Deconstraining Early Development

Until now, we have considered deconstraint to be a property of the compartment plan such that it reduces limitations on possible kinds of development following the phylotypic stage. But the compartment plan would seem to be a major constraint on the egg and stages preceding the phylotypic stage. Activating a conserved compartment plan in its precise spatial arrangement at the phylotypic stage should require a suite of highly conserved developmental processes and signals, which should be very...

Variation and Selection in Behavior

Of the more stereotyped behaviors, ant foraging is particularly amenable to quantitative study and can be analyzed in simple terms. Ants explore unfamiliar terrain or familiar terrain in which the distribution of food is constantly changing. Thus, experience with their local environment may not help them very much. Ants emerging from their nest cannot see food or smell it. There are no clues to where the food may be it could be a seed that just blew into the territory. As in microtubules,...

Constraint and Deconstraint

Disclosure of the compartment structure of multicellular animals was at once an insight into embryology and an insight into evolution. It unified our appreciation of how the organism generates the complexity of the adult, and at the same time it gave molecular clues to large-scale and small-scale evolutionary change. The discovery also exposed how unevenly conservation and change are distributed in animal evolution. Compartments are surprisingly more conserved than the anatomical differences...

Exploratory Behavior

We have seen that existing somatic adaptations can be a ready and available source for new variation when genetic change stabilizes adaptive processes at different points along their ranges. This application of the Baldwin effect has not been widely endorsed as a panacea for explaining novelty in evolution, because the kind of variation that seems most interesting in evolution is not that which causes small quantitative perturbations of existing systems. Certainly, in anatomical novelty it is...

Hemoglobin A Molecular Link Between Physiology and Evolution

Until the widespread investigations using the methods of molecular biology and biochemistry in the mid twentieth century, it was difficult to find evidence that the processes underlying somatic adaptability actually serve as a basis for evolutionary change (even though an environmental stimulus seems to be able to substitute for a genetic change). Such evidence requires a molecular understanding of the physiological or developmental process and a similar understanding of the critical...

Invisible Anatomy

If one looks at animals as one of the various life forms, and asks what is unique about them, it would have to be their large size and the varied anatomy by which their physiology and behavior are conveyed. It is not their chemistry or efficiency or resistance to harsh conditions, all of which are exceeded by bacteria, protists, fungi, and plants. Complex anatomy has not been achieved on the level of the single cell and hence emerged only with multicellularity, that is, in the last 600 million...

Environmental and Chromosomal Sex Determination

In the course of evolution, some processes move easily from environmental control to genetic control and back again. It may seem odd to think of sex determination as a response to the environment, but it is one of the clearest examples of the interchangeability of physiological and genetic control. In many organisms, fish and reptiles included, the ratio of males to females may deviate far from unity depending on environmental conditions such as temperature or social interactions. Summarizing...

Indifferent Gonads

Figure 14 Sex determination in alligators. At first the indifferent gonad develops with parts for both the testis and the ovary. Later it destroys some parts and further develops others for either testis or ovary, but not both. The Wolffian duct becomes the epididymus and vas deferens in males. The Mullerian duct becomes the oviduct and cervix in females. The temperature at a critical time decides the direction. Figure 14 Sex determination in alligators. At first the indifferent gonad develops...

Map in the Embryo

We have a double problem in the development of complex animals the anatomy of the embryo must be strewn with numerous changing signals that tell cells of that region what cell types to make. At the same time, we cannot expect so many different kinds of signals, given the relatively small number of genes and the simplicity of the egg. The answer to this embryological difficulty turns out to be an answer also to the problem of morphological evolution in multicellular animals. Its solution emerged...

Schmalhausen and the Baldwin Effect

With the aim of finding a place for somatic adaptation in evolution, James Mark Baldwin (1861-1934) and others proposed the hypothesis of organic selection, which drew on Lamarck and Darwin without conflating them. Baldwin was an early experimental psychologist at a time when psychology was separating from philosophy as a field of study. Behavior had all the elements of somatic adaptation, and Baldwin proposed that animals have broad ranges of somatic adaptability enabling them to tolerate...

Somatic Adaptation and Evolution

The lesson from Baldwin, Schmalhausen, and Waddington is that the organism has a great deal of latent novelty within its own somatic adaptability. As West-Eberhard has extended the lesson, all phenotypic novelties are reorganizations of preexisting phenotypes. In effect, the organism can express many alternative phenotypes phenotypes that are stable like the different insect castes and the male-female alternatives of sex determination, or phenotypes that are readily reversible like the...

Weak Linkage and Evolution

The selection for a small number of conserved core processes versatile enough to be used in many different contexts to support the complexity of large multicellular organisms is a product of selection for physiological adaptability. As a side effect, core processes with high adaptability have a high capacity for weak linkage. Such processes are responsive to genetic changes of regulation. They have been used in many different combinations at many different times and places in the organism's...

Weak Regulatory Linkage

Let us now delve directly into the conserved core processes that are responsible for generating most of the anatomy, physiology, and behavior of the organism. These are the processes that evolved between three billion and a half billion years ago (Chapter 2). They include metabolism, gene expression, and signaling between cells. In deepening the inquiry, we introduce the fact that all the conserved core processes possess adaptability, which they use in response to varying conditions inside the...

The Cell Response to Signals

The regulation of gene expression is one of the core processes most critical for generating phenotypic variation. The ease of remodeling the linkages that regulate genes is directly related to the ease of generating novel patterns of gene expression in evolution. For multicellular organisms, the major arena for anatomical variation is the processes of embryonic development. Spatially and temporally regulated patterns of gene expression drive these processes. It is now possible to connect weak...

The Three Pillars of Darwins Theory of Evolution

Darwin's all-encompassing theory of evolution was based on three major supports a theory of natural selection, a theory of heredity, and a theory of the generation of variation in the organism. In Darwin's view, rephrased in modern terms, organisms within populations vary genetically and consequently differ in traits that affect their capacity to contribute to the next generation. In competition with one another and facing other pressures in the environment, the most fit organisms flourish and...

The Disproof of Facilitated Genetic Variation

Subsequent years of experimentation brought no support for the direct inheritance of physiological adaptations of the organism to the environment. On the contrary, substantial evidence accumulated for the view that pangenesis did not exist. The first steps in distinguishing physiological adaptation (the subject of Lamarck's first postulate) from heritable variation (the subject of his second postulate) came in 1895 from August Weismann. He showed that it was extremely unlikely that the sperm...

Control of Gene Function

Most of biochemistry until the mid twentieth century concerned metabolism the breakdown of foodstuffs to extract energy, and the synthesis of the components of the body. Little attention was paid to how that metabolism was regulated in response to internal needs or external opportunities. Regulatory biology came of age with DNA and an understanding of protein synthesis. The early forays of molecular biology into regulation gave the first molecular picture of how time, place, amount, and...

Preface

Ignorance about novelty is at the heart of skepticism about evolution, and resolving its origins is necessary to complete our understanding of Darwin's theory. The last 150 years have seen Darwin right and Darwin wrong Darwin doubted, Darwin ignored Darwin demonized, and Darwin idolized but in the end we may have the true worth ofhis accomplishment. He came up with a single transcendent idea, variation and selection, and he demonstrated that idea through intense observation. This...