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Evolution Final Review

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BIOL 359
Jonathan Witt

Evolution Final Exam Review (Part 3) Kin Selection and Social Behaviour Social interactions take place between two individuals in a given population: the recipient is on the receiving end of said action (or behaviour). Such interactions can have an impact on the fitness of both the actor and the recipient, in one of four models: mutualism, altruism, selfishness or spite. Mutualism results in fits gains for both participants. Altruism leads to loss of fitness on the part of the actor, but a gain in fitness on part of the recipient. This trait does not seem to be adaptive, yet examples are often seen in nature. An explanation to why altruism has persisted remains to be found. Selfishness is the opposite of altruism – leads to fitness on part of the actor, but to loss of fitness on part of the recipient. Spite is characterized by fitness loss in both the actor and the recipient and is not exhibited by any species other than humans. Kin Selection Altruism was a paradox of Darwinism for a long time. However, it was later hypothesized that perhaps altruism is favored by selection in cases where it increases the fitness of closed relatives (despite the loss of fitness on part of the actor). Since an individual’s inclusive fitness is composed of their direct fitness (an individual’s direct contribution to the next generation by reproduction), as well as their indirect fitness (arising from additional reproduction by relatives that results from an actor’s actions), altruism’s existence may not be as counter-intuitive as previously believed. Importantly, indirect fitness is additional reproduction, which would not have been possible apart from altruism (or the assistance provided by the actor). The selection for and spread of alleles that increase indirect fitness is called kin selection. Quantifying Relatedness and Hamilton’s Rule In order to understand the conditions under which altruism spreads, it is useful to measure how related the actor and the recipient are. The pedigree is traced beginning with the actor and the coefficient of relatedness (r) is calculated by finding the probability that an allele in the actor and an allele in the recipient are identical by descent (ibd), or arose by replication from the same ancestral copy of an allele. In half- siblings, for example, the shared parent donates half of their genes to each sibling. The probability of these genes being the same is thus r = ½ X ½ = ¼. In full siblings, each parent donates half of their genes to both siblings and the probability of genes being ibd is r = (½ X ½) + (½ X ½) = ½. Cousins inherit half of the genes of their respective related parents. The probability of the parents sharing the same genes is r = ½. Thus the probability that cousins’ genes are ibd is r = ½ X ½ X ½ = ▯. Once known, the coefficient of relatedness can be used to determine whether or not it is reasonable to say that altruism will occur between the related pair, using Hamilton’s rule. Hamilton’s rule states that an allele for an altruistic behaviour will spread if the benefit to the recipient (in terms of number of offspring), multiplied by the coefficient of relatedness, is greater than the cost to the actor (in terms of number of offspring. This is summarized in the equation below: Br – C > 0 Thus, altruism is likely to spread if the benefit and coefficient of relatedness are large and the cost to the actor is small. Examples of Altruistic Behaviours Between Related Individuals Prairie dogs are known to raise an alarm in the presence of a predator, warning nearby prairie dogs of the danger. This behaviour can be very detrimental to the actor, since it draws the attention of the predator. This is then an example of an altruistic behaviour. An experiment was conducted to determine whether or not prairie dogs are more likely to alert their own relatives to danger. It was found that prairie dogs living in coteries with their close kin were statistically more likely to give alarms call. These results were not statistically different for the different types of relationships (e.g. parents-offspring or brothers-sisters). In addition, the probability that a prairie dog will give an alarm call decreased significantly if it was moved to a new coterie. Another kin- related behaviour was also observed in prairie dog coteries. Females were protective of their coterie and would often ward off new females, sometimes joined by other females. It was found that a female is much more likely to join the chase if related to the prairie dog whose territory is being threatened by the new female. Another example of altruism is found in populations of white-fronted bee-eaters. Birds unable to have their own offspring often remain in the nest and help other families raise their young. This has a large impact on the fitness of the offspring, as the infant mortality rates in this species are normally very high. Invariably the white- fronted bee-eaters choose to help the offspring most closely related to them. Still another example of altruism is the sperm of the wood mouse. The sperm of wood mouse form sperm trains, which allow it to move much faster to the egg. However, in order for fertilization to occur the sperm train needs to break up. To that effect, sperm near the middle and back of the train undergo normal acrosomal reactions, sacrificing themselves so that the sperm at the front can move to the egg. Tadpoles present a curious case of altruism. Two types of tadpole morphs exist – a smaller type, which mostly feeds on decaying vegetation and small insects and a larger type with a large jaw, which is cannibalistic (eats other tadpoles). To test the hypothesis that cannibalistic tadpoles are less likely to feed on their closest relatives, small tadpole morphs of salamanders were placed in a cage with one large morph. The cage contained both related and unrelated individuals to the predatory tadpole. The results showed that cannibalistic tadpoles were less likely to eat their kin. This is consistent with Hamilton’s rule: the benefit to discriminators (in number of siblings that reach maturity) was twice higher than the non-discriminators (thus, B=2). There was also no cost associated with this behaviour (C=0). Thus, regardless of the value of r, altruistic behaviour should persist, according to Hamilton’s rule. The fact that not all tadpoles are discriminators may be due to genetic drift fixing the non-discriminatory gene in certain populations. Types of Relatedness Organisms can be placed in one of three groups based on their relationship with another organism. Conspecific individuals belong to the same species. Heterospecific individuals belong to different species. Congeneric individuals (congeners) are different species belonging to the same genus. Species have evolved ways to recognize their kin and avoid paying costs for non-kin (maladaptive altruism). An example is the conspecific nest parasitism in the American coot. These birds lay eggs of different colors and different spot patterns, eggs of kin being similar and eggs of unrelated individuals being dissimilar. This is a defensive mechanism, which ensures that females do not take care of the offspring of non-related individuals. This is especially important considering the fact that the most eggs a female can raise is approximately 8 and females often sneak into other nests to lay their eggs there. The American coot accepts eggs with similar color and spot pattern, but rejects dissimilar eggs by destroying them. Eusociality Many insects exhibit an extreme form of reproductive altruism. For example, worker bees do not have their own offspring, but function entire to feed the offspring of other individuals. This is an example of eusociality. In eusocial species generations overlap between parents and offspring. Cooperative brood care is in place – special members exist who care for offspring, but never produce any of their own. There are often specialized groups of non-reproductive individuals (e.g. workers, soldiers, collectors, etc.). Eusociality is most often seen in the species of Hymenoptera (ants, bees and wasps). These species have a unique genetic system, which predisposes them to eusociality – haplodiploidy. Hymenoptera males are haploid (they develop from unfertilized eggs) and females are diploid (they develop from fertilized eggs). This results in peculiar coefficients of relatedness – females are more related to their sisters than to their own offspring (r brothers¼, rsisters, roffspring). Thus, it is natural that females would be inclined to attempt to rear more sisters, rather than to make offspring of their own. In fact, although the queen bee lays eggs that would develop into males or females in a 1:1 ratio, the ratio of females to males in Hymenoptera is 3:1, suggesting that females may actually kill eggs, which are developing into males. Although haplodiploidy is not the reason for the development of eusociality, it certainly has predisposed its evolution. Eusociality has actually arisen multiple times and is also influenced by larval care and nesting development. Parent-Offspring Conflict Parental care has probably evolved due to the fact that it increases the fitness of close relatives (the offspring). A conflict arises due to the fact that parents maximize their fitness by investing equally in offspring, while offspring maximize their fitness by receiving more parental care than the siblings. This may lead to weaning conflict and siblicide. Weaning conflict occurs when the mother attempts to chase away the weaned offspring. The offspring refuses to and tries to fight the mother. When a young is born the benefit for the mother is high and the cost is low. As time goes on the benefit of caring for the child diminishes and the costs go up. The mother wants to wean the child approximately when the benefits:cost ratio is 1. The offspring wants to prolong the weaning until the ratio has reached at least 1:2 (half siblings may even attempt to prolong the period to a ratio of 1:4). Conflict occurs between these two points. Often, the conflict may lead to siblicide – offspring killing their own siblings. This is an adaptive behaviour, since offspring are more related to themselves than they are to their siblings. Reciprocal Altruism Although often exhibited between relatives, co-operation among unrelated individuals also exists. A hypothesis to its evolution states that individuals may carry out altruistic acts for unrelated individuals provided the favor is returned. Two conditions are required for reciprocal altruism to occur: the cost to the individual must be less or equal to the benefit and a punishment must ensue if the favor is not returned. Reciprocal altruism is likely to evolve when each individual repeatedly interacts with the same set of individuals (long enough for a favor to be returned). In addition, the species must have a good memory, potential altruists must interact in symmetrical situations (the favor must be able to be repaid in full) and many opportunities for altruism must occur over a lifetime. An example of reciprocal altruism is seen in populations of vampire bats. Bats live in caves during the day and hunt for blood meals at night. This is not an easy task and many bats return to the cave hungry. Since they cannot survive without feeding for more than 48 hours, bats obtain part of the meal of another bat who regurgitates it into their mouth. The favor is later returned. Such behaviour has been observed among both related and unrelated individuals. Species Concept and Speciation A species (Latin for “kind”) is a taxonomic unit, to which taxonomists like Linnaeus assigned species. Defining species has, and continues to be controversial due to problems with the establishment of practical criteria for recognizing evolutionary independence. As a result, several species concepts exist. Species Concepts TYPOLOGICAL SPECIES CONCEPT The typological (or Morphological) species concept relies on a type specimen – a single individual representing the entire species. Specimens are considered to belong to the same species based on their morphological resemblance to the type. However, variation within species is enormous. Differences between males and females alone have often caused them to be classified as separate species. In addition, convergent evolution, development stages, reversals and parallel evolution may also pose problems to the use of this concept. The rules used to name and rename taxa in this species concept (Botanical Code, Zoological Code, etc.) are inconsistent. Despite these issues, the typological species concept is still used today, though there is an attempt to select defining characteristics, which have a genetic basis. Two major problems with using the typological species concept are cryptic species and phenotypic plasticity. Cryptic species are species, which cannot be distinguished on the basis of their morphological characteristics. Phenotypic plasticity is morphological variation, which does not have a genetic basis (environment induced morphological differences). BIOLOGICAL SPECIES CONCEPT The biological species concept defines species as groups of interbreeding natural populations that are reproductively isolated from other such groups. Individuals within a species resemble each other due to gene flow, resulting from interbreeding. The evolutionary criterion of a separate species is then reproductive isolation and interbreeding. This concept is the legal definition used in the U.S. endangered species act. The biological species concept is clearly inapplicable for asexual taxa and provides no means of classification of fossil taxa. Testing reproductive isolation also has certain practicality issues. The key problem with this concept is the fact that many species, though distinct, do hybridize (e.g. the coyote and the wolf; the mallard and the northern pintail). A local example of hybridization occurred between lake trout and speckled trout. The hybrid of the two was released in the Grand River in an effort to entice sport fishermen. However, the hybrids had lower fitness than either parent and eventually weakened the gene pool when they backcrossed to the parental species. Hybridization may be interspecific (between different species) or intraspecific (between different populations of the same species). A famous interspecific hybrid is that of the polar bear and the grizzly bear – the Grizzlar. PHYLOGENETIC SPECIES CONCEPT The phylogenetic species concept defines species as groups of populations that share a common evolutionary fate through time. Species are monophyletic groups (taxa that contain all of the known descendants of a single common ancestor). Populations must have been evolutionarily independent long enough for diagnostic traits to appear. Populations must have been evolutionarily independent long enough for diagnostic traits to appear. This species concept has been applied to the study of Eurytemora affinis (copepods). Copepods have a very broad geographic distribution – unusual for a single species. In fact, DNA sequencing revealed that there are 9 different well-defined monophyletic phylogenetic lineages, which were also reproductively isolated. This is an example of cryptic species – distinct, though morphologically identical. Elephants are another such example. Originally it was thought that only two species of elephants existed – Asian and African. However, it was noticed that different groups of African elephants associated differently with different environments (and had different behaviours). The application of the phylogenetic species concept revealed that the African elephants were in fact two separate species – forest elephants and savannah elephants. Similarly, cryptic species for giraffes have also been found. In Dr. Witt’s research he applied the phylogenetic species concept to Hyalella Azteca – a species found in fresh water habitats all over North America. He discovered that there were in fact 7 different species that could be recognized phylogenetically, but were also reproductively isolated. Up to 4 of those could be found in the same habitat. Currently he has discovered over 100 species, which has been important in the field of toxicology: EPA has used this organism to develop standards, but they are different species so may metabolize toxicants differently). The problem with phylogenetic species concept becomes obvious when the phylogenetic tree of the brown bear is considered. Taking into account that polar bears and brown bears hybridize would place them as the same species as the brown bear and make different brown bear species (e.g. W. Europe, S. Canada) separate species. Nevertheless, the phylogenetic species concept offers some advantages over other species concepts, the biological species concept for example. The biological species concept is commonly applied to natural populations, but is difficult to test. It cannot be applied to species, which hybridize freely or are asexual, or extinct (e.g. fossils). The phylogenetic species concept on the other hand can be applied to both living and extinct species and is applicable to species that reproduce both sexually and asexually. Since it is based on evolutionary independence it can be applied without direct observation of species. In defining a species scientists treat speciation as a hypothesis and apply several different species concepts and test them. There is then no perfect species concept, as each would suit different hypotheses differently. Today, about 1.4 million species have been described – a fraction of the estimated 1 to 10 billion (even up to 100 billion) species that are thought to exist. Mechanisms of Speciation Speciation occurs in three steps: isolation of the population (1), divergence of isolated populations (2) and reproductive isolation or evolutionary independence (3). Steps 1 and 2 often occur at the same time in the same place. ISOLATION OF POPULATION Speciation begins with the isolation of a population. Populations become genetically isolated and gene flow is disrupted. Three main models describe the isolation of populations: Allopatric speciation, Parapatric speciation and Sympatric speciation. Allopatric speciation occurs when a physical barrier separates a population into two, disrupting gene flow and leading to the independent evolution of the two populations. New species evolve in geographic isolation when an ancestral range splits. For example, a population of green and yellow insects belonging to the same species is separated, forming two habitats. Since each habitat selects for different characteristics and gene flow does not occur, the two populations diverge and form separate species. When later brought into contact with each other, the green and yellow insects no longer mate with each other. Isolation can occur in two ways: dispersal and vicariance. An example of dispersal is the case with Hawaiian Drosophila, which moved from island to island in Hawaii, as the islands were created through volcanic activity. Drosophila in the geographically proximate islands are most closely related – the phylogeny of the drosophila resembles the creation of the islands. Vicariance is the physical splitting up of an ancestral population. For example, an island may flood, causing its lowest parts to be submerged underwater – forming two islands. In such a case, the now isolated populations may undergo speciation. Parapatric speciation is the divergence of populations, which are in physical contact. New species evolve in a geographically contiguous population and become isolated. This usually occurs at range extremes (and due to environmental gradients). For example, the environment of a territory occupied by a species may change, causing one part to become hot dry and the other - cool and humid. Natural selection would favor different alleles in the different environments, causing divergence of the population. Oftentimes parapatric speciation is characterized by the formation of a hybrid zone between the two populations. Depending on the nature of the hybrid zone, complete divergence may ensue. Sympatric speciation is the evolution of a new species within the geographic range of the ancestral species. Disruptive selection (hybrids not being selected for) and assortative mating may induce it. Sympatric speciation was initially thought to be impossible as it occurs in the presence of gene flow. However, numerous examples have since been identified, proving that it does occur. It is most common in flowering plants, where it is most frequently caused by polyploidy. A polypoloid is instantly reproductively isolated from its diploid parents. DIVERGENCE OF POPULATIONS Once isolated, populations must diverge in order for speciation to continue. Genetic drift, mutation, natural selection and sexual selection may all cause the divergence of populations. Genetic drift is strong in very small populations and fixes different alleles at random, independently in the isolated populations. Isolation events may cause bottleneck and founder effects, which may further increase the influence of genetic drift. Natural selection can lead to divergence if one (or both) of the populations occupies a novel environment. For example, the Apple and Hawthorn maggot flies are clearly distinct populations (from protein electrophoresis studies). Apple maggot flies display a strong preference for other apple maggot flies in mating, as with hawthorn maggot flies, though there is a significant amount of gene flow between the species. The maggot flies are phytophagous insects – they are associated with a specific plant where they feed and develop from larvae into mature adults. The two species once used to be
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