Department of philosophy and history of science

Department of philosophy and history of science
Postneodarwinistic Theories of Evolution - From the Selfish Gene to Frozen Evolution Jaroslav Flegr Department of philosophy and history of science Prague 2008

Outline Darwinisms, neodarwinisms and evolution of adaptive traits by means of natural selection Model of selfish gene – a theory of evolution of adaptive traits in sexual organisms Shortcomings of the theory of selfish gene Context-dependent phenotypic expression of gene (epistasis) Context-dependent impact of biological trait on biological fitness Frequency dependent selection (ESS) Theory of frozen plasticity Here is just an outline of my lecture. First of all, I would like to speak about the classical model of Darwinian evolution, next I am going to explain why it is not applicable to the evolution in sexually reproducing organisms, the following issue is an alternative model of evolution – the selfish gene theory, then I am going to elucidate why even this theory cannot explain evolution in sexual organisms. Finally, I am going to present the theory of frozen plasticity, which, in my opinion, could explain the evolution in both asexual and sexual organisms.

The author of the classical theory of evolution is Charles Darwin
The author of the classical theory of evolution is Charles Darwin. In his theory, he explained, among other things, the origin of adaptive traits in current organisms. Charles Darwin

Darwinistic explanation of the origin of adaptive traits
An average number of offspring/parent > 1 Populations are stable in a long term Therefore, only a fraction of the young survive and reproduce Organisms in populations differ Probability of survival and reproduction (fitness) depends on properties of an individual Offspring inherit properties and fitness of their parents Therefore, useful (adaptive) traits (structures and behavioral patterns accumulate in populations His explanation seems to be rather clear and simple and many people wonder why such a simple explanation had to wait for its discovery for so long. Everybody could see that in nature, the average number of offspring produced by a parent is higher than one (or that produced by two parents is higher than two). Still, in a long-term perspective, populations are stable, they do not grow. It follows, that in any population, only a fraction of the young survive and reproduce. Everybody, or at least naturalists, could see that organisms in populations differ in many traits, in many properties. It is more or less clear that the potential for survival and reproduction (fitness) depends on the properties of individual organisms. The individuals with more adaptive traits probably produce more offspring than individuals with less adaptive traits. Offspring inherit the properties and fitness from their parents. As a result, the useful (adaptive) traits (structures and behavioral patterns) accumulate in populations.

What is wrong with darwinistic theory of origin of adaptations?
An average number of offspring/parent > 1 Populations are stable in a long term Therefore, only a fraction of the young survive and reproduce Organisms in populations differ Probability of survival and reproduction (fitness) depends on properties of an individual Offspring inherit properties and fitness of their parents Therefore, useful (adaptive) traits (structures and behavioral patterns accumulate in populations This simple mechanism is taken for granted and can be probably found in most standard biology textbooks. However, this mechanism is wrong. It can work in asexual organisms such as bacteria but not in the majority of organisms on Earth, i.e. in sexually reproducing organisms. What is wrong with Darwin’s simple model? Everything but one thing is right. In sexual organisms, there is a huge problem with the heritability of fitness.

What is wrong with darwinistic theory of origin of adaptations?
An average number of offspring/parent > 1 Populations are stable in a long term Therefore, only a fraction of the young survive and reproduce Organisms in populations differ Probability of survival and reproduction (fitness) depends on properties of an individual Offspring inherit properties and fitness of their parents Therefore, useful (adaptive) traits (structures and behavioral patterns accumulate in populations

Vanishing nature of heritability.
genotype The fitness of an organism (that reflects its viability and fertility and can be estimated, for example, from the number of offspring produced by an individual) is determined by its phenotype (combination of all morphological, behavioral and biochemical traits). The phenotype of an organism is determined by its genotype, a unique combination of gene variants, it means a unique combination of alleles. In sexual organisms, the genotype is not inherited from parents. It is unique as it is always generated de novo by random combination of genes from the two parents. This phenomenon has a very important effect, i.e. even an excellent individual showing extremely high fitness may have offspring whose fitness is average or even subnormal. Usually, within a few generations the unique combination of genes of an excellent individual will be diluted and its relatives will differ in no way from relatives of any other individual. phenotype fitness

For a long time, the problem of the vanishing heritability of fitness was something like a little sweet secret of evolutionary biology. In the seventies of the last century, this problem was openly admitted by Richard Dawkins and a new model of evolution of adaptive traits in sexual organisms was introduced in his famous book Selfish gene. Richard Dawkins

Theory of selfish gene Individual genes are (usually) transmitted from parents to offspring unchanged Variants of the same gene (alleles) differ in their influence on phenotype and fitness of an organism Successful variants of a gene are transmitted to next generation in more copies than unsuccessful variants Therefore, variants of the same gene compete for ability to program their “vehicle” to help them to be transferred in as many copies as possible Mostly, but not always, the variants try to increase biological fitness of their “vehicle” The selfish gene theory suggests that we have to concentrate our attention on the competition between variants of the same gene (different alleles) in a DNA locus, rather than on the competition between individuals in a population. Unlike the genotype, individual alleles are mostly transmitted unchanged from parents to offspring. Variants of the same gene (alleles) differ in their influence on phenotype and fitness of an organism. Successful variants of a gene are transmitted to the next generation in more copies than unsuccessful variants. Therefore, variants of the same gene compete for the ability to program their “vehicle”, it means a macroscopic organism, to help them to be transferred to the next generations in as many copies as possible. Mostly, but not always, the variants try to enhance biological fitness of their “vehicle”. It was the reason why biologists considered individual organisms, rather than individual genes or alleles, to be the major players in the evolution.

Necker’s cube In the first edition of his famous book, Richard Dawkins suggested that his model represented just a different view of the same process of biological evolution. He likened the relationship between the classical evolution theory and selfish gene theory to that between two different interpretations of Necker’s cube illusion. We cannot tell which one of these interpretations is better, whether it is the top view interpretation or the bottom view interpretation. In the next editions of his book, Dawkins admitted that in fact the two evolutionary theories are not equal as they differ in the predicted results of certain biological processes.

Blue beard model  W : WM 8 : 5 or 4 : 5 ????????????? ♀ XX ♂ XY ♀ XX
I would like you to imagine a hypothetical mammal species. As usual, the male and the female produce ten offspring, five sons and five daughters. As ten offspring is too many to feed, one son and one daughter die from malnutrition. Therefore, four sons and four daughters survive and leave the burrow. Now imagine that a new mutation occurs on the Y chromosome of a single male in the population. Let’s call this mutation the blue beard mutation and the male with this mutation the blue beard male. The Y chromosome is present in males only while females have two X chromosomes instead of one X chromosome and one Y chromosome. The blue beard male differs from normal males in parental behavior. More precisely, the blue beard male kills all of his newborn daughters. What is the result of such ugly behavior? He has no daughters but has five instead of four sons. What is the evolutionary destiny of the blue beard mutation? According to the classical evolutionary theory, it should be eliminated as the number of the blue beard male’s offspring is only five instead of eight. The biological fitness of blue beards is therefore lower than that of normal males and consequently, the blue beard mutation should be eliminated from the population. However, an opposite prediction is implied by the selfish gene theory. The blue beard mutations are located on the Y chromosomes. Therefore, they are only present in the genome of sons, and not in the genome of daughters. As a result, the blue beard males send five copies of their mutation containing Y chromosome to the next generation while the normal males only send four Y chromosome copies to the next generation. Therefore, the blue beard mutation should spread in the population, which actually occurs in nature. From this it follows that the classical evolutionary theory and the new selfish gene theory are not equal; the selfish gene theory better describes the real biological evolution.

Theory of selfish gene Explains origin of adaptations in sexual organisms Explains evolutionary origin and success of “blue beard” genes Explains evolutionary origin and success of an important category of altruistic traits Under certain conditions, an organism could transmit more copies of its genes by helping in reproduction to its relatives than by its own reproduction (helpers, bees, ants) According to Richard Dawkins, the selfish gene theory not only explains the mechanism of the origin of adaptive traits in sexual organisms, but also explains the spread of a certain category of genes that reduce an organism’s fitness. Furthermore, it explains the spread of the so-called “altruistic genes”, i.e. the genes that are responsible for altruistic behavior. Altruistic behavior reduces biological fitness of its bearer and enhances fitness of other population members. The selfish gene theory shows that, under certain circumstances, an organism could pass more copies of its genes by helping its relatives reproduce than by reproducing itself.

Theory of selfish gene is now the mainstream evolutionary theory
The selfish gene theory is now the mainstream evolutionary theory. When a classical evolutionary biologist looked for the purpose of a trait, for example, when wondering what the purpose of the armor of the well adapted hypothetical animal was, his/her question was how this trait enhanced an individual’s fitness. When a current evolutionary biologist is looking for the purpose of such a trait, his/her question is how this trait increases the number of copies of the gene variant involved in the formation of this trait. Darwin: How this trait increases fitness of an individual? Dawkins: How this trait increases a number of copies of variant of gene that is responsible for its formation?

What is wrong with the selfish gene theory?
It disregards a phenomenon of frequency dependent selection It disregards context-dependency of an impact of biological trait on biological fitness It disregards context-dependency of phenotypic expression of gene (epistasis) Unfortunately, even the selfish gene model of evolution has its serious drawbacks. It probably oversimplifies the process of biological evolution. It neglects at least three very important biological phenomena, the frequency-dependent selection, context dependency of the effect of biological traits on fitness and context dependency of the effect of a gene on biological traits.

Frequency dependent selection
A selective value of many traits depends on frequency of particular traits in population. b/2 b/2 Therefore, not the highest fitness but an evolutionarily stable strategy (ESS) wins What is a final frequency of hawks in a population (p)? Profit of hawks: ZH = p(b – c)/2 + (1 – p)b Profits of doves: ZD = 0 + (1 – p)b/2 in equilibrium: ZH = ZD p(b – c)/2 + (1 – p) b = 0 + (1 – p)b/2 p = b/c = benefit/cost b The biological value of a particular trait is often dependent on its frequency as well as on that of other traits in the population. The value is high when the trait frequency is low and either continuously or discontinuously decreases as the frequency rises. Here is the well-known hawks and doves model. Imagine a population of hawks and doves that represent two distinct behavioral strategies in combat. When two doves find a piece of food, they peacefully share it and each of them gets on average one half of the benefit. When a dove and a hawk find a piece of food, the hawk drives away the dove to get all the benefit for itself. When two hawks find a piece of food, they fight for it and each of them gets on average one half of the benefit and pays one half of the cost – the cost here means time lost and possible injuries sustained. It is a major asset to be a hawk in a population of doves. At the same time, it is an asset to be a dove in a population of hawks, especially when the cost of combat is very high. From this it follows that neither the dove nor the hawk strategy can win the competition and some equilibrium frequency of the strategies will be established. When mixed strategies are allowed, then the so-called evolutionarily stable strategy wins. Here the evolutionarily stable strategy means “behave with frequency b/c as a hawk and with frequency (1-b/c) as a dove”. Generally, the evolutionarily stable strategy is a strategy, which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare. It is clear now that this kind of stability, rather than biological fitness, is the most important criterion of evolutionary success in the competition between gene variants. Evolutionary stable strategy: to behave with probability b/c as a hawk and with probability 1-b/c as a dove. (b-c)/2 (b-c)/2

Frequency dependent selection
A selective value of many traits depends on frequency of particular traits in population. Therefore, not the highest fitness but an evolutionarily stable strategy (ESS) wins b/2 b/2 What is a final frequency of yellow chromosomes in a population (p)? Profit of red chromosomes : ZR = p(b – c)/2 + (1 – p)b Profits of yellow chromosomes: ZY = 0 + (1 – p)b/2 in equilibrium: ZY = ZR p(b – c)/2 + (1 – p) b = 0 + (1 – p)b/2 p = b/c = benefit/cost In sexually reproducing organism the competition between strategies for this kind of stability probably controls the evolution of all genes. In the previous model, the hawks and doves meet each other over a piece of food. In sexual species, pairs of alleles of different genes meet after combination of male and female sex cells in a newly formed organism. The competition between alleles therefore corresponds to that of hawks and doves shown in the model. For each allele, an equilibrium frequency exists that can be shifted, for example by selection; however, it will be restored after the end of the selection. b Evolutionary stable strategy: to behave with probability b/c as a yellow chromosome and with probability 1-b/c as a red chromosome. (b-c)/2 (b-c)/2

Effect of selection on morphological trait (body size)
end of selection body size Here we can see the result of a typical selection experiment. We can select large mice by killing all small mice. It is analogical to shooting the hawks in our model population. The pay-off matrix in the game is consequently changed since to be a hawk as well as to be an allele for small body size will become less advantageous than at equilibrium before the selection started. The frequency of such allele and therefore also the average body size of our mice decreases. However, as the frequency of this allele goes down, the advantage of being such allele (being a hawk) goes up. Therefore, our selection is decreasingly less effective and at one point our population stops responding to our selection. When we terminate our experiment, it means when we stop killing small mice or stop shooting hawks, the frequency of the given allele (of hawks) will return to the equilibrium value. It means, the population of sexual organisms under artificial or natural selection mostly behaves elastically rather than plastically. time (generations) Mayr E. 1964: Animal species and evolution. Cambridge Press, Cambridge.

Context-dependency of an impact of a trait on biological fitness
This cartoon illustrates another drawback of the selfish gene theory. The same trait, here the colts hanging pretty damn low, can, in combination with some traits, increase fitness of its bearer, while in combination with other traits, here the short arms, can dramatically reduce the fitness of an individual. Then, tell me, what am I to do with my colts hanging pretty damn low, with my pretty damn short arms!

Gene-phenotype trait relationships
pleiotropy Gene 1 Gene 2 Gene 3 Gene 4 Trait 1 Trait 2 Trait 3 Trait 4 Gene 1 Gene 2 Gene 3 Gene 4 Trait 1 Trait 2 Trait 3 Trait 4 epistasis And the last but probably not the least drawback: The relation between genes and traits is not as straightforward as supposed by most of the evolutionary models, including the selfish gene theory. Due to pleiotropy, one gene usually influences a number, sometimes even a large number, of traits and due to epistasis one trait is usually influenced by multiple genes. Moreover, the influences of particular genes on a trait are not additive. The same gene variant in the context of one combination of genes influences a given trait, for example length of arms, positively, while in the context of another combination of genes it may influence the trait negatively. Consequently, there is no such thing as a gene (allele, mutation) for short arms. The phenotypic expression of most genes is in fact genotype conditioned.

Gene-phenotype trait relationships
pleiotropy epistasis Gene 1 Trait 1 Gene 1 Trait 1 Gene 2 Trait 2 Gene 2 Trait 2 Gene 3 And the last but probably not the least drawback: The relation between genes and traits is not as straightforward as supposed by most of the evolutionary models, including the selfish gene theory. Due to pleiotropy, one gene usually influences a number, sometimes even a large number, of traits and due to epistasis one trait is usually influenced by multiple genes. Moreover, the influences of particular genes on a trait are not additive. The same gene variant in the context of one combination of genes influences a given trait, for example length of arms, positively, while in the context of another combination of genes it may influence the trait negatively. Consequently, there is no such thing as a gene (allele, mutation) for short arms. The phenotypic expression of most genes is in fact genotype conditioned. Trait 3 Gene 3 Trait 3 Gene 4 Trait 4 Gene 4 Trait 4

Problem with heritabilaty of fitness
I believe that all three disregarded phenomena have a fatal impact on the selfish gene theory which was originally suggested to solve the problem of vanishing heritability of fitness in sexual species. The selfish gene theory is better than any of the previous evolutionary theories (it explains a broader spectrum of biological phenomena); however, it fails in its major purpose, i.e. in explaining the origin of adaptations in sexual organisms. Then, tell me, how can help me that an allele is transmitted from generation to generation unchanged when it has in each subject different impact on phenotype and different impact on biological fitness!

What is wrong with the selfish gene theory?
It disregards context-dependency of an impact of biological trait on biological fitness It disregards context-dependency of phenotypic expression of gene (epistasis) It disregards a phenomenon of frequency dependent selection Therefore, it seems that we are in the same situation as were evolutionary biologists in the middle of the last century. We have a nice theory that smoothly explains the origin of adaptation in asexual organisms but that is not applicable to the majority of organisms, i.e. to sexual species. We are lucky that creationists are not yet smart enough to point out the real drawback of our theories. Selfish gene theory is better than previous evolutionary theories (it explains broader spectrum of biological phenomena), however, it fails in its major purpose, i.e. in explaining origin of adaptations in sexual organisms.

Neither Darwin nor Dawkins, mama mia, what to do next?
creationist Therefore, it seems that we are in the same situation as were evolutionary biologists in the middle of the last century. We have a nice theory that smoothly explains the origin of adaptation in asexual organisms but that is not applicable to the majority of organisms, i.e. to sexual species. We are lucky that creationists are not yet smart enough to point out the real drawback of our theories.

Origin of new species - Speciation
sympatric dichopatric Fortunately, we have now some theories and data that can solve the problem. These theories exist for more than 50 years. The theory of speciation introduced by Ernest Mayr shows that there are principally three different types of the origin of new species, i.e. speciation by splitting from an old species. The first type is sympatric speciation, with a new species emerging within the area of an old species. The second type is dichopatric speciation, with a new species arising due to splitting of the original population into two populations of similar sizes, for example as a result of the emergence of a new geographical barrier. The third type is peripatric speciation, with a new species originating from a very small number of individuals, for example, as a result of colonization of a new distinct area such as an oceanic island. This third type is the most relevant to us as this type of speciation can solve our problem with the origin of adaptation in sexual species. peripatric

Role of peripatric speciation in evolution
colonization homogenization by genetic drift population growth During the colonization of an island, only a small part of genetic variability originally present in the large population is retained by the new population. In the next generations, the new population is still small and most of the retained genetic variability fades away due to the so-called genetic drift. In a small population, chance rather than selection rules the destiny of particular organisms and particular genes. No matter whether you are a dove or a hawk – your biological success is just a question of chance. If the colonization is successful, the population has to grow up at one point. (Unsuccessful populations are not interesting from the perspective of evolution.) At that very moment, the optimal conditions for adaptations to evolve by means of selection are present. The population is large; therefore the selection, rather than chance rules the destiny of an individual. Still, the population is genetically uniform; therefore a new mutation meets the same combination of genes in all individuals and has the same influence on phenotype of the organism and its biological fitness. In such population, both the Darwinian selection of individuals for best fitness, and the dawkinsonian selection of most efficiently replicating genes could work and produce new adaptive traits. During the time, more and more mutations with frequency-dependent influence on fitness are captured in the population. Therefore, genetic variability of the population increases and an originally plastic species turns into elastic. freezing by accumalation of genetic variability adaptation by natural selection

Theory of frozen plasticity
An average number of offspring/parent > 1 Populations are stable in a long term Therefore, only a fraction of the young survive and reproduce Organisms in populations differ Probability of survival and reproduction (fitness) depends on properties of an individual Offspring inherit properties and fitness of their parents Therefore, useful (adaptive) traits (structures and behavioral patterns accumulate in populations All these holds only in asexual species or in large genetically uniform (plastic) population, ie. just after birth of species by peripatric speciation For most of time (98-99% of life of a species), the sexual species are evolutionary frozen. Therefore, the theory of frozen plasticity is, in a certain sense, a return to the original Darwinian model of evolution. The only difference is, that according to the new theory all processes described by Darwin and Dawkins could operate just after the birth of a species by peripatric speciation – as long as the species is still plastic. For most of the time (98-99% of the species existence), the sexual species are evolutionary frozen and only passively wait for such changes in their environment that cause their extinction.

What genetic experiments say
end of selection body size What is an empiric support for the theory of frozen plasticity? I have already mentioned that populations usually behave as elastic rather than plastic in most selection experiments. The existence of the so-called genetic homeostasis and return to the original state after the end of selection was described in the middle of the last century. time (generations) Mayr E. 1964: Animal species and evolution. Cambridge Press, Cambridge.

What a paleontology says
Actually, the paleontologists were the first who openly admitted the drawbacks of the current evolutionary theories. According to the classical theories, the species should accumulate changes gradually in the course of their existence. However, the paleontological data mostly suggest a quite different picture of the evolution. Most multicellular (you can read sexual) species appear in the paleontological record in their final form and remained unchanged throughout their existence. Eldredge and Gould called their model of evolution the punctuated equilibrium model and suggested several possible mechanisms for its explanation. Interestingly, a model very similar to the frozen plasticity theory was presented in their first common paper. Nevertheless, in their later works, they unfortunately rejected this model because of its alleged inconsistency with the contemporary genetic theories of evolution. time (5 milions years) Eldredge,N. 1971: Allopatric model and phylogeny in paleozoic invertebrates. Evolution, 26,

What a biogeography says
Biogeography also provides some support to the frozen plasticity theory. The comparison of the species or higher taxa living on islands and on the mainland shows that the former are on average more divergent from their original body plan than the latter. It can be explained by a higher frequency of peripatric speciation on islands than on the mainland. Ricklefs,R.E. Cladogenesis and morphological diversification in passerine birds. Nature 430, , 2004.

What molecular phylogeny says
Recent molecular studies have shown that even the molecular evolution, it means accumulation of mutations in DNA, operates in a punctuated way. A large part of mutations accumulate in time of speciation while in later phases of the species existence, the mutation accumulation rate slows down. Contrary to the predictions of the classical gradualist theories of evolution, the divergence of the evolutionary line reflects the number of speciations rather than the age of the line. Mark Pagel,* Chris Venditti, Andrew Meade: Large Punctuational Contribution of Speciation to Evolutionary Divergence at the Molecular Level Science 314, , 2006

Differences between classical and frozen plasticity theory
This table summarizes important differences between the predictions of the classical and frozen plasticity models of evolution. Asterisks denote the predictions that have already been tested and support the frozen plasticity model. I believe that the most important difference, and at the same time the take-away message of the present lecture, is expressed in point 3. According to the classical theory, all species respond to selection like a piece of plasticine, it means in a plastic way, while according to the frozen plasticity theory, 99% of species respond to selection like a piece of rubber, it means in an elastic way. Everything else follows. More details are given in my book entitled Frozen evolution. Flegr J.: Frozen evolution. Charles University, Prague 2008. Amazon, or

Conclusions Neither darwinism, nor neodarwinism can explain adaptive evolution in sexual species Selfish gene model could operate only in systems without context-dependent gene expression and context dependent fitness Theory of evolutionarily stable strategies shows that sexual species respond to selection elastically After peripatric speciation, a new species turns plastic After restoration of genetic polymorphism, the specie freezes and rest of its existence just passively waits for its extinction Frozen plasticity theory could explain broader spectrum of biological phenomena than classical evolutionary theory Here, the major points of the present lecture are summarized: Neither darwinism nor neodarwinism can explain adaptive evolution in sexual species. The selfish gene model could operate only in systems without context-dependent gene expression and context-dependent fitness. The theory of evolutionary stable systems shows that sexual species respond to selection elastically. After peripatric speciation, a new species turns plastic. As soon as genetic polymorphism is restored, the specie freezes and for the rest of its existence, just passively waits for its extinction. The frozen plasticity theory could explain a broader spectrum of biological phenomena than the classical evolutionary theory.

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