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Chapter 24 Origin of Species
Mystery of Mysteries• Speciation - origin of new species
focal point of evolutionnew species is source of biological diversity
• Macroevolution – evolutionary change beyond a single species
• Two patterns of evolutionary change:Anagenesis – (phyletic) accumulation of changes
that gradually transform a given species into a species with different characteristics
Cladogenesis – (branching) splitting of the gene pool into two or more, giving rise to new species
Biological Species Concept
• Biological Species concept – defines a species as a population whose members have the potential to interbreed and produces viable, fertile offspring
- Cannot produce viable, fertile offspring with members of other populations- emphasizes separateness of species from one
another due to reproductive barriers• Reproductive isolation – biological factors (barriers)
that impede members of two species from producing viable fertile hybrids
Reproductive Isolation
Reproductive Isolation• Prezygotic barriers – impede or hinder fertilization if mating
occursa. Habitat isolation – two species that occupy different
habitats in the same area; rarely interactb. Temporal isolation – species that breed during different
day, season, or year cannot mix gametesc. Behavioral isolation – courtship rituals or other unique
mating behaviorsd. Mechanical isolation – morphological differences
prevent successful matinge. Gametic isolation – sperm of one species may not be
able to fertilize eggs of another species
West vs. East
Behavioral isolation
Reproductive Isolation• Postzygotic barriers – prevent a hybrid from
developing into viable, fertile adulta. Reduced hybrid viability – genes from
different parents impair hybrids developmentb. Reduced hybrid fertility – hybrid may be
healthy and vigorous but sterile. Chromosome differences from two parents; meiosis will fail to produce normal gametes
c. Hybrid breakdown – first generation is viable and fertile, but next generation is impaired and possibly sterile
Other Species Distinctions
• Morphological species concept – characterizes by body, shape, size, and structural features
• Palentological species concept – morphological features of discrete species known only from fossil record
• Ecological species concept – views species in terms of ecological niche or role in community
• Phylogenetic species concept – species as a set of organisms with unique genetic history
Geographic Isolation• Allopatric speciation – gene flow is interrupted when a
population is divided into geographically isolated subpopulationsEx: rise and fall of lake water levels- can occur when descendents are isolated from parent population- barrier of separation depends on organisms mobility Ex: hawk or windblown pollen compared to squirrels or mice* true allopatric speciation would mean that populations could not interbreed successfully
Sympatric Speciation• Sympatric speciation – speciation takes place in geographically
overlapping populations- chromosomal changes and nonrandom mating
reduces gene flow• Extra sets of chromosomes or polyploidy gives rise to an
autopolyploid- more than two sets of chromosomes from a single species
- likely result of failures in cell division- offspring are normally sterile, unless that can mate with other autopolyploids (same # - tetraploids)
- Allopolyploid – a sterile hybrid that asexually propagates and becomes a fertile polyploid- new species
Ex: oats, wheat, cotton, potatoes, and tobacco all are polyploids
Sympatric Speciation
• Polyploid speciation can happen in animals depending on habitat conditions and sexual selection
Ex: Apple maggot fly whose habitat changed from hawthorne to apple trees
*Also, cichlid fish in Lake Victoria of eastern Africa, nonrandom mating or selecting on coloration
Adaptive Radiation
• Adaptive radiation – evolution of many diversely adapted species from a common ancestor after being introduced into new environmental opportunities and challenges- organisms make way to new, distant habitats, or when extinctions occur opens niches for survivors
Ex: inhabitats of Hawaiian islands
Punctuated Equilibrium
• Punctuated equilibrium – episodes in fossil record where a new species suddenly appeared, persisted unchanged, and then disappear
- Time period of the fossil record skews the perception of changes that occurred making them unnoticeable
- External anatomy may not give best interpretation of adaptive changes that occurred in species
Accumulation of Speciation Events
• Speciation can result from seemingly small events, but through continued divergence, differences accumulate
- forms basis of macroevolutionary change- processes that facilitate the change are natural
selection, mutation, genetic drift, and gene flow- accumulation of small speciation episodes accounts for
sweeping evolutionary changesEx: previous and present function of the eye
- exaptations – structures that evolve in one context but become co-opted for another function
Ex: honeycombed bones and feathers in birds
Genes that Control Development• Heterochrony –
evolutionary change in the rate and timing of developmental events
• Allometric growth – proportioning that helps give a body its specific form
*changing rates slightly can substantially change adult formEx: salamander feet
Controlling Development• Timing of Reproductive development of
somatic organs can also be altered by heterochrony
• Paedomorphosis – reproductive development accelerates compared to somatic development making the body retain juvenile body parts during sexual maturity
*contrasts ancestral species in which juvenile appearance is actually adult stage
Changes in Spatial Pattern• Homeotic genes – control
the placement and spatial organization of body parts
Ex: Hox genes that determine positional information in animal embryos- changes or
mutations can dramatically impact morphology
Species Selection
• Branching evolution can result in an evolutionary trend even if some new species counter the trend
• Species selection states that species that endure the longest and generate the most offspring determine the direction of the major trends
• Appearances of trends does not imply some intrinsic drive toward a particular phenotype
• Evolution is the result of interactions between organisms and their environments
• If environmental conditions change, those trends may stop or be reversed
Hybrid Zones
Hybrid zone – region in which members of different species meet, mate, and produce offspring of mixed ancestryOver time 3 outcomes possible:
1. Reinforcement of reproductive barriers2. Fusion or weakening of barriers to a
single species3. Stability or continued production of
hybrids even when selected against