Evolution: Speciation
This tutorial will explore the processes by which species evolve.
1. What is a species?
o A species can be defined as one or more populations of
interbreeding organisms that are reproductively isolated in nature
from all other organisms.
o Genetic divergence results when adaptation, drift, and mutation act
on populations
2. Speciation
o As populations diverge, the form similar but related species.
o When are two populations new species?
 When populations no longer interbreed, they are thought to
be separate species.
 As natural selection adapts populations occupying different
environments, they will diverge into races, subspecies, and
finally separate species.
3. Cladogenesis and anagenesis
o Cladogenesis is the situation where a small part of a parent species
gives rise to a new species
o Anagenesis is the evolution of one species into an entirely different
descendant species
4. Geographical isolation and species formation: allopatric speciation
o Populations begin to diverge when gene flow between them is
restricted.
o Geographic isolation is often the first step in this process.
o Other mechanisms may develop that further restrict reproduction
between populations: these are the reproductive isolating
mechanisms.
5. Example of geographic isolation
o Here is an example of geographic isolation.
o First, the original population started in the north and migrated
toward the south.
o Second, the population split to the east and west of the central
valley. Then, two populations began to evolve independently.
o Third, evolution of the eastern and western populations.
o Lastly, the east and west populations came back together in
Southern California but could no longer interbreed, or they
produced infertile hybrid offspring.
6. Allopatric speciation
o Gene flow is experienced by two populations.
o Geographic barriers interrupt gene flow. Variant types appear.
o Drift and different selection pressures cause divergence between
isolated gene pools.
o Reproductive isolation is present even though the geographic
barrier has been removed. Speciation is complete.
7. Sympatric speciation
o Sympatric speciation occurs when members of a population
develop some genetic difference that prevents them from
reproducing with the parent type.
o This mechanism is best understood in plants, where failure to
reduce chromosome number results in polyploidy plants that
reproduce successfully only with other polyploids.
8. Reproductive isolating mechanisms
o Geographic isolation
o Ecological isolation, or habitat isolation
o Courtship rituals, or behavioral isolation
o Seasonal or temporal differences
o Mechanical isolation
o Gamete isolation
o Hybrid sterility
9. Geographic separation
o Some barrier to gene flow between populations of a species
develops over time.
o Genetic differences build up between the populations.
o Mountain ranges, deserts, oceans, and rivers are examples of this
mechanism.
10. Habitat isolation
o If populations occupy different areas of the environment, chances
of gene flow between them are greatly reduced.
o The next several slides show examples of habitat isolation.
11. Courtship rituals or behavioral isolation
o Behavioral isolation results from differences in mating behavior
between two species.
o Numerous example occur in birds, such as peacocks and bluefooted boobies.
12. Temporal isolation
o Temporal isolation occurs when two species live in the same
location, but each reproduces at a different time of year, and so
they do not attempt to mate.
13. Mechanical isolation
o Mechanical isolation is the result of differences between two
species in reproductive structures or other body parts, so that
mating is prevented.
14. Gamete isolation
o Gamete isolation is physical or chemical incompatibility of gametes
of two different species so that they cannot fuse to form a zygote;
an egg may have receptors only for the sperm of its own species.
15. Hybrid sterility
o Hybrid sterility occurs when the hybrid offspring are sterile
o Examples include mules as sterile hybrids between horses and
donkeys, ligers as crosses between lions and tigers, and numerous
plants, such as Raphanobrassica, a cross between mustard and
radish.
16. Adaptive radiation
o The evolution of several species of finches on the Galapagos
Islands is an example of adaptive radiation, because each one has
a different way of life.
o Adaptive radiation occurs when a few individuals migrate to a new
area, then natural selection promotes different feeding habits in
different ecological habitats.
17. The pace of evolutionary change is not always slow
o The pace of evolution is often slow, so slow that all of the stages in
species formation cannot be observed.
o Studies on two related species of the fruit fly Drosophila provide
evidence for the stages in the formation of new species.
18. Pace of evolution
o Darwin’s view of evolution was that it was a very slow process,
resulting from the gradual accumulation of small genetic
differences.
o Recently, several alternative views on the pace and events in
species formation have been proposed.
o These are collectively called quantum speciation.
19. Punctuated equilibrium
o According to another model based on the fossil record, speciation
occurs rapidly over a short time, followed by a long period of little or
no change.
o “Short” means thousands or hundreds of thousands of years.
o This differs greatly from Darwin’s original view of slow and gradual
change continuing over very long periods of time.
20. Phyletic gradualism versus punctuated equilibrium
o Phyletic gradualism involves evolution of the entire species, while
punctuated equilibrium has small, isolated populations subjected to
selection pressure.
©2011 Connections Education LLC. All rights reserved.