Here is where it gets ugly…ultimate subjectivity. If you
complete this chapter and still think that a species truly exists, you missed
the whole point.
On the Road to Speciation
Reproductive Isolation Mechanisms
- What is a Species?
- The morphological species concept -
what about subtlety?
- The biological species concept -
what about asexual individuals?
- The genetic species concept - what
- The ecological species concept -
what about history?
Postzygotic Isolation (mechanisms take effect after fertilization)
- Reproductive isolating mechanisms are any heritable features of anatomy,
physiology, or behavior that prevent interbreeding between populations.
- Prezygotic Isolation (mechanisms take effect before or during
- Temporal Isolation: different groups may not be
reproductively mature at the same season, or month, or year (ie-
periodical cicadas, coral spawning).
- Behavioral Isolation: patterns of courtship may be altered
to the extent that sexual union is not achieved (for example: albatross
- Mechanical Isolation: two populations are mechanically
isolated when differences in reproductive organs prevent successful
interbreeding (ie- sage plants and bee pollinators, Great Dane &
- Ecological Isolation: potential mates may be in the same
general area by not in the same habitat where they are likely to meet (for
example: different species of Manzanita shrubs live at different altitudes
- Gametic Mortality: incompatibilities between egg and sperm
prevent fertilization (for example: chemoreception between coral larvae).
- Embryonic (larval) Mortality: gametes combine but the result is
an incompetent larvae or embryo. (Corals…)
Speciation in Geographically Isolated Populations
- Sometimes fertilization does occur between different species, but the
hybrid embryo is weak and dies.
- Hybrid sterility - In some instances the hybrids are
vigorous but sterile (example: mule produced by a male donkey and a female
- F2 breakdown - The hybrid is fertile, but the
"grandchildren" are sterile.
- Geographic Isolation
- Isolation may be sudden as in an earthquake that results in separation
- Glaciers, on the other hand, may produce conditions that yield
separate species over vast periods of time.
- Studies of enzymes from fishes on the Atlantic and Pacific sides of
the Isthmus of Panama reveal molecular differences.
- Allopatric Speciation
- In this model some physical barrier arises and prevents gene flow
- Allopatric refers to the "different lands" the two species occupy.
- Reproductive isolating mechanisms evolve in the genetically diverging
populations and will result in complete speciation when the two species
can no longer interbreed.
An archipelago is an island chain some distance away from a continent.
The finches of the Galápagos Islands are evidently ancestors of
mainland finches that invaded first a few islands and then spread to
others, diverging as time progressed.
Models for Other Speciation Routes
Patterns of Speciation
- Sympatric Speciation
- In this model, species may form within the home range of an existing
species, in the absence of a physical barrier.
- In two crater lakes of East Africa exist small fish called cichlids.
- The species in each lake are alike in their mitochondrial DNA and
unlike the species in neighboring lakes and streams.
- The lakes are small so the fish must live in sympatry.
- Polyploidy - speciation is instantaneous for plants that are
- Parapatric Speciation
- Daughter species form from a small proportion of individuals along a
common border between two populations.
- Interbreeding individuals produce hybrid offspring in this region
called a hybrid zone.
Evolutionary Trees and Rates of Change
- Branching and Unbranched Evolution
- Cladogenesis applies to populations that become isolated
from one another and subsequently diverge in different directions.
- Anagenesis is a pattern of descent in which species form
within a single, unbranched line.
- A dendrogram summarizes information
about the continuity of relationship among species.
- The gradual model of speciation is
represented by tree diagrams with branches at slight angles to each other
to show slow change over time.
- The punctuation model of speciation
is drawn with short, horizontal branches that represent abrupt periods of
speciation followed by stable periods.
- An adaptive radiation is a burst of
microevolutionary activity that results in the formation of new species in
a wide range of habitats.
- The presence of adaptive zones presents new ways of life by physical,
evolutionary, or ecological access.
Extinctions–End of the Line
Biodiversity on Oceanic Islands: its Origin and Extinction; American
Zoology, by Gustav Paulay
Human migration through time…the Neanderthal puzzle…
- Background extinction is the rather inevitable loss of species as
local conditions change over periods (usually long) of time.
- Mass extinctions are abrupt disappearances due to catastrophic, global
- Inbreeding refers to nonrandom mating among closely related
- It tends to increase the homozygous condition, thus leading to lower
fitness and survival rates.