Chapter 24 The Origin of Species
Speciation = origin of new species • Microevolution = adaptations that evolve within a population (confined to one gene pool) • Macroevolution= evolutionary change making new species or new groups of organisms
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
I. Reproductive Isolation • Species=a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring • Reproductive isolation= existence of biological factors (barriers) that impede two different species from producing viable, fertile offspring. • Hybridsare the offspring of crosses between different species (horse + donkey = mule)
Fig. 24-2 • (a) Similarity between different species • (b) Diversity within a species
A. Reproductive Barriers • Prezygotic barriers block fertilization from occurring • Postzygotic barriers prevent hybrid from forming viable fertile adult
Reproductive Barriers Between Species • PrezygoticBarriers • PostzygoticBarriers • Habitat • Isolation • Temporal • Isolation • Behavioral • Isolation • Hybrid • Breakdown • Mechanical • Isolation • Gametic • Isolation • Reduced • Hybrid Viability • Reduced Hybrid • Fertility • Individuals • of • different • species • Viable, • fertile • offspring • Mating • attempt • Fertilization • (c) • (e) • (f) • (a) • (g) • (h) • (l) • (i) • (d) • (j) • (b) • (k)
PreZygotic Reproductive Barriers • Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats
Habitat Isolation • Water-dwelling Thamnophis
Habitat Isolation • Terrestrial Thamnophis
PreZygotic Reproductive Barriers • Temporal Isolation: Species that breed at different times (day, seasons, years) • Behavioral isolation: Courtship rituals and other behaviors • Mechanical isolation: Morphological differences can prevent successful mating. • Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species.
Behavioral Isolation Occurs Without Appropriate Mating Rituals • Courtship ritual of blue-footed boobies
Mechanical Isolation: • Bradybaena with shells spiraling in opposite directions
PostZygotic Reproductive Barriers • Reduced hybrid viability -- weak offspring • Reduced hybrid fertility -- sterile offspring • Hybrid breakdown.
PostZygotic Reproductive Barrier • Mule: sterile hybrid offspring between horse and donkey
Concept 24.2: Speciation can take place with or without geographic separation • Speciation can occur in two ways: • Allopatric speciation: geographic barrier separates populations. • Sympatric speciation: no geographic barrier
Speciation • (a) Allopatric speciation • (b) Sympatric speciation
II. Allopatric Speciation • Gene flow is interrupted • Gene pool is reduced when a population is divided into geographically isolated subpopulations • Separate populations may evolve independently through mutation, natural selection, and genetic drift. • Reproductive isolation between populations generally increases as the distance between them increases. • Barriers to reproduction are intrinsic; separation itself is not a biological barrier.
Allopatric Speciation • A. harrisi • A. leucurus
Continental Drift Caused Allopatric Speciation • Mantellinae • (Madagascar only): • 100 species • Rhacophorinae • (India/Southeast • Asia): 310 species • Other Indian/ • Southeast Asian • frogs • 100 • 60 • 20 • 80 • 40 • 0 • 1 • 2 • 3 • Millions of years ago (mya) • 1 • 3 • 2 • India • Madagascar • 56 mya • 88 mya • 65 mya
III. Sympatric Speciation • Speciation takes place in geographically overlapping populations. • Polyploidyis the presence of extra sets of chromosomes due to accidents during cell division. • Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.
Sympatric Speciation via Polyploidy is Common in Plants • 2n • 2n = 6 • 4n = 12 • 4n • Failure of cell • division after • chromosome • duplication gives • rise to tetraploid • tissue. • Gametes • produced • are diploid.. • Offspring with • tetraploid • karyotypes may • be viable and • fertile.
Sympatric Speciation - Polyploidy --> Allopolyploid • Species B • 2n = 4 • Unreduced • gamete • with 4 • chromosomes • Unreduced • gamete • with 7 • chromosomes • Hybrid • with 7 • chromosomes • Meiotic • error • Viable fertile • hybrid • (allopolyploid) • 2n = 10 • Normal • gamete • n = 3 • Normal • gamete • n = 3 • Species A • 2n = 6
A. Habitat Differentiation and Sexual Selection • Sympatric speciation can also result from the appearance of new ecological niches. • New roles open for organisms to fill (new trees introduced into an area) • Sexual selection can drive sympatric speciation. • Ex. Selection for mates of different colors has contributed to the speciation in cichlid fish in Lake Victoria.
OVERVIEW: Allopatric and Sympatric Speciation • Allopatric speciation, geographic separation 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. • Sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species. • Can result from polyploidy, natural selection, or sexual selection.
IV. Time Needed for Speciation • Punctuated equilibrium= periods of apparent stasis (no change) punctuated by brief periods of rapid change. • Gradualism =slow continuous change over time in a species’ existence
Patterns in Speciation PunctuatedEquilibrium pattern • Change / Time • Gradualism pattern
Speciation - Andersen 12 min
You should now be able to: • Define and discuss the limitations of the four species concepts. • Describe and provide examples of prezygotic and postzygotic reproductive barriers. • Distinguish between and provide examples of allopatric and sympatric speciation. • Explain how polyploidy can cause reproductive isolation. • Define the term hybrid zone and describe three outcomes for hybrid zones over time.
