Speciation and Extinction
Species sometimes difficult to discern -- nature presents a continuum
ecotype: distinct population occuring in a particular habitat
geographic race: nonoverlapping ranges of subspecific category
If a lineage has been successful, will see many derived species from a common
ancestor with many adaptations.
However, the inevitable fate of a species is extinction.
Divergent vs. reticulate speciation
Morphological species concept: species recognized if morphologically
distinguishable from its close relatives
Biological species concept: population of organisms that is actually or
potentially reproductively isolated from other poulations (sensu Mayr 1963)
Phylogenetic species concept: a species is any group of organisms in which
all individuals share a unique, derived characteristic (synapomorphy)
Evolutionary species concept: recognizes each independent evolutionary
lineage as a species
Classification in flux
Microevolution and Macroevolution
microevolution tends to focus on population-level processes, whereas
macroevolution looks at major changes that occur over evolutionary time
scales presented in the fossil record.
Eldridge and Gould (1972) suggested two processes of macroevolution:
punctuated equilibrium and species selection.
Punctuated equilibrium: organisms undergo long periods of stasis (little
change) followed by rapid speciation in the fossil record.
Species selection: differential survival and speciation of species with
particular heritable traits (preadaptations to changing environments).
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Evolutionary processes of speciation:
Genetic differentiation of populations includes: mutation and genetic
recombination, genetic drift, natural selection
mutation: rates generally slow, especially in functionally constrained
genes
sexual reproduction: provides building blocks for genetic variation
genetic drift: stochastic in nature; weak force in general; changes tend
to take a long time
natural selection: strong force for change; changes can be rapid
Isolation of populations - leads to differentiation
Gene flow between populations - homegenizes populations undergoing
differentiation
Geographic isolation tends to reinforce genetic drift and natural selection,
reduce gene flow, and facilitate speciation -- especially in colonization events
to island systems.
Changes in characters over geographic range can occur with distance (e.g.,
latitudinal variation) or along an environmental gradient (cline).
Allopatric speciation
When geographic isolation prevents gene flow, allopatric speciation may
occur (divergence in different places).
Steps to allopatric speciation:
Vicariance model
1.
Widespread geographic distribution
2.
Limited gene flow among populations (isolation)
3.
Environmental heterogeneity along geographic range and differential
selective pressures (plus drift)
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Dispersal model
1.
Individuals disperse to found a new geographically isolated population
2.
Limited gene flow (isolation)
3.
Environmental heterogeneity along geographic range and differential
selective pressures (plus drift)
Parapatric speciation: both spatial segregation and spatial
differentiation initiate the process, and lead to the evolution of isolating
mechanisms betwen groups of geographically distinct but contiguous
populations.
Sympatric Speciation
Speciation where range overlap occurs -- requires different isolating
mechanisms than geographic distance; ecological or temporal segregation;
host choice, habitat selection, etc.
Disruptive selection along an environmental gradient
Host shift of herbivores or parasites
Chromosomal rearrangments
Hybridization at the homoploid level
Ecological Diversification of Species
Partitioning of the environment by competitive exclusion
Species that have different niches tend to have overlapping ranges
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Adaptive Radiation
Definition: The evolution of a diversity of ecological roles and attendant
adaptations in different species within a lineage.
Darwin (1859); Henry Fairfield Osborn (1902); Julian Huxley (1942); George
Gaylord Simpson (1944, 1953) Simpson advanced the idea of a “key
innovation” that accelerates speciation in a lineage (e.g., wings in birds).
Key conceptual issues involved in the study of adaptive radiations are:
Phylogeny, adaptation, historical ecology, speciation, genetics, development,
biogeography, tempo, predictability
Examples:
Cichlid Fishes
Galápagos Finches
Hawaiian Honeycreepers
Brocchinia (Bromeliaceae) in Tepuis of Guayana Shield
Tepuis uplifted during the late Cretaceous; have cool and very wet
climates and nutrient-poor soils.
Adaptive radiation of mechanisms of nutrient capture: carnivores,
ant-fed myrmecophytes, species with N2-fix symbionts, tank epiphytes,
non-impounding terrestrial forms; diversity of nutritional strategies is
accompanied by extensive variation in the form and nutrient uptake of
its foliar scales. Key innovation in this group was the development of a
tank habit
Polemoniaceae
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Extinction
the ultimate fate of a species
Mathematical modeling shows that the smaller a population becomes, the
lower its ratio of births to deaths, and the longer it remains at low numbers,
the more vulnerable it is to extinction.
The size of a species range can also have an affect: the larger the range, the
smaller the chance of extinction
Humans have increased the rate of extinction during this interglacial.
Mass extinctions of fossil record
K-T boundary extinctions (65 mybp)
Permian-Triassic Boundary 225 mybp
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