SPECIATION Ch. 16-3 pp. 326-330
Forming a new species • Speciation: forming a new species
Forming a new species • Speciation: forming a new species • Species: a group of organisms that can mate and produce fertile offspring
Forming a new species • Speciation: forming a new species • Species: a group of organisms that can mate and produce fertile offspring 1. same number of chromosomes 2. similar morphology (body structure)
Disruptive Selection Divergent evolution that most likely results in two new species
Mechanisms • Allopatric speciation caused by geographic isolation (physical separation of organisms)
Mechanisms Cont. • Sympatric Speciation due to reproductive isolation (barriers to successful breeding)
Mechanisms Cont. • Sympatric Speciation due to reproductive isolation (barriers to successful breeding) • Prezygotic: before fertilization (i.e breeding times or home range)
Mechanisms Cont. • Sympatric Speciation due to reproductive isolation (barriers to successful breeding) • Prezygotic: before fertilization (i.e breeding times or home range) • Postzygotic: after fertilization- infertile offspring (chromosome # doesn’t add up)
Pre or Postzygotic? • offspring die young when two species of salamanders mate – they do not make it to maturity
Pre or Postzygotic? • Two different species of birds have different mating calls so they cannot recognize each other as mates
Pre or Postzygotic? • garter snakes that live in water and one on land cannot mate although they live in the same geographic area
Pre or Postzygotic? • mules are produced by mating a horse and a donkey, but mules are sterile and cannot breed
Pre or Postzygotic? • if gametes of red and purple sea urchins do not fuse, there will be no zygote produced
Pre or Postzygotic? • although some strains of cultivated rice can be cross-mated, after a few generations, sterile offspring are created
Rates of Speciation • Gradualism: slow divergence (small changes) • Punctuated equilibrium: sudden shift in form (irregular)
Example (don’t need to write) • 1. Stasis: A population of mollusks is experiencing stasis, living, dying, and getting fossilized every few hundred thousand years. Little observable evolution seems to be occurring judging from these fossils.
Isolation: A drop in sea level forms a lake and isolates a small number of mollusks from the rest of the population.
Strong selection and rapid change: The small, isolated population experiences strong selection and rapid change because of the novel environment and small population size: The environment in the newly formed lake exerts new selection pressures on the isolated mollusks. Also, their small population size means that genetic drift influences their evolution. The isolated population undergoes rapid evolutionary change.
No preservation: No fossils representing transitional forms are preserved because of their relatively small population size, the rapid pace of change, and their isolated location.
Reintroduction: Sea levels rise, reuniting the isolated mollusks with their sister lineage
Expansion and stasis: The isolated population expands into its past range. Larger population size and a stable environment make evolutionary change less likely. The formerly isolated branch of the mollusk lineage may out-compete their ancestral population, causing it to go extinct.
Evolution appears to happen in sharp jumps associated with speciation events
Class work • Speciation Comic • Homework • Speciation Vocabulary Chart • Speciation • Allopatric speciation • Sympatric speciation • Gradualism • Punctuated equilibrium