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The evolutionary process in bird wings, especially with regard to winglets

The evolutionary process in bird wings, especially with regard to winglets



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In this answer on aviation.SE a comparison is made between the shapes of airplanes wings and the shapes of birds wings. It concludes with the following remark:

After all, no bird has winglets. Not a single one.

In addition to be a disputable assertion (the wing tips such as the eagle's could be considered akin to a "winglet" of the fanned type) this has stricken me as based on a quite wrong assumption of how evolution works.

I tried to make my point in the comments only to reach this point:

So you consider evolved wings as not mature. The winglet modification is just waiting to happen? Nature never tried it, in >100 million years of biological flight? Could be, yes. But is extremely unlikely. That settles it for me.

Am I correct in identifying this in a wrong interpretation of the evolutionary process?

As I understand the evolutionary process, the current bird wings are not necessarily perfect, are simply the version that so far has given the best advantage. The lack of "winglets" in birds cannot then be explained simply by assuming that they do not improve the wing, but it could also be that there has never been an evolutionary pressure to evolve them or that since birds flap their wings they would be detrimental instead of beneficial or whatever other reason.

Is my understanding of the evolutionary process correct? if not, where am I at fault?


As a small addendum, another user cited the "Spandrels" in comparison to the above debate, could someone explain what could have been the meaning of the comparison?


I looked up winglets so I had context for this answer. I'm interpreting winglets as the vertical tips at the end of airplane wings. If so, then you are correct. The spread primary feathers of soaring birds like eagles function as winglets (Tucker 1993). Airbus has a biomimicry web page devoted to some of the biological designs, including winglets, they incorporated into their airplane designs. Some studies suggest airplant winglets do increase efficiency (e.g., Hossain et al. 2011), but there is still some debate.

From the aviation.se answer:

Look at birds: They use two different wingtip designs. I guess we will both agree that those designs are mature after millions of years of evolution. And still there are two distinct wing tip shapes: One is the "fingered" wingtip with large, spread-out feathers, and one is the pointed tip you find from seagulls to albatrosses. Why is there not one, mature design?

This has to do with the environment they are used in. Seabirds fly in an unobstructed environment with steady winds and need to stay aloft for longer duration. All other birds have to cope with trees obstructing a straight flight path and gusts from hills, or those trees. They need to maneuver quickly and cope with gusty winds. This is helped by a reduced span and the possibility to fold the outer wing in or fan it out in an instance. Hence the fingered wingtip.

Although bird wings do tend to match his descriptions, he is not correct that these correspond entirely to ocean-going birds vs all other birds. Many inland birds have pointed wings, including swallows, nightjars, swifts, and falcons. Some seagoing birds have winglets, such as pelicans and cormorants. While wing type is shaped by natural selection in a given environment, it also reflects the evolutionary history of the birds.

There is not "one, mature design" because wings many different shapes and sizeshave evolved under varying selective pressures. For example, the long pointed wings of ocean-going albatrosses are excellent for gliding with little energy expenditure but they are lousy for taking flight or for landing. Other wings are shorter and broader, providing greater maneuverability in the woods. In this sense, there is no perfect wing. Each has evolved for different evolutionary reasons.

The main question

From the aviation.SE thread:

the fact that no random mutation has produced a winglet* in birds is in no way a demonstration that airplanes do not benefit from them, there is no logical relationship. [*even though the "fanned" design you refer, such as the eagle's, can be considered a form of optimum winglet] - Federico

followed by this response:

So you consider evolved wings as not mature. The winglet modification is just waiting to happen? Nature never tried it, in >100 million years of biological flight? Could be, yes. But is extremely unlikely. That settles it for me…

Federico is correct. The basic wing type was set long ago as birds evolved from non-avian dinosaurs. Comparisons of modern bird wings with fossils suggest that the basic structure of bird wings has not changed very much since then. The current structure may be the best that has evolved by natural selection so far but that does not mean the wing cannot be improved further. As you noted, mutations could occur in existing genes that lead to further improvements of the structure and function of the wings. If those mutations never occur, then the improvements won't occur. This is one constraint of the evolutionary process (Hoffman 2014). Natural selection can only work with existing traits, using existing genes and genetic variation.

Another consideration is that wings for powered flight have evolved independently at least four times (birds, bats, insects, pterosaurs). Each time, a different type of wing evolved. Is one better than another? Bat wings have aerodynamic properties very different from birds but function very well for bats (Hedenstrom et al. 2009). Along the lines of your argument above, each wing type perhaps is (or was for pterosaurs) optimum given the environmental circumstances and evolutionary constraints. That doesn't mean they could not be improved. If a wing evolves again in some organismal group in the distant evolutionary future, odds are very good that the structure will be different from those that have evolved so far. Perhaps that wing will be even more efficient than current wings.

Spandrels

Another user cited the "Spandrels" in comparison to the above debate, could someone explain what could have been the meaning of the comparison?

The use of "spandrels" in evolutionary biology stems back to a well-known paper by Stephen Jay Gould and Richard C. Lewontin called "The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptionist programme", published in 1979 (Gould and Lewontin 1979). A spandrel is the space that forms between two adjacent arches or between an arch and a rectangular shape. The spandrel emerges as a result of putting two arches next to each other and filling the space. It results from the joining of two arches. It was not specifically designed as part of the arch. Gould and Lewontin claim that evolutionary biologists are too quick to assign adaptive value to every trait on an organism, and argue that not every trait results directly from adaptation. Instead, they say, the trait may be a "spandrel" that emerged as a by-product of other traits. The "spandrel trait" has no adaptive value in and of itself.

The use of spandrels in the comments of the aviation discussion is not clear to me. Clearly, the person intended the evolutionary meaning, mentioning the paper by name. But I cannot tell at all the person's intent.

Edits: Incorporated comments by @Federico and @Andrew - thanks. I corrected horrendous typos and attempted to add clarity.

Citations

Hedenstrom, A. et al. 2009. Bird or bat: comparing airframe design and flight performance. Bioinspiration & Biomimetics 4 015001. doi:10.1088/1748-3182/4/1/015001

Hoffmann, A.A. 2014. Evolutionary limits and constraints. Pp. 247-252. In: The Princeton Guide to Evolution, Princeton University Press, USA.

Hossain et al. 2011. Drag analysis of an aircraft wing model with and without bird feather like winglet. International Journal of Mechanical, Aerospace, Industrial and Mechatronics Engineering 5: 30-35.

Tucker, V.A. 1993. Gliding birds: Reducion of induced drag by wing tip slots between the primary feathers. Journal of Experimental Biology 180: 285-310.


The logical assertion "winglets have not happened in a long time, therefore they are not advantageous" is incorrect.

It is possible for an advantageous trait not to evolve even when advantageous, if there is no "path" to it. The trait only occurs gradually, in small incremental steps. If intermediary steps are harmful, the trait will not occur, even if the end result would be beneficial.

(now, this does not say anything specific about winglets, just about the general argument)


One famous example of an "imperfection" that keeps existing despite a better solution being available can be seen here: https://www.youtube.com/watch?v=cO1a1Ek-HD0 (beware, video features dissection of a giraffe)

There is a nerve that connects the brain to the larinx. In fish, this nerve is somewhat short, but in the giraffe, it is enormous. The interesting thing is: the points this nerve connects are very near, even on the giraffe.

What happened was the the nerve passes below an artery. For fish, this is irrelevant. But, as larger mammals evolved, the nerve had to gradually grow to a bigger path. Evolution could not do the "reengeneering" to have a smaller nerve, because there was no immediately advantageous way to do it.


There are two possibilites with evolutionary processes: The development either never went into this direction or it brought no advantages. Besides this two possibilities the claims from the other forum are wrong. Birds (not all of them though) do have winglet-like structures. If you look at big birds, you can see feathers on the end of the wings looking like this:

These feather fulfill exactly the same purpose here. Some more information can be found here and also in the Wikipedia.


The evolutionary process in bird wings, especially with regard to winglets - Biology

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Study Notes on Termites (With Diagram)

In this article we will discuss about Termites:- 1. Features of Termites 2. Distribution of Termites 3. Social Organization 4. Caste System 5. Food and Feeding 6. Swarming and Formation of a New Colony 7. Development 8. Economic Importance.

  1. Features of Termites
  2. Distribution of Termites
  3. Social Organization of Termites
  4. Caste System of Termites
  5. Food and Feeding of Termites
  6. Swarming and Formation of a New Colony by Termites
  7. Development of Termites
  8. Economic Importance of Termites

1. Features of Termites:

1. Termites are pale-coloured and usually blind except macropterous forms.

2. Termites possess comparatively large head, chewing and biting mouth parts.

3. Well-developed eyes in reproductive castes.

4. Ocelli are either touching or away from the eyes.

5. Soft-bodied females have much en­larged abdomen.

6. Winged (alate) and wingless (apter­ous) forms are composing the colony.

7. In winged forms, the fore-wings and hind-wings are same sizes.

8. The colonies contain many castes, such as queen, workers, soldiers, etc.

2. Distribution of Termites:

The termites are broadly divided into two categories—lower termites and higher ter­mites.

The lower termites include the follow­ing families:

It includes the most primitive living termite, Mastotermes darcviniensis. It is found in Northern Australia and Papua, New Guinea. No primary queen has ever been re­corded in the field and numerous substitute queens are usually found in the nests.

They are known as dry wood termites because they live in dry wood although some dry wood termites have subterranean habits. There are 400 species and are distrib­uted throughout the world.

They are known as damp wood termites because they live in wet and rotting wood. There are about 20 species and are distri­buted in the forests of America, Eura­sia, Africa and Australia.

They are known as grass harvesting termites because they have adapted in grass land where the trees are scanty. They are found in India and Africa.

Known as subter­ranean termites except Coptotermes, Reticulitermes. Some mound-builders and aerial nesters are found in this family. About 300 species have been recorded and are distributed in most of the continents.

The higher termites include a single family, Termitidae with several sub­families.

They build generally clay nests of great height above ground (also called termitaria) that occur in tropics including Asia, Africa, Aus­tralia and South America. About 1800 species have been recorded. Examples are Amitermes, Microcerotermes, Termes, Cubitermes, Capritermes, Nasutitermes, Macrotermes, Microtermes, Odontotermes.

The lower groups of termites contain symbiotic intestinal protozoa and bacteria, and higher termites contain intestinal bacte­ria only.

3. Social Organization of Termites:

“Social organisation is a process in which single or both parents live under a shelter and perform the activities in an organised and co-operative manner for the mainte­nance of the community”.

The detailed account of termites was published first in the Royal Society of London in 1781 by Henry Smeathman. The account was based on African termites.

A typical colony possesses nymphs (semi-matured young’s), workers, soldiers and reproductive individuals of both sexes (Fig. 18.91A, B, C, D, E, and F) sometimes also several egg laying queens are included. The different types of insects, each having a par­ticular function live in a colony, called castes.

4. Caste System of Termites:

A termite colony includes two major castes—sterile castes and repro­ductive castes. The caste system and the size of the colony are controlled by a pheromone, called social hormone, which is secreted by the members of reproductive castes (e.g., king and queen).

A. Sterile castes:

They are incapable of reproduction and include usually workers and soldiers (Fig. 18.91).

They are sterile of both sexes and developed from the fertilized eggs. They constitute major numbers in a colony and occupy 80%-90% of the total number. They are small-sized individuals measuring about 6 to 8 mm in length which bear chewing mandibles and usually lack of eyes.

But workers of some harvesting species bear well developed compound eyes. They have no wings (apterous). They feed upon the wood or fungus products. Advanced ter­mites produce true workers but in primitive families (e.g., Mastotermitidae, Kalotermitidae, Termopsidae, Rhinotermitidae) the true workers are not found.

Instead, nymphs (an insect larva hemimetabolous insects, resem­bling the adult except they have no wings and reproductive organs) of various stages which are arrested in their development act all the domestic duties of the nest like the workers, called Pseudergater or false worker, found in the colonies of Kalotermitidae.

They persist as worker throughout life or can metamorphose into winged reproductive castes and also act as potential supplemen­tary reproductive castes.

Workers perform all the duties for the colony except reproduc­tion. They take care of the eggs and young, undertake the labours of food gathering, food storage and feed the nymphs, king and queens.

They take part in the building of nests and in the cultivation of fungus-garden in special chambers in the subterranean chan­nels. They also maintain the moisture in the interior of a nest of a colony. They perform defense duties in some species and also clean the other castes.

The soldiers of some species of different genera show great diversity in char­acters. They are developed from the unfertilized eggs. They are wingless and larger than the workers. Soldiers are more or less pigmented and possess large head and stout, powerful mandibles.

They are mostly devoid of eyes but occasionally appear in some species (e.g., Mastotermes, Hodotermes, etc.). They constitute 5-8% in number in a colony. Soldiers for food rely on workers. They maintain the defense of the colony.

The specialized forms of soldier termites within many species of the family Termitidae, called nasutes. They are characterized by the reduced mandibles and head which is continued as a rostrum, at the tip of which opens a frontal gland.

The secretion of this gland is acrid which helps to prevent en­emies and at the same time it dissolves hard substances including concrete structures. In some Rhinotermitidae, the soldiers are spe­cialized with a pair of large strong mandibles and highly elongated grooved labrum that contains defensive secretions, called nasutoid soldier (e.g., Nasutitermes).

B. Reproductive castes:

These are fertile and are characterised by the presence of the eyes.

From the functional point of view, the reproductive castes are of three types:

1. Macropterous or large winged or primary reproductive forms (Fig. 18.91):

These sexually matured males and fe­males have two pairs of wings longer than the body and convert kings and queens. The body is usually pigmented and may be yel­low, brown or black. The widely separated large compound eyes and paired ocelli are usually present. The royal pair or the sexu­ally active males and females of social insects are the original founders of the colony.

Usually there is a single king in a colony. It is smaller than the queen. Its func­tion is to mate intermittently and provide sperm to the queen.

The queen is also a single in a colony which is largest in size measuring 60- 80 mm in length. It lays eggs after mating and can lay more than 1000 eggs per day. Both king and queen live in a special cham­ber, called nuptial chamber.

It is widely believed that the queen or king is the primary source of a pheromone (also called social pheromone) that maintain the colony integration and the size of the colony. The colony integration is also main­tained by the exchange of food, saliva or some secretions among colony members. This process is called trophallaxis.

2. Brachypterous neotenics or short winged or substitute reproductive forms:

When the primary or original king or queen dies or gets old, its place is taken by the sexually mature male or female but in nymph form. To replace them, the nymphs of the suitable sex (male or female) attain maturity by special feeding without further moulting and become the substitute queen.

The substitute reproductives are characte­rized by the presence of short pad-like wings, ocelli and rudimentary compound eyes and small reproductive organs. The substitute reproductives vary in number in different species within colony. It is recorded fifty functional substitute queens in a colony of Hodotermes.

Grassi and Sandias reported the condi­tion of castes in the nests after examination of thousand colonies in Italy where they found none of the primary reproductives in a single colony and only substitute reproductives numbering 10-200 per nest.

3. Apterous neotenic reproductives or wingless forms or replacement repro­ductives:

If queen dies in some adverse conditions or gets old, the apterous neotenic reproduc­tives are produced. In some primitive sub­terranean species, like Reticulitermes flavipes, Reticulitermes malletei, the nymphs or ergates (workers) are without wings and possess less developed eyes, and reproductive or­gans become a functional reproductive in the colony with one or two moults.

The workers and soldiers are usually blind and live deep in the soil of the mound, so they do not require vision and always remain in close contact of food source. The reproductive adults have well developed eyes because the vision is required for flight and in search of nest sites.

5. Food and Feeding of Termites:

The eusociality of termites largely de­pends on food and feeding. After the pro­duction of workers and nymphs, the king and queen are fed by the workers. The mutual exchange of food, salivary secretion and other secretions among the members of termites are called trophallaxis.

The social insects exchange food in two ways:

(i) Stomodaeal trophallaxis and

(ii) Proctodaeal trophallaxis.

Stomodaeal trophallaxis is related to the exchange of food among the members through mouth to mouth but in proctodaeal trophallaxis, the exchange of food and symbiont through anus to mouth. In termites and cockroaches, the proctodael trophallaxis is crucial for the replacement of endosymbionts that are lost after every moult and this behav­iour is used to the origin of sociality in insects.

On the basis of feeding, termites can be divided into wood (cellulose) eating and, or litter feeding, fungus-growing and soil-feed­ing types.

(i) Wood (cellulose) eating termites:

The wood (cellulose) eating termites (e.g., Archotermopsis, Zootermopsis) feed on cellu­lose of the wood, collect either from living or dead plants. The cellulose is a polysaccha­ride, each containing three thousand sugar units and important structural component of the cell wall of plants.

The termites possess intestinal fauna of symbiotic protozoa (e.g., Trichonympha) which help to digest the cellulose, secreting cellulase enzyme. These protozoans enter the nymphs which con­sume the excrement of the adult. The recent molecular evidence indicates that termites also use own enzymes for cellulose digestion.

(ii) Soil-feeding Termites:

The soil-feeding termites are Cubitermes, Termes, Capritermes (Subfamily Termitinae, Family Termitidae) which consume large quantity of soil as a food.

In these higher termites they have lost the symbiotic intesti­nal protozoa, and only gut bacteria is found which can degrade soil organic matter that provides the termite host with nutrition. The excreta contain a mixture of clay and bro­ken down vegetable debris, used for the construction of mounds and galleries.

(iii) Fungus-growing termites:

Fungus-growing termites are small in­sects, ranging 4 to 15 mm long and belong to the subfamily Macrotermitinae, feeding on wood and plant fibres using detritus. These termites could not digest cellulose and need help digestive capabilities of fungi Termitomyces (Basidiomycetes). Macro­termitinae cultivates a fungus-garden around the colony and creates some ventilated struc­tures inside the nest in the form of fungi combs.

The termites consume cellulose-based materials like woods and vegetable debris they also eat a certain amount of fungi. These fungi help the digestion of food inside the bodies of termites that help in the breaking down of ligneous and cellulose based sub­stances.

6. Swarming and Formation of a New Colony by Termites:

Swarming is the concerted departure of the winged termites from the main colony to start new colonies, accompanying by a queen or males or females together, generally occurring during rainy season. Before mating, the new kings and queens leave the colony and fly for a while.

The kings and queens that take part in the swarming, called alates or swarmer’s. The mating flight of the royal pair is known as nuptial flight. But it is not a true nuptial flight in termites because the copulation does not take place during flight. Generally after flight the males and females fall on ground, discard their wings, choose their mates, and find a suitable place nearby for nesting.

Swarming behaviour of the opposite sexes is different in different families and genus. The behaviour pattern in primitive families like Kalotermitidae is most simple. They are crepuscular flier. Male and females fall on the ground after flight and a wait for the opposite sex.

Before mating, the wings are shed and after pairing they excavate a chamber in a suitable place. In higher ter­mite species the female usually alights first on the ground after flight and shows an attractive altitude elevating its abdomen.

The female releases a pheromone, a social hor­mone that attracts the males. Before mating, they shed their wings and dig a chamber nearby in which they take shelter and where copulation takes place.

Generally termites construct colony by digging tunnels underground (e.g., Odontotermes assumthi, Microtermes obesi, etc.) or make a large mound above ground or nests are inside wood of trees or buildings. The common termites which build clay nests of great height [Fig. 18.92(i)] above ground are Odontotermes redemani, O. obesus, etc.

A termite nest (termitarium) presents highest form of architecture and within the nest there is a special chamber where the queen and a few males are confined.

The special chamber is called nuptial chamber. A colony [Fig. 18.92(ii)] includes nursuries, chambers, royal chambers, special chambers for culti­vating fungus-gardens or storing food, guard rooms, bridges, corridors and subterranean streets and canals [Fig. 18.92(ii)].

The period of swarming occurs at differ­ent times of the year. The swarming period of Odontotermes usually occurs during rainy season one hour after and of course half an hour after rainfall. In dry-wood termites the swarming occurs in hot days during summer season. During flight most of the termites are eaten by birds, small mammals or destroyed.

7. Development of Termites:

After copulation, the queen produces a large number of eggs, most of which develop into nymphs. These nymphs develop into sexless workers, soldiers and substitute re­productive forms and may become sexual forms, if necessary. The queen at that time is specially fed and it grows to a large size.

The large size is due to enormous growth of abdomen in which huge quantities of eggs are deposited. This abnormal-sized abdo­men is known as physogastrous and the termite queen containing such distended abdomen, called physogastrous female. After copulation the queen starts laying eggs in a chamber, called royal chamber, but the egg-laying capacity is not equal in all fami­lies or species.

In primitive termites the egg-laying is intermittent and the population is small. In Coptotermes the queen lays 100 eggs per day, but in advanced termites the queen can pro­duce 30,000-40,000 eggs per day and 10 mil­lion a year.

In the primary stages of the colony the nymphs hatch from the eggs within few days and moult at least three times to become functional workers, and into soldiers after fourth moults, and also substitute reproductives develop from nymphs (Fig. 18.93).

But at the end of winter several sexual forms appear. When matured, these winged forms take colonising flight, which occur during rainy season.

The wings in them extend beyond abdomen and are membranous. After becoming functional workers and soldiers, they take charge of the nest and take proper care of their parents. They expand the nest and make adequate arrangements to accommodate the growing population.

Primary queens can survive for 15-20 years and some may live for 50 years. Aborigines in Australia have reported that the termite queens of mounds can sur­vive up to 70 years. Workers may only live for a few months. Mastotermes kings have been recorded about 16 years.

The longevity of the individuals of the different castes in the colony is an important factor in establish­ing the close relationship between successive generations which form the basis of social organization or eusociality of termites.

Termites communicate each other in a colony through chemical, acoustical and tactical signals. The sexual communication and trail following of the termites are connected each other by pheromones. There are two pheromones in termites, such as (ZZE)-(3, 6, 8)-dodecatrienol and (E)-6-cembrene, have been recognised as trail pheromones.

8. Economic Importance of Termites:

1. The workers and soldiers leave their nest at night to attack furniture, woods and books.

2. Thus they damage human properties in several ways.

3. The only way to get rid of termite menace is to destroy the queen.

4. In spite of its destructive role, the ter­mites are considered important from the point of view of agriculture.

5. Like earthworm, the termites also pul­verise the soil and make it fertile.


The proximate causes of variation in migration

Among-species variation in migratory status and, in particular, geographic variation within species may have different proximate causes. Since populations of residents, short-distance migrants, and long-distance migrants often live in different habitats, different geographical regions, or both, whether a population is migratory or resident year-round could be determined simply by environmental conditions (e.g., day length, temperature, or food availability) in the breeding area during the nonbreeding season. As a consequence, among-population variation could be a direct response to the environment. Alternatively, migration could be endogenously determined by a genetic program (box 1), and geographic variation in migration could reflect genetic adaptation to different environments.

From an adaptive perspective, we expect genetic control of migratory behavior because organisms need to leave the breeding grounds before conditions deteriorate, that is, while conditions are still good enough to allow them to build up energy reserves. Moreover, in short-lived species such as many small passerines, mean life expectancy is less than two years, and most individuals will make only one return migration. As a result, the potential gain from experience is limited. A number of experimental studies have established that in this group of birds, among-species and among-population differences in migratory behavior and in traits of the migratory syndrome—including the circannual organization, orientation, and deposition of fat and protein reserves—are largely due to genetic differences ( Berthold and Helbig 1992, Berthold 1996, 2001).

Within-population phenotypic variation in migratory behavior largely reflects genetic variation, yet nongenetic variance components, including environmental variation and variation in experience and condition, may also be important ( Pulido and Berthold 2003, van Noordwijk et al. 2006). Long-lived species such as geese, storks, or cranes migrate in groups and are guided by the oldest, most experienced individuals. In these species, the genetic program, although still present (see, for instance, Chernetsov et al. 2004), seems to play only a minor role in determining variation in migration. This cultural transmission of migration may facilitate very rapid changes in migratory behavior ( Sutherland 1998), although the adaptive response in such species is not necessarily faster than in organisms in which migration is controlled primarily by a genetic program ( van Noordwijk et al. 2006).


Evolution

Evolution: Components and Mechanisms introduces the many recent discoveries and insights that have added to the discipline of organic evolution, and combines them with the key topics needed to gain a fundamental understanding of the mechanisms of evolution. Each chapter covers an important topic or factor pertinent to a modern understanding of evolutionary theory, allowing easy access to particular topics for either study or review. Many chapters are cross-referenced.

Modern evolutionary theory has expanded significantly within only the past two to three decades. In recent times the definition of a gene has evolved, the definition of organic evolution itself is in need of some modification, the number of known mechanisms of evolutionary change has increased dramatically, and the emphasis placed on opportunity and contingency has increased. This book synthesizes these changes and presents many of the novel topics in evolutionary theory in an accessible and thorough format.

This book is an ideal, up-to-date resource for biologists, geneticists, evolutionary biologists, developmental biologists, and researchers in, as well as students and academics in these areas and professional scientists in many subfields of biology.

Evolution: Components and Mechanisms introduces the many recent discoveries and insights that have added to the discipline of organic evolution, and combines them with the key topics needed to gain a fundamental understanding of the mechanisms of evolution. Each chapter covers an important topic or factor pertinent to a modern understanding of evolutionary theory, allowing easy access to particular topics for either study or review. Many chapters are cross-referenced.

Modern evolutionary theory has expanded significantly within only the past two to three decades. In recent times the definition of a gene has evolved, the definition of organic evolution itself is in need of some modification, the number of known mechanisms of evolutionary change has increased dramatically, and the emphasis placed on opportunity and contingency has increased. This book synthesizes these changes and presents many of the novel topics in evolutionary theory in an accessible and thorough format.

This book is an ideal, up-to-date resource for biologists, geneticists, evolutionary biologists, developmental biologists, and researchers in, as well as students and academics in these areas and professional scientists in many subfields of biology.


VI. Teleology versus Deontology

Since teleology may properly apply on to conscious beings, it has played a central role in many discussions about ethics. Deontology is the leading competitor against teleology as the basis for ethical decisions. Teleological ethics, says that one’s ethical decisions should be based on final goals and ends deontology says that ethics should be based on commitments to moral principles, without regard for ends. A teleologist would say that one should kill an innocent person if that would save two other innocent lives a deontologist would say that if killing is wrong, it remains wrong, even if it could save lives.


Multiple Sequence Alignment Is a Critical Step in Phylogenetic Reconstruction from Gene Sequences

In theory, each homologous sequence could be treated as a single character trait for phylogenetic reconstruction. However, the great advantage of sequences for phylogenetic inference is that, in principle, each position in the sequence can be considered a separate character trait. For this to work, there needs to be a way of comparing individual homologous positions found in different homologous sequences. This is done by making sequence alignments, where, in essence, each sequence is assigned to a separate row in a matrix, and homologous positions in different sequences are lined up in columns. Such an alignment serves as the data matrix for phylogenetic analysis introduced earlier with the sequences in rows corresponding to OTUs, the columns corresponding to homologous traits, and the specific residue (amino acid or nucleotide base) in each sequence being the character states. It is critical to realize that the residues in one column are considered to be different states of a homologous trait. In other words, it is inferred that the residues in one column have been derived from a common residue in some ancestral sequence. This is known as positional homology.

If sequences simply evolved by changing the nucleotide found at one position to another nucleotide, alignments would not be particularly challenging&mdashall one would need to do is find the starting point for each homologous set of elements, and the rest of the residues would then simply line up downstream from the start. However, evolution is much more complex. Perhaps the most important complexity for the purpose of alignments is the occurrence of insertions and deletions. Such insertion/deletion changes can be small (e.g., involving a single position) or large (e.g., inserting a new domain in the middle of a protein). When these occur in one or more lineages in the history of the evolution of a particular element, the homologous residues in different sequences will be out of register with each other. That is, position 10 in one sequence may line up with position 45 in a homologous sequence. Thus, to make an alignment for which the positional homology still holds in alignment columns, gaps must be inserted into the alignment. If a residue were deleted in one element relative to all others, then a gap would need to be inserted in the element that included the deletion to line it up with the others. If a residue were inserted in one element, a gap would need to be inserted in all of the others to have them line up with the element with the insertion.

Figure 27.4 shows an example of how adding gaps can improve an alignment. Figure 27.4A is an alignment of hypothetical homologous genes without gaps. Note how the bases in the columns are not highly conserved. The addition of gaps in Figure 27.4B changes this, with each column containing only a single base. If the complete history of all insertions and deletions were known, the gaps in the alignments could be placed easily to make all of the homologous residues line up correctly. Of course, this information is not available in most cases, so the locations of the gaps must be inferred. Alignment algorithms are designed to slide sequences against each other in various ways to identify where and how large the gaps should be, a process that must be optimized for all of the sequences in an alignment, not just each pair.

Insertions and deletions are not the only complexity of sequence evolution that alignment methods need to deal with. For example, sections of an element can be moved from one end of an element to the other via some type of translocation. Sections from one element may move into the middle of another element. Inversions can make optimal alignments nonlinear. In part because of these complexities, there is an almost bewildering diversity of methods available for carrying out sequence alignments for phylogenetic analysis. Some of the different classes of approaches are reviewed here, including those that focus only on primary sequence and those that attempt to use secondary or tertiary structure information as a guide. Note that these same approaches are used in performing database searches as discussed in the previous section.


Defining phenotypic plasticity

From the perspective of evolutionary biology, classic and dramatic examples of phenotypic plasticity in animals include wing polymorphisms in some insects, the timing of metamorphosis in amphibians, and alternative reproductive tactics in male vertebrates – all of which exhibit complex neuro-endocrine control mechanisms that are sensitive to various environmental factors (Ketterson and Nolan, Jr,1999 Sinervo and Calsbeek,2003 Boorse and Denver,2004 Knapp, 2004 Zera, 2004). From the biomedical perspective, well-known examples of plasticity include effects of intentional physical conditioning (exercise training)(Flück, 2006) such as weight lifting, on human morphology and physiology. Various biomedical subfields use additional terminology, such as `metabolic plasticity' or`cardiac remodeling', and the molecular mechanisms underlying such processes as muscular and neuronal plasticity are the subject of intensive study [for reviews, see other articles in this issue(Flück, 2006 Hood et al., 2006 Johnston, 2006 Magistretti, 2006 Swynghedauw, 2006)]. (Many environmental insults, e.g. excessive alcohol consumption, smoking, inhalation of coal dust, can lead to `plastic' changes in organs and organ systems, but when such changes are clearly pathological they are not typically included under the rubric of phenotypic plasticity.) In plants, basic growth form is notoriously plastic, and many readers will be familiar with the differences between dandelions growing in shade versus sun [although genetic differences among clones may also be involved(Collier and Rogstad,2004)].

As with the term `adaptation' (see below), phenotypic plasticity can refer both to a process and to the outcome of that process. Phenotypic plasticity can be defined formally as the ability of one genotype to produce more than one phenotype when exposed to different environments, as the modification of developmental events by the environment, or as the ability of an individual organism to alter its phenotype in response to changes in environmental conditions (Gordon, 1992 Scheiner, 1993 Via et al., 1995 Futuyma, 1998 Freeman and Herron, 2004 Pigliucci, 2005 Rezende et al., 2005 Stearns and Hoekstra, 2005 Pigliucci et al., 2006). The range of phenotypes that a given genotype (possessed by an individual organism or by an entire clone or inbred line) may produce when exposed to a range of environmental conditions is termed its norm of reaction, and non-parallel reaction norms of different genotypes indicate the presence of genotype-by-environment interaction.

The sequence of events involved in phenotypic plasticity often includes the following components: (1) something in the environment changes (2) the organism senses that change (3) the organism alters gene expression and (4),usually, the altered gene expression yields additional observable phenotypes[e.g. see fig. 8 in Flück's paper in this issue(Flück, 2006)]. Several aspects of this scenario require amplification. With respect to (1), we may attempt to draw a distinction between environmental factors that are external or internal to an organism. Changes in ambient temperature, humidity or oxygen concentration would constitute external environmental factors, and many organisms respond to these with phenotypic plasticity that involves multiple organ systems and multiple levels of biological organization. Mechanical overload of the heart is an example of an environmental change that occurs within an organism, and it leads mainly to organ-specific changes that necessarily involve fewer levels of biological organization(Swynghedauw, 2006). Of course, external environmental `stresses' can also lead to tissue-specific responses (e.g. Cossins et al.,2006). Nonetheless, we may predict that, in general, external environmental changes will lead to more and more pervasive plastic responses as compared with internal changes. With respect to (2), some changes may occur without any formal sensing by the organism, e.g. as a result of direct (and possibly differential) effects of temperature on the rates of ongoing biochemical and physiological processes. With respect to (3), it is important to note that some plastic responses need not involve changes in gene expression (transcription) but instead could occur viaphosphorylation of existing proteins, changes in protein levels caused by variation in protein ubiquitination, or stimulation of existing microRNAs(Nelson et al., 2003 Schratt et al., 2006). For point (4) we emphasize the word `usually' because it is possible that lower level traits might change in offsetting ways such that higher level traits could show little or no apparent change. For example, it would be theoretically possible (though perhaps unlikely) for exercise training to cause an increase in maximal heart rate but a reduction in stroke volume such that cardiac output was unchanged.

Acclimation and acclimatization(Wilson and Franklin, 2002),as well as learning and memory (e.g. Magistretti, 2006), are encompassed by the most inclusive definitions of phenotypic plasticity. Therefore, environmentally induced changes may or may not be reversible,depending on the organism, trait, and when in the lifecycle and for how long the environmental exposure occurs (Hatle,2004 Johnston,2006). If the capacity for change is more-or-less fully reversible, then it may be termed phenotypic flexibility(Piersma and Lindstrom,1997).

Whether reversible or not, it is generally assumed that environmentally induced modifications are adaptive in the sense that they improve organismal function and/or enhance Darwinian fitness of the individual organisms that exhibit such effects (Nunney and Cheung,1997). In fact, this may or may not be true, and the claim that such changes will aid the organism has been termed the beneficial acclimation hypothesis (Leroi et al.,1994 Huey and Berrigan,1996 Huey et al.,1999 Wilson and Franklin,2002). In some cases, behavioral plasticity can shield lower level traits from selection (Huey et al.,2003 Price et al.,2003). At the population level, phenotypic plasticity in behavior and other traits can facilitate invasions of new habitats(Price et al., 2003 Price, 2006 Pigliucci et al., 2006). As reviewed elsewhere in this issue (Fordyce,2006), many ecological (cross-species) interactions are mediated by the phenotypically plastic responses of one or more species involved in the interaction. Some of these ecological interactions can be quite complex and difficult to predict, as when an herbivore induces a plant phenotype that in turn affects the performance of other herbivores(Fordyce, 2006)

At this point it is worth remembering that the word adaptation has numerous meanings in biology (Garland and Carter,1994 Bennett,1997). Most generally, we should keep in mind the distinction between what is often called `physiological adaptation' (environmentally induced changes that occur within individual organisms during their lifetimes,including acclimation and acclimatization) and `evolutionary adaptation'(cross-generational changes in the genetic composition of a population in response to natural selection). Physiological adaptation is one type of phenotypic plasticity, but the ability to be plastic for any particular trait may also be an evolutionary adaptation whose details vary among organisms.

As noted above, although biologists have usually assumed that physiological adaptation is adaptive in the evolutionary sense, this is not always a safe assumption because some changes will be simply the result of activation of control systems designed to do something else, and they can even be maladaptive, including various human pathologies(Nesse, 2005 Swynghedauw, 2006). In general, non-adaptive plasticity might be expected to occur any time that an organism is exposed to environmental conditions with which it is `unfamiliar'in terms of its evolutionary history. This follows from the general evolutionary principle that organisms gradually lose abilities and traits that are no longer under positive selection, well-illustrated by things like blind cave fish or flightless birds on islands that lack predators(Diamond, 1986). Thus, imagine a species that has inhabited low-elevation environments for millions of years,adapting evolutionarily to function (reasonably) well in `normal' levels of atmospheric oxygen (∼21%). If one were to expose individuals of this species to high altitude, then they might be expected to exhibit inappropriate physiological responses to reduced atmospheric oxygen. The literature on human physiological responses to high altitude, both acute and chronic, is interesting in this context because it offers conflicting views on whether and to what extent various changes are adaptive versus maladaptive, and whether long-term, high-altitude native populations exhibit evolutionary adaptations to hypoxia (e.g. Winslow et al., 1989 Beall,2001 Brutsaert et al.,2005 Norcliffe et al.,2005 Wu et al.,2005). More generally, it is worth noting that the environment that many human beings experience (including aspects of nutrition, sanitation,medicine and the so-called built environment) has changed very rapidly relative to our generation time. Concomitantly, average lifespan has increased in many countries and diseases associated with aging have become much more common (e.g. Swynghedauw,2006). Therefore, it may be expected that at least some aspects of our phenotypic plasticity may not be adaptive.


Romancing the firefly

The twinkling of fireflies heralds summer romance for these magical insects. While courting on-the-wing, male fireflies attract females' attention with bioluminescent flashes.

But new research from biologists at Tufts University's School of Arts and Sciences, published online in Proceedings of the Royal Society -- Biological Sciences, reveals that, after the lights go out, female fireflies prefer substance over flash. They seem to choose mates able to give them the largest "nuptial gift" (a high protein sperm package that helps females produce more eggs) without regard to flashes. Those generous males are also more likely to succeed in becoming the fathers of the next firefly generation.

Previous work on Photinus fireflies shows that females are very picky during the on-wing stage of courtship. These females will only flash a response toward select males that light up with especially attractive courtship flashes. After a lengthy back-and-forth exchange, the flashing stops, the lights go out, and firefly pairs spend the night together.

A night of firefly romance also includes gifts, called spermatophores, that each male donates to his sweetheart. But the next night these females are likely to mate again with a different male.

After a female has mated with several males, the big evolutionary question becomes: which male gets to pass along his genes to the next generation of firefly babies?

"Lots of people don't realize that sexual selection is happening not only before mating, but also during and even after mating," said Professor of Biology Sara Lewis, an expert on the evolutionary process of sexual selection and senior author on the paper. "Focusing on what happens after contact, we wanted to examine how much a male's success -- in both mating and fathering offspring -- depended on his flashes or on his nuptial gift offering."

Lewis and coauthor Adam South, who studied in the Lewis lab and recently received his Ph.D. from Tufts' Graduate School of Arts and Sciences, set up an experiment using infrared video and paternity testing based on firefly DNA to determine what makes certain males more successful after the lights go out.

To get the female fireflies in the mood, the researchers relied on LED lights programmed to make two kinds of flashes. Some females saw only artificial male flashes determined by previous research to be highly attractive to females others saw unattractive flashes.

Male fireflies were also split into two groups: virgins whose nuptial gifts were large since the males had never mated and males whose spermatophores were smaller because they had mated the previous night. After several minutes of courtship flashing, males and females were put together in pairs, and the Tufts biologists videotaped their close-up courtship behaviors under infrared illumination. Because they take place under cover of darkness, many of these behaviors had never before been observed.

Analysis of hours of firefly video revealed that once a female was in close quarters with a male, she was much more likely to mate with males that had larger nuptial gifts to offer, as determined by the researchers' later examination. The females didn't seem to care what kind of male flashes they had seen.

"We were surprised to discover that attractive flashes only seem to benefit males during the early stages of firefly courtship," said South. "Initially, flashes are important. Female fireflies preferentially respond to males based on temporal flash characteristics. Once males make physical contact, however, females switch to an alternative cue -- one that's related to male nuptial gift size. What makes this especially intriguing is that females have no way to directly evaluate gift size, since it's created and transferred internally."

Furthermore, when females mated sequentially with two different males, paternity testing of their offspring revealed yet another benefit for big gift-givers. Males that gave larger nuptial gifts fathered more of their mate's offspring compared with rival males. South and Lewis say that's probably because larger gifts contain more sperm.

So not unlike human romance, love remains a mystery among fireflies and first impressions are only part of the story. While a female's initial assessment of potential mates is based on males' luminescent flashes, the Tufts research shows that once a pair makes contact, sexy flashes no longer matter. Instead, it's those males that have larger nuptial gifts to give that win out with higher reproductive success. Males with limited resources may face a trade-off between investing either in sexy flashes or in costly gifts.


Watch the video: The Upturned Wing Tip of Soaring Birds (August 2022).