An Introduction to the Invertebrates / Edition 2 available in Paperback
An Introduction to the Invertebrates / Edition 2
- ISBN-10:
- 0521674069
- ISBN-13:
- 9780521674065
- Pub. Date:
- 09/21/2006
- Publisher:
- Cambridge University Press
- ISBN-10:
- 0521674069
- ISBN-13:
- 9780521674065
- Pub. Date:
- 09/21/2006
- Publisher:
- Cambridge University Press
An Introduction to the Invertebrates / Edition 2
Paperback
Buy New
$70.99Buy Used
$30.60-
-
SHIP THIS ITEM
Temporarily Out of Stock Online
Please check back later for updated availability.
-
Overview
Product Details
| ISBN-13: | 9780521674065 |
|---|---|
| Publisher: | Cambridge University Press |
| Publication date: | 09/21/2006 |
| Edition description: | New Edition |
| Pages: | 340 |
| Product dimensions: | 8.11(w) x 9.21(h) x 0.75(d) |
About the Author
Read an Excerpt
Cambridge University Press
978-0-521-85736-9 - An Introduction to the Invertebrates - by Janet Moore
Excerpt
Chapter 1
The process of evolution: natural selection
This book is about invertebrate evolution. Every account of structure and function and the adaptation of an animal to its environment is a description of the results of evolution. Not only the intricate design but also the vast diversity of animals has been achieved by descent with modification due to the action of natural selection. A process so fundamental needs to be introduced at the very beginning of the book. As the different phyla are presented, general discussion of some other topics will become necessary (and will be inserted as ‘Boxes’), but evolution cannot wait.
1.1 What was Darwin's theory of natural selection?
Our understanding of evolution dates from the publication in 1859 of Charles Darwin's great book The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Before that time, explanation of all the details of animal design in terms of a divine Creator was widely accepted, though perhaps the extraordinary variety of life (e.g. what has been termed ‘the Almighty's inordinate fondness for beetles’) was harder to explain. From very early times a few writers had postulated evolutionary theories, suggesting that different species might not all have been separately created, and further thatcomplicated forms of life could have arisen from simple antecedents by descent with modification. This however was mere speculation in the absence of support from a large array of ordered facts. What Darwin gave us was a mass of careful observations, many gathered while he was Naturalist on the voyage of HMS Beagle, from which he formulated a theoretical framework showing that evolution could have occurred by what he called ‘Natural Selection’. That the time was ripe for such a theory is shown by the simultaneous conclusions of Alfred Russel Wallace from his work in Indonesia. The cooperation of Darwin and Wallace without any competition for priority is an encouraging example of decency transcending competition.
Darwin's argument was as follows:
Living things tend to multiply. There are more offspring than parents and, if unchecked, their numbers would increase in geometrical ratio.
The progeny cannot all survive, because resources (food, space, etc.) are insufficient.
Therefore there will be competition for survival, a ‘struggle for existence’ between individuals of the same species.
Living things vary; the progeny are not all identical and some will be better equipped for survival than others.
Therefore ‘favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species’ (The Autobiography of Charles Darwin, ed. Norah Barlow, Collins 1958, p. 120).
To describe this process of natural selection Herbert Spencer used the phrase ‘survival of the fittest’. The phrase needs to be qualified if misunderstanding is to be avoided: firstly, it is not mere survival but differential reproduction that is required and, secondly, ‘fittest’ does not refer to general health and strength but to some precise advantage in particular circumstances in a particular environment. Adaptation consists in the perpetuation of such an advantage down the generations.
Here at once was Darwin's greatest difficulty. For natural selection to work, advantageous changes had to be inherited. In Darwin's time heredity was assumed to involve the blending of the features of the two parents, and Darwin was much worried by the criticism (from an engineer, Fleeming Jenkin) that any system of blending inheritance would remove the advantage in a few generations. The solution was at hand, but never known to Darwin. Gregor Mendel had already shown that heredity was particulate, but his work was not publicised until 1900.
1.2 What was Mendel's theory of heredity?
Mendel's ‘atomic theory’ of heredity was based on his experiments on crossbreeding garden peas. He deduced that hereditary factors are constant units, handed down unchanged from parent to offspring, and that these units occur as ‘allelomorphic pairs’, the two members of each pair representing two contrasting characters. At sexual reproduction when gametes (spermatozoa and ova) are formed, only one factor of each pair can enter a single gamete. When gametes fuse to form a ‘zygote’ the factors, one from each parent, are combined. One factor in a pair may be ‘dominant’ over the other, which is called
| Image not available in HTML version |
the ‘recessive’ and has no apparent effect on the organism but is maintained when it reproduces (Figure 1.1). The organism contains a very large number of such pairs (some 50 000 pairs in humans) most of which segregate and recombine independently at every sexual reproduction. Mendel's analysis explained both the basic resemblance between parents and offspring and the introduction of variation between them.
1.3 What is the cellular basis of heredity?
Early in the twentieth century, T. H. Morgan’s studies of cell structure identified Mendel's factors as ‘genes’ borne on the elongated bodies, ‘chromosomes’, contained in the nucleus of almost every cell in the body (see Chapter 15, where the contributions of studies of the fruit fly Drosophila melanogaster are discussed). All organisms develop from the division of cells which previously formed part of one or (where reproduction is sexual) two parent organisms. August Weismann first recognised that the ‘germ-plasm’ that gives rise to gametes is distinct from the rest of the body, the ‘soma’. Somatic cells divide by ‘mitosis’, the longitudinal splitting of each chromosome with self-replication of each gene so that each half chromosome has exactly the same genes as its parent (Figure 1.2a): all the somatic cells in an individual are genetically identical. Gamete-forming cells first multiply by the
| Image not available in HTML version |
same process of mitosis but then divide by ‘meiosis’, a process in which the number of chromosomes is halved and (usually) the two genes in each allelomorphic pair are separated as Mendel postulated (Figure 1.2b). The fusion of gametes combines half the genes of each parent to make a new individual. Gametes are described as ‘haploid’ since they contain half the number of chromosomes of the ‘diploid’ zygote and adult individual.
Only the gametes carry genes to the next generation. Changes often occur in the somatic cells, caused by use or disuse or by direct effects of the environment, but such changes cannot be transmitted to the offspring. Jean-Baptiste Lamarck is rather unfairly remembered mainly for his erroneous belief in the inheritance of acquired characters. Lamarckism has been typified by the idea that if giraffes stretched their necks to reach more food their offspring would be born with longer necks. A change to the body such as an elongated neck cannot directly affect subsequent generations: they can be changed only by the selection of individuals with genes promoting the growth of long necks. The ‘phenotype’, that is the organism defined by the characters made manifest, must be distinguished from the ‘genotype’ or genetic constitution, which alone can transmit changes to the offspring.
Note that the word ‘develop’ was originally used to describe two different consequences of gene action: the sequence of changes in an individual as the egg gives rise to the adult form, called ‘ontogeny’, and (on an enormously greater time scale) the process of evolutionary change, called ‘phylogeny’. We now reserve the term ‘development’ for ontogeny.
1.4 What is the origin of genetic variation?
Genes provide both the continuity and the differences between parents and offspring. The differences (‘variations’) are caused as follows:
Combination of half the genes from each parent.
Reassortment of the genes inherited from each parent. Genes borne along the same chromosome tend to be inherited together (they are said to be ‘linked’) but during meiosis there is normally some ‘crossing over’, or exchange of pieces of the split chromosomes (Figure 1.2c).
The presence of a gene does not guarantee the appearance of the character with which it is associated, because gene effects may depend on the action of other genes present. The simplest example is dominance within an allelomorphic pair, but other genes may promote, suppress or alter the effect of a gene. A character may be the product of many different genes acting together, and one gene may affect many characters; for example, genes acting early in development may transform the effects of other genes acting later. It is a dangerous oversimplification to equate a character with the gene that in part governs it. Mendel has been mistakenly described as ‘lucky’ because his choice of the peas gave a simple picture: in fact he spent a very long time experimenting to find suitable material.
Mutations occur. These may be chromosome changes, or more frequently errors in gene copying as cells divide. Sudden change in a phenotype due to mutation is rarely advantageous, as large changes tend to be lethal, but small changes may accumulate in the genotype, undetected until some change in circumstances gives them a selective advantage.
Clearly, mutation is the only one of these causes of variation that operates in asexual reproduction, where otherwise parent and offspring are genetically identical.
With the mechanism producing heritable variations understood, the picture of evolution caused by natural selection acting on random variations became firmly established. Due mainly to R. A. Fisher, the emphasis fell not on the sudden change in form of an individual but on the spread of that variation through a population. The study of natural selection at work became a matter of statistics rather than qualitative descriptions. The synthesis of Mendelian genetics and natural selection was called Neo-Darwinism or ‘The Evolutionary Synthesis’, and by the 1930s it was widely accepted. It became the unifying principle underlying all branches of biology.
1.5 What is the nature of genes?
The work of James Watson and Francis Crick and others revealed in 1953 that DNA, in the form of a double helix, is the genetic material in the chromosomes. It replicates when the cell nucleus divides, and it can be transcribed to make RNA, giving a message that in turn can be translated into assembly of amino acids to make proteins. Genes and their action can now be studied at the molecular level, which has led to an enormous increase in understanding and opportunities for manipulation. There are also new problems: for example, some at least of the changes at the molecular level may not be due to natural selection. Could there be evolutionary change due simply to chance?
1.6 What is the role of chance in evolution?
This has frequently been misunderstood by critics, some of whom regard the whole process as a combination of lucky accidents. They fail to distinguish the two stages involved. Variations arise by chance, as mutations and gene recombination occur at random. What is not at all a matter of chance is the operation of natural selection, which acts on these random variations to produce adaptation. Certainly change in gene frequencies may partly be due to chance; for example, long ago Sewall Wright pointed out that a small isolated population would contain only a few of that species’ genes and therefore these genes would become over-represented in the population. This process is called ‘genetic drift’ and cannot itself cause adaptation. It is still not clear whether ‘neutral’ (i.e. unselected) molecular evolution is important.
1.7 At what level does natural selection act?
Our understanding of evolution continues to evolve. Natural selection was at one time assumed to act for the good of the species or the group, until both experiment and theory showed that natural selection acts on individuals and cannot be shown to act on any larger entity. This at once produced new problems: a prominent puzzle is to find the advantage of sexual reproduction. Clearly this is slow and complicated compared with simple asexual multiplication, but of enormous benefit to the species because it introduces so much variation. How can this advantage apply at the individual level, by an organism being unlike its parents? The problem is unresolved, but explanations focus on the masking of harmful mutations or on the so-called ‘Red Queen’ effect: the need to run as fast as possible simply to keep up. Host and parasite, for example, engage in a continual ‘arms race’: host offspring differing from their parents have more chance of avoiding their parasites, and parasites chemically different from their parents may evade the host’s defences.
1.7.1 The unit of selection
Natural selection acts on the individual, but the effect of this action is the passing on of one set of genes rather than another. It is the relative frequency of genes that changes down the generations. As has been cogently argued by Richard Dawkins (in The Selfish Gene), the individual' s body is the vehicle for the genes, which are the replicators; individual bodies are the genes’ way of preserving the genes unaltered. Arguments about whether the individual or the gene is the ‘unit of selection’ are unprofitable: the important thing is to remember the role of each. Natural selection acts on the phenotype, not directly on the genotype. The danger of equating genes and characters must not be forgotten. Further, no gene has a fixed selective value: its effect will depend upon other genes present. Genes are now known to change surprisingly little during evolution; what changes is the regulation and expression of those genes, as will be explained and illustrated in Chapter 20.
The above brief outline may serve to introduce invertebrate evolution, but further reading (see the end of this book) is strongly recommended, to supply evidence for the above assertions and fuller discussion of these and many more facts and ideas.
1.8 What in general does evolution produce?
1.8.1 Diversity
Diversity is the product of evolution. The very long evolutionary history of invertebrates has allowed an abundance of diversity: the very name ‘invertebrates’ is revealing: they can only be united
| Image not available in HTML version |
as ‘animals other than vertebrates’ . Selection pressure caused divergence among the earliest multicellular forms, and certain body plans became successfully established, each offering particular opportunities and constraints for evolution, as this book will show. Animals sharing the same body plan are united in a ‘phylum’ (plural ‘phyla’). Within each phylum there are usually well-defined classes with characteristics that fit the animals for some particular environment or way of life, and within each class the original body form will have become modified as they exploit different habitats. This diversification from a single ancestral form is known as ‘adaptive radiation’ .
At the same time, natural selection does not only produce novelty: it also maintains and stabilises a successful structure and way of life. Nor does it only produce divergence: animals of very different ancestry may become very similar through adaptation when they solve the same problems or live in the same environment. Convergence is a widespread, often undervalued cause of resemblances, often baffling the attempt to classify animals by tracing their evolutionary history. For example, on an exposed shore the barnacles and limpets are superficially similar, being attached to rocks and covered by shells that protect them from desiccation and predatory birds (Figure 1.3). When the tide is up, the barnacle is seen to be a crustacean, shrimp-like but attached by its head and kicking its food into its mouth with its legs, while the limpet is a mollusc with a muscular foot like a snail, moving off in search of food. This is a crude example: one has only to think of the mimicry in butterflies from different families to realise the fine degree of convergence that can be produced by natural selection, superimposed upon its primary divergent effect.
Natural selection defies man-made categories: for example, in defining Platyhelminthes we state that the mouth is the only opening to the gut, yet one parasitic species has not just one anus but two. Animals are opportunists. Our categories especially meet trouble when we try to define a species, because we are trying to put firm boundaries on an evolutionary continuum. If species were incapable of changing, evolution could not have occurred.
1.8.2 Complexity
As well as diversity, complexity is a product of evolution. Primitively, multicellular forms were not very complex (note that ‘primitive’ means ‘most like the ancestral form’ , not ‘simplest’). What we call the ‘higher’ animals, those more recently produced, are on the whole very much more elaborate than their ancestors. The evolutionary pattern is clear, but must at once be modified by what we know of the evolutionary process. Complexity is not an end in itself: it will evolve where it has selective value, but not otherwise. Many simple forms survive today: one has only to look at sponges, animals extremely successful in that they are very numerous and widely distributed in the sea, yet remarkably simple in structure. A sponge is the best way of being a sponge, and natural selection has not in millions of years produced much alteration in their form. Cnidaria (anemones, corals, jellyfish, hydroids) again are simple in structure but remarkably numerous in the sea (unlike sponges they were also able to evolve great morphological diversity, as will be shown). In the more elaborate phyla the simplest animals are not necessarily the most primitive, as is clearly illustrated within the Platyhelminthes. Simplicity may be a secondary product of adaptation, and we cannot assume that a simple animal is primitive.
1.8.3 Not progress
Evolution is not directed from the outside and there is no inner directing force. The criterion for survival is immediate selective advantage, not any long-term evolutionary aim. We are being anthropocentric when we misapply the idea of progress to the evolutionary process. We like to think of all evolution leading up to humans at the apex of the evolutionary tree. This is a false picture.
1.8.4 Efficiency
This term needs careful definition. While natural selection should tend to maximise efficiency, that does not always mean maximum physical efficiency: biological efficiency can be different. For example, that a constant high body temperature enables the body' s enzymes to work at maximum speed may be physically most efficient, yet it may be more advantageous to an animal to let the body temperature fluctuate, allowing the economy of cold inactive periods. Natural selection does not necessarily generate our own idea of a perfect product. ‘Success’, another anthropocentric approach, cannot be defined in terms of complexity or position in an evolutionary tree, but rather in terms of survival, abundance and perhaps also diversity.
As we study the invertebrate phyla that are the products of the evolutionary process, we can safely ask the question ‘Why?’ (as in ‘Why is this animal so constructed?’). This is because we know that what a biologist means by such a question is ‘How has such a structure conferred selective advantage?’ Long ago when I was a student a professor said to us ‘When anyone asks me the question “Why?” I refer him to a theologian.’ I now think he was wrong – on both counts, because theologians cannot answer such questions and biologists can make the attempt.
Chapter 2
The pattern of evolution: methods of investigation
2.1 How should we classify animals?
Classification is essential to any study of animals (the first attempt is attributed to Adam) and is a necessary prelude to tracing the pattern of evolution. Systematic ordering of the products of classification (taxonomy) can be done in various ways, but most usefully it aims to produce a ‘natural’ classification, i.e. a phylogeny that reveals evolutionary history. We try to put together those animals most closely related by descent, using resemblance as the basis for our classification.
© Cambridge University Press