Chapter 19 Speciation and Macroevolution Biological Species Concept

Chapter 19: Speciation and Macroevolution

Biological Species Concept • Species consist of 1+ populations whose members are capable of interbreeding in nature to produce fertile offspring and do not interbreed with members of different species • Sexual reproduction • Reproductive isolation

Fig. 24 -2 (a) Similarity between different species (b) Diversity within a species

Reproductive Isolating Mechanisms • Prevent interbreeding between 2 species • Preserve genetic integrity • Gene flow is prevented

Reproductive barriers - prezygotic • Prezygotic – prevent fertilization • Interspecific zygote never made • Types: – Temporal – Habitat – Behavioral – Mechanical – gametic

Temporal • Different times • Day, season, year • Ex: – Fruit flies – afternoon vs. morning – Frogs – late March vs. mid-April

Fig. 24 -4 e (c) Eastern spotted skunk (Spilogale putorius)

Fig. 24 -4 f (d) Western spotted skunk (Spilogale gracilis)

Habitat • Same geographical area, different habitat • Ex: – Flycatchers • • • Open woods/farms Deciduous forest Wet thickets Coniferous forest Brushy pastures/ willow thickets

Fig. 24 -4 c (a) Water-dwelling Thamnophis

Fig. 24 -4 d (b) Terrestrial Thamnophis

Behavioral • Courtship (signals before mating) • Aka “sexual isolation” • Ex: – Nest decoration, dance, song, vocalizations

Fig. 24 -4 g (e) Courtship ritual of bluefooted boobies

Mechanical • Incompatible structures of genital organs • Ex: – Flowers adapted for different insect pollinators

Fig. 24 -4 h (f) Bradybaena with shells spiraling in opposite directions

Gametic • Egg and sperm incompatible after mating • Ex: – Aquatic animals – release egg and sperm at once; egg and sperm protein bind to each other

Fig. 24 -4 k (g) Sea urchins

Reproductive barriers - Postzygotic • Prevent gene flow when fertilization occurs – Hybrid inviability – Hybrid sterility – Hybrid breakdown

Fig. 24 -4 l (h) Ensatina hybrid

Hybrid inviability • Increased likelihood of reproductive failure after fertilization • Spontaneous abortion – genes do not interact properly

Hybrid sterility • Interspecific hybrid lives but can’t reproduce • Incompatible courtship w/ either parent species • Gametes of hybrid abnormal during meiosis – Different chromosome #’s • Female horse – 64 • Male donkey – 62 • Mule - 63

Fig. 24 -4 m (i) Donkey

Fig. 24 -4 n ( j) Horse

Fig. 24 -4 o (k) Mule (sterile hybrid)

Hybrid breakdown • Inability of a hybrid to reproduce due to some defect • F 2’s • Ex: – 2 sunflower species – 80% F 2 can’t reproduce

Fig. 24 -4 p (l) Hybrid cultivated rice plants with stunted offspring (center)

Reproductive isolation is the Key to Speciation • Speciation = evolution of a new species • 2 patterns – 1) Anagenic – 2) Cladogenic

Anagenesis • (phyletic evolution) • Relatively small, progressive evolutionary changes in a single lineage over long periods • Enough time conversion of 1 species to another • Sequence of species occurs over time without an increase in the number of species

Cladogenesis • (branching evolution) • 2+ populations of an ancestral species split and diverge, eventually forming 2+ new species • Clade = cluster of species derived from a single common ancestor • Over time increase species richness

When has speciation occurred? • Population is sufficiently different from its ancestral species that no genetic exchange can occur between them • 2 ways: – Allopatric – Sympatric

Fig. 24 -5 (a) Allopatric speciation (b) Sympatric speciation

Allopatric Speciation • Occurs when 1 population becomes geographically separated from the rest of the species and then evolves by natural selection and/or genetic drift • Most common • Geographic isolation by: – Changing of Rivers, glaciers, mountains, land bridges, lakes • Birds vs. rats – Small population migrates or is dispersed – Colonize new area – Isolated gene pool microevolution new species

Fig. 24 -6 A. harrisi A. leucurus

Sympatric Speciation • New species evolves within the same geographical region as the parent species • 2 ways: – Change in • Ploidy • Ecology

Ploidy • Polyploidy - 2+ chromosome sets – Plants – rapid speciation • Autopolyploid – multiple sets chromosomes from a single species • Allopolyploidy – multiple sets of chromosomes from 2+ species – Allopolyploid – diff # chromosomes from parents = new species • 1) extinct • 2)coexist • 3)replace parent species

Fig. 24 -10 -3 2 n = 6 4 n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2 n Gametes produced are diploid. . 4 n Offspring with tetraploid karyotypes may be viable and fertile.

Fig. 24 -11 -4 Species B 2 n = 4 Unreduced gamete with 4 chromosomes Meiotic error Species A 2 n = 6 Normal gamete n=3 Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n=3 Viable fertile hybrid (allopolyploid) 2 n = 10

Allopatric and Sympatric Speciation: A Review • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented

• In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection

Ecology • Parasitic insects – Ex: fruit maggot flies • Switched host from hawthorn tree fruits to domestic apples • Mutation disruptive selection different ecological opportunity

Evolutionary Change – rapid or gradual? 2 models • Punctuated Equilibrium – fossil record accurately reflects evolution as it actually occurs – Long periods of stasis are punctuated by short periods of rapid speciation triggered by changes in the environment – Speciation in “spurts” – Short periods evolution, long periods stability – Accounts for abrupt appearance of new species with few intermediate forms

Fig. 24 -17 (a) Punctuated pattern Time (b) Gradual pattern

• Gradualism – traditional view of evolution – Evolution proceeds continuously over long periods – Rarely observed, fossil record incomplete – Populations slowly diverge from 1 another by the gradual accumulation of adaptive characteristics within each population

Macroevolution • Dramatic changes that occur over long time spans in evolution • Attempts to explain large phenotypic changes (novelties) • Important aspects – Evolutionary novelties – Adaptive radiation – Mass extinction

Macroevolution

Adaptive radiation • Evolutionary diversification of many related species from 1 or a few ancestors in a short period • Adaptive zones: new ecological opportunities that were not exploited by an ancestor • Islands – common – fewer species there • Ex: Darwin’s finches, honeycreeper birds, silversword plants

Extinction • • End of lineage; last member of species dies Permanent Makes adaptive zones vacant Background extinction – Continuous, low-level • Mass extinction – Numerous species die at once – Adaptive radiation follows

Extinction video • Causes of mass extinction – Climate change / Earth’s temp. – Catastrophes • Comet/asteroid dust (block light) food chain disrupted, drop in temp. – Competition • Humans animal / plant habitats

Microevolution vs. Macroevolution • Genetics • Well suited survive • Chance events • “lucky” to survive • Right place, right time