The document discusses two patterns of speciation: gradualism (anagenesis) in which a species slowly changes over time through natural selection until it is considered a new species, and branching (cladogenesis) in which a species rapidly splits into two or more new species. Branching speciation occurs through geographic isolation of populations followed by the evolution of reproductive barriers between the isolated groups, preventing interbreeding. This process of branching is the basis for biological diversity as it leads to the formation of multiple new species from the original.

1. Two Patterns of Speciation
1. Gradualism (Anagenesis)
A slow, gradual accumulation of
heritable changes (adaptations) in
a population, due to many small
episodes of natural selection. So,
one species changes slowly, step-
by-step, until it looks so different
that we call it a new species.
2. Branching (Cladogenesis)
A more rapid splitting of one or
more new species from an
original species that may or may
not continue to exist. So one
species branches into two or
more new ones. This process is
the basis for all biological
diversity.

2. How does one species evolve into two or more new
species by branching?
By a 2-step process…
1. Geographic isolation - A single population gets divided
into two (or more) populations.
The members of the different populations (which are
now separated from each other) are therefore no longer
able to interact with each other.
2. Evolution of reproductive barriers - Over time, as they
adapt to their different environments due to natural
selection, the two populations evolve one or more
reproductive barriers, which prevent interbreeding.

3. Step 1: Geographic Isolation
Gene flow between two populations is
interrupted (or reduced) between two
populations because they are geographically
separated. What causes this to happen?
a) Geological Change
New lava flows, the gradual formation of a river
valley or mountain range, or the slow movement s
of tectonic plates can isolate populations if the
organisms are unable to disperse/travel between
adjacent locations (eg., Harris’ antelope squirrel
and white-tailed antelope squirrel)

4. b) Geographic isolation can also result from
colonization of a new area (as in the founder effect)
The separation of a small “founding” population from the main
population is a crucial event in the formation of new species,
especially on islands.
The combination of natural selection and genetic drift increase
the likelihood that the population will change over time, making it
less and less similar to the main population.

5. Adaptive Radiation: many diversely-adapted species
evolved from a common ancestor
• Sometimes founder populations on island groups form
different species on each island; or sometimes different
species in each valley.

6. Adaptive Radiation: many diversely-adapted species
evolved from a common ancestor
Hawaiian Honeycreepers
• 51 endemic species of honeycreepers
have evolved from 1 species of finch
that colonized Hawaii millions years
ago.
• More than 1/3 of the species are now
extinct (15 within historical times)

7. Adaptive Radiation: many diversely-adapted species
evolved from a common ancestor
30 species of plants in Hawaiian Silversword alliance evolved from
one species that colonized Kauai about 5 million years years ago.
Closest non-Hawaiian relative is a tarweed that lives on the west coast
of N. America
Online Activity 15.1

8. Time-calibrated
phylogeny of the
Hawaiian
silversword alliance
and their closest
continental relatives
Baldwin B G , Sanderson M J PNAS 1998;95:9402-9406
©1998 by The National Academy of Sciences

9. Important point to remember:
Geographic Isolation alone is not sufficient to
lead to speciation.
Even when changes in the
gene pool result in new
adaptations of an isolated
population to a local
environment, speciation may
not occur.
Speciation occurs only when
changes in their gene pools
result in the formation of
reproductive barriers
between the two (or more)
isolated populations.

10. Step 2. The formation of reproductive
barriers between species
• What are reproductive barriers?
Any morphological, physiological, or behavioral trait
that prevents different organisms from successfully
interbreeding (so they cannot produce viable, fertile
offspring).
• There are several types, which can be classified as
either pre-zygotic or post-zygotic barriers

11. Pre-zygotic Reproductive Barriers
Prevent the formation of a zygote by preventing
mating or fertilization
There are 5 types:

12. Pre-zygotic Reproductive Barriers
1. Habitat Isolation
Two species that occupy two different
habitats within the same geographical
area may never interact because they
never (or very rarely) encounter each
other.
e.g. two species of garter snakes that
live in the same area, but one is
primarily terrestrial and the other is
aquatic

13. Pre-zygotic Reproductive Barriers
• Prevent the formation of a zygote by preventing
mating or fertilization
1. Habitat Isolation
2. Temporal Isolation (a matter of timing)
Species that breed during
different times of the day,
different seasons, or different
years cannot mate.
e.g. ranges of the eastern and
western spotted skunk overlap, but
the eastern species (c) mates in late
winter and the western one (d)
mates in the fall

14. Pre-zygotic Barriers
• Prevent the formation of a zygote by preventing
mating or fertilization
1. Habitat Isolation
2. Temporal Isolation
3. Behavioral Isolation
Little or no sexual attraction between
males and females of different
species, perhaps due to unique
courtship behaviors
e.g. Eastern and western meadowlarks
are almost identical in shape, color and
habitat, but they remain separate species
because their courtship rituals differ

15. Pre-zygotic Barriers
• Prevent the formation of a zygote by preventing
mating or fertilization
1. Habitat Isolation
2. Temporal Isolation
3. Behavioral Isolation
4. Mechanical Isolation
Reproductive structures are physically
incompatible (lack of “fit”). (e.g., The differently-
shaped penises of closely-related insects
prevent cross breeding)
In plants, the pollinators may be different. (e.g.,
The differently-shaped and colored blossoms of
flowers attract different types of pollinators;
hence cross-pollination is extremely rare.

16. Pre-zygotic Barriers
• Prevent the formation of a zygote by preventing
mating or fertilization
1. Habitat Isolation
2. Temporal Isolation
3. Behavioral Isolation
4. Mechanical Isolation
5. Gametic incompatibility
Sperm from one species is unable to fertilize the eggs
of another species.
e.g. Two closely-related species of sea urchins may
breed at the same time on the same coral reef, but
their gametes are not compatible.

17. Post-zygotic Reproductive Barriers
• Occur after the formation of a zygote by preventing
the formation of healthy or fertile offspring
• There are 2 types:

18. Post-zygotic Reproductive Barriers
• Occur after the formation of a zygote by preventing
the formation of a viable, fertile adult offspring
1. Reduced hybrid viability
A hybrid zygote fails to survive embryonic or juvenile
development.
e.g. Some salamanders in the genus Ensatina
live in the same regions and habitats, where
they occasionally interbreed; however, the
hybrid offspring rarely complete embryonic
development, and those that do are frail and
do not live long.
Same is true for sympatric frogs in genus
Rana.

19. Post-zygotic Reproductive Barriers
• Occur after the formation of a zygote by preventing
the formation of a viable, fertile adult offspring
1. Reduced hybrid viability
2. Reduced hybrid fertility
Even if hybrids are healthy, they may be sterile.
If the chromosomes of the two parent species differ in structure or
number, meiosis in the hybrid offspring may fail to produce normal
gametes, preventing gene flow between the two species.
e.g. the hybrid offspring of a donkey and a horse, a mule, is healthy
but sterile

21. Concept Check
1. Why are donkeys and horses considered different species?
Their offspring (mules) are sterile (not fertile).
2. What is macroevolution?
Major evolutionary changes, often evident in the fossil record,
due to speciation and the evolution of major new adaptations
3. Give an example of a reproductive barrier that may separate
two similar species.
prezygotic: behavioral, habitat, temporal (timing), mechanical,
gametic,
postzygotic; reduced hybrid viability or fertility

22. Concept Check
4. Describe conditions that could make a new island a likely
place for adaptive radiation.
The island may consist of varied habitats with environmental
conditions that differ from one another and from other
landmasses.
Organisms may adapt to the varied conditions in different
ways, resulting in species diversity.
5. How does that branching model of evolution relate to
Darwin's theory of natural selection?
Suggests that natural selection causes most change as new
species begins. Major change is less common once a species
is established in an area.