The Nature of Species
•Any
concept of species must account for:
–Distinctiveness
of species occurring together in a single locality.
–Connection
among populations of the same species that are geographically separated.
The Nature of Species
•Distinctiveness
of Sympatric Species
–Sympatric
species are distinctive entities that are phenotypically
different, utilize different parts of the habitat, and behave separately.
–Two
species that occur together and appear to be nearly identical are termed
sibling species.
The Nature of Species
•Geographic
Variation Within Species
–Populations
within a species that occur in different areas and are distinctive may be
classified as subspecies or varieties.
•Biological
Species Concept
–If
sympatric species commonly exchange genes, their gene pools should become
homogenized.
Biological Species Concept
•Ernst
Mayr
–“groups of actually or potentially interbreeding natural
populations which are reproductively isolated from other such groups.”
•Individuals
that cannot produce fertile offspring are termed reproductively isolated, and
thus members of different species.
–Hybridization
Biological Species Concept
•Application
Problems
–Difficult
to apply the concept to populations that do not occur together in nature (Allopatric populations).
–No
meaning for asexual species.
Prezygotic Isolating Mechanisms
•Prezygotic Isolating Mechanisms prevent the formation of
zygotes.
•Ecological
Isolation
–Even
if two species occur in the same area, they may utilize different portions of
the environment and thus do not hybridize because they do not encounter each
other.
•Lions and Tigers in India
Prezygotic Isolating Mechanisms
•Behavioral
Isolation
–Many birds differ in courtship rituals which tends to keep species distinct.
•Temporal
Isolation
–Varying breeding seasons.
•Mechanical
Isolation
–Reproductive structural differences.
•Prevention
of Gamete Fusion
–Broadcasting of gametes.
Postzygotic Isolating Mechanisms
•Postgotic Isolating Mechanisms prevent the proper
functioning of zygotes after they form.
–Hybrid
inviability
–Hybrid
sterility
–Hybrid
breakdown
Reproductive Isolation and
Evolutionary Change
•Most
reproductive isolating mechanisms initially arise for some reason other than to
prevent reproduction.
•Random
Changes May Cause Reproductive Isolation
–Given
long enough periods of time, any two isolated populations will diverge due to
genetic drift.
Reproductive Isolation and
Evolutionary Change
•Adaptation
and Speciation
–Adaptation
and Speciation are probably related in many cases.
•As species adapt, they accumulate
many differences that may lead to reproductive isolation.
Geography of Speciation
•Speciation
is a two-part process:
–Identical
populations must diverge.
–Reproductive
isolation must evolve to maintain the differences.
•Allopatric Divergence is Primary
–Geographically
separated populations appear much more likely to have evolved substantial
differences leading to speciation.
Problems With the Biological
Concept
•Extent
of Hybridization
–In
recent years, greater amounts of hybridization than
previously realized between populations has been found.
•Seem to be neither experiencing
reinforcement nor losing specific identities.
•Recent survey
indicated about 10% of world’s 9,500 species are known to have
hybridized in nature.
•Adaptive
Radiation - Process producing a cluster of species, occupying a series of
similar habitats, all evolving from a recent ancestor.
•Galapagos
finches comprise four groups:
–Ground finches
–Tree finches
–Warbler finch
–Vegetarian finch
Hawaiian Drosophila
•At
least 1,250 species of Drosophila in the world; more than a quarter
found only in the
–Native
Hawaiian flies are closely associated with native plants.
•When ancestors first reached the
islands, they probably encountered many empty niches, thus facilitating
diversification and speciation.
Diversity of Life Through Time
•Trends
in Species Diversity
–There
have been 5 major mass extinctions interspersed within relatively consistent
extinction patterns.
•Most Famous - End of Cretaceous
Period (65 mya)- Dinosaurs
went extinct.
–Mammals
quickly experienced evolutionary radiation.
»Biological
diversity tends to rebound after mass extinctions.