2. What you need to be able
to do and understand:
• D.2.1 Define allele frequency and
gene pool.
• D.2.2 State that evolution involves
a change in allele frequency in a
population's gene pool over a
number of generations.
• D.2.3 Discuss the definition of the
term species.
• D.2.4 Describe three examples of
barriers between gene pools.
• D.2.5 Explain how polyploidy can
contribute to speciation.
• D.2.6 Compare allopatric and
sympatric speciation.
• D.2.7 Outline the process of adaptive
radiation.
• D.2.8 Compare convergent and
divergent evolution.
• D.2.9 Discuss ideas on the pace of
evolution, including gradualism and
punctuated equilibrium.
• D.2.10 Describe one example of
transient polymorphism.
• D.2.11 Describe sickle-cell anemia as
an example of balanced
polymorphism.
3. What is a species?
Do we have a good idea of what
defines a species?
What is the bottom line
according to the narrator?
What is the biological species
concept of a species?
From this short introduction
answer:
A group of actually or potentially interbreeding
populations, with a common gene pool, which are
reproductively isolated from other such groups
Biological species concept:
4. Why the confusion?
A group of actually or potentially interbreeding populations, with a
common gene pool, which are reproductively isolated from other
populations
Some species cannot interbreed but they are so
close in appearance that they are difficult to tell
apart. These are knows as siblings species.
Some species only reproduce asexually
-binary fission in many single celled organisms
-parthenogenesis (reproduction via self-cloning: aphids, some bees, wasps and
hornets, some fish and water fleas)
-gynogenesis (via the catalyst of a male nearby which does not actually fertilize
the eggs: komodo dragons, some sharks, some snails)
Some species occasionally mate to produce hybrids
-occasionally happens to migrating waterfowl- read this article from Ducks unlimited:
http://www.ducks.org/conservation/waterfowl-biology/waterfowl-hybrids
American gray treefrogs, Hyla chrysoscelis and Hyla versicolor. They are indistinguishable from one
another morphologically but they do not interbreed. One of them is a diploid, and the other is a
tetraploid, with twice the number of chromosomes as the other species. Their hybrids are not
viable, so they have evolved slightly different mating calls to help them avoid interbreeding with one
another.
Hyla
versicolor
Hyla chrysoscelis
5. What is a species?
A group of actually or potentially interbreeding populations, with a
common gene pool, which are reproductively isolated from other
populations
"I have come to realize that there are
different concepts of species afloat in
biology that have their own utilities. I am
an agnostic pluralist about this: there are
different concepts of species for different
endeavors.
There does not seem to be one concept
of species that meets everybody‘s needs
very well, and so I am inclined to think we
are better just leaving the term quite
loose and not making a big deal of it."
Daniel Dennett (philosopher of science)
6. What is a species?
A group of actually or potentially interbreeding populations, with a
common gene pool, which are reproductively isolated from other
populations
"Where we have sexual reproduction a
species can be objectively defined as a
group of organisms which reproduce
sexually amongst themselves but don't
reproduce with members of other
species.
Where we don't have sexual reproduction
- as in asexual species, or in fossils
where we have no idea of how they
reproduced - then there is no objective
definition of the species, and the species
just becomes like the genus, the family or
the class. It's subject to arbitrary human
decision."
Richard Dawkins
7. What is a species?
A group of actually or potentially interbreeding populations, with a
common gene pool, which are reproductively isolated from other
populations
"There has been much discussion over
the years as to whether species actually
exist in nature outside the minds of
taxonomists.
My own view is that they do exist outside
of our minds. They exist by virtue of the
evolutionary process, by virtue of the fact
that lineages go along through time and
split and evolve independently and then
split again, and that is how we get the
tree of life."
Michael Donoghue
8. What is a species?
A group of actually or potentially interbreeding populations, with a
common gene pool, which are reproductively isolated from other
populations
A good example of how similar species can tell each apart can be found in
the American cricket: Within a single habitat in the USA, as many as 30 or 40
different species of crickets may be breeding but the female cricket
recognizes the song of males of her own species and will breed only with a
male who sings that song. The song, and the female recognition of
it, constitutes a mate recognition system: the species has a specific mate
recognition system by which it can be identified.
Actually or potentially interbreeding: they must not be geographically isolated
populations
Gene pool: is the complete set of unique alleles in a species- these must be
common in the population- by changing the gene pool you change the allele
frequency.
Reproductively isolated: the inability of a species to breed successfully with
related species due to geographical, behavioral, physiological, or genetic
barriers or differences
9. Allele Frequency
a change in this means
evolution
Check this out:
http://www.allelefrequencies.net/
It ―provide one central source, freely available to all, for the
storage of allele frequencies from different polymorphic
areas in the HUMAN genome.‖
Allele frequency is the proportion of all copies of a gene that is
made up of a particular gene variant (allele). In other words it is a
measure of how common an allele is in a population. Allele frequencies
range from 0 to 1.0, or as a percentage. Evolution always involves a change
in allele frequency in a population's gene pool, over a number of generations
10. How can barriers happen
between gene pools?
Temporal isolationGeographical isolation
This can be due to either the timing
of breeding cycles (seasonal
isolation) of closely related species
or the time of day or night that
species search for mates. A good
example is from the pine trees Pinus
radiata and P. muricata, which grow
together in California. P. radiata
sheds its pollen in early February
and P. muricata in April.
P. radiata
P. muricata
A change in climate, or a
geological change such as
an orogenic event
(mountain making) or
continental drift may, over
time, split an originally
single population.
11. Read this article on new speciation genes and
how they relate to hybrid infertility
http://www.nature.com/news/2008/081211/full/n
ews.2008.1297.html
Hybrid infertility Behavioral isolation
How can barriers happen
between gene pools?
Visual and chemical signals are
important in mating. May have
frequent encounters but no mating
because male may not ―display‖ or
female may not be ―receptive‖, sex
pheromones might be confused
When placed in a wind tunnel, male moths will
fly to and attempt to mate with an object
impregnated with the pheromones of their own
species but will not respond to those of other
species.
Reduced hybrid viability and
fertility. Hybrid zygotes fail to
develop or fail to reach maturity.
Hybrids can also fail to produce
functional gametes
12. How do species form?
Speciation is the evolutionary process by which new biological species
arise.
Imagine that you are looking at a
tip of the tree of life that
constitutes a species of fruit fly.
Move down the phylogeny to
where your fruit fly twig is
connected to the rest of the tree.
That branching point, and every
other branching point on the
tree, is a speciation event.
At that point genetic changes
resulted in two separate fruit fly
lineages, where previously there
had just been one lineage.
But why and how did it happen?
The place where
you have common
ancestors
It is the formation of a new species by splitting of an existing
species.
13. A tale of two flies -
or the ―how‖ speciation happens
The scene: a population of wild fruit flies minding its own
business on several bunches of rotting bananas, cheerfully
laying their eggs in the mushy fruit...
14. Disaster strikes: A hurricane washes the bananas and the immature
fruit flies they contain out to sea. The banana bunch eventually washes
up on an island off the coast of the mainland. The fruit flies mature and
emerge from their slimy nursery onto the lonely island. The two portions
of the population, mainland and island, are now too far apart for gene
flow to unite them. At this point, speciation has not occurred—any fruit
flies that got back to the mainland could mate and produce healthy
offspring with the mainland flies.
15. The populations diverge: Ecological conditions are slightly different
on the island, and the island population evolves under different
selective pressures and experiences different random events than the
mainland population does. Morphology, food preferences, and
courtship displays change over the course of many generations of
natural selection.
16. So we meet again: When another storm reintroduces the island
flies to the mainland, they will not readily mate with the mainland
flies since they‘ve evolved different courtship behaviors. The few
that do mate with the mainland flies, produce inviable eggs
because of other genetic differences between the two
populations. The lineage has split now that genes cannot flow
between the populations.
Story from: http://evolution.berkeley.edu/evosite/evo101/VBDefiningSpeciation.shtml
17. Causes-
or the ―why‖ speciation happens
Geographic isolation
Populations are prevented from interbreeding by geographic isolation.
Geographic isolation is a common way for the process of speciation to
begin: rivers change course, mountains rise, continents drift, organisms
migrate, and what was once a continuous population is divided into two or
more smaller populations.
Any unfavorable habit will pose a barrier between populations.
18. Reduction of Gene Flow
However, speciation might also happen in a population with no specific
extrinsic barrier to gene flow. Imagine a situation in which a population
extends over a broad geographic range, and mating throughout the
population is not random. Individuals in the far west would have zero
chance of mating with individuals in the far eastern end of the range. So we
have reduced gene flow, but not total isolation. This may or may not be
sufficient to cause speciation. Speciation would probably also require
different selective pressures at opposite ends of the range, which would
alter gene frequencies in groups at different ends of the range so much that
they would not be able to mate if they were reunited.
19. Go to this tutorial and answer these
questions from the introduction, then
as a class we will watch the
animations:
1- What is allopatric speciation?
2- What is sympatric speciation?
3- What is parapatric speciation?
1-Allopatric speciation is just a fancy name for speciation by geographic isolation (ie
different geographical area).
2-Sympatric speciation does not require large-scale geographic distance to reduce
gene flow between parts of a population. Merely exploiting a new niche may
automatically reduce gene flow with individuals exploiting the other niche. This may
occasionally happen when, for example, herbivorous insects try out a new host plant
(apple maggot flies now eating a new food source – apples (introduced to America
many years ago -instead of hawthorns). It happens in the same geographical area.
3-In parapatric speciation there is no specific extrinsic barrier to gene flow. The
population is continuous, but nonetheless, the population does not mate randomly.
Individuals are more likely to mate with their geographic neighbors than with individuals
in a different part of the population‘s range. In this mode, divergence may happen
because of reduced gene flow within the population and varying selection pressures
across the population‘s range. (not part of your IB syllabus)
http://bcs.whfreeman.com/thelifewire/conte
nt/chp24/2402001.html
20. Two other examples of allopatric speciation:
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m3/s2/evm3s2_4
.htm
and
http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html
21. Data based
question
Lacewing songs
Click on the oscillograph to hear the song
Oscillograph of Chrysoperla mediterranea
Oscillograph of Chrysoperla lucasina
From: http://www.eeb.uconn.edu/people/chenry/Cryptic_songs.html
Songs are part of the process of mate
selection in members of different species
within the genus Chrysoperla (lacewings).
Males and females of the same species
have precisely the same ‗song‘ and during
the pre-mating period take turns making the
songs.
1- Compare the songs of the two
species of lacewings.
2- Explain why differences in mating
songs might lead to speciation.
3- The ranges of the two species
currently overlap. Suggest how
differences in song could have
developed:
a) by allopatric speciation
b) by sympatric speciation
C. lucasina ranges across most of Europe and eastward into western Asia, and the northern
quarter of Africa. C. mediterranea ranges southern to central Europe and across the north
African Mediterranean.
22. Polyploidy can also
lead to speciation
Polyploidy is when the number of chromosomes in a
cell becomes doubled. This can happen by a
mutation that simply makes two copies. It can also
happen when the chromosomes from two different
species are mixed (a form of sympatric speciation).
Many plants are both male and female, so they can
simply fertilize themselves. Some earthworms can
do this too.
An example is the gilia plant from the Mojave desert in California. The species
Gilia transmontana turned out to be a hybrid of Gilia minor and Gilia clokeyi. It
has as many chromosomes as the other two combined, and its flowers have an
intermediate shape. Since chromosomes are not all the same length, we can
even say which transmontana chromosomes came from which ancestor.
Gilia
transmontana
Gilia minor
Gilia clokeyi
23. Convergent vs Divergent
Evolution
Divergent
Increases the morphological
differences between species, as each
species adapts to different ecological
niches OR it is the process of two or
more related species becoming more
and more dissimilar.
Convergent
Decreases morphological differences
between species, as each species
adapts to similar ecological niches OR it
is the process of unrelated species
become more and more similar in
appearance as they adapt to the same
kind of environment. A- Divergent B- Convergent
C- Parallel evolution occurs when
two species evolve independently
of each other, maintaining the
same level of similarity. Parallel
evolution usually occurs between
unrelated species that do not
occupy the same or similar niches
in a given habitat.
24. Examples of Convergent Evolution
Old World vultures and New World vultures eat carrion, but Old World vultures are
in the eagle and hawk family (Accipitridae) and use mainly eyesight for discovering
food; the New World vultures are of obscure ancestry, and some use the sense of
smell as well as sight in hunting. Birds of both families are very big, search for food
by soaring, circle over sighted carrion, flock in trees, and have unfeathered heads
and necks.
Nubian Vulture (Torgos
tracheliotos) is an Old World
vulture
Turkey Vulture (Cathartes
aura) is a New World vulture.
See Wikipedia for
more examples
Other than wings in birds and bats
25. Examples of Divergent Evolution
The red fox lives in mixed farmlands and forests, where its red color helps it
blend in with surrounding trees. The ears of the kit fox are larger than those of
the red fox. The kit fox's large ears are an adaptation to its desert environment.
The enlarged surface area of its ears helps the fox get rid of excess body heat.
Similarities in structure indicate that the red fox and the kit fox had a common
ancestor. As they adapted to different environments, the appearance of the two
species diverged.
Red fox Kit fox
Can you think of other
examples?
26. Adaptive Radiation
It is a process of divergence where initially similar
populations become genetically different from each
other.
When a population disperses into
separate geographic locations, or
ecological niches within a
geographic range, the dispersed
populations are exposed to unique
sets of selective pressures and
therefore change from the original
ancestral species.
The most famous
example is
Darwin‘s finches
Each new species adapts to its unique environment radiating away from other
species genetically (expressed as ecological, behavioral, physiological and
morphological differences).
27. How fast does all this happen?
Gradualism
Species slowly change
through a series of
intermediate forms
Punctuated equilibrium
Long periods of relative stability in a species
are ‗punctuated‘ by periods of rapid evolution
This was the
dominant framework
in paleontology for a
long time. But, there
were gaps in the
fossil record (no
intermediate forms)
so it is hard to see
this pace of
evolution.
But according to
this version gaps in
the fossil record
might not be gaps
at all. Events such
as geographic
(allopatric)
speciation and new
niches can lead to
rapid speciation.
28. Is this an example of Gradualism or
Punctuated equilibrium?
Read this story from an article published in Nature:
http://www.telegraph.co.uk/news/uknews/1546965/Global
-warming-not-death-of-dinosaurs-led-to-rise-of-
mammals.html
29. A short video on
sickle cell anemia
When carriers have advantages that allow a detrimental
allele to persist in a population, balanced polymorphism is
at work. This form of polymorphism often entails
heterozygosity for an inherited illness that protects against
an infectious illness.
Balanced polymorphism
Deflated red
cells from
a human with
sickle-cell
anemia
Normal
human
red cells
A balanced polymorphism is a polymorphism in which the
frequencies of the characteristics remain fairly constant over time.
Polymorphism is when two or more
forms of a phenotype are
represented in high enough
frequencies to be readily noticeable.
30. Transient
polymorphismA transient polymorphism is one that is changing in
frequency over time.
Two examples, if a population with a high incidence of the
sickle cell anemia gene moves to a location where malaria
is no longer present the gene will begin to disappear
because the selective pressure in its favour has
disappeared but the pressure against it (the homozygous
form) continues. Over time its frequency will fall.
A paper looking at flowers
http://aob.oxfordjournals.org/c
ontent/early/2009/06/10/aob.m
cp141.full
Industrial melanism:
Biston betularia, the peppered moth, where allelic
frequencies continue to change over time.
As forests became soot-covered, previously rare dark
morphs became more frequent as they were selected for
over light morphs.
As forests became less soot-covered with pollution
controls, the dark moths once again decreased in
frequency to low levels as light morphs were selected for
over dark morphs.
31. “But we can’t see evolution happening, so how do we know
it’s true?”
But we do see it happening, all the time. Here is a recent
example of a new species of salamanders evolving in
California, from PBS:
32. Your assignment.. Or
How the ―_______‖ became a ―________‖
Create a story similar to the one that I did ―A tale of
two flies‖ to explain how speciation happens.
Thinking about how species develop and change over
time your job is to develop a story on speciation about
an organism of your choosing- fictional or non-
fictional. Using the proper terminology and scientific
principles on speciation (what is in your syllabus)
create a story on how any species might have
evolved. Your story can be in any format (comic
book, picture book, narrative form, children‘s
story, young adult, or in the form of a folklore or fairy
tale).
The description of the assignment and the criteria can
be found on moodle
