These proteins combine to make more complex chemicals like amino acids and enzymes, needed for the organism to function. Which of the following is correct in order of decreasing size? Cell, nucleus,chromosome, gene. Gene, chromosome, nucleus, cell. Nucleus, cell, gene, chromosome. Cell, chromosome, gene, nucleus. A lot of candidates mix up the size of chromosomes and genes, genes are small parts of chromosomes.
Which of the following could account for body mass? Genetic reasons only. Environmental reasons only. Genetic and environmental reasons. Genes can give one organism a greater body mass than another but environmental factors like diet can create differences too. Gametes contain only half the number of chromosomes other types of cells have, therefore they are what?
Diploid cells contain a full set of chromosomes. Why is plotting a line graph of foot length more appropriate than using a bar chart? Foot length is an example of continuous variation. Feet are different shapes.
Foot length is an example of discontinuous variation. Shoe size depends on foot length. Continuous variation is variation that has no limit on the value that can occur within a population. A line graph is used to represent continuous variation. Had the question been about foot shoe size rather than foot length, the answer would have been a histogram.
Foot shoe sizes have only certain values. Eye colour is an example of which of the following? Continual variation. Continuing variation. Discontinued variation. Discontinuous variation. During early speciation, when two different species are just beginning to break away from one another, reproductive incompatibility can be incomplete and "leaky"—some part of the genome may still be compatible and exchangeable.
If the species interbreeds and this selfish gene is able to be passed down, instead of becoming incompatible, "that part of the genome will become perfectly exchangeable.
In some cases a selfish gene will basically erase the build-up of incompatibilities for a part of the genome. That is, meiotic drive elements can cause incompatibilities between species if they do not experience gene flow, or they can cause a convergence of the species, if they species does experience gene flow. A major factor in determining whether or not a species is compatible hinges on whether or not there is gene flow between the species , Presgraves says. More from Biology and Medical.
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January 3, The researchers use genetic markers to track segments of the X chromosome that they move from one species of Drosophila fruit fly into a different species in order to find X-linked genes that cause male sterility.
Genetic markers that affect eye color are located on the X chromosome, so the researchers start with Drosophila mauritiana that have two genetic markers—giving them dark red eyes, left—and cross them to white-eyed Drosophila simulans. Explore further. More information: Colin D Meiklejohn et al. Gene flow mediates the role of sex chromosome meiotic drive during complex speciation, eLife DOI: Provided by University of Rochester.
Citation : What makes two species different? This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. Kappeler et al. They emphasised how variation in the components of social systems can depend on the mating system and on social organisation. In my book with Peter Gluckman, we also reviewed the plasticity literature and discussed the many different forms of plasticity, how they might have evolved and how they might affect subsequent evolution Bateson and Gluckman, Strikingly, all these extensive and varied contributions to the subject of biological plasticity overlap relatively little in terms of the scientific literature that they cover.
Anybody who wants to be completely up-to-date has a lot of reading to do in the behavioural field alone. In this article, I shall not go over ground that I have covered in detail in previous publications for example, Bateson, , I shall then consider mechanisms that appear to have evolved from repeated challenges from the environment. They do so by responding in a conditional manner so that if the environment is A, the organism gives response X, and if it is B, the organism gives response Z.
An individual whose body has been damaged in an accident or who is burdened with a mutation that renders its body radically different from other individuals may be able to accommodate to such abnormality West-Eberhard, In doing so, the individual may develop novel structures and behaviour not seen in other individuals of the same species.
Such accommodation can be particularly marked when it occurs early in development. A goat Capra aegagrus hircus born without forelimbs walked about on its hind legs and developed a peculiar musculature and skeleton Slijper, The animals have coped with an abnormality by accommodating to it, producing coordinated changes in functionally related characters.
Similarly, humans born with limb abnormalities as a result of exposure to a teratogen such as thalidomide develop strategies to cope, for example, by handling objects using their feet or teeth in ways for which others might use their hands von Moltke and Olbing, The capacity of the individual to respond to neural damage is remarkable particularly when the damage occurs early in life Bateson and Gluckman, In such cases, the brain reorganises and morphologically can look markedly different from the brain of a normal individual.
Even so, the effects on behaviour may be scarcely detectable and the plasticity at the neural level may be accompanied by robust development at the behavioural level. Although these responses may involve either structural or temporal changes in the course of development, in contrast to phenotypic accommodation, they do not entail a fundamental change in the normal pattern of development.
Thus, the phenotypic consequences are not as marked as those that involve accommodation, but may have a cost and become disadvantageous to the individual later in life Gluckman et al. If the mother is undernourished or if the placenta is not delivering optimal nutrition, the offspring may be born smaller than usual, with the consequences of greater infant mortality and lower fitness in later life resulting from persistent growth failure Gluckman et al.
In polygynous species, such as red deer Cervus elaphus , the fitness costs may be severe because a small male is less able to compete with larger males for mates and, as a consequence, has a much lower chance of fathering offspring. Nevertheless, survival means that the small male does have some possibilities for mating unobtrusively when larger males are not looking Kruuk et al. In humans, growth retardation following placental insufficiency may be associated with reduced muscle mass, bone density, adult size, and cognitive and attentive function.
These neurological effects may be related to a trade-off between investing for the long term in neural capacity and the need to expend the limited energetic supply for immediate survival Gluckman and Hanson, One of the most primitive changes in behaviour in response to experience is nonspecific. Sensitisation usually results from exposure to an alarming stimulus such as a blow-up toy snake suddenly becoming inflated , which elicits a variety of defensive or aversive reactions from the animal.
Subsequently, many other potentially aversive stimuli such as loud sounds will have the same effect even though this would not have been the case had the animal not been previously sensitised.
Thorpe classified learning into five categories: habituation, classical conditioning, instrumental conditioning, latent learning and insight learning. Some forms of learning such as behavioural imprinting, which Thorpe discussed in his chapter on insight learning, and the acquisition of song in birds may be restricted to early development, but most can take place throughout life.
Habituation is defined as a decrease in response occurring as the result of prolonged stimulation, which cannot be attributed to fatigue or sensory adaptation. The phenomenon has been described widely from single-celled organisms to humans. In some cases, the underlying process is simple, and in other cases, the experiments suggest that the subject establishes a specific representation of the stimulus in its nervous system.
Establishing a neural representation is key to the form of learning that leads to a categorisation of the sensory world. Here again such perceptual learning is found widely across the animal kingdom. In humans, it leads to the recognition of faces and places. The ability to distinguish between the vast array of objects, people and scenes experienced in a lifetime is of inestimable value and happens simply as a result of exposure.
Pavlovian or classical conditioning allows the individual to predict what will be of real significance in the confusing world of sights, sounds and smells. A different form of associative learning enables the organism to control the environment. The so-called cognitive revolution in the study of behaviour has undoubtedly greatly enriched the toolbox for understanding learning processes and is very well summarised by Shettleworth In the immune system of humans and vertebrate animals, molecular plasticity takes the form of generating new antibodies to foreign proteins that hitherto have not been encountered by the individual.
Antibodies are immunoglobulins used by the immune system to identify and neutralise foreign pathogens such as bacteria and viruses, preventing them from causing disease. The plasticity of the immune system involves selection rather than instruction as, by extremely rapid mutation and recombination, the immune system finds a match for the foreign antigen and this then sets in train rapid synthesis of the antibody from the mutated gene that provided the match.
Responses to predation or variation in food resources have provided some of the best examples of conditional responses to local environmental conditions. Some plastic responses induced in early life may have delayed benefits, so that their primary or only adaptation is expressed at a much later stage in the life cycle.
Such anticipatory responses rely on a cue in early life predicting some characteristic of the future environment. The implication of many such examples is that environmental induction provides a forecast about the conditions of the world that the individual will subsequently inhabit Bateson, In mammals, the best route for such a forecast may be via the mother.
Vole pups Microtus pennsylvanicus born in the autumn have thicker coats than those born in spring; the cue to produce a thicker coat is provided by hormonal signals from the mother before birth Lee and Zucker, The potential benefit of doing so was termed the predictive adaptive response by Gluckman and Hanson Saastamoinen et al.
When pregnant mother rats Rattus norvegicus were given restricted diets, their offspring were smaller when they were born, but if these offspring were then given plentiful food they became much more obese than the offspring of mothers given an unrestricted diet Jones and Friedman, This observation was followed by further extensive work on rats in many laboratories. Offspring born to undernourished rats developed increased appetites Vickers et al.
Even though the undernourished rats are more sedentary when kept in standard laboratory cages Vickers et al. When given a choice between pressing a lever to obtain food and running in a wheel, they are significantly more likely to run in the wheel Miles et al. This finding suggests that these offspring of undernourished mothers may attempt to find more reliable sources of food in a natural environment. Human children with lower birth weights are likely to enter puberty early Sloboda et al.
The individual relatively undernourished in early life has a preference for high-fat foods, a higher set-point for satiety and a smaller somatic phenotype—a suite of characteristics that well are adapted to limited food resources in adult life. If human foetuses respond to nutritional cues provided by their mothers, then those individuals who experience cues that indicate a plentiful environment should be adversely affected if they encounter famine later in life.
Indeed, the evidence suggests that people who enjoyed a plentiful environment in early life may be at greater risk during periods of prolonged famine than those who experienced relatively lower levels of nutrition in utero. In concentration camps and the worst prisoner-of-war camps, many reports have indicated that the physically large individuals died first while at least some of the small individuals survived Bateson, In an Ethiopian population suffering from famine, high birth weight of babies who had had mothers on a higher plane of nutrition was associated with a ninefold higher risk of rickets, which carries fitness costs during reproduction, particularly for women Chali et al.
Children born smaller are less likely, in a famine, to develop kwashiorkor, the form of infant malnutrition with high mortality that involves a lower ability to mobilise substrates. In contrast, the low-birth-weight children respond to severe undernutrition by developing marasmus, which is associated with much lower mortality Jahoor et al.
Maternal forecasting by induction of a specific developmental trajectory is thought by many researchers to be important in human biology for example, Bateson, ; Gluckman and Hanson, ; Sandman et al. The individual benefits, it is argued, by adjusting the trajectory of his or her development so that the developed phenotype is most likely to match the anticipated environment.
In general, a cue from the mother suggesting a future environment with relatively scarce resources leads to a more economical body form and a bias towards insulin resistance, thereby capturing the higher-energy fat-dense foods when they are available Gluckman et al. A full discussion of the predictive adaptive response in humans is provided elsewhere Bateson et al.
Although the evidence provides strong grounds for supposing that humans exhibit conditional plasticity, extreme nutritional impoverishment of the mother can have long-term effects that are maladaptive Gluckman and Hanson, Plasticity takes many different forms. The processes involved in the start of a sensitive period generally correspond with changes in the ecology of the developing individual. These changes are linked to developmental processes of regulation and cellular replication.
The processes that bring the sensitive period to an end may reflect the passage of normal growth and temporal constraints on development in other related processes. Sometimes the terminating processes are related to the gathering of crucial information and, except in extreme circumstances, do not shut down until that information has been gathered.
In these cases, the ending of the sensitive period reflects the variable opportunities for gathering such domain-specific information in the real world. For example, in cold weather, ducks brood their hatched young for longer than in warm weather, and the ducklings delay the process of learning the characteristics of their mother Bateson and Martin, A limit must be set on such flexibility, however, because so much else has to be done in development.
If the relevant information remains unavailable for too long, the individual may eventually have to settle for less than the best and develop without acquiring that information. These sexual preferences are for partners that are slightly different from those individuals usually close kin in natural conditions with which the animal is already familiar Bateson, Although the preferences are generally robust, they can change under special conditions such as those associated with high levels of stress Bateson and Martin, The acquisition of songs by birds also starts early in life.
The typical pattern is for the young male bird to listen to and memorise sounds made by his father and other males during the first few months after he has hatched. The following spring he produces a range of sounds and, by degrees, settles on songs he has heard before. When he is mature, he uses his songs to defend his territory and attract females. Although avian song-learning often occurs early in development and is irreversible, in some species such as the canary Serinus canaria , the repertoire changes in each breeding season, and in others, such as the European blackbird Turdus merula , it is added to each season Marler and Slabberkoorn, Examples of plasticity include coping with disruption of normal development, different phenotypic outcomes generated by different cues early in development, learning and the plasticity found in the nervous and the immune systems.
A key question is whether these vastly heterogeneous phenomena have anything in common with each other. Plasticity operating at different levels of organisation often represents different descriptions of the same process.
Underlying behavioural plasticity is neural plasticity, and underlying that is the molecular plasticity. The plasticity of the immune system, with its long-lasting effects similar to memory, relies on a selective process in development.
In contrast, at the level of the whole organism, the processes of learning that change behaviour seem to involve instruction. Whether the same selective process could be involved in any or all of the myriad examples of learning is, however, much more controversial. In the case of associative learning, for example, a cue from the environment instructs the individual about the causal nature of its environment, providing a link between something that is biologically significant and something that had hitherto been neutral.
As yet, the underlying mechanism involving changes in neural connectivity is not readily attributable to a process that involves selection. That said, the process of strengthening reinforcement of one of many different possible actions has often been likened to the Darwinian process of variation, differential survival and inheritance in evolution Skinner, ; Snell-Rood, ; Heisenberg, In many cases of conditional plasticity that abound across the animal and plant kingdoms, the individual starts its life with the capacity to develop in a number of distinctly different ways.
The individual has the potential to express a phenotype that is adaptive in the appropriate context. The particular phenotype is triggered by a feature of the environment in its environment—whether it is the odour of its predators, the available quality of food or the presence of other males. Whether or not the numerous examples of plasticity are necessarily or even plausibly related, they are all of great biological and psychological significance even when they refer to pathologies.
Plasticity can be viewed in many ways and along many different dimensions. The temporal dimension, the dimension of different organisational levels, the mechanistic dimension of whether plasticity involves selection or instruction and the functional and evolutionary issues are all part of the picture.
A multidimensional view is essential if the ways in which the organism responds to environmental cues and challenges are to be understood.
However, the understanding needs to be broadened to take account of the ways in which plasticity is constrained and regulated.
A substantial body of evidence indicates, then, that individuals of the same species, the same age and the same sex may differ strikingly in their phenotypes. More recently, epigenetics has become mechanistically defined as the molecular processes by which traits defined by a given profile of gene expression can persist across mitotic cell divisions, but which do not involve changes in the nucleotide sequence of the DNA Carey, Developmental biology had already paved the way for understanding what is an obvious feature of ontogeny.
While all cells within the body of a multicellular organism contain the same genetic sequence information, each lineage has undergone specialisations to become a skin cell, hair cell, heart cell and so forth. These phenotypic differences are inherited from mother cells to daughter cells. The process of differentiation involves the expression of particular genes for each cell type in response to cues from neighbouring cells and from the extracellular environment, and the suppression of others.
Genes that have been silenced at an earlier stage remain silent after each cell division. Such control of gene expression provides each cell lineage with its specific characteristic. As these epigenetic marks are faithfully duplicated across cell division, stable cell differentiation results. The general principles of differentiation apply at higher levels of organisation and are involved in mediating many aspects of developmental plasticity seen in intact organisms. The processes involved in gene expression and suppression can be transmitted from one generation to the next.
Before this was demonstrated, it was already apparent that extragenetic inheritance processes are important in inheritance. These include cytoplasmic effects, parental effects, ecological legacies, behavioural traditions and cultural inheritance Mousseau and Fox, ; Odling-Smee et al. Many of the factors that influence individual development, be they social or ecological, have been amassed by the activities of multiple individuals over multiple generations cultural knowledge, ecological legacies.
Some of these influences on development may stretch back a long way in time. The presence of animal burrows, moundsand dams—or, on a larger scale, changed atmospheric states, soil states, substrate states, or sea acidity Meysman et al.
The evidence for transmission of the epigenome across generations in both animals and plants is substantial Gissis and Jablonka, After mother rats had been injected with an endocrine disruptor, lowered spermatogenic capacity and several adult-onset diseases were observed over four successive generations in their male descendants.
These effects were accompanied by altered DNA methylation patterns in the germline Anway et al. In the plant Arabidopsis , epigenetic inheritance can occur over at least eight generations Johannes et al. In the nematode worm Caenorhabditis elegans , stable transmission across generations depends on gene regulation by microRNA Rechavi, Rassoulzadegan found that injection of RNA from sperm from one strain of mice into wild-type embryos led to a distinct phenotype in the offspring, which was in turn transmitted to their progeny.
Mouse embryos injected with a microRNA that targets an important regulator of cardiac growth developed hypertrophy of the cardiac muscle, which was passed on to descendants through at least three generations without loss of effect Wagner et al. The costs of evolving plastic processes are not known, even though the costs of maintaining them are likely to be low van Buskirk and Steiner, Plasticity is conserved across all multicellular taxa Bateson and Gluckman, Given that evolutionary change in response to Darwinian evolution can be rapid, this suggests that a fitness advantage exists to sustaining plasticity in some aspects such as those affected by normal variation in levels of nutrition, predation and stress.
Adaptability plasticity may confer the potential to cope with a wider range of environments than would otherwise be possible, and also to sustain fitness when environmental conditions fluctuate, particularly when the environment changes relatively slowly. Sultan et al. They demonstrated that offspring of the generalist species showed adaptive and plastic responses to drought, such as larger root systems, which were not found in the offspring of the species that was specialised to live exclusively in a moist environment.
Some forms of plasticity involve rapid responses to environmental challenges. Learning processes in all their different manifestations provide the obvious examples. The adaptive advantages of learning and memory confer additional capacity to recognise and avoid predation, to identify nutritional resources, to undertake sexual reproduction and so on.
The evolution of learning mechanisms evidently began early in the history of life and may be uncovered by a comparative approach. The process of sensitisation to environmental conditions is found in organisms that are extremely ancient. Even this form of plasticity found in simple organisms requires the ability to categorise information provided by the environment.
A process similar to behavioural imprinting in birds has been described in the nematode worm C. The worms respond preferentially to food odours to which they have been exposed in early life. With such a capacity to distinguish between different types of stimuli in place, a plausible case can be made for the evolution of associative learning from sensitisation Wells, ; Kandel and Schwartz, East African Acridoid grasshoppers deposit black melanin in their cuticle if the reflectance of the ground is low when they hatch out, as it would be after a savannah fire Rowell, As a consequence of this mechanism, most grasshoppers are green in periods without fires and most are black in periods after fires when the savannah is blackened.
The question therefore arises: under what conditions would the plasticity of the grasshopper evolve? If fires were infrequent and quickly followed by rain, it might be disadvantageous ever to be black. Conversely, if fires were frequent it might be advantageous to remain black at all times. In between these two extreme conditions, it would be highly advantageous to be capable of matching colour to the background.
Whether or not the plasticity evolved would depend on whether individuals appeared that were capable of making the switch. If plasticity appeared and spread through the population, it might disappear again if the probability of fires dropped and an energetic cost was associated with the propensity to be plastic. In the case of the freshwater crustacean, Daphnia pulex , Darwinian evolution has provided the young animals, still developing within the brood pouch of their mother, with the capacity to anticipate future conditions.
The presence of a predatory midge in the water causes the young to form a defensive helmet and long tail spine. In the absence of the midge or, more accurately, the chemical remains of Daphnia killed by the midge , the young do not develop the armour.
The benefit of not doing so is that the non-helmeted females are able to devote their resources to making many more eggs in adulthood Laforsch et al. The trade-off between forming a helmet or not is between present survival and future reproduction. In the presence of the predator, the balance swings towards devoting resources to improving the chances of survival, and in the absence of the predator the balance swings the other way towards producing more offspring.
The maintenance of the capacity for such flexibility will depend on historical conditions. If predators had always been present and the capacity for changing the course of development carried a cost, this capacity would almost certainly have been lost. Conversely, it would almost certainly have been lost if predators that could be deterred by armour were never present. In the Pennsylvanian meadow, vole coat thickness in the offspring is dependent on whether the mother experiences lengthening or shortening periods of daylight during pregnancy Lee and Zucker, The mechanism is plausibly an evolved adaptation arising from correlated seasonal changes in temperature.
Those mothers that did not signal the future conditions of the environment to their unborn offspring would have had a lower reproductive success than those mothers that did.
Correspondingly, those offspring that failed to respond to the maternal cue would have been less likely to survive after birth than the responsive ones. Where the cue is regular, such as seasonal change, it will have high fidelity.
Selecting an appropriate developmental trajectory in response to the cue carries little risk and much advantage. When the cue reliability is less than perfect, the evolutionary benefit of a plastic response to environmental conditions must be greater than the cost of producing an inappropriate phenotype Moran, ; Lachmann and Jablonka, ; Sultan and Spencer, The delay between the detection of an inductive cue and the full expression of the phenotype is sometimes lengthy, as in the human case.
A complex body cannot be built in a trice and adaptations to particular environmental conditions are often complex. This lag explains, in part, why a phenotype—once developed—cannot be readily changed and a mismatch of phenotype to environment can therefore arise if the forecast of local conditions proves incorrect. A further conceptual issue concerns the optimal time lag between detecting a cue that predicts a given set of environmental conditions and the phenotypic response to that cue. A hasty response might mean that conditions could change again before the adaptation becomes relevant.
Left too late, and the capacity for plastic change might be exhausted or the adaptation may not be developed in time to be effective. Conditional plasticity may confer the potential to cope with a wider range of environments than would otherwise be possible, and also to sustain fitness when environmental conditions fluctuate, particularly when the environment changes relatively slowly.
When environments remain constant over long periods of time, the benefits of developmental plasticity are lost. The likelihood of loss would become greater if the underlying mechanisms of developmental plasticity were energetically costly to maintain. A central question in considering evolutionary change driven by plasticity is whether the transmitted epigenetic markers could facilitate genomic change Johnson and Tricker, Many authors have argued that the plasticity of epigenetic processes provides a substrate of phenotypic variation on which Darwinian evolution can act Pigliucci, , West-Eberhard, , Moczek et al.
Giving a central role to development in evolutionary processes has prompted researchers to wonder whether, and how, developmental systems fashion evolutionary outcomes. In most experimental studies, the environmental stimulus producing an epigenetic change is only applied in one generation. This might be enough since work on yeast Saccharomyces cerevisiae suggests that an environmental challenge can permanently alter regulation of genes Braun and David, In natural conditions, the environmental cues that induce epigenetic change may be recurrent and repeat what has happened in previous generations.
This recurring effect might stabilise the phenotype until genomic reorganisation has occurred Jablonka and Raz, ; Bateson and Gluckman, Alternatively, the induced epigenetic changes that mediate adaptive plasticity might have biased the sites of subsequent mutation Jablonka and Lamb, ; Bateson and Gluckman, ; Bateson, Variation at these sites may throw up phenotypes, some of which are adaptive and subject to Darwinian evolution.
This is one way that plasticity might lead to evolutionary change. Organisms and proto-organisms were arguably immobile in the initial stages of biological evolution. However, as they evolved, they would soon have become active. Although migration can be highly adaptive, the possibility of movement into a novel environment raises a key conceptual point in understanding how plasticity and behaviour can drive evolutionary change. Development depends on the constancy of many genetic and environmental conditions.
If any of these conditions changes, as can happen to environmental conditions when organisms are mobile, the characteristics of the organism can also change.
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