Download Speciation and Extinction

Speciation and Extinction
Definitions
• Speciation – The origination of new species
• Extinction – The death of all organisms in a species
• Adaptations – Specific morphological changes as a
result of environmental pressures
• Taxonomy – Discipline that assigns names to
organisms and classifies biological diversity (Linnaeus
binomials)
What is a species?
What is a species?
Revisiting a few species concepts
Morphological
Biological
Phylogenetic
Morphological Species Concept
Species can be distinguished from each other by
morphological traits
“Classical species concept”
Morphological SC limitations
High plasticity
Morphological SC limitations
Undetected speciation
African bush elephant
African forest elephant
Biological Species Concept
Groups of interbreeding natural populations
that are reproductively isolated from other
such groups
Biological SC limitations
Asexual reproduction
Biological SC limitations
Spatially disjunct populations/species:
-Reproductively isolated due to spatial segregation,
but may be reproductively compatible
-Lab tests
Biological SC limitations
Hybridization: different species often hybridize
naturally (very common in plants)
Coulter pine
Jeffrey pine
Biological SC limitations
Fossils: impossible to know for sure if similar
looking species interbreed
Phylogenetic Species Concept
“the smallest diagnosable cluster of individual
organisms within which there is a parental
pattern of ancestry and descent” (Cracraft 1983)
Clusters must be:
1) Monophyletic (ancestor + all descendants)
Phylogenetic Species Concept
“the smallest diagnosable cluster of individual
organisms within which there is a parental
pattern of ancestry and descent” (Cracraft 1983)
Clusters must be:
1) Monophyletic (ancestor + all descendants)
2) derived through an evolutionary process of
descent from an ancestral lineage
3) diagnosable through examination of derived
characters
Phylogenetic
Phylogenetic tree
Phylogenetic SC limitations
Can inflate the number of species if species
designations are based on a few characters
The genetics of a species may not always fully
explain the differences in species.
Variations below the species level:
Subspecies - morphologically and genetically
different populations
– Geographically isolated
– Trinomial Latin name
Lodgpole pine (Pinus contorta)
Distribution of subspecies
Contorta
Latifolia
Murrayana
Variations below the species level:
Ecotype – genetic subpopulations adapted to specific
physical environmental factors (interfertile with other
ecotypes of same species)
– Differences are genetically based
– Differences may be morphological, physiological or
phenological
– Ecotypes occur in distinct habitats
– Genetic differences are adaptive (enhance
survival/reproduction)
Example: Achillea experiment (Clausen et al. 1948)
How does a new species “emerge”?
• New species evolve from preexistent species
• Speciation processes
– Cladogenetic speciation
– Reticulate speciation
– Phyletic speciation
new species originate from a
splitting event in which each
branch is a species
Time
Speciation Processes
Cladogenetic speciation
new species results from
interbreeding of two existing
species
Time
Reticulate speciation
ancestral species transforms
into a single descendant
species
Time
Phyletic speciation
From White 2008
Time
Time
Why could it be difficult to distinguish past
cladogenetic and phyletic speciation?
When can we say speciation happens?
• When two populations of one species become
sufficiently different so they are no longer the
same species (this will partly depend on the
species concept that you choose)
• The divergence of an ancestral
species into 2 or more daughter
species requires genetic change
among populations
Brief genetics review
• Diploid organisms have 2
homologous copies of each
chromosome (2n)
• Allele – one of two or more
alternative forms of a gene located
at a single point (locus) on a
chromosome
-Homozygote: alleles same
-Heterozygote: alleles different
• Populations contain individuals with
different alleles
Mechanisms of genetic differentiation
• Mutation
• Genetic drift
• Natural selection
• Gene flow
Mutation
– Random changes in DNA
– Source of all new alleles
– If beneficial, allele
frequency increases
– Negative & neutral
mutations also occur
Genetic Drift
– Change in allele frequencies that occurs entirely
from chance
– Largest effects in small,
isolated populations
Jaguars
Example
Genetic drift
1. Genetic drift acts faster and has more drastic results in
smaller populations
2. Can reduce genetic variation in populations
3. Genetic drift can contribute to speciation
Natural Selection
Genetic traits that enhance the survival and
reproduction (fitness) of carriers relative to other
individuals in the population will increase in
frequency over time
Peppered moth, Manchester, UK
www.ibri.org/Books/ Pun_Evolution/Chapter3/3.2.htm
Gene Flow
Movement of alleles within a population or between
populations caused by the dispersal of gametes or
offspring
How does gene flow affect the progression of
natural selection?
Adaptation and gene flow
• Niche of species not fixed
• Expect adaptations at periphery of distribution
• So, why don’t the peripheral populations of all
species adapt to local conditions resulting in
continual expansion of range?
Deer Mice, Geographic Variation
Geographic modes of speciation
Allopatric
Sympatric
Parapatric
Allopatric speciation
Geographic isolation cuts off gene flow between
populations and generates reproductive barriers
Allopatric speciation modes:
– Vicariance
– Jump dispersal
Allopatric speciation: Vicariance
Dispersal barrier forms
isolating populations of the
species
e.g. rising sea level, plate
tectonics, mountain building,
glaciation, climate change
From White 2008
Allopatric speciation: Vicariance
Gondwanaland breakup and Ranidae family
Madagascar
India
Allopatric speciation: Jump dispersal
A few individuals cross
a preexisting barrier
and start a new
isolated population
1
2
3
From White 2008
Allopatric speciation: Jump dispersal
1
2
3
From White 2008
Allopatric speciation: Jump dispersal
Galapagos tortoises
Video
Dispersal or Vicariance?
• Vicariance usually results in multiple species
diverging at the same time
• Dispersal only one (or a few) species diverging
at the same time
Allopatric speciation
What if populations get together again?
Fail to interbreed or produce fertile descendants
Interbreeding produces fertile hybrids and backcross
to parental populations
Interbreed but hybrids are less fit so natural selection
acts against the hybrids (reinforcement)
Sympatric speciation
Populations of ancestral
species overlap extensively
throughout population
differentiation
From White 2008
Sympatric speciation
Cichlids in Lake Malawi, eastern Africa
Parapatric speciation
Overlap occurs only
partially & in zone of
marginal fitness (i.e.
isolating forces operating)
Sympatric and parapatric speciation
How to overcome gene flow in a population?
Fitness
1. Disruptive selection: selection that favors the
extreme traits in a population
Trait
Apple flies
Apple
Hawthorns
Sympatric and parapatric speciation
How to overcome gene flow?
2. Chromosomal changes (e.g. polyploidy)
Gen 1
2n
fuse
3n
Gen 2
n
3n
fuse
4n
n
Allopatric vs. Sympatric speciation
• Allopatric speciation likely most predominant
• Sympatric/parapatric speciation likely occurs
more commonly than suspected but hard to
detect
Speciation is successful…what now?
Adaptive Radiation – divergence of a taxon into
a number of different forms and adaptive zones
E.g. silversword plant family
Hawaiian Honeycreepers
Adaptive radiation,
trophic specialization
Extinction!
“The extinction of species and of whole groups of
species, which has played so conspicuous a part in
the history of the organic world, almost inevitably
follows on the principle of natural selection; for old
forms will be supplanted by new and improved
forms.” - Darwin
Extinction
• All species eventually go extinct
• Over 99.9% of all species that have existed are
now extinct
• Counterbalances speciation
Animals Driven to Extinction
Passenger
pigeon
Great auk
Dodo
Dusky seaside
sparrow
Aepyornis
(Madagascar)
Old magazine illustration of hunters shooting Passenger Pigeons
(Ectopistes migratorius). Note the density of the flight. (From
copy in Schorger, 1955.)
Historical range of passenger pigeon. The most similar species in
existence today is probably the mourning dove (Zenaida macroura)
Two kinds of extinction
• Background extinction
– Continuously operating
– Low rate of extinction
• Mass extinction
– High rate of extinction (~75%)
– Short period of time (100,000s-millions years)
– Affects many lineages
Mass
Extinction
Events
K-T
Background
Extinction
Foote 2003
Few possible causes of mass extinctions
• Ice Ages
– Rapid change in climate
– Large areas uninhabitable
• Asteroids
– Catastrophic damage
– Debris/climate
• Volcanoes (massive volcanoes)
– Change in climate via particulates & SO2
Five or six mass extinctions?
Dramatic changes in fossil record
Eg – Cretaceous-Tertiary event ("K-T extinction")
Until 1980s - climate
P-Tr
O-S
Late D
Tr-J
K-T
K-T extinction
1980 – Asteroid hypothesis
1. Rock strata enriched with Iridium
2. Selective extinctions
3. Crater
6th mass extinction
Rate of extinctions
• Magtoday < Magprior
• Ratetoday = Rateprior
Causes of 6th Mass Extinction
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Habitat destruction
Habitat fragmentation
Pollution
Overharvesting
Invasive species
Climate change
Human overpopulation