Download The origin of species.

The origin of species.
AP chapter 22
Miss Kim Jensen
2002-2003
Summary
• The origin of species or speciation is the basis of the
evolution of biological diversity.
• Anagenesis or phyletic evolution, involves the
transformation of a lineage of organisms into forms
different enough to be considered new species.
• In cladogenesis or branching evolution, new species arise
from parent species that continue to exist.
• Cladogenesis is both the more common pattern of
evolution and the process that increases biological
diversity.
• Evolutionary theory attempts to determine the mechanism
by which new species originate.
The species problem
Taxonomists often find that their classification of local
species corresponds to the taxonomy of a region
• Biological species _
• morphospecies –
Mayr (1942) goes beyond
anatomical features.
physical differences and
considers reproductive
• Species are most often
and genetic isolation to be
characterized by their
the basis for separating
physical form or
species.
morphology.
• Have the potential to
interbreed and produce
fertile offspring.
• The largest unit in which
gene flow is possible.
Reproductive barriers
http://gort.canisius.edu/~dehn/chpt24.htm
Reproductive Barriers - Reproductive Isolation
prevents pop. from interbreeding
1. Pre-zygotic barriers - impedes mating between
species or hinders fertilization of ova if mating
occurs
– a. geographic or habitat isolation - species live in
same general area, but habitat preferences Are so
different that they rarely mate or cross fertilize
each other
• example: Scarlet oak lives in swamps, poorly drained
bottom land, with acidic soils; black oak well drained soils
Prezygotic preproductive
barriers. Cont.
– b. temporal isolation - breed during different times
of the day or different seasons of the year
– c. behavioral isolation - most important in animals,
courtship patterns different for different species
– d. mechanical isolation - differences in structure of
genital parts restricts gametic transfer- recently
found to be less effective
– e. chemical or gametic isolation - due to difference
in structure of flower parts - attracts different
pollinators
Reproductive Barriers
2. Post-zygotic barriers - prevent zygote from
developing into a viable, fertile adult
– a. hybrid sterility (reduced hybrid fertility) - sterile
hybrid generally occurs during development of
reproductive organs/structures or during meiosis
when hybrid matures
– b. hybrid inviability (reduced hybrid viability) development of embryo stops - typical stages of
susceptibility are at. first cleavage or gastrulation
when many genes in the hybrid are suddenly
turned on. These events can't be properly
coordinated & development stops
– c. hybrid breakdown or weakness - weak,
abnormal or sterile progeny in 2nd generation,
even though first generation is ok
The biogeography of Speciation
• The evolution of reproductive barriers is the
critical event in the origin of new species.
• When the gene pool of a population becomes
separated from other populations, the isolated
population can follow its own evolutionary
course as a result of selection, genetic drift and
mutation.
• Speciation mechanisms can be grouped by
biogeographical factors: Allopatric speciation and
Sympatric speciation. Speciation occurs faster in
smaller populations
•Allopatric speciation occurs when the gene pool of
a population becomes segregated geographically
from other populations
Sympatric speciation occurs when a subpopulation
becomes reproductively isolated in the midst of its parent
population.
The mechanisms leading
to speciation can be
• 1. Natural selection (multiple
niche polymorphism)
• 2. Genetic drift (founder effects,
bottlenecks)
• 3. Non-random mating (sexual
selection)
Genetic Mechanisms of
Speciation
• Speciation by divergence – This does not
exactly favor reproductive barriers; genetic
isolation may be a secondary consequence of
adaptations to different environments.
• Speciation by peak shifts – Adaptive peaks
occur where a population’s gene pool is at an
equilibrium that maximizes fitness.
Punctuated Equilibrium
• Punctuated equilibrium is an evolutionary theory
advocated by Stephen Jay Gould and Niles Eldredge.
This theory has been hotly debated for the past 27 years.
• Evolution occurs in a stepladder fashion, with fits and
burst of development (morphological change).Such
bursts are followed by long periods of stasis.
• These periods of stasis may last for millions of years.
• This contrasts with Darwin’s original theory, which
offered that evolution was a gradual process that led to
the perfection of species.
Four points of evidence that point towards
punctuated equilibrium
1. Individual Cases show that branching occurs, rather that
phyletic transformation. An example is that ancestors
outlive their descendents, showing that evolution is not
a steady process.
2. The majority of evolution cannot be accounted for by
gradual development. 90 percent of cases can only be
accounted for by punctuated equilibrium.
3. Unstable environments account for rapid genetic
change, whereas stable environments allow for the
possibility of gradual change.
4. Tests show that punctuated equilibrium is predominant.
Gradualists
• Some gradualists maintain, however, that fossils
show stasis only in external anatomy and that
changes in internal anatomy, physiology and
behavior go unrecorded.
• Even reports of long periods of particularly
complete set of trilobite fossils, Sheldon found a
gradual change in aspects of their morphology,
which challenged earlier categorizations of the
youngest and oldest fossils in each evolutionary
lineage as different species.
Taxonomy: names organisms, assigns hierarchical
relationships
Populations: the individuals of a species in a
(contiguous) area
Cladogenesis: evolution and differentiation of
species lineages
• http://www.geo.arizona.edu/Antevs/ecol43
8/evolglos.html#grad