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Ch 24: The Origin of Species (Intro (Speciation- (process by which 1…
Ch 24: The Origin of Species
24.1: The Biological Species Concept emphasizes reproductive isolation
Biological Species Concept
-
species group of population whose members can interbreed and produce in nature and
produce viable, fertile offspring
gene flow b/w populations holds a species together genetically
Limitations:
no way to evaluate reproductive isolation of fossils
emphasizes absence of gene flow
Reproductive Isolation
- existence of biological barriers which impede species from inter breeding
hybrid
- offspring which result from interspecific formation
Two types of Reproductive Isolation:
Pre-zygotic
Habitat Isolation
- organism occuy 2 diff habitats
Mechanical Isolation
- morphological difference prevent successful completion (ex. mouse and elephant)
Gametic Isolation
- sperm unable to fertilize egg of another species
Temporal Isolation
- species breed @ diff times of day, seasons, years, etc
Behavioral Isolation
- courtship rituals that attract mates unique to species
After mating is attempted
Post-zygotic Barrier
Reduced Hybrid Viability
genes of diff species interact in ways that impair hybrid's development
Reduced Hybrid Fertility
hybrids produced but are sterile, infertile
ex. Mule, Liger
Hybrid Breakdown
first generation viable and fertile
hybrid's offspring=feeble or sterile
Other Definitions of Species
Ecological Species Concept
- views species in terms of ecological niche
advantage
disadvantage
Phylogenetic Species Concept
- defines species as smallest group on phylogenetic tree which share a common ancestor
advantage
disadvantage
Morphological Species Concept
- distinguishes species through body shape or other features
advantage
disadvantage
24.2: Speciation can take place w or w/o geographic separation
Allopatric ("Other Country") Speciation
gene flow interrupted when population divided into
geographically isolated
sub populations
separated gene pools diverge--(mutations, natural selection, genetic drift); populations evolve independently
reproductive isolation may occur as by-product
Sympatric("Same Country") Speciation
occurs in species that live in same geographic area
can occur when gene flow reduced by:
Sexual Selection
- when females choose males based on appearance
Cichlid fish in Lake Victoria-- mate choice based on male breeding coloration= reproductive barrier
Habitat differentiation
subpopulation exploits new habitat or resource not used by parent population
Polyploidy
- accident during cell division resulting in too many chromosomes
far more common in plants
Two types of polyploidy observed:
Autopolyploid
- more than 2 chromosome sets derived from single species
Allopolyploid
- more than 2 chromosomes derived from 2 diff species
2 diff species produce an infertile hybrid that can asexually reproduce, as plants do. Later, various mechanisms may turn sterile hybrid into fertile polyploid, called an
alloployploid
If barriers taken away b/w populations in both Allopatric and Sympatric Speciation no interbreeding can take place
24.3: Hybrid zones reveal factors that cause reproductive isolation
Hybrid zone
- region where members of diff species meet and breed, producing some offspring w/ mixed ancestry
Hybrid Zones over Time
Fusion
- weakening of reproductive barriers
Stability
- continued production of hybrids
Reinforcement
- strengthening of reproductive barriers
Hybrids have reduced fitness compared to parent species
Hybrids often have reduced fitness compared to parent species
24.3: Speciation can occur rapidly or slowly and can result from changes to many or few genes
Broad patterns in fossil record and studies using morphological and molecular data can be used to access time interval b/w speciation events
Fossil record includes many episodes in which new species appears, remains unchanged, then disappears
Punctuated equilibria
- periods of apparent status punctuated by
sudden
change
other species appear to have changed more
gradually
Genetics of Speciation
depends on species; can occur from change in few or many genes
Macroevolution
= cumulative effect of many speciation and extinction events
Intro
Speciation
-
process by which 1 species splits into 2 or more
explains shared characters b/w different species (common ancestor)
origin of species
Bridge bw/ micro and macro evolution
Microevolution
- changes in allele frequencies in a population over time
Species
- "kind" or "appearence"
Macroevolution
- broad pattern of evolution above the species level
