Chapter 24: Origin of Species and Macroevolution Flashcards
What is macroevolution?
Evolutionary changes that result in the formation of new species.
Over time, how does reproductive isolation result in the formation of a new species?
If two populations of organisms become reproductively isolated (i.e., no longer interbreed with each other), then they are no longer exchanging genetic material (DNA). The two populations may evolve independently until their genomes and behaviors become so different that they are now two distinct species.
Approximately 3.5 million years ago the Isthmus of Panama formed. This new landmass severed the connection between the Pacific Ocean and the Caribbean Sea. An ancestral porkfish population was split in two by this event; the two populations no longer interbreed and are two distinct species. These species most likely arose by which type of speciation?
Allopatric speciation. The original population was split into two different populations that were geographically isolated by a physical barrier and therefore no longer able to interbreed.
The Northern leopard frog (Rana pipiens) and Southern leopard frog (Rana utricularia) are morphologically similar (i.e., similar in appearance), but their mating calls sound very different. Based on this, what prezygotic barrier is responsible for reproductive isolation between these two different frog species?
Behavioral isolation. Northern frogs prefer the mating call of their own species and vice versa, so northern frogs tend to breed together and southern frogs tend to breed together.
American sycamore (Platanus occidentalis) and European sycamore (Platanus orientalis) are tree species that will interbreed if planted nearby but will not normally interbreed because they occur on different continents. What type of prezygotic barrier is normally responsible for reproductive isolation between the two species?
Habitat isolation. The 2 species live on different continents, so they do not come into contact with each other to reproduce.
Two species of cricket have similar ranges, but one breeds in spring, while the second one breeds in fall. What prezygotic barrier is responsible for reproductive isolation between the two species?
Temporal isolation. The two species breed at different times of the year, so they cannot reproduce with each other.
There are large and small sea urchins in a particular area. Both species release eggs in the same area at the same time, but there is no interbreeding because sperm from the large urchins is unable to penetrate eggs from the small urchins and vice versa. What type of prezygotic barrier maintains reproductive isolation?
Gametic Isolation. The gametes from the two different species cannot fuse.
If goats are crossed with sheep, embryos will form but cease development and spontaneously abort. What type of post-zygotic barrier maintains reproductive isolation?
Hybrid inviability: the hybrid does not develop past the embryonic stage.
A mule is a hybrid produced from a male horse and a female donkey. Mules are sterile (cannot produce offspring). What type of post-zygotic barrier maintains reproductive isolation between the horse and donkey?
Hybrid sterility: a hybrid (the zorse) is viable, but sterile (i.e., it cannot reproduce), so it is an evolutionary dead end.
A liger is a hybrid from a cross between a male lion and a female tiger in captivity. Female ligers are fertile (can reproduce), but subsequent offspring are of fragile health and low fitness. What type of post-zygotic barrier maintains reproductive isolation between tigers and lions?
Hybrid breakdown. The hybrid is viable and fertile, but subsequent offspring are of low fitness and would not survive in the wild.
What factors are considered when deciding whether two organisms are members of the same species or members of a different species?
1) Morphological traits: how similar the organisms are in structure and appearance.
2) Reproductive isolation: whether or not two organisms can interbreed to produce viable and fertile offspring (if they do not then they are reproductively isolated).
3) Molecular traits: how similar the genomes are (e.g., DNA sequences and chromosome structure and number).
4)Ecological factors: whether or not organisms share the same habitat and feeding habits.
What is allopatric speciation? What are 2 ways that allopatric speciation can occur?
A population becomes geographically isolated from another population, so the two populations become reproductively isolated (cannot interbreed due to habitat isolation). Over time the populations evolve into two different species. Allopatric speciation can occur due to:
- the formation of a physical barrier
- geographical distance if a founder population migrates to a new location.
What is sympatric speciation? What are 3 ways that sympatric speciation can occur?
Populations within the same geographical area diverge into two or more species despite there being no geographical barriers between them. Sympatric speciation can occur due to:
- Changes in chromosome structure and number: if two different organisms develop a different chromosome number or different chromosome structures then resulting offspring (if they are even viable) will not likely be able to produce gametes for reproduction because homologous chromosomes cannot pair during meiosis. Therefore, organisms are restricted to breeding with other organisms that have the same chromosome number.
- Members of a population may occupy different local environments: this could lead to diversifying selection (we discussed diversifying selection in chapter 24), which selects for two different phenotypes within a population. Eventually, the two different populations in the different local environments may evolve to be different enough that they no longer interbreed; they will then evolve independently, eventually becoming two different species.
Two species of three-spined stickleback fish live in a single lake in British Columbia and exist nowhere else. They are closely related but occupy different niches. These species most likely arose by what type of speciation?
Sympatric speciation. The two species are not geographically isolated as they occupy the same lake.They likely diverged into two different species due to occupying two different niches within the same lake. This likely resulted in diversifying selection with one population becoming adapted to one niche and the other population becoming adapted to a different niche. Over time the populations became different enough that they stopped interbreeding and diverged into different species.
What is a prezygotic barrier that promotes reproductive isolation? How do each of the following mechanisms act as prezygotic barriers?
- Habitat isolation
- Temporal isolation
- Behavioral isolation
- Gametic isolation
- Mechanical isolation
A prezygotic barrier is a barrier that prevents the formation of a zygote (the cell formed when egg and sperm fuse after sexual reproduction).
- Habitat isolation: two species occupy different habitats, so they never come into contact with one another and therefore do not reproduce.
- Temporal isolation: two species may occupy the same habitat, but they reproduce at different times of the year, and therefore do not reproduce with one another.
- Behavioral isolation: members of different species have different mating behaviors. A species is not familiar with, or does not prefer, the mating behavior of a different species. For example, female eastern meadowlark birds only like the song of males from the same species, they do not like the song of males from the western meadowlark species and vice versa.
- Gametic isolation: fertilization cannot occur between the egg from one species and the sperm from another species; the two gametes are not able to fuse. Gametic isolation is common in plant species and aquatic species. For example, several aquatic species release their gametes into the water, and the gametes can only fuse with gametes from the same species.
- Mechanical isolation: body structures just do not fit together to allow reproduction.
