Unit 6 Lesson 7: Environmental Change Flashcards
There have been five mass extinction events that have fundamentally altered the evolutionary path of species on Earth:
- 444 million years ago, during the Ordovician period, most animals on Earth were sea creatures. Over the course of about a million years, a severe ice age lowered sea levels. When the water disappeared, silicate rock was exposed. Silicate rock reacted chemically with the atmosphere to lower the level of carbon dioxide in the air. The loss of carbon dioxide cooled the planet. The result: 86% of all species went extinct.
- While some life forms survived the first great extinction, many went extinct during the second extinction which occurred about 373 million years ago, in the late Devonian period. One possible reason for this huge extinction was the development of land plants. Plant roots stirred up the soil, releasing nutrients into the water. The nutrients may have triggered algae growth. Algae sucked oxygen out of the water. Without oxygen, many ocean-dwelling species went extinct.
- 251 million years ago, at the end of the Permian period, the “great dying” occurred, and 96% of all species went extinct. In fact, the great dying almost extinguished life on Earth. The event was the result of several events that occurred around same time. A massive volcano sent carbon dioxide into the air, warming the climate. Bacteria, stimulated by the warmth, produced massive amounts of toxic methane. The temperatures became extremely high while the ocean became acidic. All coral died off, as did most life forms.
- At the end of the Triassic period, about 200 million years ago, 80% of species went extinct. It is unclear why this happened.
- Probably the best-known extinction event occurred 66 million years ago during the Cretaceous period (during the time of dinosaurs). A meteor s impact, which caused a dust cloud to cover the planet, was a major factor in the extinction of 76% of life on Earth
How does a population of one species evolve into an entirely new species? There are three ingredients for the formation of a new species, or speciation:
evolution, time, and reproductive isolation.
Extinction events, of course, result in loss of species. At the same time, however, they also encourage different species to
evolve
speciation
formation of new and distinct species as a result of evolution
There are three ingredients for the formation of a new species, or speciation: evolution, time, and reproductive isolation. What does this mean?
In other words, you need changes to occur to the genetic makeup of the population over time, but you also need the population to remain reproductively isolated from other populations of the species for that evolution to follow a unique path.
Geographic Isolation:
The geography of an ecosystem can change over time. Forests shrink while grasslands expand. Rivers can erode valleys into mountains. Lakes can dry up, separating into two bodies of water instead of just one. These geographic changes can create barriers that are difficult if not impossible for different populations of the same species to cross. When this happens, the two populations become reproductively isolated. Over time, the combination of reproductive isolation and evolution creates such a difference between the genetic makeup of the two populations that it is not possible for the two populations to interbreed and create fertile offspring. At that point, the two populations have become separate species.
First, it is important to understand the term species. There are several definitions for the term species, so how do biologists determine when a new species has emerged from an existing species?
In evolutionary biology, scientists decide that a new species has emerged when members of two isolated populations can no longer interbreed and produce fertile offspring. For example, you find two populations of closely related squirrels in an ecosystem. If you try to mate an individual from each population together and the two individuals cannot have offspring or the offspring they do have cannot have offspring, which means it is sterile or not fertile, then you can conclude that these two populations have become different species.
Example of Geographic Isolation
For example, there are two closely related species of antelope squirrel that live in the Grand Canyon region, but they are separated by the canyon. Based on DNA analysis, the two species are very closely related, and most evolutionary biologists believe they started off as two populations of the same species that were isolated by the formation of the Grand Canyon.
Habitat Isolation:
: Populations that live in the same ecosystem can also become isolated when the populations choose to live in different habitats ecosystem. For example, in one ecosystem, you have a forested area that surrounds a lake. One population settles within the forest while the other population tends to live near the edges of the lake. Over time, this preference for habitats causes the two populations to isolate reproductively and evolve different characteristics. Once the two populations can no longer breed together and produce fertile offspring, they have become different species.
Example of Habitat Isolation
One of the classic examples of this include the big cats in Africa. Lions and tigers can mate and produce offspring called ligers. These offspring are sterile and cannot produce offspring of their own. Therefore, lions and tigers are considered separate species. Evolutionary biologists believe that these two species evolved from the same species with populations that preferred different habitats. In the case of the populations that eventually evolved into lions, these individuals preferred the grasslands, whereas the populations that would eventually evolve into tigers preferred the jungles.
Behavioral Isolation:
Populations that live in the same ecosystem can also become isolated because the populations develop behaviors or mating rituals that prevent them from finding mates from the other population. This is especially common in bird and insect species.
Example of Temporal Isolation
The eastern and western spotted skunks are very similar in appearance, but they differ in when their mating periods occur. Eastern spotted skunks tend to mate during the late winter to early spring while western spotted skunks usually mate during the late summer to early fall. Because of this separation in mating periods, the two populations rarely interbreed. Over time, this reproductive isolation combined with evolution has created enough genetic differences between the two populations that they have become different species. This is also very common in plants where different species of flowers only open during certain times of the day or night.
Example of Behavioral Isolation
For example, in the Galápagos Islands, the blue footed boobie has a very special mating dance that the couple must perform before they will mate with one another. A bird species that does not know this mating ritual will not be able to mate with an individual of this species. Firefly species have unique lighting patterns that differentiate one species from another. Females will only mate with males that display a specific lighting pattern.
Temporal Isolation:
: Populations that live in the same ecosystem can also become isolated because the populations begin to mate at different times. For example, there are two closely related species of skunk in the United States that evolutionary biologists believe started as one species. The eastern and western spotted skunks are very similar in appearance, but they differ in when their mating periods occur.
Mechanical Isolation:
: Populations that live in the same ecosystem can also become isolated when populations develop variations in sexual morphology preventing individuals from mating together. This is very common in flower species where the shape of the flower evolves to fit a certain type of pollinator.
Example of Mechanical Isolation in flowers
For example, one population of a flower species may evolve to favor bees as pollinators while another evolves to use birds. Over time, this leads to enough physical differences in the flower shape that the same pollinator could not pollinate the different flower populations, which makes the two populations different species.
Example of Mechanical Isolation in animals
This type of isolation also happens in animals as well. Damsel flies, which are insects that resemble dragonflies, have very different penis shapes from species to species. This difference in sexual morphology prevents different Damsel fly species from mating with each other.
Gametic Isolation:
: Finally, two populations that live in the same ecosystem can become reproductively isolated when genetic mutations cause the gametes, or sex cells, of the two populations to become incompatible. During the process of fertilization when the gametes come together, there are mechanisms that check that the gametes are of the same species. This is especially common in marine species which typically release sperm and egg directly into the ocean and rely on ocean currents to bring the gametes together. This type of isolation prevents gametes of different species from fertilizing and producing nonviable offspring.
What was the “great dying”?
The “great dying” was a massive and catastrophic extinction event that occurred about 251 million years ago.
Explain the impact of extinction events on evolution.
During extinction events, large numbers of species die off. At the same time, however, changes in the environment lead to the evolution of new species with traits that make it easier for them to survive and reproduce in a new climate.
Which of the following is an example of temporal isolation?
Members of a particular species are isolated from one another as a result of deforestation.
Some members of related species are awake during the day while others are awake only at night.
A river is dammed. As a result, fish cannot swim down river to spawn.
A group of insects is blown away from an island toward a mainland.
Some members of related species are awake during the day while others are awake only at night.
Animals that experience temporal isolation are awake at different times.
What is the connection between fossil fuel use and evolution?
Fossil fuels are increasing the level of carbon dioxide in the atmosphere. This is leading to a warmer atmosphere. A warmer atmosphere has, in the past, led to rising sea levels and extinctions. Extinctions are one major driver of evolutionary change.
What are some of the pros and cons of green energy?
The pros are that green energy does not add carbon dioxide to the atmosphere and too much carbon dioxide leads to global warming and climate change. The cons are that when we switch from fossil fuels to green energy sources, some people will lose money and jobs. Green energy can also be more expensive than fossil fuel.
