Although the limbs of vertebrates have diverged functionally into the wings of bats, the arms of humans, the forelimbs of reptiles and the fins of whales, they are nevertheless homologous: the general skeletal structure is similar in each, despite large differences in individual bone size and shape Fig. In contrast, the common ancestor of humans and fruit flies did not have any limbs, so our limbs and the limbs of the fly are independently evolved and not homologous.
Comparative anatomy of vertebrate limbs. The general skeletal structure of vertebrate limbs is similar in each species, despite large differences in individual bone size and shape reflecting the different functions.
This is because the form of an organism is made during its embryonic development by following developmental programmes encoded in our genes. These programmes need to be adjusted to change anatomy, and adjustments can only be made on what is already there.
Any changes made in earlier developmental programs will have effects on all later programs. This is called developmental constraint [ 1 ]. Insect wings are likely to have evolved from appendages on the exoskeleton of their ancestors that are absent in our lineage, so they cannot be altered to form wings. Although in principle one might evolve to fly in a different way—bats and birds have both independently evolved wings from their forelimbs, what we call convergence—in this case other constraints are in operation.
To evolve wings like those of bats, we would have to lose the current function of our hands and arms, which seems an unlikely evolutionary path to take. When the bones of vertebrates that fly are studied it is clear that they have undergone adaptations to allow flight, with the evolution of hollow or very slender bones.
Morphology is a very useful way of understanding evolutionary processes. Strikingly finch-like beaks could be induced in chick embryos by manipulating these signaling pathways [ 2 ]. Understanding morphology, and how that morphology is created in the embryo developmental biology , can illustrate how it is possible to modify structures and thereby suggest mechanisms that may underlie evolutionary change evodevo. No, while the above examples are compelling examples for the importance of morphological change at the micro level, morphology can be very useful in understanding changes that gave rise to different groups of animals, i.
For example in our lab we are interested in the morphological and developmental changes giving rise to the evolution of mammals. This work involves comparing embryonic development with the fossil record. To understand mammalian evolution we need to be able to accurately identify what defines a mammal—but this is somewhat difficult, especially in evolutionary history as observed in the fossil record.
Most of the specialisations mammals have are shared by other groups, and so are not on their own sufficient to identify a mammal. Mammals belong to the aminote clade—tetrapod vertebrates that protect their developing embryos—either in an egg or in the mother—in a membrane called the amnion.
Other amniotes include the birds and reptiles, and one needs to be able to distinguish mammals from their amniote relatives.
It is likely that the common ancestor of mammals and birds was cold blooded, so the presence of endothermy in these two groups is another example of convergent evolution.
Most mammals have live births; however, some reptile species such as Zootoca vivipara and Pseudemoia entrecasteauxii also give birth to live young, while the extant monotreme species the platypus and two echidna species lay eggs but are still mammals. All mammals produce milk and most have fur, but these features are not useful since they are not usually preserved in fossils. However, a useful defining feature to identify mammals and distinguish them from other amniotes like reptiles and birds is a specialised middle ear and jaw joint—and this is often easier to find in the fossil record.
The ears of reptiles, birds and mammals are made up of three components. These are the outer ear through which sound in the form of vibrating air enters the head, the inner ear in which sound is converted into neuronal signals by vibration of hair cells lining the cochlea, and the middle ear that sits between the two structures.
The middle ear is an impedance matching apparatus that facilitates the transmission of sound from the air low impedance to the liquid filled inner ear high impedance. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material, in the near universality of the genetic code, and in the machinery of DNA replication and expression.
In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences. This is exactly the pattern that would be expected from descent and diversification from a common ancestor. DNA sequences have also shed light on some of the mechanisms of evolution. Key Points Fossils serve to highlight the differences and similarities between current and extinct species, showing the evolution of form over time.
Similar anatomy across different species highlights their common origin and can be seen in homologous and vestigial structures. Embryology provides evidence for evolution since the embryonic forms of divergent groups are extremely similar. The natural distribution of species across different continents supports evolution; species that evolved before the breakup of the supercontinent are distributed worldwide, whereas species that evolved more recently are more localized.
Molecular biology indicates that the molecular basis for life evolved very early and has been maintained with little variation across all life on the planet. Key Terms homologous structure : the traits of organisms that result from sharing a common ancestor; such traits often have similar embryological origins and development biogeography : the study of the geographical distribution of living things vestigial structure : genetically determined structures or attributes that have apparently lost most or all of their ancestral function in a given species.
Evidence of Evolution The evidence for evolution is compelling and extensive. The species depicted are only four from a very diverse lineage that contains many branches, dead ends, and adaptive radiations. One of the trends, depicted here, is the evolutionary tracking of a drying climate and increase in prairie versus forest habitat reflected in forms that are more adapted to grazing and predator escape through running.
Since then, as the number of equid fossils has increased, the actual evolutionary progression from Eohippus to Equus has been discovered to be much more complex and multibranched than was initially supposed. Detailed fossil information on the rate and distribution of new equid species has also revealed that the progression between species was not as smooth and consistent as was once believed. Although some transitions were indeed gradual progressions, a number of others were relatively abrupt in geologic time, taking place over only a few million years.
The series of fossils tracks the change in anatomy resulting from a gradual drying trend that changed the landscape from a forested habitat to a prairie habitat. Early horse ancestors were originally specialized for tropical forests, while modern horses are now adapted to life on drier land.
Successive fossils show the evolution of teeth shapes and foot and leg anatomy to a grazing habit with adaptations for escaping predators. The horse belongs to the order Perissodactyla odd-toed ungulates , the members of which all share hoofed feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure.
This means that horses share a common ancestry with tapirs and rhinoceroses. Later species showed gains in size, such as those of Hipparion , which existed from about 23 to 2 million years ago. The fossil record shows several adaptive radiations in the horse lineage, which is now much reduced to only one genus, Equus , with several species. Homology is the relationship between structures or DNA derived from the most recent common ancestor. A common example of homologous structures in evolutionary biology are the wings of bats and the arms of primates.
Although these two structures do not look similar or have the same function, genetically, they come from the same structure of the last common ancestor. Homologous traits of organisms are therefore explained by descent from a common ancestor. If we go all the way back to the beginning of life, all structures are homologous! Homology in the forelimbs of vertebrates : The principle of homology illustrated by the adaptive radiation of the forelimb of mammals.
All conform to the basic pentadactyl pattern but are modified for different usages. The third metacarpal is shaded throughout; the shoulder is crossed-hatched. In genetics, homology is measured by comparing protein or DNA sequences. Homologous gene sequences share a high similarity, supporting the hypothesis that they share a common ancestor.
Homology can also be partial: new structures can evolve through the combination of developmental pathways or parts of them. As a result, hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and some of shoots.
Homologous sequences are considered paralogous if they were separated by a gene duplication event; if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous. A set of sequences that are paralogous are called paralogs of each other. Paralogs typically have the same or similar function, but sometimes do not.
It is considered that due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions. Homology vs. This is because they are similar characteristically and even functionally, but evolved from different ancestral roots. Paralogous genes often belong to the same species, but not always. For example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are considered paralogs. This is a common problem in bioinformatics; when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.
The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately rather than being present in the last common ancestor. Bat wings and bird wings evolved independently and are considered analogous structures.
Genetically, a bat wing and a bird wing have very little in common; the last common ancestor of bats and birds did not have wings like either bats or birds. Wings evolved independently in each lineage after diverging from ancestors with forelimbs that were not used as wings terrestrial mammals and theropod dinosaurs, respectively. It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons.
In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm not a wing in the last common ancestor of tetrapods. Analogy is different than homology. Although analogous characteristics are superficially similar, they are not homologous because they are phylogenetically independent. Analogy is commonly also referred to as homoplasy. Convergent evolution occurs in different species that have evolved similar traits independently of each other.
Sometimes, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. Convergent evolution describes the independent evolution of similar features in species of different lineages.
The two species came to the same function, flying, but did so separately from each other. Both sharks and dolphins have similar body forms, yet are only distantly related: sharks are fish and dolphins are mammals.
Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming have been favored. Thus, over time, they developed similar appearances morphology , even though they are not closely related.
One of the most well-known examples of convergent evolution is the camera eye of cephalopods e. Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye.
Eye evolution : Vertebrates and octopi developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot 4 where the nerves pass through the retina. This means that octopi do not have a blind spot. Convergent evolution is similar to, but distinguishable from, the phenomenon of parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics; for example, gliding frogs have evolved in parallel from multiple types of tree frog.
Traits arising through convergent evolution are analogous structures, in contrast to homologous structures, which have a common origin, but not necessarily similar function. The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies.
Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions.
The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes.
Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs. Another example of a structure with no function is the human vermiform appendix.
These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings which may have other functions on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals.
Vestigial appendix : In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function. There are also several reflexes and behaviors that are considered to be vestigial. Prior to canalization, unless all of the phenotypes swept over by an individual in development keep the robot motionless, there will be intervals of relatively superior and inferior performance. Evolution can thus improve overall fitness in a descendant by lengthening the time intervals containing superior phenotypes and reducing the intervals of inferior phenotypes.
However, this is only possible if such mutations exist. We have found here that such mutations do exist in cases where evolutionary changes to one trait do not disrupt the successful behavior contributed by other traits.
For example, robots that exhibited the locally optimal trotting behavior Fig. Brief ontogenetic periods of rolling behavior Fig. The key observation here is that only phenotypic traits that render the agent robust to changes in other traits become assimilated, a phenomenon we term differential canalization. This insight was exposed by modeling the development of simulated robots as they interacted with a physically realistic environment.
Differential canalization may be possible in disembodied agents as well, if they conform to appropriate conditions described in Supplementary Discussion. This finding of differential canalization has important implications for the evolutionary design of artificial and embodied agents such as robots.
Computational and engineered systems generally maintain a fixed form as they behave and are evaluated. However, these systems are also extremely brittle when confronted with slight changes in their internal structure, such as damage, or in their external environment such as moving onto a new terrain 25 , 26 , Indeed, a perennial problem in robotics and AI is finding general solutions which perform well in novel environments 28 , Our results demonstrate how incorporating morphological development in the optimization of robots can reveal, through differential canalization, characters which are robust to internal changes.
Robots that are robust to internal changes in their controllers may also be robust to external changes in their environment Thus, allowing robots to change their structure as they behave might facilitate evolutionary improvement of their descendants, even if these robots will be deployed with static phenotypes or in relatively unchanging environments. Our approach addresses this challenge, because differential canalization provides a mechanism whereby static yet robust soft robot morphologies may be automatically discovered using evolutionary algorithms for a given task environment.
Furthermore, future soft robots could potentially alter their shape to best match the current task by selecting from previously trained and canalized forms. This change might occur pneumatically, as in Shepherd et al.
We have shown that for canalization to occur in our developmental model, some form of paedomorphosis must also occur. However, there are at least two distinct methods by which such heterochrony can proceed: progenesis and neoteny. Although a superior phenotype can materialize anywhere along the ontogenetic timeline, late onset mutations are less likely to be deleterious than early onset mutations. This is because our developmental model is linear in terms of process, and interfering with any step affects all temporally-downstream steps.
Since the probability of a mutation being beneficial is inversely proportional to its phenotypic magnitude 32 , mutational changes in the terminal stages of development require the smallest change to the developmental program. Indeed progenesis was observed most often in our trials Fig. Finally, we would like to note the observed phenomenon of increased plasticity prior to genetic assimilation. Models of the Baldwin effect usually assume that phenotypic plasticity itself does not evolve, although it has been shown how major changes in the environment can select for increased plasticity in a character that is initially canalized In our experiments however, there is no environmental change.
Despite great interest in sensitive periods, the adaptive reasons why they have evolved are unclear In our model, increasing the amount of morphological development increases the chance of capturing an advantageous static phenotype, which can then be canalized, once found.
However, a phenotype will not realize the globally optimal solution by simply maximizing development. This would merely lengthen the line on which development unfolds in phenotypic hyperspace n -dimensional real space. The developmental model described herein is intentionally minimalistic in order to isolate the effect of morphological and neurological change in the evolutionary search for embodied agents.
The simplifying assumptions necessary to do so make it difficult to assess the biological implications. For example, we model development as an open loop process and thus ignore environmental queues and sensory feedback 36 , We also disregard the costs and constraints of phenotypic plasticity 38 , By removing these confounding factors, we hope these results will help generate novel hypotheses about morphological development, heterochrony, modularity and evolvability in biological systems.
For smaller voxels, it is necessary to implement damping into their actuation to avoid simulation instability. Current length is broken down into its constituent parts for a single voxel, under each treatment, in Supplementary Fig. We employed a standard evolutionary algorithm, Age-Fitness-Pareto Optimization 40 , which uses the concept of Pareto dominance and an objective of age in addition to fitness intended to promote diversity among candidate designs.
A single evolutionary trial maintains a population of thirty robots, for ten thousand generations. Every generation, the population is first doubled by creating modified copies of each individual in the population.
The age of each individual is then incremented by one. Next, an additional random individual with age zero is injected into the population which now consists of 61 robots. Finally, selection reduces the population down to its original size 30 robots according to the two objectives of fitness maximized and age minimized.
We performed the above procedure thirty times to produce thirty independent evolutionary trials. That the number of trials is the same as the population size within each trial is an admittedly confusing coincidence. The mutation rate is also evolved for each voxel, independently, and slightly modified every time a genotype is copied from parent to child. These 48 independent mutation rates are initialized such that only a single voxel is mutated on average. There is a negligible difference between Evo and Evo-Devo in terms of the expected number of parent voxels modified during mutation Supplementary Fig.
The amount of development in a particular voxel can range from zero in the case that starting and final values are equal to 1.
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