Revisions of darwin's theory

REVISIONS OF DARWIN’S THEORY

Neo-Darwinism

The most serious weakness in Darwin’s theory was his failure to identify correctly the mechanism of inheritance. Darwin saw heredity as a blending phenomenon in which the hereditary factors of parents melded together in their offspring. Darwin also invoked the Lamarckian hypothesis that an organism could alter its heredity through use and disuse of body parts and through the direct influence of the environment. August Weismann rejected Lamarckian inheritance by showing experimentally that modifications of an organism during its lifetime do not change its heredity, and he revised Darwin’s theory accordingly.

We now use the term neo-Darwinism to denote Darwin’s theory as revised by Weismann. Mendelian genetics eventually clarified the particulate inheritance that Darwin’s theory of natural selection required. Ironically, when Mendel’s work was rediscovered in 1900, it was considered antagonistic to Darwin’s theory of natural selection. When mutations were discovered in the early 1900s, most geneticists thought that they produced new species in single large steps. These geneticists relegated natural selection to the role of executioner, a negative force that merely eliminated the obviously unfit.

Emergence of Modern Darwinism:

The Synthetic Theory

In the 1930s a new generation of geneticists began to reevaluate Darwin’s theory from a mathematical perspective. These were population geneticists, scientists who studied variation in natural populations using statistics and mathematical models. Gradually, a new comprehensive theory emerged that brought together population genetics, paleontology, biogeography, embryology, systematics, and animal behavior in a Darwinian framework. Population geneticists study evolution as a change in the genetic composition of populations. With the establishment of population genetics, evolutionary biology became divided into two different subfields. Microevolution pertains to evolutionary changes in frequencies of different allelic forms of genes within populations. Macroevolution refers to evolution on a grand scale, encompassing the origins of new organismal structures and designs, evolutionary trends, adaptive radiation, phylogenetic relationships of species, and mass extinction. Macroevolutionary research is based in systematics and the comparative method. Following the evolutionary synthesis, both macroevolution and microevolution have operated firmly within the tradition of neo-Darwinism, and both have expanded Darwinian theory in important ways.

MICROEVOLUTION:

GENETIC VARIATION AND

CHANGE WITHIN SPECIES

Microevolution is the study of genetic change occurring within natural populations. Occurrence of different allelic forms of a gene in a population is called polymorphism. All alleles of all genes possessed by members of a population collectively form the gene pool of that population. The amount of polymorphism present in large populations is potentially enormous, because at observed mutation rates, many different alleles are expected for all genes. Population geneticists study polymorphism by identifying the different allelic forms of a gene present in a population and then measuring the relative frequencies of the different alleles in the population. The relative frequency of a particular allelic form of a gene in a population is called its allelic frequency. For example, in the human population, there are three different allelic forms of the gene encoding the ABO blood types). Using the symbol I to denote the gene encoding the ABO blood types, I A and I B denote genetically codominant alleles encoding blood types A and B, respectively. Allele i is a recessive allele encoding blood group O. Therefore genotypes I A I A and I Ai produce type A blood, genotypes I B I B and I B i produce type B blood, genotype I AI B produces type AB blood, and genotype i i produces type O blood. Because each individual contains two copies of this gene, the total number of copies present in the population is twice the number of individuals. What fraction of this total is represented by each of the three different allelic forms? In France, we find the following allelic frequencies: I A ! .46, I B ! .14, and i ! .40. In Russia, the corresponding allelic frequencies differ ( I A ! .38, I B ! .28, and i ! .34), demonstrating microevolutionary divergence between these populations. Although alleles I A and I B are dominant to i, i is nearly as frequent as I A and exceeds the frequency of I B in both populations. Dominance describes the phenotypic effect of an allele in heterozygous individuals, not its relative abundance in a population of individuals. We will demonstrate that Mendelian inheritance and dominance do not alter allelic frequencies directly or produce evolutionary change in a population.

MACROEVOLUTION: MAJOR

EVOLUTIONARY EVENTS

Macroevolution describes large-scale events in organic evolution. Speciation links macroevolution and microevolution. Major trends in the fossil record are clearly within the realm of macroevolution. Patterns and processes of macroevolutionary change emerge from those of microevolution, but they acquire some degree of autonomy in doing so. The emergence of new adaptations and species, and the varying rates of speciation and extinction observed in the fossil record go beyond the fluctuations of allelic frequencies within populations. Stephen Jay Gould recognized three different “tiers” of  time at which we observe distinct evolutionary processes. The first tier constitutes the timescale of population genetic processes, from tens to thousands of years. The second tier covers millions of years, the scale on which rates of speciation and extinction are measured and compared among different groups of organisms. Punctuated equilibrium is a theory of the second tier, explaining the occurrence of speciation and morphological change and their association over millions of years. The third tier covers tens to hundreds of millions of years, and is marked by occurrence of episodic mass extinctions. In the fossil record of marine organisms, mass extinctions recur at intervals of approximately 26 million years. Five of these mass extinctions have been particularly disastrous. The study of long-term changes in animal diversity focuses on the third-tier timescale.

Speciation and Extinction

Through Geological Time

Evolutionary change at the second tier provides a new perspective on Darwin’s theory of natural selection. Although a species may persist for many millions of years, it ultimately has two possible evolutionary fates: it may give rise to new species or become extinct without leaving descendants. Rates of speciation and extinction vary among lineages, and lineages that have the highest speciation rates and lowest extinction rates produce the greatest number of living species. The characteristics of a species may make it more or less likely than others to undergo speciation or extinction events. Because many characteristics are passed from ancestral to descendant species (analogous to heredity at the organismal level), lineages whose characteristics increase the probability of speciation and confer resistance to extinction should dominate the living world. This species-level process that produces differential rates of speciation and extinction among lineages is analogous in many ways to natural selection. It represents an expansion of Darwin’s theory of natural selection. This expansion is particularly important for macroevolution if one accepts the theory of punctuated equilibrium, which states that the evolutionarily important variation occurs primarily among rather than within species.

References : Hickman.2008.INTEGRATED PRINCIPLES OF ZOOLOGY, FOURTEENTH EDITION. New york. McGraw-Hill Companies

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GALERY

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Introduction of Zoology

Zoology, the scientific study of animal life, builds on centuries of human inquiry into the animal world. Mythologies of nearly every human culture attempt to solve the mysteries of animal life and its origin. Zoologists now confront these same mysteries with the most advanced methods and technologies developed by all branches of science. We start by documenting the diversity of animal life and organizing it in a systematic way. This complex and exciting process builds on the contributions of thousands of zoologists working in all dimensions of the biosphere.

General Properties of Living Systems

The most outstanding general features in life’s history include chemical uniqueness; complexity and hierarchical organization; reproduction (heredity and variation); possession of a genetic program; metabolism; development; environmental interaction; and movement.

1. Chemical uniqueness. Living systems demonstrate a unique and complex molecular organization. Living systems assemble large molecules, known as macromolecules, that are far more complex than the small molecules of nonliving matter. These macromolecules are composed of the same kinds of atoms and chemical bonds that occur in nonliving matter and they obey all fundamental laws of chemistry; it is only the complex organizational structure of these macromolecules that makes them unique. We recognize four major categories of biological macromolecules: nucleic acids, proteins, carbohydrates, and lipids. These categories differ in the structures of their component parts, the kinds of chemical bonds that link their subunits together, and their functions in living systems. The general structures of these macromolecules evolved and stabilized early in the history of life. With some modify cautions, these same general structures are found in every form of life today. Proteins, for example, contain about 20 specific kinds of amino acid subunits linked together by peptide bonds in a linear sequence. Additional bonds occurring between amino acids that are not adjacent to each other in the protein chain give the protein a complex, three-dimensional structure A typical protein contains several hundred amino acid subunits. Despite the stability of this basic protein structure, the ordering of the different amino acids in the protein molecule is subject to enormous variation. This variation underlies much of the diversity that we observe among different kinds of living forms. The nucleic acids, carbohydrates, and lipids likewise contain characteristic bonds that link variable subunits. This organization gives living systems both a biochemical unity and great potential diversity.

2. Complexity and hierarchical organization. Living systems demonstrate a unique and complex hierarchical organization. Nonliving matter is organized at least into atoms and molecules and often has a higher degree of organization as well. However, atoms and molecules are combined into patterns in the living world that do not exist in the nonliving world. In living systems, we fi nd a hierarchy of levels that includes, in ascending order of complexity, and has its own internal structure, which is also often hierarchical. Within the cell, for example, macromolecules are compounded into structures such as ribosomes, chromosomes, and membranes, and these are likewise combined in various ways to form even more complex subcellular structures called organelles, such as mitochondria. The organismal level also has a hierarchical substructure; cells combine to form tissues, which combine to form organs, which likewise combine to form organ systems.

ZOOLOGY AS A PART OF BIOLOGY

Animals form a distinct branch on the evolutionary tree of life. It is a large and old branch that originated in the Precambrian seas over 600 million years ago. Animals form part of an even larger limb known as eukaryotes, organisms whose cells contain membrane- enclosed nuclei. This larger limb includes plants, fungi and numerous unicellular forms. Perhaps the most distinctive characteristic of the animals as a group is their means of nutrition, which consists of eating other organisms. Evolution has elaborated this basic way of life through diverse systems for capturing and processing a wide array of food items and for locomotion. Animals are distinguished also by the absence of characteristics that have evolved in other eukaryotes. Plants, for example, use light energy to produce organic compounds (photosynthesis), and they have evolved rigid cell walls that surround their cell membranes; photosynthesis and cell walls are absent from animals. Fungi acquire nutrition by absorption of small organic molecules from their environments, and their body plan contains tubular fi laments called hyphae; these structures are absent from the animal kingdom.

            Some organisms combine properties of animals and plants. For example, Euglena is a motile, single-celled organism that resembles plants in being photosynthetic, but it resembles animals in its ability to eat food particles. Euglena is part of a separate eukaryotic lineage that diverged from those of plants and animals early in the evolutionary history of eukaryotes. Euglena and other unicellular eukaryotes are sometimes grouped as the kingdom Protista, although this kingdom is an arbitrary grouping of unrelated lineages that violates taxonomic principles.

PRINCIPLES OF SCIENCE

Nature of Science

We stated in the first sentence of this chapter that zoology is the scientific study of animals. A basic understanding of zoology therefore requires an understanding of what science is, what it is not, and how knowledge is gained using the scientific method. Science is a way of asking questions about the natural world and sometimes obtaining precise answers to them. Although science, in the modern sense, has arisen recently in human history (within the last 200 years or so), the tradition of asking questions about the natural world is an ancient one. In this section, we examine the methodology that zoology shares with science as a whole. These features distinguish sciences from activities that we exclude from the realm of science, such as art and religion.

 Despite an enormous impact of science on our lives, many people have only a minimal understanding of the nature of science. For example, on March 19, 1981, the governor of Arkansas signed into law the Balanced Treatment for Creation-Science and Evolution-Science Act (Act 590 of 1981). This act falsely presented “creation-science” as a valid scientifi c endeavor. “Creationscience” is actually a religious position advocated by a minority of the American religious community, and it does not qualify as science. The enactment of this law led to a historic lawsuit tried in December 1981 in the court of Judge William R. Overton, U.S. District Court, Eastern District of Arkansas. The suit was brought by the American Civil Liberties Union on behalf of 23 plaintiffs, including religious leaders and groups representing several denominations, individual parents, and educational associations. The plaintiffs contended that the law was a violation of the First Amendment to the U.S. Constitution, which prohibits “establishment of religion” by government. This prohibition includes passing a law that would aid one religion or prefer one religion over another. On January 5, 1982, Judge Overton permanently

enjoined the State of Arkansas from enforcing Act 590. Considerable testimony during the trial dealt with the nature of science. Some witnesses defined science simply, if not very informatively, as “what is accepted by the scientific community” and “what scientists do.” However, on the basis of other testimony

by scientists, Judge Overton was able to state explicitly these essential characteristics of science:

1. It is guided by natural law.

2. It has to be explanatory by reference to natural law.

3. It is testable against the observable world.

4. Its conclusions are tentative and therefore not necessarily the final word.

5. It is falsifiable.

Pursuit of scientific knowledge must be guided by the physical and chemical laws that govern the state of existence. Scientific knowledge must explain what is observed by reference to natural law without requiring intervention of a supernatural being or force. We must be able to observe events in the real world, directly or indirectly, to test hypotheses about nature. If we draw a conclusion relative to some event, we must be ready always to discard or to modify our conclusion if further observations contradict it. As Judge Overtonstated, “While anybody is free to approach a scientific inquiry in any fashion they choose, they cannot properly describe the methodology used as scientific if they start with a conclusion and refuse to change it regardless of the evidence developed during the course of the investigation.”

Science is separate from religion, and the results of science do not favor one religious position over another. Unfortunately, the religious position formerly called “creation science has reappeared in American politics with the name “intelligent- design theory.” We are forced once again to defend the teaching of science against this scientifically meaningless dogma.

Scientifi c Method

 These essential criteria of science form the hypotheticodeductive method. The first step of this method is the generation of hypotheses or potential answers to the question being asked. These hypotheses are usually based on prior observations of nature or derived from theories based on such observations. Scientif c hypotheses often constitute general statements about nature that may explain a large number of diverse observations. Darwin’s hypothesis of natural selection, for example, explains the observations that many different species have properties that adapt them to their environments.

On the basis of the hypothesis, a scientist must make a prediction about future observations. The scientist must say, “If my hypothesis is a valid explanation of past observations, then future observations ought to have certain characteristics.” The east hypotheses are those that make many predictions which, if found erroneous, will lead to rejection, or falsification, of the hypothesis. The scientific method may be summarized as a series of steps:

1. Observation

2. Question

3. Hypothesis

4. Empirical test

5. Conclusions

6. Publication

 form a critical first step in evaluating the life histories of natural populations.  might cause the observer to question whether rate of larval growth is higher in undisturbed populations than in ones exposed to a chemical pollutant. A null hypothesis is then generated to permit an empirical test. A null hypothesis is one worded in a way that would permit data to reject it if it is false. In this case, the null hypothesis is that larval growth rates for crabs in undisturbed habitats are the same as those in polluted habitats. The investigator then performs an empirical test by gathering data on larval growth rates in a set of undisturbed crab populations and a set of populations subjected to the chemical pollutant. Ideally, the undisturbed populations and the chemically treated populations are equivalent for all conditions except presence of the chemical in question. If measurements show consistent differences in growth rate between the two sets of populations, the null hypothesis is rejected. One then concludes that the chemical pollutant does alter larval growth rates. A statistical test is usually needed to ensure that the differences between the two groups are greater than would be expected from chance fluctuations alone. If the null hypothesis cannot be rejected, one concludes that the data do not show any effect of the chemical treatment. The results of the study are then published to communicate findings to other researchers, who may repeat the results, perhaps using additional populations of the same or a different species. Conclusions of the initial study then serve as the observations for further questions and hypotheses to reiterate the scientifc process.

References : Hickman.2008.INTEGRATED PRINCIPLES OF ZOOLOGY, FOURTEENTH EDITION. New york. McGraw-Hill Companies

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