SpeciationThe Process of

Evolution

Speciation• Formation of a new species• Species:

– a population that can breed freely and produce fertile offspring

• Speciation often occurs when part of the population is isolated from another part– Selective pressures of the environment in

one area may be different from pressures in another area

What is a Species?• Definition :• Morphospecies - based on appearance• Biologic species - a population that can

breed freely and produce fertile offspring• The largest unit of population in which gene

flow is possible• Limitations:

– doesn’t work for asexual organisms– extinct life forms– populations that are geographically isolated -

sometimes call subspecies

• No clear answer; idea is arbitrary

Patterns of Speciation

• Fossil record shows 2 patterns:• Anagenesis (phyletic evolution)

– the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species

• Cladogenesis – branching evolution; budding of one or

more new species from a parent species that continues to exist.

Anagenesis vs. Cladogenesis

Causes of Speciation• Speciation often occurs when part of

the population is isolated from another part

• Geographic Isolation – most common– a physical barrier develops (changing

course of a river; separation of an island)– Selective pressures in one area are

different from pressures in another area

• Reproductive Isolation– another form of isolation

Isolation

Geographic Isolation• Biogeography of Speciation • Classified according to geographic relationship

between new and old species• Sympatric

– population becomes reproductively isolated in the midst of the parent population

– ranges of new and old species overlap.

• Allopatric – species are physically separated– more likely in small populations

• Adaptive radiation is allopatric :– emergence of numerous species from a common

ancestor that spreads to several new environments.

Allopatric vs. Sympatric

Allopatric Barriers

Geographic Isolation

Reproductive Isolation

• Example: organisms breed at different times

• Reproductive barriers are of 2 types:• Prezygotic

– before the formation of fertilized eggs– impedes mating or fertilization

• Postzygotic – after

Reproductive Isolation

Prezygotic Isolation• Impedes mating or fertilization• Habitat isolation

– not geographically separated, but occupy different niches within an area, e.g. trees versus ground

• Temporal isolation – breed at different times

• Behavioral isolation– don’t produce appropriate courtship signals

• Mechanical isolation – anatomically incompatible

• Gametic isolation – mating occurs but gametes rarely fuse to form

zygotes

Behavioral Isolation: Courtship Barrier

Postzygotic Barriers

• Hybrid inviability – offspring don’t make it

• Hybrid sterility – e.g. mules

• Hybrid breakdown – F2 are sterile or weak

Introgression

• Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species

• Increases variation• Rare

– 2 species remain distinct

Post Speciation Evolution• Divergent Evolution

– Process by which related organisms become less alike

– occurs after speciation– at first 2 new species are very similar, but

over time become more & more different.

• Adaptive radiation is a special type of divergent evolution– Many new species from a single parent

species

Adaptive Radiation

Timing of Evolution• Most scientists accept natural selection as the

process of evolution• The timing is controversial• Gradualism

– the traditional view– a slow, steady accumulation of changes, leads to

new species

• Punctuated Equilibrium– long periods of inactivity followed by big jumps

• Fossil record provides evidence that the pace of evolution varies– The same evidence is used to support different ideas– Could be some of both

Gradualism vs. Punctuated Equilibrium