these animals will interbreed with one another€¦ · macroevolution part ii allopatric speciation...

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Macroevolution Part II:

Allopatric Speciation

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Looks Can Be Deceiving!

• These meadowlarks lookvery similar yet they are notthe same species.

• By contrast, these brittle stars look very different from one another, but they are the same species.

One of the outcomes of evolution is the formation of new species. What is a species? Allow the students come up with a definition and debate its merit. Several definitions of macroevolution also exist depending on the textbook you use. • Evolving complex structures such as an eye. • Speciation • Evolution of whole communities.

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Definition of Species

• A species is a group of interbreeding organisms that produce viable, fertile offspring in nature.

• Members of a species will interbreed with one another but not other organisms outside of the species. (At least most of the time!)

Note the italics on “viable”! Hybrid animals exist which also makes the information in the second bullet need some qualifiers! Now is a good a time as any to see if students know about hybrids at all and/or can list any. This site has photos of the “Top Twenty” most popular animal hybrids: http://www.buzzfeed.com/toyota/the-20-coolest-hybrid-animals-3d8x A zebroid (zebra and equine) and donkra (donkey and zebra) are pictured. Hybrids will be revisited later as well.

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Macroevolution vs. Microevolution

• Macroevolution is evolution on a scale of separated gene pools.

• Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population.

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Asexual Species

Even though asexual groups do not exchange genes, they do form recognizable groups.

Most have evolved from a sexual species. Only those whose phenotypis best adapted to the environment, will continue to survive. However, itmakes them less adapted to environmental change.

Dandelions are asexual. The pollen sterile and the egg is diploid.

Asexual Species

If there is no interbreeding, how can this be a species? Even though asexual groups do not exchange genes, they do form recognizable groups. Most have evolved from a sexual species. Only those whose phenotype is best adapted.

Macroevolution Part II Allopatric Speciation TEACHER NOTES

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Ring Species

• A ring species is a connected series of neighboring populations, each of which can interbreed with closelysited related populations, but for which there exist at least two "end" populations in the series.

• These end populations are too distantly related to interbreed, though there is a potential gene flow betweeach "linked" species.

• Such non-breeding, though genetically connected, "enpopulations may coexist in the same region thus closina "ring".

Examples forthcoming…

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Ring Species • Ensatina escholtzi is a

salamander ring species that has a range along the coast and inside range of California.

• All along this range, the salamanders interbreed, but the salamanders on the ends of the ring do not interbreed.

• Their groupings are called subspecies.

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Ring Species

• The blue zones represent where interbreeding is occurring.

• So are there is gene flow all along the salamander’s range, yet the ends of the rings do not interbreed. Are they the same species?

If the ends of the rings, cannot interbreed how can this be the same species? (See next slide…)

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Ring Species

They become a subspecies. http://www.flickr.com/photos/davemedia/6243040930/lightbox/


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Limited Interbreeding

• Each Canis species will interbreed with the domestic dog but not readily with one another.

• This is true, even when given the opportunity to do so. Thus, they are not the same species since they do not interbreed in nature.

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These animals will interbreed with one another but on a limited basis. So are they the same species? No Why not? They do not produce VIABLE offspring. Why not? Often it has to do with meiosis, specifically, the chromosomes do not pair evenly during meiosis, or gamete formation. There is a stage during meiosis that has to occur called homolog pairing and crossing over, which can not occur if chromosomes are not the same size or shape.

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Limited Interbreeding• Tigers and lions will interbreed in captivity, but they

do not interbreed in nature.

• Lions form groups or prides and live in the grasslands

• Tigers are more solitary and live in the forests.

• Tiglon are products of male tigers and female lions.

• Ligers are the opposite cross.

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These animals will interbreed with one another but on a limited basis. So are they the same species? No. How do we know? The offspring are not viable.

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I. Anagenesis- is the accumulation of changes in one species that leads to another species. It is the lineage of a species. Over time a species may accumulate enough changes that it is considered a species that differs from the ancestral species.

II. Cladogenesis- is the budding of one or more new species from an ancesteral species that continues to exists. This results in biological diversity.

Cladogenesis vs. Anagenesis

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Allopatric vs. Sympatric Speciation

Allopatric speciation-Speciation occurs because a given group has been separated from the parent group, usually because of a geographic separation as time goes by.

Sympatric speciation-speciation occurs even though the two groups are still living in the same area.

Graphic-Campbell From Latin: Allo = “different” and Patric = Father (meaning “fatherland” in this case), so Allopatric translates as “different fatherland” meaning the varieties evolved in different geographical locations. Sym = “same” so Sympatric translates as “same fatherland”


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Allopatric Speciation• First, geographic isolation occurs. This is an extrinsic isolating

mechanism.

• The two populations must become isolated geographically fromone another.

• If the groups become sympatric again one of two things result

Graphic-Campbell An extrinsic is a property is one that is not dependent on the organism in question. It is an external factor.

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Allopatric Speciation1. They become separate species, as evidenced by the fact they c

no longer interbreed.

2. They can still interbreed, thus they remain the same species.

Islands produce some of the most profound examples of speciation due to geographic isolation.

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1. The population that left the original group will hava different allelic make-up than the original speciesthus experiencing the “founder effect”.

2. The two groups will continue to experience different mutations.

3. The two groups will now experience genetic drift and different selection pressures due to living in separate and perhaps different environments.

Why does speciation occur after geographic isolation

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Adaptive Radiation

• The classic adaptive radiation example involves the finches of the Galapagos Islands.

• There are 14 different species of finches and 13 main islands, 3 smaller islands, and 107 rocks and islets.

These finch populations are often referred to as “Darwin’s Finches”. Also, you can find varying numbers of the different species throughout the literature. Note that only 10 of the species are pictured here.

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Adaptive Radiation

These finch populations are often referred to as “Darwin’s Finches”. This slide shows a common South American ancestor and 14 other species that “radiate” from the original immigrant finch.


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Adaptive Radiation

• One would expect that each island would have only one species, however, each island has more than one species of finch and larger islands may have as many as ten.

• The process one species inhabiting a new area and evolving into several new species is called adaptive radiation.

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The Amazing Galapagos Islands

One would expect one species of finches on each island but that is not true. Each island has at least three species of finches and some islands as many as 10 species.

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Adaptive Radiation• Lets suppose that finch species A, from South

America migrates to an island .

• Finch species A would undergo speciation into finch species B due to one or more of the following: – The Founder effect

– Varying selection pressures

– Varying mutations

Explain that there are the three most common “reasons” speciation occurs.

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Adaptive Radiation

• Now let’s suppose that some of the new finch species B migrate over to a second island. (speciation) & (migration)

• The finches in this new environment are geographically isolated from the other island and nowwill evolve into finch species C for the same three basic reasons. (Founder effect, varying selection pressures, or varying mutations.)

Graphic Campbell

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Adaptive Radiation• Now some of the newly evolved finch species C make their

way to yet another new island.

• Guess what? Yep! Once again finch species C will evolve intofinch species D (not shown yet) for the same three reasons. (Founder effect, varying selection pressures, or varying mutations.)

• But suppose some of species C make it back to the first island (The plot thickens…)


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Adaptive Radiation• Obviously, species C is different from finch species B thus

they can no longer interbreed back on the original island.

• Finch species C may or may not evolve into another speci

• If there is a niche similar to that of the second island, the selection pressure may also be similar and species C may slow to change.

• So, both first and second islands will have species C. Thethird island will have a new species D.

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Adaptive Radiation

• Now lastly, lets suppose that finch species D from thethird island returns to the first and second islands. (& )

• On the second island finch species D does not change because it finds a niche similar to the third island so no selection pressure is exerted upon it.

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Adaptive Radiation

• Alas, the first island has no such niche. Now, there exists a selection pressure on finch species D causing to evolve (character displacement) into species E.

• As a result, the first island now has three different species of finches. Two of which are not found on other islands (B & E). Each species has a distinct habitat with different food sources. This process is called adaptive radiation and most commonly involveislands.

Character displacement refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur but are minimized or lost where the species’ distributions do not overlap. This pattern results from evolutionary change driven by competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

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The Amazing Galapagos Islands

So, NOW we understand how it is possible that each island has morthan one finch species. Some islands actually have as many as 10 species. Examine the map once more.


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Differences are found among beaks and feathers of the finch

Darwin found 14 different speof finches inhabiting these islawhich are a result of adaptive radiation.

There are finches that eat seedcacti, insects and other interesfoods.

He also observed adaptive radiation among the tortoises mocking birds.

The Amazing Finches From the Galapagos Islands

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2. Warbler finch

3. Large insectivorous tree finch

4. Medium insectivorous tree finch

5. Mangrove finch

6. Small insectivorous tree finch

7. Large cactus ground finch

8. Cactus ground finch

9. Cocos finch

10. Woodpecker finch

11. Large ground finch

12. Sharp-beaked ground finch

13. Medium ground finch

14. Small ground finch

The Amazing Finches From the Galapagos Island

Finches 1-9 are all tree finches. Finch #1 is vegetarian and the others insects but they differ in the types of insects they each eat which is also reflected in the size of their beaks. On islands where two or more of these finches exists, the differences in the beak size is even more pronounced. Finch #2 is in a genera all its own as its characteristics are more warbler like. Finch #8 has its own genera as it is the only finch found on the Cocos island 700 km from the Galapagos Islands. The second large groups of finches are ground finches 10-14. Some live on seeds and the others on cactus flowers. Smaller beaks are specialized for small seeds and larger beaks for large seeds.

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Example of Natural Selection• During droughts in the Galapagos Islands, larger seeds are mor

abundant. Finches with slightly larger beaks have an advantagsince they are able to crack larger seeds.

• Thus, natural selection favors finches with larger beaks. These finches are more likely to survive and pass those genes on to thnext generation. A study conducted by Peter and Rosemary Grover a 20 year period confirmed these assertions.

Why would larger seeds be more abundant during a drought? They can store more water! Peter and Rosemary Grant conducted a study of finches for over 20 years on an islet Daphne Major. They studied the medium ground finch, which is a seed eater. Its medium beak is very good a cracking small seeds which are produced in abundance during wet years. During droughts, larger seeds remain. Those finches with slightly larger beaks are at a advantage to crack larger seeds. Their study concluded that natural selection selects for those larger beaks during droughts. Those birds with larger beak are more likely to survive and pass those genes on to the next generation.

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• So, two populations of organisms are not the same species unless they can interbreed, and produce viabfertile offspring in nature.

• Each Prezygotic and Postzygotic barrier listed lefexplains HOW speciation occurs.

How Does Speciation Occur

Emphasize to students that these are the mechanisms of speciation. They explain HOW speciation occurs. Revisit the “why’s” of speciation addressed in the first few slides of this presentation.


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• Ecogeographic Isolation – Two populations have become so specialized for survival in different environments, that once the geographical barrier is removed the two species will neveagain interbreed as one species. The adaptations for survival itheir geographic locations prevent gene flow.

Prezygotic isolations (do not result in a zygote) are intrinsic isolating mechanisms. Graphic http://www.google.com/imgres?q=Plantus+occidentalis+(sycamore)&hl=en&noj=1&tbm=isch&tbnid=qbC7HYS29yBgOM:&imgrefurl=http://www.fcps.edu/islandcreekes/ecology/american_sycamore.htm&docid=7iiMHTDkfpwtTM&imgurl=http://www.fcps.edu/islandcreekes/ecology/Plants/sycamore/platanus_occidentalis2.jpg&w=239&h=360&ei=xABrUJeaEsHUqgHPnoHQDg&zoom=1&iact=rc&dur=324&sig=115159201469248240313&page=1&tbnh=157&tbnw=104&start=0&ndsp=18&ved=1t:429,r:12,s:0,i:111&tx=42&ty=88&biw=1350&bih=719

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Prezygotic: Ecogeographic Isolation

• The Plantus occidentalis (sycamore tree) is found in the eastern United States and the Plantus orientalis(oriental plane tree) is found in the Mediterranean are

• They can form fertile hybrids when artificially crossedbut are so different from one another that neither tree can survive in the other's habitat.

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Habitat isolation- twspecies have developa preference for two different habitats. Evif the species becomesympatric, the probability that they will meet and mate islow.

Prezygotic: Habitat Isolation

Example: Bufo woodhousei and Bufo americanus are two closely related toads. B. woodhousei prefers to reproduce in the quiet wateof a stream whereas B. americanus prefers to reproduce in shallow rain-pools. As a result, they remain separate species.

Another example: The dragonflies Progomphus obscurus and Progumphus alachuensis are closely related. In places where their range overlaps, P. obscurus reproduces in streams whereas P. alachuensis reproduces in lakes.

Another example: Two species of buckthorn shrubs, Ceanothus thysifbrus and Canothus denatus have ranges that overlap in California but C. thysifbrus is found in moist hills with good soil and C. denatus is found in drier, poor, and shallow soil.

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Prezygotic: Seasonal Isolation

Seasonal isolation- the two species have developed different times the year to mate.

Example: There are four species of frogs from the genus, Rana, eacof these frogs mates at different times of the year so that if they aresympatric, no interbreeding occurs.

Another example- Pinus radiata and Pinus muricana-are 2 closely related pine trees in California. While capable of cross breeding, they seldom do but because P. radiata sheds its pollen February and P. muricana shed its pollen in April.


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Behavioral isolation- If courtship behavior changes during separation, then sympatric mating will not occur and two new speciare formed.

Example: Twelve fiddler crab species inhabit a certain beach in Panama. Males of each species have distinctive mating displays whinclude waving claws, elevating the body, and moving around the burrow.

Another example: There are four species of leopard frogs (Rana). These frogs look very much alike, but they have different mating calls. When they hybridize, the embryos are deformed.

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Prezygotic: Mechanical Isolation

Mechanical isolation- There is a physical or biological structure thaprevents mating. For example differences in the size or fit of genitamay not allow mating. This can be found in certain snails, insects aplants.

Example: The Bradybaena shown are two different species of snailbecause the shells spiral in opposite directions, thus they are unablemate with one another.

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Prezygotic: Gametic IsolationGametic Isolation: The gametes are shed simultaneously but something physical or chemical prevents the sperm from fertilizing the egg.

Example: Many sea urchin species shed their gametes at the same time, but remain evolutionarily distinct.

The formation of hybrid zygotes is prevented because the surface proteins of the ovule (the "lock") and sperm, or male gametes (the "keys") of different species do not fit together.

Graphic Campbell Another example- When Drosophila virilis and Drosophila americanus mate, the sperm are immobilized so that they never reach the eggs. In other cross mating of Drosophila the walls of the vagina swell and kill the sperm before they reach the eggs. Point out to the students that this is a waste of gametes. Also, in the case of tobacco plants there are 66 species, cross pollination results in no fertilization since the sperm nucleus never reaches the egg in the female gametophyte.

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Postzygotic: Developmental Isolatio

Developmental isolation- If fertilization occurs, the development othe embryo can be irregular and is thus spontaneously aborted.

Example: Sheep belong to the genus Ovis and have 54 chromosomes, while goats belong to the genus Capra and have 60chromosomes. When goats and sheep mate, they produce embryosthat die prior to birth.

The next isolating mechanisms apostzygotic meaning the zygote indeed formed. Energy and resources are wastedin producing gametes and subsequent zygote production, yno offspring.

Another example-The eggs of fish can be fertilized by other fish species sperm due to external fertilization. This results in malformed embryos.


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Postzygotic: Hybrid Inviability

Hybrid inviability- A hybrid is produced, but often does not make it to reproductive age because it is weak, irregular, etc.

Example: When tobacco hybrids are successful, they often form tumors. These tumors are located in their vegetative parts. Oftenno flowering occurs, thus no reproduction occurs.

occurs after mating species overcome pre-zygotic barriers (behavioral, mechanical, etc.) to produce a zygote. The barrier emerges from the cumulative effect of parental genes; these conflicting genes interfere with the embryo’s development and prevent its maturation. Most often, The hybrid embryo often dies before birth. However, sometimes, the offspring develops fully with mixed traits, forming a frail, often infertile adult. This hybrid displays reduced fitness, marked by decreased rates of survival and reproduction relative to the parent species. The offspring fails to compete with purebred individuals, limiting genes flow between species. Graphic http://www.google.com/imgres?q=tumors+found+on+tobacco+plants&hl=en&sa=X&biw=812&bih=667&tbm=isch&prmd=imvns&tbnid=5JA7A-8h6BrZqM:&imgrefurl=http://web.williams.edu/Biology/Faculty_Staff/lbanta/lbanta.shtml&docid=xeUYYhBnBa_JTM&imgurl=http://web.williams.edu/Biology/Faculty_Staff/lbanta/Images/tumor.gif&w=308&h=200&ei=j4lrUOL5BKLe2AX1v4H4Cw&zoom=1&iact=hc&vpx=522&vpy=376&dur=7172&hovh=160&hovw=246&tx=125&ty=101&sig=115159201469248240313&page=1&tbnh=108&tbnw=166&start=0&ndsp=12&ved=1t:429,r:11,s:0,i:108

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Postzygotic- Hybrid Sterility

Hybrid sterility- some hybrids produce superior offspring but thoffspring are sterile.

Example: A mule is the result of female horse crossed with a madonkey. Mules are sterile, thus there is no potential for gene flowIn terms of evolution it is a dead end. The horse is on the left, thdonkey is in the center and the mule is on the right.

Just for fun: Ever heard “stubborn as a mule”? Evidently mules are actually smart and independent or “superior” thinkers! A male donkey is known as a “jack” and a female donkey (or ass) is known as a jenny. So, yes—a “jackass” is indeed a male donkey. A burrow is a feral donkey. A mule is the offspring of a male (jack) donkey and a female horse. A hinny is the offspring of a cross between a female donkey and a male horse and is rare.


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Postzygotic: Selective Hybrid Eliminatio

Selective hybrid elimination or hybrid breakdown occurif two species are sympatric and can hybridize, and theiroffspring can reproduce.

One of the following two things will happen:

1. The hybrids are as viable or as fit as the parents and gene flow will occur and the two species will becomone again.

2. The hybrids are weaker or have lower fitness than thparents and will be selected against.

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Postzygotic: Selective Hybrid Eliminatio

• Natural selection will select for those individuals that will mate with their own species and the hybrids will die out. The competition between the two species will cause character displacement.

• Example: the offspring of rice hybrid are not as fit as the parents. Crosses between the purebred parents will be favored.

The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

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Summary of Prezygotic Barriers

When allopatric speciation occurs, usually more than one isolation mechanism also occurs and more than one trait will change betweenthe two populations.

Be sure to tell the students that some of the terminology changes from text to text. Seasonal isolation is also known as temporal isolation. Clarify that extrinsic mechanisms are external mechanisms and intrinsic mechanisms directly affect the organism. Geographic isolation is extrinsic and the ten prezygotic and postzygotic mechanisms are intrinsic. Students should now know WHY speciation occurs upon geographic isolation and HOW speciation occurs after geographic isolation.

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Summary of Postzygotic Barriers

Postzygotic barriers keep two populations distinct, thus they are nolonger the same species and can no longer interbreed to produce viable, fertile, offspring in nature. Again, when two population aallopatric and changes occur, most likely more than one of the 10 barriers will occur in the population leading to speciation.

Graphic Campbell Hybrid sterility is reduced hybrid fertility. Hybrid breakdown is selective hybrid elimination.


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Created by:

Carol LeiblScience Content DirectorNational Math and Science


these animals will interbreed with one another€¦ · macroevolution part ii allopatric speciation...

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