Evolution

This page discusses the increasingly complex behaviours exhibited by evolutionary systems.

The first section discusses replication and how the potential for beneficial mutation will result in evolution. Behavioural processes that are fundamental to evolutionary systems are then explained, such as repair mechanisms, information transfer, altruism, mutualism and competition.

The second section investigates how the environmental conditions of Earth have determined biological evolution via a number of key adaptations. Simple autocatalytic systems have improved information storage (DNA) and catalytic activity (enzymes). The development of the cell and multicellular adhesion has eventually resulted in nervous systems, learning and culture.

1. Transduction between states conserves energy.
2. The probability of transduction depends on the number of potentially stable states.

The stability of a given state is defined by the probability of transduction to and from other states.

Any state which increases the transduction of other states into replicas of itself shall be referred to as a replicator.

The environment of a replicator includes any state that influences replicator stability.

The ontological nature of replicators will not be discussed until Section 2. It should be noted that a replicator may not refer to a distinct unit or individual, but can include complex systems, the constituents of which may not exhibit replicative properties when separated. The behaviour discussed in this section can therefore be applied to non-biological evolutionary systems such as artificial life and memetics.

The division between a replicator and its environment is defined by heritability and may not be distinct. Phenotypic interactions are those between a replicator and its environment. As with all other replicator properties, heritability is environmentally determined.

Some environments may promote variation of the replicator itself or errors in replication accuracy. In simple replicative systems this is likely to prevent further replication. The rate at which such lethal mutations occur can be subtracted from the replicative reaction rate to give replicative success, or the growth rate of the replicator population.

There is a probability that replicative properties will be retained by the replicator following a mutation.

Neutral mutations are those which have no effect on replicative success. However, the neutrality of a mutation can change with environmental variation.

When a non-lethal mutation occurs, a new replicator population is created. Darwin (1859) states that in a system involving hereditary units, which can vary in terms of replicative success, evolution is inevitable.

However, a replicator which can only undergo costly or neutral mutations will not evolve, it will simply be maintained by selection. Therefore the variation of replicative success must include potentially beneficial mutations for evolution to occur. This potential for beneficial mutation is often termed evolvability.

The adaptability of a replicator can be defined as its capacity to survive environmental variation. This can be divided into short-term adaptability, involving homeostatic responses, and long-term adaptability, or evolvability. As an evolutionary system ages, increasingly higher-level selection processes determine which replicator populations survive (Dawkins 1989). The rate of adaptation to such processes will depend on the pressure exerted by long-term environmental variation.

The evolutionary rate, and hence adaptability to long-term environmental variation, is determined by mutational and replicative rates.

Evolution relies on mutation, yet mutation heavily reduces success. Therefore, a threshold exists for an optimum evolutionary rate, dependent on the cost of mutation and environmental variation. Repair systems will evolve to reduce the most severe mutations and hence optimize this threshold.

Causes of Sexual Reproduction

Repair - The high risks of mutation exert a very strong selective pressure for the evolution of repair mechanisms. Mutation can only be detected by comparison with a back-up copy or template. Fortunately, replicative systems are full of such copies in the form of replicas, and comparisons between them can maintain stability.

It can be assumed that the majority of mutations are deleterious through loss of function as opposed to a costly by-product. Therefore, long-term association between two replicators will result in dominance of the normal phenotype over recessive mutants.

This diploid method of repair is a potential reason for the evolution of sexual reproduction.

Muller’s Ratchet hypothesis argues that over time weakly negative mutations will build up within each path of replicative descent (Muller 1932). This is more probable with slow replicative rates where selection cannot occur fast enough to remove such mutants.

The cost of mutation will be reflected by the investment into repair mechanisms. These will evolve in order to control costly mutations. Depending on its complexity, variations in the cost of mutation may exist within the replicator. Therefore, repair mechanisms may target the parts of a replicator where mutations are most costly.

Controlled Variation - At the other extreme, areas of a replicator in which mutation consistently leads to replicative benefit may undergo controlled variation. This can result in competition between the variable region and the rest of the replicator.

The high risks of variation limit this behaviour to only occur where pressure for controlled variation is very high.

The run-away selection involved in parasite-host interactions leads to increased specificity of a parasite for its host. This exerts pressure for variation within the host, as parasite activity will target common host phenotypes. One method for controlling such variation could involve splicing with other replicators.

This pressure for variation is another potential reason for the evolution of sexual reproduction (Hamilton 1980).

Such a method for obtaining variation also allows a choice of mate. Mechanisms for communication and recognition of successful variants would evolve.

Sexual reproduction is extremely significant to the study of zoology as it occurs in the majority of metazoans.

The ultimate causes of sexual reproduction are subtle, probably involving methods for repair and variation. However, the effect of sexual reproduction on replicator interactions is very significant. (Biological terms are introduced in brackets)

Replicators (genomes) with the potential to splice with each other are said to be of the same species. In order to prevent the separate evolution of each replicator line a process of recombination must occur. This process must be randomized (crossing over) to prevent exploitation of such a system.

Recombination divides genomes into smaller replicatory units (genes), by definition becoming the replicators themselves. Replicatory success is now dependent on cooperation with potential replicator partners (gene pool). Replicators compete with variants (alleles) depending on their compatibility within the pool (e.g. the same locus). Crossing over favours proximity between mutually beneficial alleles (linkage).

Replicators involved in sexual reproduction are under pressure to find a beneficial mate (intersexual selection). The benefit of potential mates is determined by compatibility within the pool. The communication of success between potential mates leads to a hypothetical perfect mate. This is reminiscent of Plato’s theory that variation within a species represents the imperfect shadows of a perfect animal. Arbitrary characteristics can become desirable in sexual selection and even lead to a run-away process.

Predicting relatedness becomes an important factor for determining kin and levels of altruism within family groups. Altruistic behaviour will depend on the probability of relatedness. Competition within a species is very high and therefore altruism is often restricted to close relatives.

In asexual reproduction, the success of offspring is equally important as that of the parents and hence division usually occurs. As relatedness to offspring is reduced so is the amount of parental investment.

Speciation - Genetic flow tends to diffuse selection pressure within a species. A speciation threshold will exist when strong selection pressures compete with a low rate of flow. Speciation usually occurs when flow is resisted by geographical barriers.

Parapatric speciation – partial resistance to gene flow is combined with contrasting selection pressures.

Sympatric speciation – contrasting selection pressures are strong enough to cause divergence without any initial resistance to gene flow.

Anisogamy - Investment in gametes can be of two successful strategies: low investment in many gametes (male) or high investment in few gametes (female). Both strategies are likely to become dependent on one another and will result in anisogamy. The greater investment by females results in a biased operational sex ratio. Hence, males tend to compete with each other (intrasexual selection) for receptive females.

The offspring of a sexually reproducing pair will only contain half of each parent replicator. This is often referred to as the twofold cost of sex. However, each parent only has to invest half as much in each offspring, so the only costs of isogamous sexual reproduction are those involved in finding a mate and the mating process itself.

This confusion usually arises because the majority of sexually reproducing organisms are anisogamous, and hence is usually referred to as the twofold cost of males (Maynard-Smith 1978).

Environmental variation within a population may be reflected in replicative success. Therefore, within a species, variation will be determined by complex interactions between genes and environment.

Meiotic Drive - A sexually reproducing system can be exploited by outlaw replicators that increase the probability of their descent into offspring (Dawkins 1982). Such segregation distorters attack competing alleles either directly during meiosis, or target gametes which contain them (including prevention of syngamy). This is effectively intra-allelic sexual selection and can potentially exert as strong pressures as other forms of sexual selection.

Diploid parental genes require an equal probability of descent of each allele into their offspring and therefore compete with haploid gametic genes.

Segregation distorters provide a useful tool for flooding natural populations with artificial genes. For example, a vector organism could be given a gene for parasite resistance with meiotic drive properties. This could have devastating consequences for the parasite population.

Evolutionary systems ultimately involve information regarding replicative success in past environments.

Information is a form of energy and therefore acquisition of useful information can improve replicative success. Environmental interactions provide information about the environment allowing a replicator to optimize its response. Behaviour is therefore determined by the specificity and sensitivity of sensory processing and the efficacy of response. Effective signaling mechanisms must account for the expected accuracy of perceived information. Communication is the transfer of information and includes signaling (originating from a replicator) and reception (utilized by a replicator).

Negative feedback from phenotypic interactions can provide stability at an optimal rate. This is known as homeostasis and acts at almost every level of replicative systems.

The optimal rates for phenotypic interactions represent the desired state of a replicator. Deviation from this state regulates interaction rate via an error signal.

Positive feedback loops are unstable unless regulated by negative feedback. Combining positive and negative feedback allows complex signaling systems to model environmental information.

The use of feedback mechanisms to eliminate environmental interference can create clock mechanisms. This allows an environmentally independent method of temporal control. It is now thought that the size and accuracy of biological clocks have reached limits imposed by the Heisenberg Uncertainty Principle.

Storage of information provides memory of past interactions that can regulate current ones.

The adaptability/diversity of homeostatic mechanisms will be determined by environmental variation. Adaptability is often costly and in constant environments specialization is favoured.

A significant part of replicator environments will be replicas and, following mutation, other populations of replicators. The interactions between different populations are dynamic and complex (Game theory). Stable interactions are known as an evolutionary stable strategy (ESS) and new phenotypes have to overcome this stability to survive. The interactions between evolutionary systems can result in run-away selection pressures.

Emergent network properties involve the behaviour of complex systems that cannot be predicted from the constituent properties of the system. This would directly contradict the reductionist nature of science.

The influence of replicator behaviour on the success of its replicas is termed indirect fitness. Inclusive fitness represents a combination of direct and indirect fitness.

Hamilton’s Rule - altruistic genes will only spread if the direct cost is less than the indirect benefit (Hamilton 1964).

Altruism is any costly behaviour that benefits another replicator. The pressure generated by replica interactions is kin selection. Parental care is a form of kin selection involving direct descendents.

The efficacy of altruism is dependent on its specificity towards kin as opposed to parasitic non-altruists. This leads to run-away selection of kin recognition and parasite evasion.

Therefore, the amount of altruism displayed between individuals will depend on the probability of relatedness.

When the success of one population is increased by that of another, pressure exists to optimize this interaction. A threshold will exist for how much one population can gain from another without influencing it in some way. Further optimization requires either a negative or positive influence.

If one population benefits from the by-product of another, then optimizing the success of the host population may indirectly benefit your own. A bi-directional positive effect will result in symbiosis up to a threshold level of interaction. The selection pressures for symbiosis is dependent on the strength of indirect positive interaction.

The indirect flow of replicative success is heavily influence by the number of populations involved. The introduction of a third benefactor allows exploitation of mutual systems.

The reciprocity of mutualism is the temporal delay between the reception of benefits by those involved. Increasing reciprocity involves a greater risk and slower rate of benefit transfer.

If one population reduces the success of another (bottom-up control), there is pressure for the inhibited to evade or retaliate. As with mutual effects, these indirect phenotypic interactions are heavily determined by the number of populations involved and their interaction strengths. Evasion becomes favourable over retaliation the more populations that are involved.

Reciprocal inhibition is unstable and such interactions will be selected against. However, if a population gained from a reduction of host success, then this parasitic behaviour would be favoured. The dependency of a parasite on specific host success (top-down control) limits the amount of negative pressure it can exert. Infectivity of a parasite reduces this dependency and allows increasing virulence to develop.

Obligate parasitism occurs when the replicative properties of a population become completely dependent on those of another. Extreme forms of obligate parasitism involve hijacking host replicative mechanisms, sometimes by structural association between replicators. The pressure for host evasion/retaliation can be reduced if the parasite can offer benefits to its host.

Mutation may result in competition within a replicator itself. By definition, this results in two new replicators (although they may be structural associated). If one is an obligate parasite of the other it is known as an outlaw. Survival of such outlaw replicators depends on a threshold between its self-benefit and the cost to the host.

Predation - If parasite success is independent of host success, predation may occur. This involves complete loss of prey success. Such polar interactions favour run-away selection although it is biased for prey evasion (life/dinner principle - Dawkins & Krebs 1979).

The predation of two populations on a single prey population is a common form of indirect competition. Such populations are in the same trophic level. The level of competition between any two populations depends on the number and strength of phenotypic interactions they share. Unique combinations of such interactions are called a niche.

A prey population will gain from being surrounded by another population in a similar predatory niche (dilution/selfish herd). There will be a proximity threshold determined by resource competition.

Trophic Cascades - Energy will be lost from any system, therefore the long-term survival of an evolutionary system is dependent on its energy resources. An evolutionary system must eventually be able to adapt to new energy resources, as no resource is infinite. Such autotrophic systems will be parasitized by heterotrophs. Autotrophic efficiency represents ultimate donor control on any biological system (commensalism). The efficiency and diversity of heterotrophic parasitism determines energy flow in a trophic cascade and how many levels it can sustain.

It should be noted that competition can occur via communication pathways. This exerts pressure for determining the legitimacy of a signal. Modes of communication often evolve that cannot be deceptive due to physical laws (e.g. sound frequency signaling size).

In the previous section, the fundamental behaviour of evolutionary systems has been discussed. The nature of these systems is ultimately environmentally determined and contains information regarding past environments.

Ultimate - the selective pressures that have acted upon these mechanisms to preserve successful replicators.

Early evolutionary adaptations are of the most significance. The evolution of a certain trait will determine the constraints on further adaptation.

Evolution is forced onward by selection pressures and resisted by mutational constraints. The influence of a mutation depends on its primary effect and flow through phenotypic systems.

The commonality of a trait reflects the pressure for it to evolve and the lack of constraints for its evolution. Convergent evolution represents different mutational pathways resulting in the same trait.

The evolution of similar traits is less independent if it evolved from a common precursor. For example, true flight independently evolved four times from a flightless ancestor; in the Insecta, Aves, Chiroptera and probably Pterosauria. However, the three latter groups evolved wings from the same forelimb precursor.

Therefore, the determination of fundamental evolutionary patterns (Kauffman 1985) from the commonality of independently evolved traits (Dawkins 2004) must take such precursors into account.

Geographical isolation provides natural experiments for recent evolutionary divergence. However, the nature of evolutionary systems that emerge from a given environment is extremely unpredictable. The fundamental behaviour discussed in the previous section may not occur due to constraints.

The nature of extraterrestrial life must be discussed with great care. For example, Dawkins (2004) writes of how “Earlier generations of scientists would have treated the weather and the chemical composition of the atmosphere” as factors “which life cannot influence” having previously stated a number of factors which he believes “life cannot influence”. However, there is no limit on the influence of life on its environment.

Some cosmological theories suggest variation of the physical laws within the universe. Predictions involving extraterrestrial life are limited only by the human imagination.

When atoms interact with each other, there is a probability that a reaction will occur depending on each atoms mass, charge and velocity.

A catalyst increases the probability of a reaction without changing its own chemical structure.

The replicative reaction rate of an autocatalyst depends on the concentration of reactants and other environmental factors such as temperature and pressure.

The presence of liquid water on the surface of the Earth is of significant evolutionary importance. In a solid state, replicative patterns cannot develop between the atoms, as they are not free to move. In a gaseous state, rate of diffusion is heavily influenced by mass. Although gaseous autocatalysts can occur, it is unlikely they could reach a large enough size for mutation to be non-lethal. An autocatalytic system of many gaseous components could potentially reach the complexity required for evolution.

The liquid state provides a balance between diffusion rate and mass. The polar properties of water further increase the likelihood of autocatalysis by acting as a general catalyst. A polar liquid provides semi-stable pools of charge variation. This allows relatively large polar molecules (potential autocatalysts) to dissolve and interact

The possibility of replicative systems emerging from electromagnetic fields should not be ignored. It is also possible that the universe itself may have replicative properties.

As a catalyst remains unaffected by the reaction it catalyses, polymerization of autocatalysts may not hinder replication. Polymerization may itself be the replicative reaction if the reactants of an autocatalyst are identical subunits. Large polymers will replicate and diffuse slower. However, polymerization offers molecular stability, and the formation of a circular polymer is likely to be the most stable arrangement.

Variation within subunits can vary charge distribution along the chains axis. The use of variation between subunits is modularity, and is common in biological systems. The ratio of subunits will optimize relative to their availability. Charge interactions between subunits causes variation in three-dimensional structure. Structural plasticity depends on the variation of available subunits and the chains flexibility. Structural variation can increase affinity, stability and diffusion rate. Those subunits involved in a catalytic reaction are referred to as the active site.

The interactions between subunits that translate primary structure into tertiary structure are extremely complex. The processing power required to predict such interactions are well beyond modern computer simulations.

The “like repels like” and “opposites attract” nature of electromagnetism favours palindromic replicatory patterns.

The majority of Earth replicators consist of deoxyribonucleic acid (DNA). The double-helix structure is highly stabile but proteins are required for replication.

Ribonucleic acid (RNA) provides greater flexibility than DNA and displays greater autocatalytic properties.

Amino acid chains provide the greatest catalytic flexibility but are relatively unstable. Some modern proteins display replicative properties (prions).

RNA World Theory suggests that RNA was the original replicator that evolved to use protein flexibility for catalysis and the DNA double helix for storage of genetic information (Poole et al 1997). It is possible that the significance of RNA in modern biochemistry has been underestimated.

The ancestral autocatalytic systems of life may not have involved any of the modern biological molecules.

Behavioural complexity of individual autocatalytic molecules is limited to configurational processing. However, the behaviour of an autocatalytic system could become extremely complex.

Symbiosis involves structural association between mutual populations. Strong mutual interactions will favour an optimum population ratio and hence optimize relative replication rates. Structural association will favour simultaneous replication of individual replicatory units. Association at the genetic level creates a new genome.

The simplest RNA-based autocatalytic system was created by Sol Spiegelman of Columbia University. This was only 550 base pairs in length and could evolve from raw materials in the presence of a replicase enzyme (Dawkins 2004).

It is becoming increasingly apparent that polynucleotides are not the only part of biological systems involved in inheritance. It is possible that some structures only exhibit partial heritability, blurring the genotype-phenotype division even further.

Examples of epigenetic activity include colostrum immunoglobulins, DNA methylation, histone acetylation and RNA interference. Genomic imprinting is a form epigenetics in which genetic expression varies depending on the gamete/parent of origin.

Internal development within the mammalian uterus allows hormonal signaling between the mother and child. Translation of the maternal environment into developmental change can predispose the child for such conditions. For example, people who were in their last trimester of gestation during the Dutch famine of 1944 later suffered from a higher incidence of diabetes as a result of the rapid change in calorific availability (Ridley 2003).

Bonding between replicators and non-replicative molecules may enhance catalytic capabilities (enzyme complex) or structural stability. Peptide chains provide greater flexibility and charge diversity. The translation of the replicative chain into a protein chain reduces constraints on catalytic manipulation of the environment.

It is possible that replicator structure became extremely complex before the evolution of the cell membrane.

Polynucleotide to protein translation is universal among biological systems. This replicative method determines the effect of a mutation. Protein synthesis represents the majority of genetic interaction with the environment (although the significance of RNA is becoming increasingly apparent). The involvement of a particular genomic region in the synthesis of a protein can be extremely complex:

Exon regions are directly translated into proteins. Surrounding regions (introns and promoters) interact with proteins and regulate exon activity. Some genomic regions are involved in secondary and tertiary structure and their mutation could influence distant exon activity. Junk DNA does not influence specific genetic expression but may be involved in structural stability or repair mechanisms.

Mutation in an exon region can influence genetic expression, but may also change protein structure. In polycistronic translation, control regions may also determine protein structure.

As most proteins are enzymes involved in catalysis of a specific reaction, the phenotypic effect of a mutation is usually variation of this reaction rate. This variation can then affect other exon activity either directly, or via signal pathways. The type of mutation can be as important as its location. The entire system is subject to environmental interference.

Increasing the number of base pairs involved in a phenotypic interaction could potentially buffer against the negative effects of mutation.

The commonality of cistron duplication and silencing could represent increased protein evolvability as this allows a relatively controlled method of divergence and variation.

The number of proteins involved in controlling behavioural traits determines the distribution of phenotypes within a population. Single protein control will result in distinct phenotypes for each allele. Increasing the number of proteins involved will tend to a normal distribution of phenotypes.

Investigating the advantages of such control strategies is important in understanding the ultimate causes of their evolution.

The creation of free catalytic molecules (enzymes) may increase success depending on diffusion rates. The smaller the volume a replicator inhabits the greater the influence of its own products. The ability to contain products within a smaller volume increases the benefit they provide. It is possible that protein-based life evolved within naturally small volumes (e.g. porous rock). Such containment is dependent on selectivity as essential reactants must be accessible. Creation of a barrier in order to retain useful products allows a successful catalytic environment to be maintained.

Phospholipids exhibit polar polarity. Their fatty acid chains are non-polar and hence hydrophobic. Units with a conical hydrophobic region will associate as micelles. Phospholipids have cylindrical hydrophobic region and hence stabilize as a spherical bilayer. This prevents diffusion of polar molecules. As most organic molecules are polar, the production of such a barrier is likely to restrict the supply of replicative reactants unless transport proteins are present. It is possible that “inside-out” membranes could evolve in a non-polar liquid.

As well as creating an enclosed volume, the phospholipid bilayer itself provides a two-dimensional plane to which polar molecules can be attached via a non-polar anchor. The evolution of the phospholipid membrane may have begun with the replicator spanning the bi-layer in this way. It is possible that biological systems evolved within naturally occurring membranes.

A selective barrier allows greater control over the local environment of a replicator and the specialization of proteins for metabolism and processing. Sensation and response now occurs at the cell membrane, with varying levels of control from deeper within the cell.

Some of the simplest cellular organisms are Nanoarchaeum equitans and Mycoplasma genetalium with a genome of around 500 000 base pairs. This codes for around 500 proteins (and some RNA) of which between 250-350 are essential for reproduction.

Communication within a population of cells (quorum sensing) allows specialization of activities to provide an optimum environment. Apoptosis is a good example of such behaviour; when resources are limited, cell suicide may increase overall reproductive success.

The formation of a multicellular organism by cell adhesion allows rapid communication and increased structural stability. Optimal cell size within a multicellular organism will depend on structural stability, nuclear distribution and cellular processing.

Optimum genetic control over a colony requires a germ cell line from which new colonies develop. However, it is not always optimal for each organism to produce germ cells. Eusocial behaviour and the use of sterile castes is effectively the division of a multicellular colony into more than one individual per germ cell line.

The development of distinct organisms (individuals) allows improved motile coordination.

Chemical reactions alter the kinetic energy of the molecules involved. An organism can utilize this mechanical force to move relative to its environment (this includes intracellular and extracellular circulation).

Motion will therefore increase environmental variation, and hence even random motion can be coupled with negative environmental factors to give a beneficial response.

Metazoan motility requires rapid communication across long distances that cannot be attained through diffusion or circulation. The selective permeability of a membrane allows charge gradients to exist across it. Charge variation will flow rapidly along membranes allowing fast communication between via cells. Linking membrane transport to charge sensitivity allows amplification of such signals.

The use of inhibition, excitation, feedback, divergent cascades and convergent comparison makes neural communication fundamentally similar to chemical communication.

Neural processing involves the translation of sensory input into motor output. In a variable environment it may be beneficial to modify such translation based on previous outcomes.

The nervous system is a colony of rapidly communicating cells. Structural development of the system determines the potential for connectivity. Learning represents variation in this neural connectivity (long-term potentiation and NMDA receptors).

“Clean Slate” theories of learning mechanisms are fundamentally flawed. Complete plasticity of a system is a logical impossibility. The heritability of behaviour is inevitable.

Certain associations will require less stimulation for learning then others. For example, rats will associate nausea with food consumption more rapidly than an electric shock.

Temporal specialization of plasticity results in critical periods of learning (imprinting). This is useful for kin discrimination.

The adaptability of a system can be increased using common feedback mechanisms. Pleasure and pain allows complex associations to form between behavioural responses and inferred success.

Large organisms with a slow reproductive rate are less adaptable to environmental variation in an evolutionary sense. However, the ability to overcome unexpected problems using neural processing introduces a new form of adaptability.

The brain represents a specialized unit of sensory convergence. Association between senses allows increasingly complex methods of behavioural homeostasis. Sensory selectivity and association can result in extremely complex pattern recognition systems.

Animal behaviour can be defined as the motor output of the central nervous system, particularly the brain. By comparing internal/external environmental factors with a desired state, deviations can be converted into motivated behaviour. Decisions are based on motivational value. Memory provides access to past environments, and in primates this has become a major decision making factor. This is enhanced by social learning.

Electrical communication requires cellular adhesion. Individual nervous systems can communicate via pre-established sensory pathways. Social learning not only extends the sensory environment of an individual but can also reduce processing through categorization and language.

Learned information can substitute inherited information about how to respond to the environment. Behavioural patterns that can be learned do not have to be stored genetically. Communication between kin allows inheritance of such information and is effectively a form of epigenetics. The relatively few cistrons of mammals/birds could be explained by these alternative methods of information transfer.

The evolution of intelligence can be described as a progressive shift from inherited information of past environmental success to receiving successful information from the present environment. Therefore, variable environments favour intelligence.

However, social learning allows inheritance of information from kin, resulting in a new form of rapidly evolving neural constructs. Whether this seemingly progressive shift is inevitable from chemical evolution is of great significance to the possibility of extraterrestrial intelligence.

Theories as to why intelligence evolves are often divided into social and ecological causes although it is likely that both play some part:

Social interactions can be very complex. For example, in a simple case of alliance formation only three individuals are involved; a potential ally deciding whether to help an oppressed individual against an aggressor. In determining the potential benefit of such an alliance, the individual must consider the present cost of such an interaction compared with the future benefit. However, such costs and benefits depend on its past interactions with the aggressor and the oppressed. As most individuals in a social group will be related then the level of relatedness must also be considered for each interaction. Alliances between many individuals (e.g. dolphin “super-alliances”) could involve extremely complex cost/benefit calculations.

Reciprocal social interactions require memory and individuals who have an effective memory can exploit those who do not. Deception (Machiavellian intelligence) and reconciliation are as important as cooperation in social behaviour.

In a social environment it is advantageous to be able to predict the response of others. Such a method of thinking could culminate in a “cause and effect” understanding which may explain human intelligence. For example, basic physics could be a form of anthropomorphism of inanimate objects.

Predicting the response of others could involve an individual understanding that they think in similar ways (Theory of Mind).

It is becoming clear that many social animals have an understanding of third-party relationships (Boyd 1997). This includes recognizing dominance ranks and kinship between other individuals. In response to aggressive behaviour, primates are known to target inferior relatives of the aggressor (redirected aggression).

Ecological theories include high spatial and temporal variation (e.g. uneven distribution of seasonal food resources) and foods that require complex processing. It is likely that such ecological niches have existed for a long time and that increasingly intelligent animals are gradually able to exploit increasingly complex resources. Social learning improves such a system through the inheritance of knowledge and the formation of cultural traits.

Costs of social interactions include mate/resource competition and parasite infectivity.

Social and cultural interactions lead to extreme environmental complexity that masks the ultimate causes of human behaviour. However, cultural behaviour not only originated from biological adaptations, but is itself an evolutionary system ( Dawkins 1976).

A number of arguments have been given as to why memes will not have the same evolutionary properties as genes:

This confusion seems to have been emphasized by misunderstanding Dawkin's Selfish Gene Theory as referring to specific genetic material when it effectively does the opposite. Information regarding successful replication evolves, its storage as nucleotide bases or electrical connections is ultimately irrelevant.

“Cultural traits cannot be described as independent units” - neither can phenotypes.
“Interactions between cultural traits and their causes are very complex” - so are genotype-phenotype interactions.

A valid argument that requires investigation is the potential for memetic control by an individual. This has parallels with controlled genetic variation although memes are likely to be varied in order to benefit the individual, as opposed to memetic success.

Even the controlled production of successful memes (e.g. marketing) can be seen as an effectively random process because determination of success is extremely unpredictable.

Meme theory could be strengthened if evolutionary theory was used to predict memetic behaviour, for example, memetic equivalents of repair, reproduction, altruism, mutualism or competition.

Culture is not restricted to Homo sapiens, and studies of cultural traits and inheritance in other primates and cetaceans may aid social anthropological understanding.

Monozygotic twin studies are useful tools for determining the heritability of a trait. However, a number of factors must be taken into account. The environmental variation between separated twins is difficult to determine. Separated children are often adopted by relatives.

Controls involving dizygotic twins and other siblings could help to eliminate these problems. The significance of sharing development within the womb is becoming more apparent.

Also, the rarity of monozygotic twins seriously limits the accuracy of such studies. It is possible that studies involving non-twin siblings would provide better results if the greater sample size made up for the lower relatedness.

The relationships between genes, phenotype and the environment are extremely complex but relevant to all aspects of biology. Animal behaviour is a form of phenotypic interaction and hence determined by genetic interactions with the environment.

Metazoans are colonies of cells controlled by chemical interactions that not only determine, but are also mirrored in their behaviour. Early molecular and cellular adaptations are of great significance to the nature of modern biological systems.

The increased complexity and rapidity of neural communication, and ultimately its incorporation into social networks, has resulted in the evolution of intelligence and the unique behaviour of our own species.

A number of concepts require further investigation due to their biological significance or the challenges they pose to present evolutionary theories:

Evolvability – The existence of long-term selection processes is fundamental to understanding modern biological systems. This includes controlled variation and indirect altruism.

Control Mechanisms - The phenotypic effect of a mutation is determined by the method of genetic control. The similarities between chemical, neural and social communication and the command hierarchies they establish should be investigated.

Epigenetics – The distinction between hereditary information and its environmental interactions (the genotype/phenotype divide) is becoming less clear. This may include social learning.

Individualism – Describing biological systems in terms of distinct organisms is becoming less relevant to evolutionary theories. Symbiosis and parasitism blur the boundaries between organisms and the definition of life itself. A clear distinction between an organism and its environment is not inevitable.

Memetics – The evolution of neural (as opposed to genetic) information is especially relevant to human behaviour. Evolutionary theory should be able to make novel predictions of memetic evolution.

Sexual Reproduction – The ultimate causes of sexual behaviour are not yet understood but are relevant to the majority of metazoan behaviour. Intra-allelic competition (segregation distorters & meiotic drive) is proving to be an important branch of sexual selection. Its potential for controlling natural gene pools should not be overlooked.

Emergent Behaviour – The discovery of unpredictable emergent properties would fundamentally change our understanding of reality and the nature of science.

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