Evolution3
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Transcript of Evolution3
SpeciationThe Process of
Evolution
Speciation• Formation of a new species• Species:
– a population that can breed freely and produce fertile offspring
• Speciation often occurs when part of the population is isolated from another part– Selective pressures of the environment in
one area may be different from pressures in another area
What is a Species?• Definition :• Morphospecies - based on appearance• Biologic species - a population that can
breed freely and produce fertile offspring• The largest unit of population in which gene
flow is possible• Limitations:
– doesn’t work for asexual organisms– extinct life forms– populations that are geographically isolated -
sometimes call subspecies
• No clear answer; idea is arbitrary
Patterns of Speciation
• Fossil record shows 2 patterns:• Anagenesis (phyletic evolution)
– the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species
• Cladogenesis – branching evolution; budding of one or
more new species from a parent species that continues to exist.
Anagenesis vs. Cladogenesis
Causes of Speciation• Speciation often occurs when part of
the population is isolated from another part
• Geographic Isolation – most common– a physical barrier develops (changing
course of a river; separation of an island)– Selective pressures in one area are
different from pressures in another area
• Reproductive Isolation– another form of isolation
Isolation
Geographic Isolation• Biogeography of Speciation • Classified according to geographic relationship
between new and old species• Sympatric
– population becomes reproductively isolated in the midst of the parent population
– ranges of new and old species overlap.
• Allopatric – species are physically separated– more likely in small populations
• Adaptive radiation is allopatric :– emergence of numerous species from a common
ancestor that spreads to several new environments.
Allopatric vs. Sympatric
Allopatric Barriers
Geographic Isolation
Reproductive Isolation
• Example: organisms breed at different times
• Reproductive barriers are of 2 types:• Prezygotic
– before the formation of fertilized eggs– impedes mating or fertilization
• Postzygotic – after
Reproductive Isolation
Prezygotic Isolation• Impedes mating or fertilization• Habitat isolation
– not geographically separated, but occupy different niches within an area, e.g. trees versus ground
• Temporal isolation – breed at different times
• Behavioral isolation– don’t produce appropriate courtship signals
• Mechanical isolation – anatomically incompatible
• Gametic isolation – mating occurs but gametes rarely fuse to form
zygotes
Behavioral Isolation: Courtship Barrier
Postzygotic Barriers
• Hybrid inviability – offspring don’t make it
• Hybrid sterility – e.g. mules
• Hybrid breakdown – F2 are sterile or weak
Introgression
• Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species
• Increases variation• Rare
– 2 species remain distinct
Post Speciation Evolution• Divergent Evolution
– Process by which related organisms become less alike
– occurs after speciation– at first 2 new species are very similar, but
over time become more & more different.
• Adaptive radiation is a special type of divergent evolution– Many new species from a single parent
species
Adaptive Radiation
Timing of Evolution• Most scientists accept natural selection as the
process of evolution• The timing is controversial• Gradualism
– the traditional view– a slow, steady accumulation of changes, leads to
new species
• Punctuated Equilibrium– long periods of inactivity followed by big jumps
• Fossil record provides evidence that the pace of evolution varies– The same evidence is used to support different ideas– Could be some of both
Gradualism vs. Punctuated Equilibrium