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Chapter 27

BIO SCI 94 Chapter Notes - Chapter 27: Zygote, Helianthus Annuus, Zygosity

Biological Sciences
Course Code
Catherine Loudon

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Chapter 27- Bio 94
Four evolutionary process: natural selection, genetic drift, mutation, & gene flow.
- If gene flow ends, allele freq in isolated populations = free to diverge = pop begin to evolve
independently of each other.
Speciation results from genetic isolation and genetic divergence.
- Gene isolation: results from lack of gene flow
- Genetic divergence - occurs because selection, genetic drift, and mutation proceed
independently in the isolated populations.
Speciation: splitting event that creates two or more distinct species from a single ancestral
- Speciation complete --> new branch added to tree of life
Species: an evolutionarily independent pop or group of pop.
3 most common ways to indentify species
1. Biological species concept
2. Morphospecies concept
3. Phylogenetic species concept
Biological Species concept (BSC)
- main criterion for indentifying species is reproductive isolation
- NO gene flow occurs between populations that are reproductively isolated from each
If two diff pop can't interbreed/fail to make fertile offspring = considered distinct species.
- Ernst Mayr promotes BSC b/c criterion of rep isolation allows for clear proof of
evoluationary independence.
Mechanisms that stop gene flow b/w populations
1. prezygotic isolation: prevents indiv. of diff species from mating
2. postzygotic isolation: offspring of matings b/w members of diff species don't
survive or reproduce.
Criterion of Rep Isolation CAN'T be evaluated in fossils or in species that reproduce
Morphospecies Concept
- Researchers identify evolutionary independent lineages by diff in size, shape, or other
morpho features
- PRO: when biologists don't have data on gene flow, it is equally applicable to sexual,
asexual, or fossil species.
1. may lead to naming of 2 or + species when there's only 1 polymorphic species with
diff phenotypes.
2. can't identify cryptic species, which differ in traits other than morphology
3. Morphological features used to distinguish species are subjective/biased
Phylogenetic Species Concept
-indentifies species based on evolutionary history of populations
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- reasoning behing this concept: uses Darwin's theory: all species related by common
ancestry: All species form monophyletic (one-tribe) group
- Monophyletic group: also called a CLADE/LINEAGE, consists of ancestral pop, all of
descendants, and only those descendants.
- Mono groups indentified by traits called synapomorphies (unique forms): trait found
in certain groups of organisms and their common ancestor, but is missing in more distant
EX: Fur/lactation = synapo's that indentify mammals as mono group
Synapo: are homologous traits that can be indentified at genetic, developmental, or structural
Under PSC, species defined as smallest mono groups on tree of life: phylo species: made up
of pop that share one or more synap.
1. can be applied to any population( fossil, asexual, or sexual
2. logical because diff species have diff synap only if they're isolated from gene flow/ have
evolved independently
CON: carefull estimated phylogenies are available only for a tiny subset of pop on the tree of
Critics of PSC say
that it would probably lead to recognition of many more species than morpho or BSC
Proponents counter that saying the recognition of #'s of species might better reflect extent
of life's diversity
Subspecies: populations that live in discrete geographic areas, have distinguishing features,
such as coloration or calls, but aren't considered distinct enough to be called separate species:
these subspecies can interbreed if geographical barriers to their isolation are removed.
- Speciation begins when gene flow b/w populations is reduced, causing genetic isolation
- Genetic isolations happens when populations become geographically separated = allopatry
So speciation that begins with geographic isolations is allopatric speciation.
- Geographic isolation occurs in two ways: dispersal or vicariance, physical
splitting of a habitat.
- Biogeography- study of how species and populations are distributed geographically
Two evolutionary processes could be responsible for change in beak size (Grants observed
finches on the island of Daphne Major):
1. The founder event made a colonizing population that happened to have particularly
large beaks relative to the migrant population. This is an example of genetic drift.
2. N.S. in the new enviro could favor alleles associated with large beaks
- Dispersal and colonization, followed by genetic drift & N.S., are thought to be responsible
for speciation in Galapagos finches and other island groups
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