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