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Development and Evolutionary Change. Development and Evolutionary Change. Introduction Evolution and Development Regulatory Genes and Modularity: Modifying Morphology Plant Development and Evolution Environmental Influences on Developmental Patterns Learning: A Modification of Development.
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Development and Evolutionary Change • Introduction • Evolution and Development • Regulatory Genes and Modularity: Modifying Morphology • Plant Development and Evolution • Environmental Influences on Developmental Patterns • Learning: A Modification of Development
Introduction • Fish that can change their sex in response to their social environment, such as anemonefish, demonstrate that an organism’s development is not determined entirely by its genes. • The phenotypes of adult organisms are the result of complex interactions between basic genes, gene products, and the environment.
Evolution and Development • Charles Darwin’s idea of characterizing evolution as “descent with modification” led to the recognition that the results of evolution could be visualized as a “tree of life.” • He explained similarities among organisms by their descent from a common ancestor. • Differences among organisms were explained as the result of natural selection, which adapted them to different environments.
Evolution and Development • Darwin recognized and showed that similarities among embryos could be used to infer the relationships among groups of organisms. • Using similarities in larval forms as a basis, Darwin was able to conclude that barnacles are crustaceans.
Figure 21.1 Similarities In Early Developmental Stages Can Be Used to Infer Relationships
Evolution and Development • Late in the twentieth century, the fields of genetics and embryology came together to form the new discipline of evolutionary developmental biology. • Evolutionary developmental biologists investigate how the course of evolution has been influenced by heritable changes in the development of organisms. • Many of the genes regulating development are highly conserved, meaning their sequences have changed very little throughout the evolution of multicellular organisms.
Evolution and Development • Many of the genes that regulate the development of very different animal species are remarkably similar. • For example, many of the same genes are involved in the development of the compound eyes of fruit flies and the camera-like eyes of house mice. • The genes involved in eye development in these two species are so similar that the fruit fly cell that normally develops into part of a leg will form an eye when a mouse Pax6 gene is expressed in it.
Figure 21.2 The Mouse Pax6 Gene Causes Eye Development in Drosophila
Evolution and Development • The same set of homeobox genes provides the positional information along the anterior–posterior axis of the body in both human and insect embryos. • For example, the Drosophila gap genes ems, tll, and otd, as well as the homologous genes of vertebrates are expressed in the anterior regions of the brain.
Evolution and Development • Mutations of genes involved in development can result in abnormal differentiation during development. • The bithorax mutation in insects, for example, results in the development of two sets of forewings instead of one pair. • When the expression of certain vertebrate Hox genes is altered, vertebrae that normally develop into lumbar vertebrae instead develop instead into thoracic vertebrae.
Evolution and Development • The enormous amount of variation of morphological forms found in animals is underlain by a common set of instructions that have been conserved in thousands of species. • The vast differences in morphological form that result from similar genetic instructions means that these instructions alone cannot be entirely responsible for an organism’s morphology.
Regulatory Genes and Modularity:Modifying Morphology • Developing embryos exhibit modularity—they are made up of self-contained units that can be changed independently of the other units, or modules, that compose the organism. • There are two ways in which changes in genes that regulate development can lead to important morphological changes: • Mutations in genes that regulate developmental processes • Changes in the time or place of expression of developmental regulatory genes • The modular nature of most organisms makes both of these pathways of evolution easier.
Regulatory Genes and Modularity:Modifying Morphology • Insects provide examples of how mutations in genes that regulate segmentation can lead to the evolution of morphological changes. • For example, the homeotic gene Ultrabithorax (Ubx), which is found in all organisms. • The insect Ubx gene has a mutation not found in other arthropods. • The Ubx protein produced from this mutated gene is expressed in the abdomen of insects, where it represses the expression of the distal-less (dll) gene, which is essential for leg formation. • As a result of the Ubx repression of the dll gene, insects do not form legs on their abdomens.
Figure 21.5 A Mutation Changed the Number of Legs in Insects
Regulatory Genes and Modularity:Modifying Morphology • The evolution of webbed feet in ducks provides an example of an altered spatial expression pattern of a regulatory gene. • A gene encoding a protein called bone morphogenetic protein 4 (BMP4) is expressed in the spaces between the developing bones of the toes and instructs the cells in those spaces to undergo apoptosis, destroying the webbing between the toes. • Ducks express a BMP inhibitor protein called Gremlin in their webbing cells. • This protein prevents the BMP4 protein from signaling for cell death in the webbing, resulting in a webbed foot.
Figure 21.6 Changes in gremlin Expression Correlate with Changes in Hindlimb Structure
Regulatory Genes and Modularity:Modifying Morphology • Modularity allows the relative timing of two different developmental processes to shift independently of one another. • This process is known as heterochrony and has been widely studied in salamanders. • Two species of Bolitoglossa illustrate heterochrony. • The webbing between the feet of most salamander species disappears as the animals mature. • If expression of genes that dissolve the webbing is slowed, the digits don’t grow, and “juvenile” webbed feet result. • These feet can act like suction cups, opening an arboreal way of life.
Regulatory Genes and Modularity:Modifying Morphology • Modularity also allows structural changes to evolve via gene duplication. • If a gene is duplicated, the new copy can evolve a new function without disrupting the organism as long the other copy is performing its original function.
Plant Development and Evolution • Rapid progress has been made during the past decade in identifying the genes that regulate growth and cell differentiation in plants. • The sequencing of the complete genome of the thale cress, Arabidopsis thaliana, has provided much of this information. • About 1,500 of the nearly 26,000 Arabidopsis genes code for transcription factors. • Over half of the known families of transcription factors are found in all eukaryotes, but many others are found only in plants.
Plant Development and Evolution • Plants and animals share many regulatory genes, but plants differ from animals in important ways: • Plant cells do not move relative to one another. Changes in the shape of a developing plant result from cell proliferation and elongation. • Future reproductive cells are not set aside early during plant development. Plants produce clusters of undifferentiated, actively dividing cells called meristems throughout their lives. • Plants have tremendous developmental plasticity. Plants can change their development in response to environmental conditions.
Plant Development and Evolution • Members of the MADS box and homeobox families of genes encode transcription factors that regulate developmental processes in both plants and animals. • Plants and animals share many of the genes that regulate their development, even though they have been evolving separately for a long time. • This is in part due to their modular construction which allows different parts of their bodies to change independently of one another.
Plant Development and Evolution • Plants have greater developmental plasticity than animals do because plasticity is especially valuable for a sessile organism. • The combination of repeated production of meristems and developmental plasticity compensates for being sessile.
Environmental Influences on Developmental Patterns • The idea that the environment plays an important role in the development of organisms was downplayed until recently. • Developmental biologists tended to study small organisms that develop rapidly and do not change dramatically under controlled conditions. • It is now known that the development of many organisms is very sensitive to environmental conditions. • A single genotype may encode a range of phenotypes under different environmental conditions.
Environmental Influences on Developmental Patterns • Signals from the environment can be divided into two major types: • Environmental signals that are accurate predictors of future conditions. It is expected that the developmental processes of organisms respond adaptively to these signals. • Environmental signals that are poorly correlated with future conditions. Organisms are unlikely to respond to these signals.
Environmental Influences on Developmental Patterns • Developing organisms respond to signals such as day length, temperature, and precipitation in such a way that the adults they become are adapted to the predicted conditions. • The West African butterfly Bicyclus anynana has a dry-season form and a wet-season form with different wing coloration. • The temperatures experienced during pupation determine which form of adult butterfly will be produced. Temperature influences the expression of the distal-less gene.
Figure 21.9 Development of Eyespots in Bicyclus anynana Responds to Temperature
Environmental Influences on Developmental Patterns • The moth Nemoria arizonaria provides another example of developmental plasticity in response to seasonal changes. • The spring larvae of this moth feed on and resemble oak flowers; the summer larvae feed on oak leaves and resemble small oak branches. • Spring caterpillars have been experimentally converted to summer caterpillars by feeding them oak leaves. • A chemical in the oak leaves probably induces them to develop into the twiglike summer form.
Figure 21.10 The Spring and Summer Forms of a Caterpillar Differ
Environmental Influences on Developmental Patterns • Some organisms need help from another species to complete their development. • For example, house mice raised in microbe-free environments do not have the bacteria that normally colonize their gut. • These gut bacteria induce gene expression in the mouse intestine, which is essential for normal capillary development.
Environmental Influences on Developmental Patterns • Many changes to an organism’s environment are not as certain as signals such as day length or wet- and dry-seasons. • Despite this uncertainty, if the changes have occurred frequently during the evolution of a species, developmental plasticity may allow individuals to respond to them. • The presence or absence of active predators is an example of one of these uncertain environmental signals.
Environmental Influences on Developmental Patterns • Water fleas (Daphnia), for example, increase the size of the “helmets” on the top of their heads when they encounter the predatory larvae of the fly Chaoborus. • Helmet induction occurs if the Daphnia are exposed to water in which the fly larvae have been swimming. • Offspring that are developing in the abdomens of mothers with induced large helmets are born with large helmets. • The tradeoff for this defensive advantage is that Daphnia with large helmets produce fewer eggs.
Figure 21.11 Predator-Induced Developmental Plasticity in Daphnia
Environmental Influences on Developmental Patterns • Tadpoles of the spadefoot toad can respond developmentally if their pond begins to dry up while they are growing. • Some of the tadpoles respond to crowding in a shrinking pond by developing a wider mouth and powerful jaw muscles. • These tadpoles complete their development rapidly before the pond dries up by eating other tadpoles.
Environmental Influences on Developmental Patterns • Plants respond developmentally to light availability. • In low light conditions, plant cells elongate so that the plants become spindly and are more likely to reach a patch of brighter light than if they were to remain compact. • Because they have meristems, plants can continue to respond to light as long as they grow.
Environmental Influences on Developmental Patterns • Organisms generally ignore environmental signals that are poorly correlated with future conditions. • Plants, for example, produce seeds that will germinate in future years with different and unknowable conditions. • Plant seed sizes remain relatively constant in spite of changing environmental conditions. • Seed size is adjusted to the average conditions encountered by plants over many generations.
Environmental Influences on Developmental Patterns • Organisms cannot be expected to have evolved appropriate responses to environmental signals that they have not encountered before. • This is an important problem because human societies have changed the environment in so many ways. • One way humans change the environment is through the release of new chemical compounds. • Understanding how chemicals affect development is important because it may help in the development of less harmful substitutes.
Learning: A Modification of Development • Learning allows an individual to adjust its behavior to the physical, biological, and social environment in which it matures. • Learning is especially important in species with complex social structures, in which individuals must learn the identities and characteristics of their associates and adjust their behavior accordingly. • The field of evolutionary developmental biology is generating many new insights with which to understand the evolution of life.
