AP Bio Speciation PDF

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AP Biology Unit 7 Speciation...

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Principles of Biology Unit 7: Macroevolution Speciation Diverging Populations -In microevolution, we saw how allele frequencies in a population can change in a way that results in the population splitting into distinct groups that have different characteristics. One way for this to happen is If the different portions of a population experience different selective pressures that cause different traits to be more successful in one region compared to another (diversifying selection). -Diversifying selection can most readily take place in populations that are distributed over a wide area that contain different habitants. -For example white-tailed deer are a very successful species that can be found from Northern Canada to South America. Different traits probably helped deer be successful when they live in dry environments with little vegetation, such as that in the desert plains, than when they live in tropical woodlands. In fact, scientists have recognized several different species of deer. Subspecies are different portions of a larger population that can be distinguished from one another, but can still exchange genetic information by interbreeding or gene flow. -Assortative positive mating can be seen in many different species and often involves mating behavior. -For example, jumping spiders have distinct coloration and males perform elaborate displays to attract females. Birds have similar rituals, in addition to songs, gifts, and dance. -When two populations become different enough, they can be considered to be different species. For example, three different species of bluebirds can be found across the United States and Mexico. Each of these species has distinct different appearances. The ranges where these birds can be found sometimes overlap. But the species display assortative mating and only occasionally interbreed to produce viable offspring. The fact that they can produce offspring that are viable at all raises the question of how to determine if two populations are separate species or not. When Do Populations Become Different Species? -One method for distinguishing species is to use the morphology or the physical traits of the organisms to distinguish between one and another. This was the approach used by Carl Linnaeus, a naturalist in the 1700s and 1800s, who was the first person who tried to categorize and name all living things using physical traits to distinguish different species. This has the advantage of being relatively straightforward, does not need expensive equipment, and can be done with dead specimens. However, it has a disadvantage of sometimes being wrong. -For example, if you’re not a bird expert, you might assume that the following birds belong to different species because they look different from one another. However, these are both indigo buntings, and this is an example of sexual selection causing sexual polymorphisms where

males appear different from females. Males that have bright feathers are more successful at reproducing than males that have darker feathers. Females have brown feathers because they don’t face the selective pressure. -From first glance, you might assume that the following butterflies are the same species. However, the one on the left is a viceroy and the one on the right is a monarch. Monarchs are poisonous to birds and their bright coloration warns birds away. The viceroys are probably tasty, but birds tend not to eat them because they look like monarchs. The difference between these species can be more easily seen if you look at their caterpillars. Monarchs have brightly colored caterpillars, while viceroy caterpillars look like bird poop, which most likely protects them from being eaten as well.

Biological Species Concept -A second method for distinguishing between species is called the biological species concept, which says that two populations are separate species if they do not interbreed naturally, even if they have the opportunity. These two species are said to be reproductively isolated from each other and do not exchange alleles. In other words, there is no gene flow between the two populations. There are several different ways in which populations can become reproductively isolated and these are broken down into two categories. Pre-zygotic, which takes place before fertilization, and post-zygotic, which takes place after fertilization. -Pre-Zygotic Isolation -One of the more common ways for reproductive isolation to take place is geographic, when two populations are separated by a physical barrier that prevents them from interacting. Physical barriers do not need to be large to separate populations, especially for small organisms. -For example, the Kaibab squirrel is found in the northern rim of the Grand Canyon while a different population called Abert’s squirrel is found on the southern rim. These two populations don’t interbreed because the canyon between them prevents interaction. -Organisms can also be separated by the habitats in which they live. -For example, the ranges of lions and tigers used to overlap, and these animals can interbreed and produce viable offspring. However, tigers prefer to live in dense wooded areas, while lions prefer to live in open grasslands, so these two populations rarely interact. Small organisms can live very close to each other as well and still be isolated. -Some species are isolated when they reproduce.

-For example, some species of cicadas emerge from the ground and mate every 13 years, while other species emerge every 17 years, meaning that these two species will only ever interact every 221 years. Some species are isolated by very short periods of time. For example, some species reproduce at dawn while others reproduce at sunset. -Species can selectively mate can be separated by behavior. -For example, fireflies attract mates using flashes of light. Different species flash different patterns and will not respond to the wrong pattern. -For some species, it is physically impossible for them to interbreed. -For example, dogs are all one species, but if a disease killed all dogs except for Great Danes and Chihuahuas, these two populations could become separate species because it would become physically impossible for them to interbreed without help. Even small differences can cause difficulties. The following two species of snail can’t interbreed because their shells coil in separate directions.

-Some species are isolated because their gametes can’t interact. -For example, marine species are often broadcast spawners, meaning that they release their gametes into the seawater. It is possible for different species to spawn at the same time, which means that gametes from each of these species are all mixed together. To ensure that sperm and eggs find the correct partners, these cells have proteins on their surfaces that allow them to recognize and interact only with the corresponding cells from the same species. -Post-Zygotic Isolation -Hybrid Inviability -In some cases, gametes from different species are able to fuse, but the resulting zygote is not viable. It is not able to develop and produce viable offspring because the two parent species are too different from one another. This is called hybris inviability. -Hybrid Sterility -In other cases, the parent species are closely related and can interbreed to produce healthy offspring, but because of genetic differences, these offspring are sterile. For example, horses and donkeys can interbreed to produce a hybrid called a mule. However, because horses have 32 pairs of chromosomes and

donkeys have 31 pairs, the resulting mules carry three unmatched chromosomes. The chromosomes and mules can’t be divided equally during meiosis, and so these animals can’t produce viable gametes....