Investigation 3 – Concept Day
Adaptation: Investigation 3
In this Investigation, we will discuss how adaptation to environmental pressure leads to natural selection and evolution of a species. We will also briefly discuss extinction.
We will end by looking at how ancient forms of animals can be found in the geological record or the fossil record.
- Environmental pressure is a change in an organism’s environment that selects for organisms with slight differences in a trait that give that organism a survival or reproductive advantage. Remember that when an organism survives to reproduce, its alleles continue on in the population. On the other hand, if an individual does not survive to reproduce, its alleles will not be passed on in the population.
- Some examples of environmental pressure are listed on this slide. New predators would exert pressure on a population that it preys upon by decreasing their numbers. Organisms that contain alleles that allow them to escape predation better would be more likely to survive and reproduce.
- Any environmental change that reduces the food supply can act as severe pressure. This was the case with the penguin example in Investigation 2. If a new competitor for food encroaches upon a population, this can also serve as an environmental pressure.
- A disease may exert pressure by selecting for organisms that are resistant to it and killing those individuals that are susceptible. This environmental pressure can result in a population of organisms that are largely immune to the disease if it does not drive the entire population to extinction.
- Climate change and environmental pollution can also be strong environmental pressures on plant and animal populations. We will discuss a well-known case of pollution and natural selection later.
- We have used the term natural selection already in previous discussions. Natural selection is sometimes referred to as “survival of the fittest”.
- Environmental pressures often give an organism with a favorable trait an advantage over other organisms without the trait (the dense bone penguins, for example). The organism with the trait is selected by the pressures and is able to survive and reproduce, but the organism without the trait cannot thrive and may eventually be forced into extinction.
- Several examples of natural selection of traits are listed on this slide but there is an infinite number of others.
- For the next several slides we will turn our attention to one of the most studied and exemplified cases of natural selection ever. We will discuss the peppered moth of England.
- The peppered moth has two natural forms, a light peppered color and a much darker, almost entirely black form. These phenotypes are, of course, dictated by the moth’s genotype. The dark color allele is the dominant allele and the light color allele is recessive. Thus, as shown, the dark form of the moth can be either homozygous dominant or heterozygous dominant while the light form can only have a homozygous recessive genotype.
- Prior to the Industrial Revolution, in the Manchester area of England, almost all of the peppered moths were light-colored and thus were able to blend in with the light grey bark and light-colored lichen (an algae and fungus, plant-like organism) that also grew on these trees. Bird predators could easily see the dark form of the moth when it rested on the trees. The photo on the right of the slide shows both a light and dark form of the peppered moth on a tree. Not surprisingly, birds preferentially detected and fed on the dark form of the moth. This drove the frequency of the dominant, dark allele way down in the population.
- The Industrial Revolution began in the mid-eighteenth century. It was a time of massive expansion of manufacturing and energy production associated with the burning of enormous amounts of coal. The entire countryside was covered in dark soot from the coal stacks. In addition, the sulfur oxide pollution from coal-burning killed the lichen on local trees. A combination of factors thus turned the light-colored trees in the vicinity a dirty black color.
- Under these conditions, the light color peppered moth became much easier for birds to detect than their dark counterparts (see photo on right). As a result, birds began to prey predominantly on the white, recessive form of the moths. The frequency of the dark dominant allele skyrocketed while the recessive white allele crashed to very low levels.
- Fortunately, by the mid-1950s, industry found methods of decreasing coal pollution and trees began to recover to their natural light colors. The lichen came back with the result of decreased coal pollution as well. As a result, today the light color peppered moths once again dominate in Manchester parks and the countryside.
- Thus, environmental pressure can have a major impact on the genetic composition of organisms and populations of organisms. Genetic variety is one of the best ways nature has of giving a species increased chances of surviving.
- We will now briefly turn to the concept of evolution, which is a slow progressive change in organisms that occur over very long periods of time, often millions of years, due to environmental pressure and natural selection. Two specific examples of evolution are depicted on this slide.
- Horses have evolved to become larger and larger over a period of about 50 million years. They were originally small, dog-sized, forest-dwelling creatures.
- The other example of evolution is the increase in the brain size of modern humans over their prehuman ancestors. The distinctive and protruding “brow ridge” of pre-human species is partly due to the small amount of skull above the eyebrows that, in modern man, contains a very large brain. A modern human has a cranial capacity (brain size) of about 1,400 milliliters. Prehumans had brains half that size or less.
Because evolution works through progressive changes in animals and plants, it is quite natural that various groups of animals or plants seem more related to one another than to other groups. Scientists have used similarities and differences between organisms as a means of classifying plants and animals.
- This slide depicts a very simple and incomplete classification of animals based on similarities and differences. Notice that the two major divisions are vertebrates (animals that have a backbone/spinal column) and invertebrates (animals that do not have a backbone).
- Notice that the invertebrates contain organisms as simple as single-cell protozoans to much more complex animals such as crustaceans and insects.
- In such organizational hierarchies, the nearer the animals are to each other, the more closely they are related to each other. Thus, a fish and an amphibian are more closely related than a fish and an insect.
- Finally, notice that this slide only includes animals. A similar graphic could be created for the relationships between plants.
- This slide is included so that you can appreciate that there are relationships between modern animals and extinct animals. Scientists are able to use fossil records and molecular biology techniques to study the relationship between living species and extinct species.
- This is a very interesting slide because it accentuates that modern paleontology places birds as direct decedents of dinosaurs. Notice that both Deinonychus and Coelophysis lived during the time of the dinosaurs. In fact, both Coelophysis and Deinonychus would have been extinct before Tyrannosaurs rex even appeared on Earth!
- The important point of this slide is that the relationship between animals is not restricted to only modern, living animals but includes animals that no longer roam the Earth. Even though these animals may be gone, they are nonetheless related to many living species and some of their alleles are still present in modern populations.
- This slide shows the embryonic development of a number of different types of animals. The point is that even though the fully formed animals (far right) may look completely different from one another, one can see that during development, inside the egg or mother, there are many more similarities than one might guess.
- The similarity of embryonic development sometimes helps scientists in studying the relationship between organisms.
- While the peppered moth story resulted in very reduced dark and then light allele frequencies depending on environmental pressures, the species was nonetheless able to adapt due to its genetic variation and thus far has survived and avoided extinction.
- But what would have happened if prior to the Industrial Revolution all of the dark moths were eaten by predators and the dominant dark allele disappeared from the population? With the onset of uncontrolled coal burning and the darkening of trees, the dark phenotype would not have been available and there would have been no way for the homozygous recessive white moths to be anything but white. They would stick out against the black trees and provide easy feeding for their predators. When the last of these homozygous recessive white pepper moths was consumed, the species would become extinct.
- Extinction means that the genotype of a previously living organism simply no longer exists. Extinction of a species does not mean that it changes into another species – that would be evolution. Extinction means that the organism’s genome is gone – gone forever. From the point of its extinction forward, no sign whatsoever of the species will be found in the fossil record.
- We now turn our attention to the geological or the fossil record. Much of what we know about previous life forms on Earth comes from finding evidence of their existence buried beneath the surface.
- The formation of sedimentary rock, as shown in this slide, preserves the fossils of ancient plants and animals more or less in the order in which they died. That is, deeper layers of sediment were laid down prior to more recent layers. Thus, in undisturbed rock layers, the deeper one digs, the further back into time one goes!
- In addition to modern dating methods, sample collections from deeper and deeper layers have given us a look at the many different forms of life that have existed on our planet. Such scientific work is performed by paleontologists. Their work has unfolded amazing stories of the struggle of life to survive on Earth. How cool would it be to hunt dinosaur bones for a living!
- In this slide, we will try to imagine what the fossil record of our two penguin types from Investigation 2 might look like. We will assume that the evolutionary process we described for the light- and dense-bone penguins occurred over a long period of time, as most adaptation and evolution events do. Remember, as we dig deeper, we are going back into time.
- In this slide, the dense bone, deep-diving penguins are colored white and the less dense penguins are blue. We also have the two fish species that served as the penguins’ food. The thinner fish is the shallow-swimming species and the stockier fish is the deep-swimming species. Note that, while the actual pictures of the penguins and fish are shown here, in actuality only remnants of their fossilized bones would be found.
- On the next slide, we will count the number of fossils in each layer of rock.
- In this slide, we have counted the number and kind of fossils found at each layer of rock. Layer 1 is the top layer and therefore represents the most recent time. Layer 4 is the deepest layer shown in this slide.
- The chart to the right indicates the number of fossils of each penguin type counted at each layer of the geologic record. This data is used to construct a line graph on the next slide to follow the fossil record over time.
- This final slide simply graphs the data from the fossil record shown on the previous two slides. The x-axis depicts time. Time passes from left to right as indicated by the arrow. Therefore, modern times are at the right and ancient times are at the left. The 4 rock layers are indicated on the timeline. In real life, these layers could be quite accurately dated by radiocarbon and other methods and we would use real-time on this axis. The y-axis depicts the number of fossils at each rock layer and is, therefore, a reflection of the abundance of organisms over time.
- Let’s consider the two fish first. It is clear that over time there was a decline in the shallow-swimming fish population. On the other hand, the population level of the deep-swimming fish has remained relatively constant over the time period encased in the fossil record we have.
- Turning to the penguins, one can see that as the shallow-swimming fish population decreases, so does the light-bone penguins. This decrease begins at the third rock level and continues to the present.
- The dense bone penguin population has not changed much over the time studied in this fossil record.
Note: It would be interesting to have a discussion with your class at this point to compare the penguin story and experiments from Investigation 2 with this fossil record. How does it make sense? Are there any other observations that can be made?