Based on Pough et. al, 1989. Vertebrate Zoology - 3rd Ed. MacMillan
Hickman et al., 1996. Integrated Principles of Zoology - 10th Ed. William C. Brown Publishers
Question - what is a vertebrate? Before we get to evolution, I want to discuss this question just a little.
Vertebrates range in weight from 0.1 gram to 100,000 kg, and in length up to 60m. They are found in all habitats on the planet, and exhibit virtually all types of feeding strategies. The name vertebrate implies the presence of a vertebral column (and associated brain, spinal cord, etc.). There are over 50,000 extant species, and about 500,000 extinct species. Examine the characteristics of vertebrates on the "Tree of Life".
Some examples of vertebrates:
The groupings above divide the vertebrates up, but you should be aware that the classification system is rapidly changing. Not long ago, the vertebrates were divided into 5 groups: fish, amphibians, reptiles, birds, and mammals. It was recognized however, that many of these groups contained organisms that were not particularly closely related to each other. For instance, a crocodile is more closely related to a bird than it is a turtle, yet under the old system both crocodiles and turtles were placed together in the Reptilia. Taxonomists are now beginning to embrace a system called cladistics. Application of cladistic rules to vertebrate nomenclature will mean that the groups above will change even more in the future. A complete list of the taxa we will use this semester is found in your syllabus and in a separate web page.
As an example of how the current classification system works, let us consider the box turtle:
Classification of the turtle:
Kingdom Animalia (Plantae, Fungi, Protista, Monera)
The formal name, or specific epithet, is: Terrapene carolina (Linnaeus, 1758). The genus name is given first, and is always capitalized. The species name is never capitalized, despite what you see in poorly done advertisements trying to look scientific. These two names are always set off, preferably through italics, although underlining will do in a pinch. The specific epithet ends with the authority, the person who first described the species, and the date of description. If the organism is placed in a different genus than the one it was originally placed in, the authority is placed in parentheses. In zoology, family names always end in -dae.
Let's consider the map turtle, Graptemys geographica (Le Seur) 1827. This species is of note to us in Marietta, since Le Seur passed through Marietta along with a group of other noted "American" (Leseur was an immigrant from France) thinkers on their way to found the community of New Harmony, Indiana. They passed through Marietta in January of that year. This species was probably collected by Leseur during the next summer in Indiana. An accounting of the "Boatload of Knowledge" can be found in the December, 1989 edition of The Ohio Journal of Science.
Common names are a bit of a problem for zoologists. People living in different areas may have different common names for the same species, or they may misidentify species. For instance, how many people do you know that instinctively classify all turtles as "snappers"? Scientists prefer the scientific names for precision and for the fact that they work no matter what the language.
This digression into classification was necessary, because it helps to frame the scope of our study, and because the various taxonomic units are the "building blocks" of evolution. We need to know a little bit about them so that later examples will make sense. It is also important to note that any classification system is a human construct, and that it will be imperfect. Nature is more complicated than we are capable of dealing with. Classification systems and rules are attempts to put order (often where none exists) into a system so that we can look for patterns. Further, knowledge of classification systems is necessary for our study of evolution because our modern classification systems attempt to incorporate evolutionary relationships. In our modern classification systems, we attempt to link those groups which evolved from a common ancestor.
Before we can explore the main topic here - evolution - we need to define a few of the terms that will crop up in the discussion.
Charles Darwin was an English naturalist who was able to accompany the HMS Beagle, a ship of the Royal Navy, on a surveying trip around the world in 1831. On that trip, Darwin was exposed to many new creatures and fossils, and his ideas about how they all came into being began to form. It is important to note that the concept of evolution (the changing of organisms from one species into another) over time was very much in the minds of scientists of Darwin's day, and there were several competing theories. Perhaps most important was the development of a new theory in geology - uniformitarianism - which predicted that the world was much older than had been previously thought. Darwin received a copy of the book advancing uniformitarianism from its author, Charles Lyell, just before he sailed. On the journey, Darwin witnessed the great diversity of animal and plant life. Several key observations stick out:
the tortoises of the Galapagos Islands; with slightly different species on each island, each adapted to the peculiarities of each individual island.
the finches of the Galapagos Islands. These birds had a wide array of feeding habits. All finches, they had changed over time to fill the ecological niches normally filled by a variety of different types of birds.
fossils in the Andes mountains of South America. The fossils were striking because fossils of seafloor organisms were found high in the mountains, attesting to vast geological change. There was also apparent development in individual species within the fossils.
Darwin returned to England in 1836 and, over the next several years, wrote accounts of his travels. He also read an article by Thomas Malthus, who argued that it was a natural tendency for populations to grow beyond what their environments would support. This was the final piece in the puzzle, and Darwin's theory of evolution through natural selection began to come into its final form. He would spend the next 20 years refining it and gathering additional evidence (obviously Darwin didn't have to worry about publish or perish!). He was startled to find, in 1858, that another English naturalist working in the tropics had come to many of the same conclusions Darwin had. Alfred Wallace wrote to Darwin to share his theory with him. Darwin graciously forwarded both his own and Wallace's paper to the Journal of the Linnaen Society and they were published together in 1858. Darwin followed up with the massive The Origin of Species (which he called an "abstract") in 1859. Technically, both men should receive equal credit for the idea, although it is obvious from the records of the time that Darwin conceived of the idea first and was able to supplement it with a mass of data. Darwin's thinking can be summarized from its basis in two facts and three deductions from those facts (your book uses a more complicated system on page 168):
Darwin's theory appeared before Mendel's work in genetics were published in 1866 (and even then, Mendel's work was ignored for almost 40 years). Darwin could formulate his theory without knowing how genetics worked, but the later discovery of how organisms pass on their characteristics to their offspring proved to be compatible with Darwin's theory. This leads us to some more terms we need to know:
Natural selection is the mechanism by which evolution occurs in Darwin's theory. Natural selection can act on several different levels:
Usually, when talking about natural selection, we are discussing individual organisms which survive and reproduce on the merits of their genes, and thus we are mostly concerned with individual selection, although the other two types exist.
Types of natural selection:
Natural selection can act on a group of organisms belonging to an individual species (a population) in one of three ways. You might want to consult your text on page 175 for figures that may help explain this discussion. All three are based on the assumption that for most traits there is a natural range of variation - say from light to dark or from bare to hairy.
directional selection - this occurs when natural forces select against members of the population which exhibit one extreme of the range of variation. An example would be cold weather selecting against furless mammals in favor of hairy ones.
stabilizing selection - driving force of allopatric speciation. Here, selection acts against both extremes, making the whole population more homogeneous. In our mammal example, both naked and furry mammals might be selected against in a climate where there are extremes of temperature.
disruptive selection - driving force of sympatric speciation. In this type of selection, pressure is placed on members in the middle of the spectrum, forcing them to the extremes. In our mammal population, this would lead to mammals with no hair or with long hair. Over enough time, these differences (along with others) might accrue to the point where the two groups would no longer interbreed.
Actually, there is another type of selection known as sexual selection. In sexual selection, females or males choose mates based on certain characteristics. Organisms having those characteristics are more likely to mate. Sexual selection is interesting because it can go against survival; it is important to remember that in evolution both survival and reproduction are equally important. Examples of sexual selection gone nuts include the tail feathers of the male peacock (yeah, that will help them survive in a jungle with tigers, the females, of course are drab brown) or the now extinct Irish Elk with its huge antlers.
Again, more terms:
Fitness - measure of the reproductive success (of an allele or individual)
Inclusive fitness - fitness gained by the copies of your genes carried by relative's kin. Sometimes one can maximize one's own fitness by foregoing reproduction to help kin rear offspring; this is called altruistic behavior.
Types of variation :
Evolution of the many diverse forms of life on Earth must involve more than simple change in organisms. A species is basically a group of organisms that only breed with each other. Species, then evolve together as natural selection acts to promote characteristics (and thus the genes that create the characteristics) and suppress genes that produce characteristics which hinder survival and reproduction. How species form (speciation) is one of the key questions of evolution.
speciation - the separation of a population into distinct breeding groups via natural selection and other genetic processes, including chance.
Speciation, then, is caused by natural selection (and other forces) and leads to the production of species:
biological species - groups of actually (or potentially) interbreeding natural populations which are reproductively (genetically) isolated from other such groups.
The biological species, in turn, remain separate because there is no effective gene flow between them:
gene flow - the mixing of alleles in a population through breeding. If effective gene flow exists, then speciation cannot occur.
Gene flow is prevented by reproductive isolating mechanisms:
Reproductive isolating mechanisms - keep organisms of different species from interbreeding. We will consider two basic types of reproductive isolating mechanisms, those that act to prevent mating, and those that prevent future offspring if mating does occur.
Note that the reproductive isolating mechanisms described above are progressive in nature; if two organisms surmount one barrier they face a next one, and so on. It's a wonder anything reproduces!
Effects of population size on evolution:
In evolution, as we already mentioned, there are forces other than natural selection at work. One of these is chance, and it is the size of the population that in large part determines how much of a role chance will play (as opposed to natural selection).
genetic drift - the tendency for random chance to change allelic frequencies within a population - pronounced in small populations.
The random chance might be the result of natural forces such as floods or storms. Imagine a population into which one mutant has been born with superior survival abilities. Those new abilities might be useless in the face of a major flood, however, and the mutant might be swept away. Genetic drift is more pronounced in small populations because in these populations the chance of losing an important gene is greater.
Over the long run, the current population size may not be critical in the species' evolution. Often low population size is. This is due to two related effects:
Very small populations only have room for a few types of alleles; therefore whenever a population passes through a period with a small population, many alleles are filtered out of the population. It may be a very long time before mutation (or immigration) restores the "lost" alleles to the population.
adaptive radiation - the evolution into many different niches by a single species (through speciation) - usually requires colonization of an ecologically void area (island) A good example is the case of the Galapagos finches - Figure 9-22 pg. 166
There are two main models of speciation:
Allopatric (literally "different places") speciation occurs when two populations are separated for a long period of time. Over that time, genetic differences arise due to either chance or natural selection (natural selection is more important if the populations are large and the habitats are different). If the changes are great enough, they may include some reproductive isolating mechanisms which will keep the populations separate even if they encounter each other. In fact, if they do encounter each other, lowered fertility of matings between the two populations will cause natural selection to encourage greater differences and additional reproductive isolating mechanisms. This is known as character displacement. On large continents, rivers, mountain ranges, deserts, etc. may form the barriers that keep developing species separate.
Some species do arise without physical separation between habitats. This is fairly common in plants, where polyploidy is more easily handled (plants often have various methods of asexual reproduction - runners, fragmentation, etc.). In animals, it may occur when a mutation leads to a small group of animals becoming reproductively isolated from others. Imagine a small insect which feeds on a certain plant, and finds its mates on that plant. If some individuals are born with a mutation that causes them to be drawn to feed on another plant, they will spend all their time on the new plant, and will only mate with others having the same mutation. Over time. genetic drift and natural selection will cause them to look different than the population which stayed with the original plant, and one species will have become two. This is sympatric speciation ("same place").
While biologists who work with real, whole animals and plants are solidly convinced of the validity of evolution, they disagree about some of the fine points
how often does allopatric speciation occur?
what is the relative importance of chance in evolution?
is individual selection more important than genic selection? .
and so on. One of the largest debates is over the pace of evolution. Darwin's view, and the view of most scientists for a long time, was that evolution was a slow, steady process. More recently, scientists such as Stephen J. Gould and Niles Eldridge have proposed that evolution proceeds in spurts separated by long periods without change (stasis). This concept is known as punctuated equilibria (see figures 9-24, and 9-25 on pg. 167). Many biologists today would agree with Gould and Eldridge, although this in no way refutes Darwin's basic theory - it is simply a clarification and elaboration.
Please read all of Chapter 9 in the text. Don't worry about dates except for the ones mentioned here. Know the names and ideas of the pre-Darwinian evolutionary thinkers, and be able to show what separates their ideas from Darwin's. Ignore the section on evolutionary trends on page 157-158. It is wrong. The real story of the horses is not the trend towards larger size; this is only a freak occurence in this one lineage; as the book points out many lineages were lost, and several of these had trends towards smaller size. Pay attention to bold-faced terms, and keep a dictionary of them by writing out the definitions from the glossary.
Stephen J. Gould is one of the best thinkers and writers on evolutionary theory today. His monthly column in Natural History is a "must read" for biologists and educated laypeople alike. He has combined these writings in a series of books, any of which is highly recommended. Several of his other books, however, are more thematic and accessible. I would particularly recommend Full House (1996, Harmony Books) and Wonderful Life (1989, W.W. Norton). You should be aware that Gould is a baseball fanatic and often illuminates evolutionary concepts with examples from the world of baseball.
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