in most species.
Ie. temperature, rainfall, elevation etc…
Cline: a gradual variation correlated with geography.
Subspecies:
It is not the degree of morphological resemblance that determines whether or not
two populations belong to the same species, but rather whether they are
reproductively isolated. (some situations, the members of the two populations are
of the same species, but are incapable of producing viable offspring.
THE PROCESS OF SPECIATION:
Divergent Speciation:
 Geographic isolation
b. Allopatric:
Sympatric:
THREE Factors that make geographically separate populations DIVERGE:
1.
2.
3.
Reproductive Isolation:
As two populations diverge, they accumulate differences that may, over
time, lead to characteristics that may prevent the two from interbreeding IF or
when they meet again…Some of these isolating mechanisms that can evolve are:
c. Ecogeographic isolation:
d. Habitat Isolation:
e. Seasonal isolation
(see Fig. 18.9, mating seasons of five species of frogs, all sympatric.)
f. Behavioral Isolation: Ch. 38 expands on this…
Ie. Jocelyn Crane and fiddler crabs:
g. Mechanical Isolation
h. Gametic Isolation:
The above mechanisms do not ‘cost’ the individual organism. The
isolation is either imposed or generated internally from existing behavior,
physiology and morphology.
The following, exact a significant cost from the individual, as a selection
pressure favoring evolution of the more efficient mechanism of
reproductive isolation already mentioned.
i. Developmental Isolation
j. Hybrid Inviability
k. Hybrid Sterility
l.
Selective Hybrid Elimination:
b. If those offspring and their progeny are vigorous, and adaptive like
their parental forms, then the two original populations will not
remain distinct.
c. If those offspring and their progeny are less well adapted, then the
parental forms, they will be eliminated. Only limited gene flow
will occur through the hybrids. The parental populations are
separate species.
The last four mechanisms isolate species from one another effectively, but costly.
Metabolic resources are spent in non-viable embryos, or sterile young. Mistake
matings waste gametes. Selection favors individuals whose behavior,
morphology or physiology reduces the chance of mismatch in the first place.
Gene combinations that lead to correct mate selection will increase in frequency
until eventually all hybridization ceases. This tendency of closely related
sympatric species that will diverge rapidly to REDUCE the chances of
hybridization is character displacement.
See table 18.1. Typically, more than one of these isolating mechanisms contribute
to keeping two species apart.
SPECIATION BY POLYPLOIDY AND CHROMOSOMAL CHANGE:
Speciation by polyploidy and chromosomal change:
Sympatric speciation:
POLYPLOIDY is one example. (common in plants, almost 70%of
plant species arose this way.
AUTOPOLYPOLIDY:
Ie
ALLOPOLYPLOIDY
BOTH of the above have been important to new crop plants, to create new
varieties (polyploids)
PROBLEMS:
Nonchromosomal sympatric speciation:
So, we have geographic isolation and polyploidy (chromosomal
changes)
Can this occur without polyploidy? In other words are there other means of
sympatric speciation?
a. habitat preference:
b. host specificity: ie.
c. Sexual imprinting: (also in Ch. 38)
Divergent Evolution, the evolutionary splitting of species into many separate
descendant species, has been as frequent as extinction. (only .1% of all species
ever to exists is in our present ecosystem)
Adaptive Radiation: An example of RAMPANT divergent evolution:
Finches on the Galapagos Islands.
HOW did the descendents split into 14 species???
Because of the geography of the 25 separate islands, and the fact that Finches will
not readily fly across them, a population on any one of the islands is effectively
isolated from the populations on the others.
Show 18.16…if two populations being isolated from each other, geographically,
would eventually evolve into separate species, then if they colonized the same
island, they would NOT interbreed.
If members of two species A and B coexisted on the same island, they could not
continue to exist on the same food supply
RATES of evolutionary divergence is not constant.
How Constant and Important is Competition??
REMINDER: Barriers that contribute to reproductive isolation can arise by
chance (genetic drift), by natural selection, or by a combination of the two.
Competition plays a big role in the formation of the species, even in
reproductively isolated populations, as they may not be able to coexist indefinitely
if competing for the same food. The superiority of one will lead to the eventual
extinction of the other.
NICHE RULE: no two species occupying the same niche can long coexist.
CHANCE though, now takes a larger role in causing species to diverge:
1. Crises:
2. ecological changes that alter the competitive “landscape”.
Example of above:
Chance: based on consequences of poor rainfall, lack of food resources,
simply by chance….
What seemed to be an adaptive equilibrium, was upset by a chance disturbance,
and a population with altered characteristics emerged….
EVOLUTIONARY BOTTLENECK.
-Although most believe that competition is the major force leading to speciation
over time, studies also show that rare “boom or bust” events have been important
in the evolution of some species.
THEORY!!!! Punctuated Equilibrium: Hypothesis based on evidence of natural
catastrophes affecting allelic frequencies of populations. (mode and tempo of the
speciation) Could also be the result of major genetic alterations (ie. polyploidy,
deletions, translocations and hybridization) that affect the control genes, which can
change the expression of groups of other genes.
This versus the Darwinian theory of ‘gradualism’, or accumulation of
morphological changes over time.
The Cambrian Explosion: example of rapid evolutionary radiation. The ‘sudden’
appearance of complex animals, as older fossils are nearly all single cell animals,
occurred during the Cambrian Period, 550 million years ago.
FAUNA: animals that typify a given place or period.
 How did these animals, mostly arthropods, come into existence so quickly?
PROBABLY, the tempo of speciation lies in the middle of gradual and
punctuated theories.
CHANCE??? Read the paragraph on p. 511-512…if things didn’t fall just right, or
happen at a certain time… perhaps we would not be the dominant species of
today’s world…..hmmmmmmm
PROBLEMS IN IDENTIFYING SPECIES:
Although most populations can be identified according to our definition of
species…there are some exceptions:
1. Asexuals:
ie. dandelions
2. Fossil species:
3. Populations v. species:
4. Allopatric Species:
PHYLOGENY: Determining ancestry (evolutionary history) and defining
groups (lots of discussion among biologists…)