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Lecture 5: Ch 27

Terms in this set (28)

Biological species concept
-Identifies species based on reproductive isolation between populations (they don't breed and produce viable, fertile offspring)
-Advantages: Reproductive isolation = evolutionary independence
-Disadvantages: Not applicable to asexual or fossil species; difficult to assess if populations do not overlap geographically
Prezygotic isolation
Prevents individuals of different species from mating
Postzygotic isolation
The offspring of matings between members of different species do not survive or reproduce
Temporal
Populations are isolated because they breed at different times
Habitat
Populations are isolated because they breed in different habitats
Behavioral
Populations do not interbreed because their courtship displays differ
Gametic barrier
Matings fail because eggs and sperm are incompatible
Mechanical
Matings fail because male and female reproductive structures are incompatible
Hybrid viability
Hybrid offspring do not develop normally and die as embryos
Hybrid sterility
Hybrid offspring mature but are sterile as adults
Morphospecies concept
-Identifies species based on morphologically distinct populations
-Advantages: Widely applicable
-Disadvantages: Subjective (researchers often disagree about how much or what kinds of morphological distinction indicate speciation); misidentifies polymorphic species; misses cryptic species
Polymorphic species
A species that has two or more distinct phenotypes in the same interbreeding population at the same time
Cryptic species
A species that cannot be distinguished from similar species by easily identifiable morphological traits
Phylogenetic species concept
-Identifies species based on smallest monophyletic group on phylogenetic tree
-Advantages: Widely applicable; based on testable criteria; it is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently
-Disadvantages: Relatively few well-estimated phylogenies are currently available
Monophyletic group
Consists of an ancestral population, all of its descendants, and only those descendants (snip test)
Synapomorphy
-A trait that is found in certain groups of organisms and their common ancestor, but is missing in more distant ancestors
-Monophyletic groups are identified by synapomorphies
Allopatric speciation
-Speciation that begins with geographic isolation
-Geographic isolation of populations via dispersal and vicariance produces genetic isolation due to the interruption of gene flow
Allopatric speciation by dispersal
1. Some individuals disperse from their population and colonize a new habitat
2. New population begins to diverge due to mutation, genetic drift, and selection
3. Eventually the two populations are genetically isolated from one another

Colonists establish a new population in a novel location.
Allopatric speciation by vicariance
1. Chance event physically separates population into subgroups (involves a physical barrier)
2. Isolated populations begins to diverge due to mutation, genetic drift, and selection
3. Eventually the two populations are genetically isolated from one another

A widespread population becomes fragmented into isolated subgroups.
Sympatric speciation
-Speciation that occurs even though populations live within the same geographical area
-But gene flow would easily overwhelm any difference among two populations created by genetic drift and natural selection
Sympatric speciation by disruptive selection (external event)
-Even though sympatric populations are not geographically isolated, they may become reproductively isolated by adapting to different ecological niches via disruptive selection

1. Sympatric individuals live in the same geographic area
2. New population begins to diverge due to selection
3. Eventually the two populations are genetically isolated from one another
Sympatric speciation by polyploidization (internal event/chromosomal mutation)
-Polyploidy occurs when an error in meiosis or mitosis results in a doubling of the chromosome number
-Two types: autopolyploid and allopolyploid
-Self fertilization can happen in both types
-Polyploid individuals are reproductively isolated from the original diploid population and thus evolutionarily independent, because breeding between diploids and tetraploids generally result in sterile offspring (three homologous chromosomes can't synapse and separate correctly during meiosis)
-Speciation by polyploidization is virtually instantaneous; it is fast, sympatric, and common
Autopolyploid
-"same-many-form"
-Individuals are produced when a mutation results in a doubling of chromosome number and the chromosome all come from the same species
-Key event leading to autopolyploidy is nondisjunction during meiosis, resulting in diploid gametes rather than haploid gametes
-Diploid gametes could combine to form tetraploid offspring
Allopolyploidy
-"different-many-form"
-Individuals are created when parents belong to different species mate and produce an offspring with two different sets of chromosomes
-More common than autopolyploid

Ex.
-Diploid parents (2n = 6 and 2n = 4)
-Each undergo meiosis and produce haploid gametes (n)
-One haploid gametes from each species undergo fertilization and produce a hybrid offspring that is sterile (supposed to make dipoid but instead makes a haploid offspring because no homologous chromosomes; each chromosome is different)
-Error in mitosis in hybrid offspring and chromosome number doubles
-In allopolyploidy offspring (effectively 2n = 10), now each chromosome has a homolog and meiosis can take place to form gametes for the next generation
Reinforcement of divergence (possible outcome of secondary contact between populations)
If hybrid offspring have low fitness, natural selection favors the evolution of traits that prevent interbreeding between the populations
Hybrid zone formation (possible outcome of secondary contact between populations)
Hybridzation occurs in a well-defined geographic area; this area may move over time or be stable
Extinction of one populations (possible outcome of secondary contact between populations)
If one population or species is a better competitor for shared resources, then the poorer competitor may be driven to extinction
Creation of new species (possible outcome of secondary contact between populations)
If the combination of genes in hybrid offspring allows them to occupy distinct habitats or use novel resources, they may form a new species
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