Investigation 1 – Concept Day
Adaptation: Investigation 1
In this Investigation, we wish to introduce you to the concept of adaptation. We wish to emphasize that adaptation occurs over very long periods of time in both animals and plants.
Once you understand what adaptation is, we want to ensure that you conceive of adaptation on both a genetic and molecular genetics level. This is essential to truly understand larger concepts like natural selection and evolution.
- In this slide, we focus on the common bullfrog. The definition of adaptation is included at the upper left,
Adaptation: Traits that allow plant or animal species to survive in a particular environment.
- The key to understanding the force behind adaptation is to consider that there is pressure to survive in nature; there is competition for resources between species that live in the same environment. This competition may be for food, shelter, escaping predators, and so on. If there were no environmental pressure applied to a species, there would be little need for it to adapt.
- The frog pictured in this slide has a number of adaptations that allow it to survive and thrive in its environment. Eyes and nostrils on the top of its head permit the frog to keep almost its entire body submerged while still able to see and breathe above the surface. Such adaptations help conceal it from bird predators as well as to make it difficult to detect by its prey. This is particularly important since this type of frog lays in wait for its meals, which must come close enough to it to be captured. Green color and camouflage patterns also hide it from both its predator and prey in a similar manner. Not easily seen in this photo, the green color and camouflage pattern are only found on the top (dorsal) surface of the frog. Its underside (ventral surface) is a light color without a pattern so that predatory fish cannot easily see the frog from below – it tends to blend in with the sky above.
- In addition, the frog’s eyes have evolved for exceptional night vision when it feeds and mates and its hind feet are webbed for swimming.
- The frog is a dramatic and very clear example of adaption. Many of its features or “traits” are obvious and make sense for its aquatic, carnivorous life. However, even if traits are less obvious in other species, we can be sure that all animal and plant species have in some ways adapted to their environment or they would have likely disappeared and become extinct long ago.
- Adaptation is not limited to animals. All life on Earth is subject to adaptation and the gradual change associated with evolution. In this slide, we see a water lily plant of the genus Nymphaea.
- Specific adaptations to cope with its freshwater aquatic environment include the buoyancy of the water lily’s stem and leaves. This permits the broad leaves, which are themselves adaptations to increase the photosynthetic surface area, to float on the water’s surface. Not only does this capture a maximal amount of the Sun’s photons for energy, but it also creates dark shadows in the water below the lilies that prevent protons from reaching any plants that may attempt to grow near it.
- Finally, one cannot help but notice the beautiful flower of the Nymphaea. This adaptation of a buoyant flower permits pollination by insects on the surface. Even the flower itself represents a more basic adaptation that took place during the Triassic Period some 200 million years ago when dinosaurs were abundant. Prior to that time, no flowering plants existed. The flower was an adaptation to increase reproduction, genetic variation, and survival.
- Perhaps the most famous case of adaptation was described by Charles Darwin based on notes and collections while serving as a naturalist aboard the British ship H.M.S. Beagle from 1831 to 1836.
- Darwin noticed that the beaks of the finch on the Galapagos Islands, off the coast of Ecuador (South America), were different based on their distinct feeding habits. Finch that fed on very hard-shelled seeds had thick short beaks. Finch that fed on small seeds protected deep within thistles had slimmer and longer beaks. These birds are only found on the isolated islands that Darwin visited, located some 905 km (563 mi) from mainland South America. Thus, he thought it likely that the huge variety of finch that he saw (at least 13 different species) all originated from one ancestral species that arrived at the island many years ago.
- The means by which Darwin’s finches evolved was through a process of adaption to the many different and specific environmental niches found on the islands. Darwin extended his finding to suggest that species that competed with each other obtained structural adaptations, like beak shape, to win in such competitions for survival (“survival of the fittest”). It was such beneficial adaptations passed from generation to generation that suggested the slow progressive change in plants and animals that Darwin called evolution.
- Up to this point, we have discussed what adaptation is and have considered a few specific examples. However, these examples don’t explain the mechanism by which adaptations occur. Sadly, Charles Darwin, the father of natural selection, adaption, and evolution, died without knowing the story that follows over the next several slides.
- To really understand adaptation, we must think in terms of cell biology and molecular genetics. This slide shows a form of a slide similar to those you saw previously in the CELL Genes and Proteins. It is a good review to establish the context of chromosomes and genes.
- Chromosomes are located in the nucleus of both plant and animal cells. The chromosomes, in turn, contain the cell’s DNA.
- This slide shows that genes are located at specific locations on the “arms” of chromosomes. They are shown as colored bands on the chromosome. Notice that each chromosome consists of two chromosomes: one derived from the mother of the organism and the other from the father. Centromeres are important structures for mitosis and meiosis.
- The genes on the individual chromosomes are referred to as alleles. Alleles are essentially either the mother’s or father’s form of any particular gene. Alleles of a gene can be identical (the two black alleles at the top of each chromosome in this example) or contain a slightly different version of the allele (all the other allele pairs in this example).
- The presence of non-identical alleles for genes provides the organism with genetic variation. For example, a particular gene may determine eye color. The alleles for the eye color gene may be identical or different, depending on the parent’s genetic composition. We will see later why this is important for the adaptation and survival of a species.
- This slide is included to remind you that genes and their alleles are simply defined stretches of DNA. Because of mutations in the coding region of the DNA molecule (those areas of DNA that direct the sequence of a protein), there can be differences in the sequence between the two alleles of the same gene. We will see shortly that different alleles of the same gene may dictate different traits in the individual.
- This is another review slide to remind you of the basic steps in molecular biology. It graphically shows that the nucleotide sequence of the DNA molecule in the cell nucleus is used, through the process of transcription, as a template to form mRNA molecules.
- The RNA molecules pass into the cytoplasm and are bound by ribosomes there. This is followed by the translation of the triplet RNA codons into a long, linear sequence of amino acids that, when completed, form an entire protein molecule. It is the protein molecules that are used by the cell to perform all cellular functions and determine traits in the individual such as eye color, height, and many thousands of other traits that make each organism unique.
- Therefore, the DNA sequence of each allele on the chromosome ultimately determines specific traits.
- This slide describes two classes of alleles: dominant and recessive alleles. A dominant allele contains a DNA sequence that will produce a trait that masks another (recessive) trait for the same characteristic. An allele in a frog chromosome that codes for the production of green skin pigment would likely be a dominant allele.
- A recessive allele, on the other hand, is an allele that produces a trait that is masked by a trait coded by a dominant allele DNA sequence.
- When the alleles of any given gene are the same, either both dominant or both recessive, they are referred to as homozygous (the Greek term ‘homo’ meaning the ‘same’).
- Finally, when the alleles of any given gene are different, one dominant allele and one recessive allele, they are referred to as heterozygous (the Greek term ‘hetero’ meaning ‘different’).
- In the chromosome in this slide, only the black gene at the very top is homozygous. All the other genes shown are heterozygous. Unless more information is provided, we do not know if the genes are dominant or recessive.
- In addition, many traits are not produced by a simple dominant or recessive allele mechanism. Rather, the selection of traits may be the result of a more complex interaction of alleles or genes. One example would be traits inherited by incomplete dominance, which occurs when the trait produced is the intermediate of two alleles. Another example is co-dominance when the trait is the result of equal contributions of each allele.
This slide shows the potential combinations of alleles for the gene at the bottom of the chromosome illustrations. Notice that one chromosome originated from the organism’s mother and one from the organism’s father. The circle with the cross is female. These symbols for female ( ♀ ) and male ( ♂ ) are derived from the astronomical signs for Venus and Mars, respectively.
- In this example, we will focus on only the gene at the bottom of the chromosome illustrations indicated by the arrow and dashed line. The blue allele represents the dominant trait. In this hypothetical example, it indicates light bone density. On the other hand, the white allele represents the recessive trait, a more dense bone structure.
- The first chromosome on the left is homozygous dominant. This designation indicates that both alleles for the bone density trait are the same and both are dominant.
- The second chromosome is homozygous recessive. This designation indicates that both alleles for the bone density trait are the same and both are recessive.
- The third chromosome is heterozygous dominant. This designation indicates that the alleles for the bone density trait are different. Importantly, if there is one dominant and one recessive allele, the dominant trait will always be expressed.
- Finally, the fourth chromosome is also heterozygous dominant. The only difference from the third chromosome pair in terms of the bone density gene is that the dominant allele is derived from the paternal rather than the maternal side. However, this doesn’t matter, since there is one dominant and one recessive allele, the dominant trait will be expressed.
- The actual traits that are observed for each individual represented by the four different chromosome pairs are shown in the green box at the bottom. Notice that the only condition that leads to the recessive trait (dense bone) for the bone density gene is the homozygous recessive case (second from left). This is because it is the only chromatid pair that does not contain at least one dominant allele.
- The allele composition of the chromosomes is known as the genotype. The actual trait that is expressed, after considering dominant and recessive mixes (in this example, bone density) is known as the phenotype.
- The previous slide introduced the concept of genotype and phenotype. This slide simply reiterates these concepts.
- On the left is a karyotype of a human female. It shows all of the chromosomes in her cells. There are 23 pairs of chromosomes. Because she is female, her 23rd chromosome is composed of two “X” chromosomes. A male would have one “X” and one “Y” chromosome in his 23rd chromosome pair. The actual genetic composition (or genotype) of the chromosomes will determine the ultimate phenotype of the individual – that is, what the traits of the organism will be.
- On the right of this slide, a small human female is pictured. She is composed of thousands of different genetic combinations derived from her mother and father. For example, she has red hair and blue eyes. These are phenotypes that result from the many thousands of genes housed in the 23 chromosomes located in the nuclei of each of her cells.
- This final slide is included to introduce you to a simple procedure, the Punnett Square, to look at the genotype and phenotype of any genetic cross (mating).
- Let’s begin with the cross at the top of the slide. Here we have a cross between a homozygous dominant parent and a homozygous recessive parent. In a Punnett Square, the alleles for a particular gene from one parent are separated on the left of a four-box table and the alleles for the other parent are separated and placed on the top of the table. In this case, the homozygous dominant is placed on the left and the homozygous recessive is placed on top. We then simply transpose one allele from each column and one from each row into each of the four boxes. This gives us the only four possible genetic combinations of offspring from this cross or mating. Notice that in this cross, any offspring produced would be heterozygous dominant for this trait. All genotype combinations formed are heterozygous (one blue allele and one white allele). This Punnett Square demonstrates that all of these potential “offspring” will express the dominant phenotype of light bones as a phenotype.
- The middle cross mates two heterozygous dominants. As can be seen in the Punnett Square, the cross results in four possible combinations: one homozygous dominant (blue-blue), two heterozygous dominants (blue-white), and one homozygous recessive (white-white). Because of dominance, there is only a one in four possibility that an offspring would have the dense-bone recessive phenotype, the homozygous recessive, because it is the only combination of the cross that lacks even a single copy of the dominant blue allele.
- It is important to understand that the results of mating or crosses represent the chance that certain genotypes and phenotypes will occur, not that four offspring will result, and that each will have one of the four genotype combinations illustrated in the cross. For example, in the middle cross, let’s assume that each time the parents mate, one offspring is produced. The first time the parents mate, the offspring has a 25% chance that it will have the homozygous dominant genotype, a 50% chance that it will have the heterozygous genotype, and a 25% chance that it will have the homozygous recessive phenotype. However, if the parents produce 4 separate offspring at four separate times, each of these offspring may all be homozygous recessive, or they may all be homozygous dominant or all heterozygous dominant. Each time the parents mate, the same four genotype combinations are equally possible.
- We will go over dominance and recessive alleles again in Investigation 2.