Chapter 24 Origin of Species

Mystery of Mysteries Speciation - origin of new species focal point of evolution new 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

Prezygotic barriers – impede or hinder fertilization if mating occurs a. Habitat isolation – two species that occupy different habitats in the same area; rarely interact b. Temporal isolation – species that breed during different day, season, or year cannot mix gametes c. Behavioral isolation – courtship rituals or other unique mating behaviors d. Mechanical isolation – morphological differences prevent successful mating e. 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 adult a. Reduced hybrid viability – genes from different parents impair hybrids development b. 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 subpopulations Ex: 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 changes Ex: 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 form Ex: 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 ancestry Over time 3 outcomes possible: 1. Reinforcement of reproductive barriers 2. Fusion or weakening of barriers to a single species 3. Stability or continued production of hybrids even when selected against