sign in sign up
<<
Home , Adaptive radiation, Allopatric speciation, Hybrid (biology), Ring species, Sympatric speciation

Looks Can Be Deceiving! Definition of vs.

• These meadowlarks look • A species is a group of interbreeding • Macroevolution is on a scale of very similar yet they are not that produce viable, fertile offspring in nature. separated pools. the same species. • Members of a species will interbreed with one • Macroevolutionary studies focus on change that another but not other organisms outside of the occurs at or above the level of species, in species. (At least most of the time!) contrast with microevolution, which refers to • By contrast, these brittle smaller evolutionary changes (typically stars look very different from described as changes in frequencies) Macroevolution Part II: one another, but they are within a species or . the same species. Allopatric

2 3 4

Ring Species Asexual Species Ring Species Asexual Species • escholtzi is a salamander ring species Even though asexual groups do not • A ring species is a connected series of neighboring • The blue zones represent exchange , they do , each of which can interbreed with closely that has a range along the where interbreeding is recognizable groups. sited related populations, but for which there exist at coast and inside range of occurring. California. Most have evolved from a sexual least two "end" populations in the series. species. Only those whose • These end populations are too distantly related to • All along this range, the • So are there is is best adapted to the environment, interbreed, though there is a potential gene flow between salamanders interbreed, but all along the salamander’s will continue to survive. However, it each "linked" species. the salamanders on the range, yet the ends of the makes them less adapted to ends of the ring do not environmental change. • Such non-breeding, though genetically connected, "end" rings do not interbreed. populations may coexist in the same region thus closing interbreed. Are they the same Dandelions are asexual. The is a "ring". • Their groupings are called sterile and the egg is diploid. species? . 5 6 7 8 Ring Species Limited Interbreeding Limited Interbreeding vs. • Each species will • and will interbreed in captivity, but they I. Anagenesis- is the accumulation interbreed with the do not interbreed in nature. of changes in one species that domestic but not • Lions form groups or prides and live in the grasslands. leads to another species. It is the readily with one another. lineage of a species. Over time a • Tigers are more solitary and live in the forests. species may accumulate enough • Tiglon are products of male tigers and female lions. changes that it is considered a • This is true, even when • are the opposite cross. species that differs from the given the opportunity to do ancestral species. so. Thus, they are not the II. Cladogenesis- is the budding of same species since they do one or more new species from an not interbreed in nature. ancesteral species that continues to exists. This results in biological diversity. 9 10 11 12 Allopatric Speciation Allopatric vs. Why does speciation occur after geographic isolation? • First, geographic isolation occurs. This is an extrinsic isolating 1. They become separate species, as evidenced by the fact they can Allopatric speciation- mechanism. no longer interbreed. Speciation occurs because a 1. The population that left the original group will have • The two populations must become isolated geographically from given group has been separated 2. They can still interbreed, thus they remain the same species. one another. a different allelic make-up than the original species, from the parent group, usually thus experiencing the “founder effect”. because of a geographic • If the groups become sympatric again one of two things result… separation as time goes by. 2. The two groups will continue to experience different . Sympatric speciation- 3. The two groups will now experience speciation occurs even though and different selection pressures due to living in the two groups are still living in separate and perhaps different environments. the same area. Islands produce some of the most profound examples of speciation due to geographic isolation. 13 Adaptive Radiation Adaptive Radiation The Amazing Galapagos Islands

• The classic adaptive radiation example involves the • One would expect that each of the Galapagos Islands. island would have only one • There are 14 different species of finches and 13 main species, however, each island islands, 3 smaller islands, and 107 rocks and islets. has more than one species of 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.

Adaptive Radiation Adaptive Radiation Adaptive Radiation Adaptive Radiation

• Lets suppose that finch species A, from South • Now let’s suppose that some of the new finch species • Now some of the newly evolved finch species C  make their • Obviously, species C is different from finch species B thus America migrates to an island . B migrate over to a second island.  (speciation) & way to yet another new island.  they can no longer interbreed back on the original island. • Finch species A would undergo speciation into finch  (migration) • Guess what? Yep! Once again finch species C will evolve into • Finch species C may or may not evolve into another species. species B due to one or more of the following: • The finches in this new environment are finch species D (not shown yet) for the same three reasons. (Founder effect, varying selection pressures, or varying • If there is a niche similar to that of the second island, the – The Founder effect geographically isolated from the other island and now mutations.) selection pressure may also be similar and species C may be – Varying selection pressures will evolve into finch species C for the same three slow to change. • But suppose some of species C make it back to the first island. – Varying mutations basic reasons. (Founder effect, varying selection pressures,  (The plot thickens…) or varying mutations.) • So, both first and second islands will have species C. The third island will have a new species D.

Adaptive Radiation Adaptive Radiation The Amazing Galapagos Islands The Amazing Finches From the Galapagos Islands

• Now lastly, lets suppose that finch species D from the • Alas, the first island has no such niche. Now, there Differences are found among the third island returns to the first and second islands. ( exists a selection pressure on finch species D causing it and feathers of the finches. & ) to evolve () into species E.  Darwin found 14 different species • On the second island finch species D does not of finches inhabiting these islands • As a result, the first island now has three different which are a result of adaptive change because it finds a niche similar to the third species of finches. Two of which are not found on radiation. island so no selection pressure is exerted upon it. other islands (B & E). Each species has a distinct There are finches that eat seeds, with different food sources. This process is cacti, insects and other interesting called adaptive radiation and most commonly involves foods. islands. He also observed adaptive radiation among the tortoises and So, NOW we understand how it is possible that each island has more mocking birds. than one finch species. Some islands actually have as many as 10 species. Examine the map once more.

The Amazing Finches From the Galapagos Islands Example of How Does Speciation Occur? Prezygotic: Ecogeographic Isolation • During droughts in the Galapagos Islands, larger seeds are more • So, two populations of abundant. Finches with slightly larger beaks have an advantage organisms are not the same • Ecogeographic Isolation – Two populations have become so since they are able to crack larger seeds. species unless they can specialized for survival in different environments, that once the geographical barrier is removed the two species will never • Thus, natural selection favors finches with larger beaks. These interbreed, and produce viable, again interbreed as one species. The for survival in finches are more likely to survive and pass those genes on to the fertile offspring in nature. 1.Vegetarian Finch 8. Cactus ground finch next generation. A study conducted by their geographic locations prevent gene flow. 2. Warbler finch 9. over a 20 year period confirmed these assertions. • Each Prezygotic and 3. Large insectivorous tree finch 10. Postzygotic barrier listed left 4. Medium insectivorous tree finch 11. Large ground finch explains HOW speciation 5. finch 12. Sharp-beaked ground finch occurs. 6. Small insectivorous tree finch 13. 7. Large cactus ground finch 14.

Prezygotic: Ecogeographic Isolation Prezygotic: Habitat Isolation Prezygotic: Seasonal Isolation Prezygotic: Behavioral Isolation • The Plantus occidentalis (sycamore tree) is found in Habitat isolation- two the eastern United States and the Plantus orientalis species have developed (oriental plane tree) is found in the Mediterranean area. a preference for two different . Even • They can form fertile hybrids when artificially crossed if the species become but are so different from one another that neither tree sympatric, the can survive in the other's habitat. probability that they will meet and mate is low. Seasonal isolation- the two species have developed different times of Behavioral isolation- If courtship behavior changes during Example: Bufo woodhousei and Bufo americanus are two closely the year to mate. separation, then sympatric mating will not occur and two new species related toads. B. woodhousei prefers to reproduce in the quiet water Example: There are four species of from the , Rana, each are formed. of a stream whereas B. americanus prefers to reproduce in shallow of these frogs mates at different times of the year so that if they are Example: Twelve fiddler crab species inhabit a certain beach in rain-pools. As a result, they remain separate species. sympatric, no interbreeding occurs. Panama. Males of each species have distinctive mating displays which include waving claws, elevating the body, and moving around the burrow. Prezygotic: Mechanical Isolation Prezygotic: Gametic Isolation Postzygotic: Developmental Isolation Postzygotic: Inviability

Gametic Isolation: The are shed The next isolating mechanisms are simultaneously but something physical or postzygotic meaning the zygote is chemical prevents the sperm from indeed formed. fertilizing the egg. Energy and resources are wasted in producing gametes and Example: Many species shed subsequent zygote production, yet their gametes at the same time, but remain no offspring. evolutionarily distinct. Mechanical isolation- There is a physical or biological structure that Developmental isolation- If fertilization occurs, the development of - A hybrid is produced, but often does not prevents mating. For example differences in the size or fit of genitalia the embryo can be irregular and is thus spontaneously aborted. make it to reproductive age because it is weak, irregular, etc. may not allow mating. This can be found in certain snails, insects and The formation of hybrid zygotes is Example: belong to the genus Ovis and have 54 . prevented because the surface proteins of Example: When tobacco hybrids are successful, they often form the ovule (the "lock") and sperm, or male , while belong to the genus Capra and have 60 tumors. These tumors are located in their vegetative parts. Often Example: The Bradybaena shown are two different species of snails gametes (the "keys") of different species do chromosomes. When goats and sheep mate, they produce embryos no flowering occurs, thus no reproduction occurs. because the shells spiral in opposite directions, thus they are unable to not fit together. that die prior to birth. mate with one another.

Postzygotic- Hybrid Sterility Postzygotic: Selective Hybrid Elimination Postzygotic: Selective Hybrid Elimination Summary of Prezygotic Barriers

Selective hybrid elimination or hybrid breakdown occurs • Natural selection will select for if two species are sympatric and can hybridize, and their those individuals that will mate with offspring can reproduce. their own species and the hybrids will die out. The One of the following two things will happen: between the two species will cause character displacement. Hybrid sterility- some hybrids produce superior offspring but the 1. The hybrids are as viable or as fit as the parents and offspring are sterile. gene flow will occur and the two species will become • Example: the offspring of Example: A is the result of female crossed with a male one again. hybrid are not as fit as the parents. Crosses between the . are sterile, thus there is no potential for gene flow. 2. The hybrids are weaker or have lower than the In terms of evolution it is a dead end. The horse is on the left, the parents will be favored. donkey is in the center and the mule is on the right. parents and will be selected against. When allopatric speciation occurs, usually more than one isolation mechanism also occurs and more than one trait will change between the two populations.

Summary of Postzygotic Barriers

Created by: Carol Leibl Science Content Director National Math and Science

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