Download Species and Their Formation

Species and Their Formation
Patterns of Speciation
Biological Species
• What are biological species?
– fundamental units of classification
Biological Species
• Species definitions
– several exist
• Morphological species
–species are discrete morphological units
»members and non-members are
distinguishable
but
Biological Species
• Morphological species
– some problems exist
• some reproductively distinct species are
not easily distinguishable
and
• some morphospecies freely interbreed
therefore
• animal species are generally defined by
their reproductive patterns
Biological Species
• The Biological Species Concept
– Ernst Mayr (1940)
“Species are groups of actually or potentially
interbreeding natural populations which are
reproductively isolated from other such
groups.”
Biological Species
• “Species are groups of actually or potentially
interbreeding natural populations which are
reproductively isolated from other such
groups.”
groups: collections of local populations
actually or potentially: are or could be if in
close proximity
natural: not in captivity or under coercion
reproductively isolated: prevented from
genetic exchange
Biological Species
• A “Biological Species” is a group of
individuals that shares a gene pool.
Biological Species
• The BSC fits some organisms better than
others.
– most plant species are morphologically
distinguishable
– many plants freely interbreed with clearly
distinct morphospecies
– many plants reproduce asexually, almost
exclusively
Species Formation
• Evolutionary change occurs according to one
of two patterns
1. anagenesis
• change over time in a single lineage
• may produce a new species
Species Formation
• Evolutionary change occurs according to one
of two patterns
2. cladogenesis
• interruption of gene flow between two
segments of a population
–the two groups evolve independently
–the two gene pools cannot exchange
information
Species Formation - Cladogenesis
Figure 24.3
Species Formation
• Evolutionary change occurs according to one
of two patterns
– cladogenesis
• changes that occur may prevent
interbreeding when (if) the two groups are
reintroduced
Species Formation
• Three modes of cladogenesis
1. allopatric speciation (a.k.a. geographic
speciation)
• speciation due to physical barriers
–land barrier to aquatic species
–water barrier to land species
–habitat barrier to fastidious species
–distance barrier to mobile species
water
may be
a barrier
to
land organsims
Figure 24.4
distance may be a barrier to mobile organisms
Figure 24.5
distance
may
be a
barrier to
mobile
organisms
Figure
24.6
Species Formation
• Three modes of cladogenesis
1. allopatric speciation
• a population may be divided
• divided groups may experience different
evolutionary agents
• once reintroduced, they may be
reproductively incompatible
• if gene flow does not resume, they are
distinct species
Species Formation
• Three modes of cladogenesis
2. sympatric speciation occurs without physical
separation
• most often through polyploidy
–autopolyploidy
»formation of diploid gametes
»self or infraspecies fertilization
»tetraploid offspring
• new population can’t interbreed effectively
with diploid parent population
Figure 24.7
Species Formation
• Three modes of cladogenesis
2. sympatric speciation occurs without
physical separation
• most often through polyploidy
–allopolyploidy
»formation of a diploid hybrid
»asexual reproduction
»formation of diploid gametes
»self or infraspecific fertilization
Allopolyploidy
diploid &
tetraploid
populatiion
Distributions
Figure 24.8
Species Formation
• polyploidy is common among plants
– ~70% of flowering plants, 95% ferns are
polyploid
– polyploid species can spread rapidly
– polyploid species can be more successful
than their diploid parents
Species Formation
• sympatric speciation among animals is less
common, but not unknown
– more common is strict habitat selection and
mating behavior
• sympatric picture-winged fruit flies
reproduce on different fruits
Hyla
versicolor
a tetraploid
frog
Species Formation
• Three modes of cladogenesis
3. parapatric speciation occurs between two
adjacent populations without a physical
barrier
– e.g. plant populations differing in tolerance
to heavy metal toxicity
Reproductive Isolation
• conditions or mechanisms that prevent gene
flow between two populations
• geographically separated populations may still
be “potentially interbreeding”
Figure 24.9
Reproductive Isolation
• conditions or mechanisms that prevent gene
flow between two populations
• geographically separated populations may still
be “potentially interbreeding”
• reintroduced populations may be unable to
reproduce for many different reasons
– reproductive isolating mechanisms may
operate before fertilization (prezygotic) or
after fertilization (postzygotic)
Prezygotic Reproductive Isolation
• Spatial: two new species have come to prefer
different habitats
• Temporal: two species have adopted
reproductive periods that do not overlap
• Mechanical: sizes/shapes of reproductive
organs have become incompatible
• Gametic: gametes cannot fuse because of
chemical incompatibility
• Behavioral: the other is not seen as a mate
Figure 24.10
spatial isolation
reinforced by
behavioral
differences
third-party behavioral isolation
Figure 24.11
Postzygotic Reproductive Isolation
• Hybrid zygote abnormality: developing
embryos die or produce abnormal adults
• Hybrid infertility: normal adult hybrids are
sterile
• Low hybrid viability: low survival rates of
hybrids
Incomplete Reproductive Isolation
• separated populations may be reintroduced
before complete isolation has occurred
– hybrids may be vigorous; the populations
may merge
– hybrids may be weak; isolation may
continue to strengthen
Figure 24.13
Reproductive Isolation
• reproductive isolation does not require
extensive differentiation
speciation
without
extensive
genetic
variation
Figure
24.14
Speciation Rates
• Factors affecting speciation rates
1. species richness
• the more species are part of a lineage
–more opportunity for polyploidy
–more opportunity to be separated by a
barrier
Speciation Rates
• Factors affecting speciation rates
2. species range
• the larger the range of a species
–more likely to be fragmented by a
barrier
–more likely that isolated subpopulations
will experience different conditions
Speciation Rates
• Factors affecting speciation rates
3. Dispersal rates
• species that do not disperse well will be
separated by relatively small barriers
Speciation Rates
• Factors affecting speciation rates
4. sexual selection
• species that discriminate among potential
mates - engage in non-random mating
Speciation Rates
• Factors affecting speciation rates
5. environmental change
• environmental change may increase or
decrease available habitat
Speciation Rates
• Factors affecting speciation rates
6. ecological specialization
• discontinuous populations more easily
become isolated
Speciation Rates
• Factors affecting speciation rates
7. generation times
• short generation times: more generations
per unit time
• more generations: more rapid response to
evolutionary agents
Evolutionary Radiations
• evolutionary radiations: rapid speciation with
low extinction
– following mass extinctions
– following colonization of a new habitat
• low competition rates
• low predation
• abundant resources
Evolutionary Radiations
• common on the Hawaiian Islands
– descendants of U.S. west coast tarweeds
• silverswords occupy all major habitats and
exhibit growth forms not represented in
the ancestral populations
H
a
w
a
i
i
a
n
r
a
d
i
a
t
i
o
n
Figure 24.16
Species and Their Formation
• we know the results of macroevolution from
the study of fossils and relics
– the mechanisms of macroevolution are
inferred to be those of speciation
extrapolated over many millions of years
Species and Their Formation
• Likewise – multiple factors influence microevolution
• assortative mating, small population size,
migration, high mutation rate, directional
natural selection, sexual selection
– macroevolution may depend on additional
processes that are
• invisible to short term studies
or
• processes that are rare or one-time events.