Download Chapter 18 Speciation

Chapter 18
Speciation
What is a Species?
• The morphological species concept expresses
the following:
– Species, in its simplest interpretation means
“kind”
• The biological species concept relies on
reproduction to define relatedness of species
– Ernst Mayer says, “Species are groups of
interbreeding natural populations that are
reproductively isolated from other such groups.”
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• 1. Linnaeus separated species based on morphology, i.e., their traits
differed; Darwin saw that similar species are related by common
descent.
2. Ernst Mayr (1942) developed the biological species concept: a
species is a group of actually or potentially interbreeding populations
that are reproductively isolated from other such groups.
3. The biological definition of a species says that the members of one
species interbreed and have a shared gene pool, and each species is
reproductively isolated from every other species.
4. Gene flow occurs between populations of one species but not
between populations of different species.
5. Biochemical genetics uses DNA hybridization techniques to
determine relatedness of organisms; the phylogenetic species
concept uses DNA/DNA comparisons.
Speciation
• Is the attainment of reproductive isolation,
but genetic changes between population of
the same species can be countered by gene
flow.
• 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
Reproductively Isolating Mechanisms
• Any heritable features of body form,
functioning, or behavior that prevent
interbreeding between genetically divergent
populations
Prezygotic mechanisms take effect
before or after fertilization
• 1- Behavioral Isolation: patterns of courtship may
be altered to the extent that sexual union is not
achieved.
• 2- Temporal Isolation: Different groups may not
be reproductively mature at the same season, or
month, or year.
• 3- Mechanical Isolation: Two populations are
mechanically isolated when differences in
reproductive organs prevent successful
interbreeding
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• 4- Ecological Isolation: Potential mates may be
in the same general area by not in the same
habitat where they are likely to meet
• 5- Gametic Mortality: Incompatibilities
between egg and sperm prevent fertilization
Postzygotic Mechanisms: takes effect
after fertilization
• Sometimes fertilization occurs between
different species, but the hybrid embryo is
weak and dies.
• In some instances the hybrids are vigorous but
sterile
– Example: Mule produced by a male donkey and
female horse
Allopatric Speciation Defined
• Model some physical barrier arises and
prevents gene flow between populations
• Reproductive isolating mechanisms evolved in
the genetically diverging populations and will
result in complete speciation when the two
species can no longer interbreed
• First postzygotic isolation occurs and then
prezygotic isolation occurs.
Geographic Isolation
• Isolation may be sudden as in an earthquake
that results in speciation of species
• Studies of enzymes from fishes on the
Atlantic and Pacific sides of the Isthmus of
Panama reveal molecular differeneces
Allopatric Speciation on Archipelagos
• Archipelagos is an island chain some distance
away from a continent
• The finches of the Galapagos Islands are
evidently ancestors of mainland finches that
invaded first a few islands and then spread to
others, diverging as time progressed
Sympatric Speciation
• 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
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• Polyploidy is the inheritance of three or more
of each type of chromosome due to improper
separation of chromosomes during meiosis
and mitosis.
– Speciation is instantaneous for plants that are
polyploid
– Polyploid animals are rare because of dosage
compensation which regulates the level of gene
expression in chromosomes.
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
Branching and Unbranching 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
Evolutionary Trees and Rates and
Change
• Evolutionary Trees- summarize information about
the continuity of relationship among species
• Gradual model of speciation- is represented by
tree diagrams with branches at slight angles to
each other to show slow change over time
• Punctuation Model of speciation- drawn with
short, horizontal branches that represent abrupt
periods of speciation followed by stable periods
Adaptive Radiation
• 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.
• The case of Darwin's finches illustrates the
adaptive radiation of 13 species from one
founder mainland finch.
Extinctions
• Extinction is the rather inevitable loss of
species as local conditions change over
periods (usually long) of time.
• Mass extinctions are abrupt disappearances
dues to catastrophic, global events