Evidence from Living Organisms
By examining fossils and by determining their relative and absolute ages, scientists have collected evidence that supports the theory that species changed over time. Further evidence is derived from living organisms. In order to determine if species change scientists compare common ancestry, structure, biochemistry, and development of organisms alive today. As you read this section, study this evidence and critically evaluate whether it indicates that species may have arisen by descent and modification from ancestral species. Evidence of Common Ancestry
If species change over time, then scientists should be able to cite examples showing that a group of living species may have come from a common ancestor. Let us examine one of many cases for which this seems to be true. Gracing the islands of Hawaii is a family of birds commonly called the Hawaiian honeycreepers. All Hawaiian honeycreepers have similarities in skeletal and muscle structure that indicate they are closely related. However, each of the Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists suggest that all 23 honeycreeper species apparently arose from a single species that migrated to Hawaii.
If a bat, a human, an alligator, and a penguin all evolved from a common ancestor, then they should share common anatomical traits. In fact, they do. Compare the forelimbs of the human, the bat, the penguin, and the alligator. Find the humerus, radius, ulna, and carpals in each forelimb. Though the limbs look strikingly different on the outside and though they vary in function, they are very similar in skeletal structure. More significantly, they are derived from the same structures in the embryo. Structures that are embryologically similar, but have different functions, are called homologous structures. Though these animals look different, a comparison of homologous structures indicates that they are quite similar. This suggests that these animals evolved from a common ancestor.
Tuesday, October 28, 2008
Speciation
Speciation is the evolutionary process by which new biological species arise. There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout.
Defining a Species
A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.
For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.
That definition of a species might seem cut and dried, but it is not—in nature, there are lots of places where it is difficult to apply this definition. For example, many bacteria reproduce mainly asexually. The bacterium shown at right is reproducing asexually, by binary fission. The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually.
Also, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups—but in some areas, they hybridize. Should they be considered the same species or separate species?
If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count them as different species? There are lots of other places where the boundary of a species is blurred. It’s not so surprising that these blurry places exist—after all, the idea of a species is something that we humans invented for our own convenience!
Defining a Species
A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.
For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.
That definition of a species might seem cut and dried, but it is not—in nature, there are lots of places where it is difficult to apply this definition. For example, many bacteria reproduce mainly asexually. The bacterium shown at right is reproducing asexually, by binary fission. The definition of a species as a group of interbreeding individuals cannot be easily applied to organisms that reproduce only or mainly asexually.
Also, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups—but in some areas, they hybridize. Should they be considered the same species or separate species?
If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count them as different species? There are lots of other places where the boundary of a species is blurred. It’s not so surprising that these blurry places exist—after all, the idea of a species is something that we humans invented for our own convenience!
Classification of living things
Linnaean taxonomy is a method of classifying living things, originally devised by (and named for) Carolus Linnaeus, although it has changed considerably since his time. The greatest innovation of Linnaeus, and still the most important aspect of this system, is the general use of binomial nomenclature, the combination of a genus name and a single specific epithet to uniquely identify each species of organism. For example, the human species is uniquely identified by the binomial Homo sapiens. No other species of organism can have this binomial. Prior to Linnaean taxonomy, animals were classified according to their mode of movement.
All species are classified in a ranked hierarchy, originally starting with kingdoms although domains have since been added as a rank above the kingdoms. Kingdoms are divided into phyla (singular: phylum) — for animals; the term division, used for plants and fungi, is equivalent to the rank of phylum (and the current International Code of Botanical Nomenclature allows the use of either term). Phyla (or divisions) are divided into classes, and they, in turn, into orders, families, genera (singular: genus), and species (singular: species).
Though the Linnaean system has proven robust, expansion of knowledge has led to an expansion of the number of hierarchical levels within the system, increasing the administrative requirements of the system (see, for example, ICZN), though it remains the only extant working classification system at present that enjoys universal scientific acceptance. Among the later subdivisions that have arisen are such entities as phyla, superclasses, superorders, infraorders, families, superfamilies and tribes. Many of these extra hierarchical levels tend to arise in disciplines such as entomology, whose subject matter is replete with species requiring classification. Any biological field that is species rich, or which is subject to a revision of the state of current knowledge concerning those species and their relationships to each other, will inevitably make use of the additional hierarchical levels, particularly when fossil forms are integrated into classifications originally designed for extant living organisms, and when newer taxonomic tools such as cladistics and phylogenetic nomenclature are applied to facilitate this.
There are ranks below species: in zoology, subspecies and morph; in botany, variety (varietas) and form (forma). Many botanists now use "subspecies" instead of "variety" although the two are not, strictly speaking, of equivalent rank, and "form" has largely fallen out of use.
Groups of organisms at any of these ranks are called taxa (singular: taxon) or taxonomic groups.
Tuesday, September 30, 2008
How do genes work?
Most genes contain the information needed to make functional molecules called proteins. (A few genes produce other molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.
During the process of transcription, the information stored in a gene’s DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.
Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which “reads” the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three bases that does not code for an amino acid).
The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the “central dogma.”
During the process of transcription, the information stored in a gene’s DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.
Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which “reads” the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three bases that does not code for an amino acid).
The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the “central dogma.”
What is a gene?
A gene is an operational region of the chromosomal DNA, part of which can be transcribed into a functional RNA at the correct time and place during development. Thus, the gene is composed of the transcribed region and adjacent regulatory regions.We know that it is a chain of chemical makeup consisting of the letters A, T, C, and G. Genes are the information that tell your body how to look, and even control the way you laugh or smile. Every human and animal have genes inherited from their parents. A parent passes on traits to their children when they are born. Genes are lined up on chromosomes which are in pairs. Chromosomes and genes are made of deoxyribonucleic or DNA.
Chromosomes are found inside the nucleus of the cell. Although quite small a nucleus is like a very thick and specific dictionary. It just fits inside of a cell that of course requires a microscope to see. Humans are made up of 46 chromosomes and 23 chromosomal pairs. Half of these chromosomes come from one parent and half come from the other parent. In meiosis.
Each gene has a special job to do. It carries blueprints — the instructions — for making proteins in the cell. Proteins are the building blocks for everything in your body. Things like bones, mucsles, and skin are made of proteins. Those proteins help our bodies grow, work properly, and stay healthy. Scientists today estimate that each gene in the body may make as many as 10 different proteins.
Chromosomes are found inside the nucleus of the cell. Although quite small a nucleus is like a very thick and specific dictionary. It just fits inside of a cell that of course requires a microscope to see. Humans are made up of 46 chromosomes and 23 chromosomal pairs. Half of these chromosomes come from one parent and half come from the other parent. In meiosis.
Each gene has a special job to do. It carries blueprints — the instructions — for making proteins in the cell. Proteins are the building blocks for everything in your body. Things like bones, mucsles, and skin are made of proteins. Those proteins help our bodies grow, work properly, and stay healthy. Scientists today estimate that each gene in the body may make as many as 10 different proteins.
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