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Species definitions, complications, isolating barriers,
Terms in this set (31)
General concept of species
Species form boundaries for spread of alleles, have independent evolutionary trajectories and are the smallest evolutionary independent group.
Species are organisms with similar physical characteristics.
Advantages for morphospecies concept
Widely applicable. Useful for extant species and fossils.
Disadvantages for morphospecies concept
Criteria for species classification varies among taxonomists. Problems with divergence; how much divergence should be considered? Problems with cryptic species (2 or more species morphologically identical). Problems with polymorphic species ( 2 or more genotypes).
Biological Species concept
Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other groups.
Reduction or prevention of gene flow between populations because of genetically determined differences between them (not geographically separated).
Advantages of Biological species concept
Clear, testable hypothesis. Can help answer how and why species arise. emphasize the evolution of reproductive isolation.
Disadvantages of Biological species concept
Cannot be applied to asexual organisms. Is not useful to explain anagenesis. We cannot know if present species is reproductively isolated from ancestors. Is unclear under what conditions species are considered reproductively isolated because many species can reproduce under controlled conditions. It is difficult to implement, must cross species in order to know.
Phylogenetic species concept
A species is the smallest monophyletic group of common ancestry, recognizable by a unique combination of character states. Must have been evolutionary independent for a sufficient time.
Populations or groups of populations that contains all known descendants of single common ancestors.
Advantages of phylogenetic species concept
Applies to all life forms. Easy to implement (just sequence DNA).
Disadvantages of Phylogenetic species concept
Species is considered whenever genetic differences exist, even if is a single base pair. Species with horizontal gene transfer will result in misclassification.
Complications in classifying species
Asexual lineages can often be misclassified due to polyploidy. If hybrids are possible, does this means that parental species are the same species? Hybrids are adapted to narrow zones and often have low fitness, are they another species? Imprinting in birds. Cryptic species are identical morphologically but not genetically. Polymorphic species-many alleles or variations of the same gene, are each a different species? Ring species are related by descent from common ancestor, but is because they interbreed in a ring form, are they the same species?
Biological differences that act as barriers that prevent gene flow.
I. Premating barriers
Features that impede transfer of gametes to members of other species. Ecological isolation and Potential mates do not mate.
A. Ecological isolation
Potential mates do not meet, although in sympatric. Temporal isolation, habitat isolation and immigrant inviability.
1. Temporal isolation
Population breed at different seasons or times of day.
2. Habitat isolation
Populations breed in different habitats in the same general area.
3. Immigrant inviability
Immigrants between populations do not survive long enough to interbreed.
B. Potential mates meet but do not mate
Sexual (behavioral or ethological) isolation. Pollinator isolation in plants.
II. Postmating, prezygotic barriers
Mating or gamete transfer occur, but zygotes are not formed. Mechanical isolation, Copulatory isolation, Gametic isolation and Immigrant inviability.
A. Mechanical isolation
Copulation occurs, but no transfer of male gametes takes place because of failure in reproductive structures.
B. Copulatory isolation
Failure of fertilization because of behavior during copulation or genitalia fail to stimulate properly.
C. Gametic isolation
Competition between conspecific and heterospecific gametes impedes proper transfer of gametes or fertilization.
III. Postzygotic barriers
Hybrid zygotes are formed but have reduced fitness. Extrinsic and Intrinsic.
Hybrid fitness depends on context. Ecological inviability and behavioral sterility.
1. Ecological inviability
Hybrids do not have an ecological niche in which they are competitively equal to parent species.
2. Behavioral sterility
Hybrids are less successful than parents species in obtaining mates.
Hybrid fitness is low because of problems that are relatively independent of environmental context. Hybrid inviability, Hybrid sterility.
1. Hybrid inviability
Developmental problems reduce survival.
2. Hybrid sterility
Inability to produce viable gametes or incapacity to perform normal courtship.
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