2. 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
3. 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
4. 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.
6. 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
8. 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.
12. 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
14. 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
16. Postzygotic Barriers
• Hybrid inviability
– offspring don’t make it
• Hybrid sterility
– e.g. mules
• Hybrid breakdown
– F2
are sterile or weak
17. Introgression
• Alleles pass a reproductive barrier
when a fertile hybrid mates with a
parent species
• Increases variation
• Rare
– 2 species remain distinct
18. 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
20. 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
