Biodiversity, Questionnaire Example

Charles Darwin’s theory states that the characteristics of populations of organisms change through time and that natural selection has led to great diversity from one or a few original forms. These are two related concepts: evolution (change through time) and divergence (one species becoming two). Describe and define evolution and natural selection and how, acting in concert, the evolutionary changes that occur can lead to increased numbers of species.

Evolution of species is all genetic variations that occur slowly from generation-to-generation over time. Although, the genetic changes can either be a positive or a negative, the key to evolution is the descent through genetic inheritance. If my answers are correct feel free to use what you like. Evolution of species is the genetic variations that occur over time from generation to generation. Basically, evolution is the change in the genetic makeup of a population over time. The population changing over time is due to some individuals in the population not reproducing and therefore, their genes are not being inherited in the next generation. The force that drives evolution is referred to as natural selection. Natural selection is the differential reproduction of genotypes, or genetic makeup, and is one of several ways in which genetic variations in a population can occur. Other variations in population occur through mutation, migration and genetic drift. In regard to evolution, natural selection ultimately determines which individuals in a population survive and reproduce. Darwin believed that the changes in populations of species where repeatedly checked by environmental factors, such as disease and weather, which would kill individuals before they were able to reproduce. Therefore, certain individuals were “selected” for the ancestors of future generations. That is where the term “natural selection” came from. Darwin also noted that there are variables within natural selection, such as fitness, that aid in natural selection. For instance, some individuals with certain genotypes may reproduce more offspring than other individuals allowing their genetic makeup to be passed on to future generations. The individual that is capable of reproducing and carrying their genes or alleles to future generations is referred to as having a high fitness. As stated in the coined phrase from Darwin, “Survival of the fittest”. Individuals in a population that are not able to reproduce begin to disappear from the population as well as their genes and alleles. These individuals are considered to have a low fitness. The result in the change that occurs in the genetic makeup or gene pool of the population over time is referred to as evolution. Therefore, both evolution and natural selection work together to form new species. New species arise because the population evolves enough differences in their genetic makeup in comparison to their ancestors and therefore, the population can be considered a different species than its original ancestor.

Although Darwin knew of Gregor Mendel’s research on plant hybridization (1865), he never included Mendelian concepts in the later revisions of “On the origin of species”, originally published in 1859 but with revisions by Darwin through the sixth edition in 1872 and further editorial revisions in 1876. Darwin believed in “blending inheritance”. Mendel’s research implied that inheritance of variety from generation to generation did not blend, but was discrete (particulate), and was predictable and fixed according to probability theory.

Describe Mendel’s theory and how this countered Darwin’s theory as Darwin presented it but how it solved a major Darwinian problem that variety should disappear through “blending”. Include in your answer:

1. a) Description of the concepts of “blending” and “particulate” inheritance.
2. b) How Mendel’s research showed that the characters of organisms were associated with particulate “factoren” (now we call them “genes”) and that varieties did not blend.
3. c) Definitions for genotype, phenotype, segregation, recombination, allele, homozygous, heterozygous, dominant, recessive

One of the setbacks to the understanding and acceptance of Darwin’s theory of natural selection was the belief in the hypothesis of blending of inheritance. It was thought that the different traits that were inherited to the offspring from the parental generation were a blend of traits from the parental generation. If this were correct, it would decrease the genetic variation in a population, which would not support Darwin’s theory of natural selection. Charles Darwin also introduced another theory called, “pangenesis”. He described hereditary as “particles” that were in our bodies and were affected by environmental things that happened to us during our lifetime. He proposed that the particles traveled though our bodies through our blood system and entered our reproductive cells so that it could be inherited to the next generation.

Mendel’s work showed that genes are inherited as individual units, not from blending inheritance. Mendel was able to illustrate that certain traits showed up in future generations of offspring without the blending of both parental traits from his experiments using pea plants. Mendel chose to use common pea plants because they were able to be grown easily and in large quantities. In addition, their reproduction could be worked. For instance, the pea plants have both male and female reproduction parts and can either self-pollinate or cross-pollinate with a different plant. Mendel selected particular plants with certain traits in order to observe the results of offspring. His experiments showed that different parental alleles on a gene could produce an in-between phenotype expression in the offspring; however, he also showed that the original allele form or trait in the parent could be passed on to the third and fourth generation. He illustrated that if the blending inheritance were true, all of the offspring would have a mixture of the gene. For example, the offspring’s pea flowers from his cross pollination of pea plants were either purple or white, not a blending of the colors. During this process he was able to find the principles of heredity; however, he did not discuss his perspective on evolution and therefore, Charles Darwin and other scientists in that time period still struggled with the questions of how the characteristics in individuals were inherited. Mendel discussed his results in numbers in which he kept track of which generations the plants belonged to, how many different types of genotypes of offspring were created by hybrids and the ratios of the different genotypes in order to figure out the dominant and recessive alleles. The genotypes are the genetic makeup of an individual organism. Mendel was able to visualize the genotype through the resulting phenotypes (physical characteristics) of the offspring.

Mendel was also able to figure out the patterns of dominance and recessive alleles, segregation and inheritance of a single gene from each parent. So essentially he discovered that alleles, different forms of a gene, make up the genotype. An individual who has a dominant allele will show phenotypic characteristics whether it is homozygous or heterozygous. While an individual with recessive alleles will only show the phenotypic characteristics when it is in a homozygous state. To be in a homozygous state they must possess two of the same alleles of that gene. In heterozygous they possess two unlike alleles of that gene.

Mendel also discovered what he referred to as principle of segregation by creating ratios to represent passed particles from parent to offspring using a probability theory. In segregation, the alleles separate from each other during reproduction. In addition to segregation, recombination of alleles also became apparent. Some of the offspring inherited new allele combinations. These offspring were referred to as recombinants. Recombination occurs in sexual reproduction during the crossing over of DNA from one chromosome to another and increases genetic variation. Therefore, all of Mendel’s work supported Darwin’s theory of natural selection because it explained how some individuals might cause changes in a population.

Describe the structure and function of the chromosome in bacteria and in higher, eukaryotic life forms. Include in your answer:

1. the molecular nature of DNA and how this structure encodes proteins
2. the nature of the evidence that DNA is the source of heritable information
3. descriptions of the various types of mutations that increase variety and ultimately are selected “for or against”

Fred Griffith was a scientist in the 1920’s who provided the first evidence that genes were not composed of proteins. He was initially working with bacteria pneumonia in mice and was working with two different strains, which contained two different types of genetic material. One strain caused the production of external capsules that protected them from animal immune response and the other strain did not produce capsules and did not protect; therefore, one was fatal and the other not fatal to the mice. During his research, he found that the non-fatal bacteria could be transformed into the fatal type. He discovered this because when the fatal bacteria were killed and injected into the mice containing the non-fatal bacteria, some of the mice still died. He theorized that some of the genetic material from the fatal bacteria in the mice had entered the living non-fatal bacteria, which became referred to as bacterial transformation. Bacterial transformation occurs when genetic material from one bacteria transfers to another. The next question was how did it transfer? Another scientist, Oswald Avery, found that the transformation occurred through DNA, not protein or other substances. This meant that treatment with DNA gave a new trait to an organism that it did not have before, implying that DNA is the genetic material. Additional evidence that DNA was the genetic material came from research using bacteriophages. Bacteriophages are viruses that enter bacteria. The phage or virus takes over the metabolism of the bacteria and produces new phages to infect additional bacteria. The scientists Hershey and Chase were able to tell the difference between the phages protein and the DNA through radioactive isotopes. In their studies, new DNA from original viruses was found in the new generation of viruses indicating that DNA was in fact the genetic material.

DNA is the molecule that carries a cell’s genetic information. The DNA molecule is composed of three different parts: 1) a five-Carbon sugar referred to as deoxyribose, 2) one to three phosphates groups bonded to the fifth carbon sugar, 3) and one of four nitrogen containing bases (thymine, cytosine, adenine, or guanine). The nucleotide bases are linked together in a strand of DNA which forms a string of phosphate and sugar groups, called the sugar-phosphate backbone. The bases attach to one side of the sugar-phosphate backbone. Watson and Crick later described the model of DNA as consisting of two strand of DNA that looked like a ladder, with the ladder’s sides being the sugar-phosphate backbones of the two strands and the steps being the bases. It was also indicated that adenine was always paired with thymine and guanine with cytosine through a hydrogen bond, making the bases in the pairs complementary. In addition, Watson and Crick indicated that the DNA molecule strands were antiparallel and that the ladder was twisted into a double helix. Furthermore, Watson and Crick showed that the double-stranded DNA molecule was structured in a way that allowed it to copy genetic material. For instance, the two strands of complementary base pairs gave the information needed to produce the next strand. This came to be known as semiconservative replication where one new double helix contained one old strand and one new strand. Therefore, in order for replication to occur, the strands are separated. The separation occurs through DNA helicase enzymes. DNA polymerase enzymes then grabs a template strand and moves along the strand making a complementary DNA strand and add nucleotides one by one.

It was also described that DNA in both prokaryotes (bacteria) and eukaryotes (animal cells) contained the double helix structure. The only difference was that the prokaryote or bacteria DNA is circular; whereas, the eukaryotic DNA is organized into chromosomes, each with a DNA double helix. The chromosome in the eukaryote contains DNA that is associated with numerous proteins and forms what is called chromatin. The chromosomes proteins in eukaryotes are referred to as histone and non-histone chromosomal proteins. The histone proteins are made of amino acids and the non-histone proteins function as structural proteins for organization and regulation of DNA. In addition, the eukaryotic cells contain more DNA than the prokaryotic cells. There is also a difference in DNA replication in the prokaryote versus the eukaryote. In the prokaryote, the circular DNA attaches to the plasma membrane where replication originates. From this point on the plasma membrane, the replication forks move in opposite directions around the circular molecule until they meet each other. The plasma membrane at the replication fork separates the new DNA molecules. Some prokaryotes have several small circular DNA molecules called plasmids in addition to the one larger circular DNA molecule. During replication, the small plasmids are also replicated at the same time as the larger DNA molecule. In eukaryotes, replication occurs at numerous locations at replication forks on the chromosome. The replication forks move away from each other until the reach another replication fork.

Mutations can also occur to DNA. Mutations are inheritable alterations in DNA. They can result during the replication process, spontaneous transformation, or the inability to repair damage that occurred to DNA molecules. Once the DNA has been mutated it copies the mutated DNA. There are several types of mutations that occur from changes in the DNA, such as substitution, insertion, deletion, or inversion of nucleotides, breakage of chromosome fragments, attachment of one or part of a chromosome to another, loss of a chromosome, extra copy of a chromosome or duplication of a chromosome. Although mutations cause a change in DNA, a small amount of mutation can be an advantage because it produces genetic variation. It is from genetic variation that evolution occurs. As the mutations are expressed in the organisms, natural selection can occur and some mutations can persist to future generations making the organisms different than their ancestors.

Describe the many lines of evidence from a wide variety of fields that support the theory of evolution. Include in your answer:

1. evidence from at least the fields of plate tectonics, biogeography, stratigraphy, fossils and molecular biology
2. how each of the five tenets of Hardy-Weinberg population genetics support evolutionary theory
3. the reasons why a population divided and isolated on different islands will necessarily evolve and become different species over time.

There are many lines of evidence from a wide variety of fields that support the theory of evolution to include biogeography, plate tectonics, stratigraphy, fossils, molecular biology, Hardy-Weingberg law and isolation on islands. Evidence of evolution from biogeography, the study of the geographical distributions of organisms, was noticed by Darwin during his travels. Darwin began to ask questions during his travels of why the Galapagos Islands, which was a small group of islands, contained more species of finches compared to the South American continent. In addition, he wondered why marsupial mammals were found in only Australia and South America. Furthermore, he wondered why similar but different species were found on different continents. Darwin believed that these questions could be explained through evolution. He theorized that from ancestors that descendent populations could radiate into other locations and adapt to the new environment resulting in new species, more formally known as adaptive radiation.

The fossil record also provides evidence from evolution. Fossils allowed scientists to trace the origin of mammals from reptiles and dinosaurs to ancient birds. The stratigraphy of the earth provides evidence of evolution as well. Many of the fossils are formed in several layers of the rock formations. Geologists found that the bottom layer or stratum was composed of the older fossils. The age of the fossils is found using Carbon 14 dating. The stratum layers contained different types of fossils and different ages, providing evidence of evolution. Plate tectonics supported evolution as well and provided evidence of the overlapping fossil assemblages on different continents. This explains how similar species were found on different continents and shows how they evolved to survive in that environment.

Evidence of evolution is also provided through molecular biology. At the cellular and molecular level, plants and animals have the same types of organelles, nucleic acids and cytochrome c in the DNA. The Hardy-Weinberg law in genetics also supports evolution. There are five tenets that support the theory: 1) No net mutation (no mutation in genes), 2) No mating preferences (mating is random), 3) Large size (the population is large), 4) No gene flow (population is isolated) and 5) No selection (no genotype has an advantage over another). Basically, the law explains that if all of these conditions were true in a population, that evolution would not occur.
Evidence from islands provided evidence of evolution as well. Darwin found species that were found nowhere else, even on islands close to each other. The Galapagos Islands in particular had numerous species found nowhere else with similar species living on different islands. Darwin theorized that species from the mainland moved to other islands and evolved to cope with the different environment. He proved his theory using the 13 species of Galapagos finches. The different finches adapted to living in different habitats and to eating different foods and evolved different types of beaks.

References

Arms, K. and Camp. P.S. (1995). Biology (4^th Edition). Harcourt Brace & Company, Orlando, Florida. 1108pgs.

McClean, P. (1997). Eukaryotic Chromosome Structure. North Dakota State University. http://www.ndsu.edu/pubweb/~mcclean/plsc431/eukarychrom/eukaryo3.htm

O’Neil, D. (2011). Mendel’s Genetics. Retrieved on February 15, 2012 from: http://anthro.palomar.edu/mendel/mendel_1.htm

Qual, P., Simison, B., Wong, B., Peters, S. (2011). Intro to the Science of Living Things. Bio 11 Course online notes. Retrieved on February 20, 2012 from: http://ib.berkeley.edu/courses/bio11/SimisonMay3.pdf