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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
Undetected speciation
African bush elephant African forest elephant
Morphological SC limitations
Biological Species Concept
Groups of interbreeding natural populations that are reproductively isolated from other such groups
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)
Biological SC limitations
Jeffrey pine Coulter 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
Tim
e
Tim
e
Tim
e
Speciation Processes Cladogenetic speciation new species originate from a
splitting event in which each branch is a species
Reticulate speciation new species results from
interbreeding of two existing species
Phyletic speciation ancestral species transforms
into a single descendant species
From White 2008
Tim
e
Tim
e
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
A few individuals cross a preexisting barrier and start a new isolated population
Allopatric speciation: Jump dispersal
1
2
3
From White 2008
From White 2008
1
2
3
Allopatric speciation: Jump dispersal
Galapagos tortoises
Allopatric speciation: Jump dispersal
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
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)
Allopatric speciation
Sympatric speciation
Populations of ancestral species overlap extensively throughout population differentiation
From White 2008
Cichlids in Lake Malawi, eastern Africa
Sympatric speciation
Parapatric speciation
Overlap occurs only partially & in zone of marginal fitness (i.e. isolating forces operating)
Trait
Fitn
ess
Sympatric and parapatric speciation
How to overcome gene flow in a population?
1. Disruptive selection: selection that favors the extreme traits in a population
Apple flies Apple Hawthorns
Sympatric and parapatric speciation
How to overcome gene flow?
2. Chromosomal changes (e.g. polyploidy)
2n n fuse
3n
Gen 1 Gen 2
3n n fuse
4n
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
Foote 2003
Background
Extinction
Mass
Extinction
Events
K-T
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
O-S Late D
P-Tr
Tr-J K-T
K-T extinction
1980 – Asteroid hypothesis
1. Rock strata enriched with Iridium
2. Selective extinctions
3. Crater
6th mass extinction
• Magtoday < Magprior
• Ratetoday = Rateprior
Rate
of extinctions
Causes of 6th Mass Extinction
• Habitat destruction
• Habitat fragmentation
• Pollution
• Overharvesting
• Invasive species
• Climate change
• Human overpopulation
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