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Lecture Outline
The Case of the Road-Killed Snails
A. Sometimes it is easy to see how an animal that flies long distances would be able to
take its genes from one place to another, but what about the slow-moving snail?
B. Yet even snails confined to relatively small areas show genetic variation that could
possibly lead to eventual speciation many years hence.
18.1
On the Road to Speciation
A. What is a Species?
1. The morphological species concept expresses the following:
a. Species, in its simplest interpretation, means "kind."
b. Attempting to determine whether similar, yet different, animals are the same
species by appearance (phenotype) is not reliable due to the subtle variations
that are displayed.
2. The biological species concept relies on reproduction to define relatedness of
species.
a. Ernst Mayer says, "Species are groups of interbreeding natural populations
that are reproductively isolated from other such groups."
b. As good as it is, this definition is troublesome for organisms that are nonsexually reproducing and those known only from fossils.
3. Speciation is the attainment of reproductive isolation, but genetic changes
between populations of the same species can be countered by gene flow.
4. Genetic divergence is the process whereby local units of a population become
reproductively isolated from other units and thus experience changes in gene
frequencies between them.
B. Reproductive Isolating Mechanisms
1. Reproductive isolating mechanisms are any heritable features of body form,
functioning, or behavior that prevent interbreeding between genetically
divergent populations.
2. Prezygotic mechanisms take effect before or during fertilization.
a. Behavioral Isolation: patterns of courtship may be altered to the extent that
sexual union is not achieved (for example: albatross courtship rituals).
b. Temporal Isolation: different groups may not be reproductively mature at the
same season, or month, or year (for example: periodical cicadas).
c. Mechanical Isolation: two populations are mechanically isolated when
differences in reproductive organs prevent successful interbreeding (for
example: floral arrangements in sage plants discriminate between different
bee pollinators).
d. 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 and habitats).
e. Gametic Mortality: incompatibilities between egg and sperm prevent
fertilization (for example: signals to pollen grains to begin growing toward
the egg).
3. Postzygotic mechanisms take effect after fertilization.
a. Sometimes fertilization does occur between different species, but the hybrid
embryo is weak and dies.
b. In some instances the hybrids are vigorous but sterile (example: mule
produced by a male donkey and a female horse).
18.2
Speciation in Geographically Isolated Populations
A. Allopatric Speciation Defined
1. In this model some physical barrier arises and prevents gene flow between
populations.
2. Allopatric refers to the “different lands” the two species occupy.
3. Reproductive isolating mechanisms evolve in the genetically diverging
populations and will result in complete speciation when the two species can no
longer interbreed.
B. The Pace of Geographic Isolation
1. Isolation may be sudden as in an earthquake that results in separation of species.
2. Glaciers, on the other hand, may produce conditions that yield separate over vast
periods of time.
3. Studies of enzymes from fishes on the Atlantic and Pacific sides of the Isthmus of
Panama reveal molecular differences.
C. Allopatric Speciation on Archipelagos
1. An archipelago is an island chain some distance away from a continent.
2. 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.
18.3
Models for Other Speciation Routes
A. Sympatric Speciation
1. In this model, species may form within the home range of an existing species, in
the absence of a physical barrier.
2. Literally means "together with others in the homeland."
3. In two crater lakes of East Africa exist small fish called cichlids.
a. The species in each lake are alike in their mitochondrial DNA and unlike the
species in neighboring lakes and streams.
b. The lakes are small so the fish must live in sympatry.
4. Polyploidy is the inheritance of three or more of each type of chromosome due to
improper separation of chromosomes during meiosis or mitosis.
a. Speciation is instantaneous for plants that are polyploid.
b. Polyploid animals are rare because of dosage compensation which regulates
the level of gene expression in chromosomes.
B. Parapatric Speciation
1. Daughter species form from a small proportion of individuals along a common
border between two populations.
2. Literally means "near another homeland."
3. Interbreeding individuals produce hybrid offspring in this region called a hybrid
zone.
18.4
Patterns of Speciation
A. Branching and Unbranched Evolution
1. Cladogenesis applies to populations that become isolated from one another and
subsequently diverge in different directions.
2. Anagenesis is a pattern of descent in which species form within a single,
unbranched line.
B. Evolutionary Trees and Rates of Change
1. Evolutionary trees summarize information about the continuity of relationship
among species.
2. The gradual model of speciation is represented by tree diagrams with branches at
slight angles to each other to show slow change over time.
3. The punctuation model of speciation is drawn with short, horizontal branches that
represent abrupt periods of speciation followed by stable periods.
C. Adaptive Radiations
1. An adaptive radiation is a burst of microevolutionary activity that results in the
formation of new species in a wide range of habitats.
2. The presence of adaptive zones presents new ways of life by physical,
evolutionary, or ecological access.
D. Extinctions—End of the Line
1. Extinction is the rather inevitable loss of species as local conditions change over
periods (usually long) of time.
2. Mass extinctions are abrupt disappearances due to catastrophic, global events.