

The Great Debate: Darwinism Today

Selected Notes from a ten week course by Caspar Hewett

Introduction: Evolution by Natural Selection

evolution n. the doctrine according to which higher forms of life have gradually arisen out of lower

natural selection evolution by the survival of the fittest with inheritance of their fitness by the next generation

gene, n. one of the units of DNA, arranged in linear fashion on the chromosomes, responsible for passing on specific characteristics from parents to offspring. –adj. genic of or relating to a gene. –ns. genome the full set of chromosomes of an individual: the total number of genes in such a set; genotype genetic or factorial constitution of an individual: group of individuals all of which possess the same genetic constitution. Chambers 20th Century Dictionary

A gene is life's way of remembering how to perpetuate itself. Genethics, Suzuki & Knudtson (1990)

A gene is any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection. The Selfish Gene, Dawkins (1976).


Genes are the basic vehicle of biological inheritance. They have a chemical memory which is recorded in the internal structures of a family of biological molecules, the nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

A human cell contains 46 strands of DNA, called chromosomes, in its nucleus, 22 matched pairs and two sex chromosomes. This DNA contains the information necessary for synthesising proteins and for regulating cellular processes.

DNA molecules have two distinct strands which are held together by weak hydrogen bonds to form a double helix. Each strand is made up of a series of small molecules called nucleotides. Only four nucleotides are used, and they are the same in all animals and plants. They each contain a deoxyribose sugar, a phosphate, and one of four kinds of nitrogen-containing bases, which are usually denoted by the first letters of their names: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).

Each base has distinctive chemical characteristics which allow it to bond with only one of the other three bases. Thymine will combine only with Adenine and Cytosine will combine only with Guanine. Thus there are four possible combinations; (T-A), (A-T), (C-G), (G-C). This is extremely important in that it causes the two strands to be complementary to each other - when they separate, each strand acts as a template to form its complement.

DNA encodes the genetic information for producing proteins in the order of the bases along the DNA. The proteins themselves are made up of amino acids. A three-base sequence along the DNA contains the instructions to make one amino acid. RNA provides the mechanism though which DNA transmits its message to the cytoplasm of the cell.

RNA differs from DNA in that it has ribose sugars and it has another base, Uracil, which replaces T alongside A, G and C. Uracil bonds with adenine forming (U-A) and (A-U) combinations. RNA is usually a single-stranded molecule forming a single helix.

Some Definitions

A protein is any member of a group of complex nitrogenous substances that play an important part in the bodies of plants and animals. Proteins are compounds of carbon, hydrogen, oxygen, nitrogen, usually sulphur, often phosphorus etc.

Amino acids are fatty acids which are the building blocks of proteins.

A gamete is a reproductive cell. In the case of sexual reproduction, an egg-cell or sperm-cell.

A species is ‘a group of individuals sharing common characteristics, a group (sometimes rather arbitrarily defined) of closely allied mutually fertile individuals showing constant differences from allied groups, placed under a genus (biol.)’ (Chambers 20th Century Dictionary)

Speciation is the fragmentation of one species into two or more descendant species.

Genetic drift is a chance mechanism that alters gene frequency from generation to generation without the guiding determinism of natural selection (due to Sewall Wright).

Taxonomy is the classification of plants and animals into a detailed hierarchy consisting of phyla, classes, orders, families, genera, species and sub-species or varieties.

A strategy is a term used for a pre-programmed behavioural policy adopted by an animal, i.e. it is presumed not to be conscious.

An Evolutionarily Stable Strategy (ESS) is a strategy which, if most members of a population adopt it, cannot be bettered by an alternative strategy.

Convergence is a term used to describe instances where species from different genetic lineages develop similar physical characteristics

A phenotype is an observable characteristic of an organism produced by the interaction of its genes and environment e.g. eye colour, having four legs, inherited behavioural patterns.

A deme is a local breeding population of a species.

An avatar is a local economic population of a species.

soma is a term for the body of an organism excluding the germ cells.

The Tree of Life

Darwin and Natural Selection

Charles Robert Darwin published On The Origin of Species by Means of Natural Selection in 1859. He wanted to establish three things;

  1. The notion of evolution. That all organisms are descended from a common ancestor.
  2. That life on Earth evolved by the process of natural selection.
  3. Life has existed for the incredible length of time necessary for the diversity of life which exists today to have evolved in this way.

Although there were other theories of evolution around at the time, Darwin’s great contribution was the provision of the mechanism by which it took place. The consensus at the time was that evolution had not taken place. Species were considered to be distinct and static rather than as interrelated and changing. Darwin’s vision was one in which change is inevitable and continuous. Species might seem stable but are actually constantly evolving themselves out of existence.

Darwin’s knowledge of animal husbandry (in particular breeding pigeons) and selective plant breeding informed his idea of natural selection. Through selective breeding, bloodlines can be changed fairly quickly. Darwin saw that the attributes of those individuals who were more successful at surviving and breeding than others would spread through the population by a similar but slower process. Those better suited to a particular environment in any generation would have an advantage that would ensure that the next generation contained a high percentage of their offspring. Organisms should then become well adapted to the environment in which they live.

Darwin, of course, did not know what the vehicles of heredity are (genes) nor its mechanics. The concept of natural selection depended only on the basic observations that individual organisms differ, and that they tend to resemble their parents.

Mendelian Genetics

The foundation for the field of genetics was laid by the Augustine monk Gregor Mendel in the nineteenth century. Before he published his work, the vehicle of inheritance was thought to be blood-borne substances which mingled together to form offspring. Individuals were seen as being the average of their parents. Mendel’s work proved otherwise. In 1865 he published the results of his experiments with selective breeding using hybrid varieties of the common garden pea, Pisum sativum.

He took advantage of two useful properties of peas:

  • Each plant has both male and female organs. This means that any female flower can be fertilised by pollen from any male. This includes the possibility of self-fertilising plants.

  • They have true-breeding lines within which every individual looks the same. Different lines have different physical characteristics such as stem length, seed texture and colour.

Mendel discovered, by following inheritance of a number of selected physical characteristics, that inherited traits seemed to be controlled by pairs of discrete hereditary ‘particles’. Adult plants had two copies of each particle, but pollen and egg carried only one. These ‘particles’, of course are what we describe as genes today.

Mendelian cross showing dominance

Mendel also found that certain genes seemed to exert a more decisive influence on inherited traits than others. These genes are described as dominant (the gene for yellow peas, smooth seed) and those they dominate are described as recessive (green peas, wrinkled seed). Since each pea plant has two complete sets of genes, each trait could be determined by either of two possible alternative genes, or alleles. The plant is described as homozygous if the alleles are the same and heterozygous if they are different.

Mutation and Discontinuity

Mendel's work was largely undiscovered until the beginning of the twentieth century, and as such was completely unknown to Darwin. However, with the rediscovery of Mendel’s work in the early part of the century came a decline in the perceived importance of natural selection. This was reinforced by the discovery of mutation by DeVries. An alternative theory of evolution which emphasised the importance of mutation was proposed and was thought by many to supersede the theory of evolution by natural selection. Early experiments in genetics showed mutations having large-scale effects, which seemed to confirm the existence of rapid, discontinuous evolutionary change. However this had to be tempered with the observation that large mutations were often harmful and sometimes lethal.

There are echoes of this idea in the work of the geneticist Richard Goldschmidt in the 1930s and 1940s. He observed significant discontinuity when studying the genetic variation of the silkworm moth. He attempted to explain this by invoking large-scale mutations or "hopeful monsters". Goldschmidt and the other saltationists were ridiculed by the Neo-Darwinists for their ideas and provided a focus around which the Neo-Darwinists united.

Theodosius Dobzhansky, also writing in the 1930s stressed the importance of geographic and reproductive isolation, another aspect of discontinuity. Dobzhansky argued that when a species divides into two or more discrete reproductive communities natural selection will tend to hone the adaptations of each community to their particular environment, which would tend to minimise variation within new species. Dobzhansky went even further in saying that isolation between species is a direct outcome of the evolutionary process.

Neo-Darwinism – The Modern Synthesis

The 1920s saw the emergence of Neo-Darwinism, which was founded by Ronald Fisher, J. B. S. Haldane and Sewall Wright. They reconciled the idea of evolution by natural selection with the discontinuous, particulate nature of genes. This was the essence the modern synthesis of Darwin's theory and Mendelian genetics.

The new synthesis continued to develop in the 1940s, notably with Julian Huxley’s, Evolution: The Modern Synthesis (1942) and Bernhard Rensch’s, Evolution Above the Species Level (1947). Natural selection was seen as the dominant force shaping evolutionary change. Rensch expressed the view that nothing in biological nature suggests that any evolutionary processes other than natural selection work on the natural genetics of variation within populations.

Modern Neo-Darwinism – The Selfish Gene

Richard Dawkins epitomises what is described by some as the ultra-Darwinian view. In his 1976 book The Selfish Gene he presents the theory of evolution by natural selection in a very general way as an algorithm. First he introduces the concept of a replicator, which is anything which is capable of making replicas of itself. He argues that the two conditions necessary for the evolutionary process to begin are the existence of a replicator and the possibility of small errors being made in the copying process. Once it exists a replicator can generate copies of itself indefinitely. Mistakes in the copying process will lead to a situation where there is more than one variety of replicator in the population. Some new varieties will be less effective at self-replicating than the old, some will not be able to replicate themselves at all, but some new varieties will be better than the old - This may be because they produce copies of themselves more quickly than their predecessors (fecundity), because they survive for longer and thus are able to self-replicate for longer (longevity), or because they are less prone to errors in the copying process (accuracy). Whatever the reason, such a replicator's descendants will naturally come to dominate the population. The ‘fittest’ replicator then can be thought of as the one of which there are the most surviving copies.

As the population of replicators grows there will be increasing competition between them. This will lead over time to the evolution of increasingly elaborate ways of surviving and replicating. Certain characteristics will be selected for, i.e. replicators which have them will be more abundant than those that do not. The success of a replicator will depend on its environment, and especially on the effects other replicators have on it and on its environment. Mutually compatible replicators will evolve and perhaps larger vehicles which house the replicators will follow. This does indeed appear to have been the case on earth.

The formation of the first replicating molecule was an ‘exceedingly improbable’ accident. However, once such a molecule existed, the process above - evolution by natural selection - began. At some point in evolutionary history mutually compatible replicators created vehicles - cells and, later, many-celled bodies. The replicators of today are DNA and we - that is humans, animals and plants - are their survival machines!

Dawkins thus invites us to think of the body as a colony of genes. He emphasises that the basic unit of natural selection is best regarded as the gene.

It is worth noting that we often use purposive language when we speak about natural selection, for example ‘the robin evolved wings for flying’ does not imply any purpose in the sense that someone designed the wings with a purpose in mind - It is accepted that natural selection is a blind process, but it is wise to be wary of using this type of language, especially when discussing animal behaviour.

Animal Behaviour - Altruism

Dawkins tells us to expect ‘ruthless selfishness’ in a successful gene. However, he spends much of The Selfish Gene discussing altruistic behaviour and how it evolved. His argument hinges on the idea that the selfish gene is not a specific piece of DNA, but is rather ‘all replicas of a particular bit of DNA, distributed throughout the world.’ This emphasis on the distributed nature of the gene leads to the observation that, if an animal has some way of knowing that copies of its own genes reside in other individuals, then its genes have good reason to programme that animal to behave altruistically towards those individuals. Often what appears to be altruistic behaviour is actually gene selfishness. Thus ‘there are special circumstances in which a gene can achieve its own selfish goals best by fostering a limited form of altruism at the level of individual animals.’ This is the argument behind kin selection and, less obviously is applied to a range of seemingly altruistic behaviour that depends on mutual benefit of the individuals involved, both within species and across boundaries between species.

A simple replicator

Conflicts of interest in the family

  • There is a real conflict of interest between parent and child. In practice, some compromise is usually reached between the ideal situations of the parent and its offspring.

  • Genes in the bodies of children are selected for their ability to outsmart their parents, and genes in the bodies of parents are selected for their ability to outsmart their offspring.

Resources invested by parents on offspring

  • Food
  • Energy and time expended obtaining food and maintaining the nest or home
  • Time spent teaching children
  • Risks taken to protect young

Definition: Parental Investment (PI) is any investment by a parent in one of her (his) offspring that increases the chance that the offspring will survive at the expense of that parent’s ability to invest in any other offspring (alive or yet to be born).

Sibling Rivalry

Dawkins states that much of child behaviour is characterised by selfish greed. However, in the case of monogamous species, the relation of an individual child to its siblings is the same as that of its mother, ½. That is, on average, the child will share half of its genes with any one of its siblings. Thus we would expect the child to want some of the parental investment to go to its brothers and sisters, especially if they are contemporaries. Theory would predict that the child will stop grabbing extra at that point where the resulting cost to its sibling is twice the benefit to itself.


There is no genetic reason for a mother to have favourites. She has the same relatedness to all of her children – they each have half of her genes. Thus her optimal strategy is to have as many children as possible that she can successfully raise to survive to reproductive age.


Weaning is another example where there is a conflict of interest between mother and child. The disagreement is essentially one over timing. We would expect the mother to want to wean child when she reaches a point when her PI could be better spent on having another child, or even on other relatives such as grandchildren, nieces, nephews. In contrast, the child will not want to be weaned until the cost to its unborn sibling is twice the benefit to itself.

Conflict between mates

  • In the past sexual behaviour was sometimes considered as a co-operative venture undertaken for the good of the species or at least for mutual benefit of the two partners. In contrast, from the selfish gene perspective (following the work of R.L. Trivers), the relationship of sexual partners is seen as one based on mutual mistrust and exploitation!

  • There is a fundamental asymmetry between the sexes in most species. Male gametes (sperm) are more mobile, smaller and more numerous than those of females (eggs). This means that a female can have only a limited number of offspring, whereas a male can have a virtually unlimited number, provided that he can find females willing to mate with him.

  • In some fungi and other primitive organisms there are not two distinct sexes, this is known as isogamy. Rather than having two types of sex cells, sperms and eggs, all of the gametes are the same – isogametes. A new individual is formed by the fusion of two isogametes, each produced by meiotic division.

  • There are a variety of reasons why the asymmetry between the sexes may have evolved from an isogamous state (See exercises).


The asymmetry discussed above puts the female at a disadvantage. She invests much more than the male initially and thus is open to desertion by her mate. If there is any chance that the female will successfully rear their young without his help the male’s best strategy is to leave and find another mate. Thus we would expect to find a bias towards lack of paternal care to evolve, and it is observed in some species. However there are many other selection pressures which could act against this tendency.

Desertion strategies

Given that the male has deserted, the female can adopt one of a number of strategies:

  • Deceive another male – There would then be selection pressure against gullibility in males. This can be one reason why a prolonged period of courtship can benefit a male – so that he can be sure that any children she produces are his.

  • She can abort - This is likely to be a preferred option if the child has only just been conceived. If she is not likely to deceive another male this strategy could be to mutual advantage.

  • Rear the young herself - which is most likely to pay if they are quite old.

  • Counter-desert - It could pay either partner to be the first to desert!


In considering the possibility of being deserted the female can play her ace – she can refuse to copulate. Dawkins proposes that, in choosing a mate, a female has two main strategies open to her.

  • Domestic-bliss strategy : The female looks for males who exhibit signs of fidelity and domesticity. One way she can achieve this is by being coy – the male has to prove his faithfulness and perseverance. Courtship rituals can represent some considerable investment by the male before copulation. In the case of some birds the male has to build a nest before mating, which then represents a direct investment by the male in the eggs! Breeding with last season’s mate also a good strategy for a female.

  • He-man strategy : The female goes for the best genes, accepting that she will get no help from the father. She refuses to copulate with all but the best males. In this situation only the best males would reproduce. The female would look for evidence of ability to survive and reproduce – qualities such as strong muscles, long legs, age. It is worth noting that once females begin to select their mates on the basis of a useful attribute such as long legs, then we would expect natural selection to favour that attribute purely because it is attractive to females.


  • Previous investment by either parent in their offspring is no deterrent to desertion provided there is a good chance that those offspring will survive without them!

  • Behavioural strategies such as the domestic-bliss strategy often depend on recognition between individuals and thus is one of many reasons why many animals recognise other individuals.


The notion of stasis is based on the observation that species appear to change very little throughout their existence. Darwin acknowledged the abrupt appearance of species in the fossil record and their apparent stability in the sixth edition of On The Origin of Species. Darwin had his own reasons for emphasising gradualism in the evolutionary process – he was trying to establish the notion of evolution itself! The existence of stasis is seen by those in the ‘naturalist’ camp, notably Stephen Jay Gould and Niles Eldredge, as requiring explanation; there is no doubt that natural selection is the motor by which adaptive change takes place but, they argue, the question remains why adaptive change takes place when it does.

Habitat Tracking

Given an environmental change there are three possible outcomes for a species:

  • A geographical move
  • Adaptation to the new environment.
  • Extinction

The notion of habitat tracking is simply based on the observation that when environmental change takes place, it is common for species to move geographically – finding a suitable habitat elsewhere. According to Niles Eldredge, adaptation is the least likely of the three outcomes above, with extinction in second place!

Punctuated Equilibria

Niles Eldredge describes punctuated equilibrium as "a molding, in essence, of the pattern of stasis with the recognition that most evolutionary change seems bound up with the origin of new species - the process of speciation." The importance of the theory is that it tells us something about the context of evolutionary change - why evolutionary change happens when it does. He and Stephen Jay Gould introduced the theory in their 1972 paper Punctuated equilibria: an alternative to phyletic gradualism in an attempt to deal with some of the problems for evolutionary theory highlighted by palaeontology:

  • Persistent patterns of little or no change are found within fossil samples in a particular location (stasis)

  • Evolutionary novelty usually appears abruptly in the fossil record and it is very difficult to know whether an organism evolved at the location where its fossil was found or whether it came from elsewhere.

  • Only a tiny percentage of all organisms ever appear in the fossil record!

Eldredge and Gould were inspired by the work of Dobzhansky and Mayr on species and the speciation process, especially the idea that descendant species derive from their ancestral species through geographic isolation. Paterson's Specific Mate Recognition System provides further justification for the idea.

Eldredge and Gould begin their 1972 paper by noting how difficult it is to develop new theory when our interpretation of facts depends heavily on old theory: ‘New pictures must cast their influence before facts can be seen in different perspective.’ They state that the picture of ‘phyletic gradualism’ has dominated palaeontology and that, because of this, all breaks in the fossil record are treated as imperfections in the record rather as real phenomena requiring explanation. They identify the following as the tenets of phyletic gradualism:


  2. The transformation is even and slow.


  4. The transformation occurs over all or a part of the ancestral species’ geographic ranges.

Eldredge and Gould argue that, if evolution does indeed occur in this gradualist fashion, then the fossil record would be expected to ‘consist of a long sequence of continuous, insensibly graded intermediate forms linking ancestor and descendant.’ In practice there are usually morphological breaks found in postulated phyletic sequences, but these are traditionally put down to the imperfections of the fossil record. This approach ‘renders the picture of phyletic gradualism virtually unfalsifiable.’ Further, even the term "morphological break" presupposes the continuous nature of such sequences. Thus, they argue that only an alternative picture is capable of shedding light on cases in which breaks are found and to treat them as real phenomena.

They advance the theory of allopatric (or geographic) speciation as providing an alternative picture. This is a theory that developed through the study of modern species’ distribution, ecology and behaviour. According to this theory ‘new species can arise only when a small local population becomes isolated at the margin of the geographic range of its parent species.’ These peripheral isolates evolve into a new species if isolating mechanisms develop which make reproduction impossible with the parent species should their paths cross at a later date. This means that we would not expect new fossil species to appear in the same place as their ancestors! Speciation relies on the limited gene pool of the small initial population to speed up morphological change – large gene pools tend to be conservative. This suggests a picture of (relatively) rapid change initially followed by stability once the new species is established. Thus ‘it is likely that the two species will display their greatest difference when the descendant species first appears.’ This pattern is borne out by the fossil record – which can thus be taken as much more reliable than tradition dictates.

Eldredge and Gould see the picture of phyletic gradualism as inadequate to explain speciation because it ‘fails to recognize that speciation is primarily an ecological and geographic process.’ The theory of punctuated equilibria considers speciation to be a component of the evolutionary process: ‘The history of evolution is not one of stately unfolding, but a story of homeostatic equilibria, disturbed only "rarely" (i.e., rather often in the fullness of time) by rapid and episodic events of speciation.’ Eldredge describes punctuated equilibria as providing ‘the context - the boundary conditions - for much of what really matters in terms of adaptive change in evolutionary history.’ Thus speciation is the key to understanding why we find long periods of stasis being interrupted by shorter periods of (relatively) rapid evolutionary change.

Eldredge and Gould found themselves accused of proposing a form of saltationism. To explain why this is not the case Eldredge draws attention to the time-scales relevant to palaeontology:

  • Most species of marine vertebrates last between 5 million and 10 million years.

  • Terrestrial animals do not tend to last quite as long, mainly thanks to the more frequent environmental changes experienced on land.

  • The fossil record only has a resolution of tens of thousands of years.

  • Eldredge estimates the time required for speciation to be five to fifty thousand years.

In Darwin's Dangerous Idea (1995) Daniel Dennett questions whether there is anything new in the punctuated equilibrium thesis. Like Eldredge, he draws attention to the problem of time-scale. The left hand diagram in Figure 1 represents the orthodox Neo-Darwinian view, while the right hand diagram represents punctuated equilibria. The horizontal axis represents changes in design and the vertical axis is time. On close inspection – when the time-scales are not quite so vast – Dennett questions whether there is any real difference between these two views, especially when Gould has clearly stated that he does not propose saltations – speciation takes place gradually – the apparently horizontal sections of the right hand diagram are actually a ramp over the speciation period.

Figure 1

Figure 2. Differing views of a geological instant (after Dennett)

Dennett argues that, rather than challenging gradualism itself, what Eldredge and Gould were really saying is that most of the time evolution is not even gradual – it is at a standstill. This interpretation is borne out when Eldredge stresses that there is no need to imagine that large-scale mutations would be required for speciation to take place - Natural selection, provided it acts on enough heritable variation, should be a good enough motor to affect the evolutionary change observed.

Figure 3. Anagenesis and cladagenesis (after Dennett)

Thus what they are really challenging is "constant speedism" which is not the orthodox view. In fact, Darwin himself noted how little change species seem to undergo once formed. Dennett goes on to argue that ‘the fossil record could only show periods of stasis that suggest that evolution is not even gradual.’ He also points out the problem that, by definition species can only be identified if they exhibit some modicum of stasis, so again it is impossible for the fossil record to show otherwise.

Dennett uses the Figure 3 to illustrate how it is impossible to distinguish between anagenesis and cladogenesis. Anagenesis is change without speciation: the left hand figure shows a single species undergoing periods of rapid change in design followed by periods of stasis. Cladogenesis is change via speciation, illustrated by the right hand figure. He uses this as a counter to Eldredge and Gould's argument that horizontal steps (representing changes in design) are steps of speciation.

What are species?

As we have already seen, Darwin set out to destroy the notion of species as having a real existence in order to establish the idea that evolution, with the aid of its motor natural selection, had indeed taken place. He established that species are neither eternal nor immutable, but rather that they had evolved over time. So, what are species?

Ernst Mayr characterised them as ‘groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.’ He believed that species are real entities, although his was a static view – he went along with the orthodox view that species routinely evolve themselves out of existence. Dobzhansky and Mayr argued that there was no contradiction between the idea that evolution by natural selection had occurred and that species are real discrete entities. They came up with a biological species concept which defines a species in terms of its reproductive cohesiveness.

Sewall Wright saw species as being made up of many semi-isolated populations, usually living in a variety of habitats over a large area. There are inevitably some habitats in a large area in which no members of a particular species live, thus there is inherent disjunction in the distribution of species. This, of course, means that there is less opportunity for exchange of genes between populations. Wright imagined these semi-isolated populations as evolving semi-independently – each population would experience slightly different effects of genetic drift and, more importantly, different populations would be integrated into different ecosystems and so would have different selection pressures. Of course some populations die out and some populations meet and merge so we would not necessarily expect large-scale directional change across a species.

Ernst Mayr asked how likely it is that members of a species would be able to breed with their ancestors of millions of years before. He was highlighting the question of whether, if they could not breed, they could be considered to be the same species, again drawing attention to the fleeting nature of species.

In contrast, Niles Eldredge argues that the existence of stasis suggests that species have a real existence in time. Even when there is substantial evolutionary change it is still true that lineages persist. Thus Eldredge emphasises continuity. He draws attention to the analogy of species and individuals suggested by the work of Michael Ghiselin. Species have births, they grow and change (evolve) and ultimately die (become extinct). Niles Eldredge describes species as reproductive systems. He notes that reproduction between two species does not usually occur and that even closely related species cannot blend in with their closest relatives the way populations within a species can. Once speciation has occurred the variation present in the fledgling species will be conserved along with any evolutionary novelties they develop.

Note on Systematics and Cladistics

There is a detailed pattern of nested resemblance in life on earth. Humans share a whole series of characteristics with other mammals (hair, placentas, three bones in the middle ear etc.); we share with our more distant relatives, birds and reptiles, the amniote egg; and so on down to the level of RNA, which is common to all life. This fact is of course the strongest evidence that life has evolved from a common ancestry.

Phylogenetic systematics is a set of rules for analysing this pattern of resemblance developed by the entomologist Willi Hennig. George Gaylord Simpson and Ernst Mayr attacked this approach for taking no account of adaptation, paying attention only to genealogical lineages or clades and described Hennig’s system as cladistics. For all the criticism of cladistics, the real strength of this approach is that it does not rely on a set of assumptions about the evolutionary process. Genealogical patterns are reconstructed purely through the study of nested patterns of resemblance. This has, in practice, led to a more rigorous approach to the study of adaptation: Cladistics has made it easier to separate examples of evolutionary convergence from examples of true relatedness.

The existence of convergence is one of the best arguments for adaptation. Convergence is described by Niles Eldredge as ‘the development of similar structures independently in different lineages’. A good example is that of sharks and porpoises. They have a very similar shape and are similarly well suited to life as marine hunters. Yet sharks are fish and porpoises are mammals – they evolved from quite different lineages. Thus, there is a very strong case for their common streamlined shape to be described as an adaptation. It is obvious that it would incorrect to classify sharks and porpoises together despite their resemblance. Cladistics has made it easier not to make such mistakes!


In general, when speciation takes place there is a marked difference in physical characteristics, geographical distribution or both. Dobzhansky and Mayr highlighted the importance of reproductive isolation in speciation and cited geographic isolation as its primary cause.

Much later, Hugh E. H. Paterson (1985) came up with the idea of the Specific Mating Recognition System (SMRS). He argued that the factors connected with successful mating, for example finding and recognising potential mates, mating itself, production of offspring that survive long enough to reproduce themselves, constituted a single integrated system. Reproductive systems are about finding mates and reproducing successfully. Thus recognition plays an important part in reproduction. Should the mating signals of a population, be they physical characteristics or behaviour, change, organisms who belong to that population would be unlikely to mate with those from another population simply because they would fail to recognise each other as potential mates. Speciation then has occurred, by definition. Thus speciation itself is an accident since the barriers to reproduction are an accidental outcome of changes within a population. For Paterson, then, only reproductive attributes count in speciation. Eldredge argues that, since the dynamic behind adaptation is continued reproductive success within local populations, there is no reason to think that natural selection would favour speciation, so speciation is decoupled from adaptive change. He thinks that speciation is critical to conserving the results of both natural selection and genetic drift and thus must be explained in order to fully understand the evolutionary process.

Daniel Dennett, in contrast, thinks it impossible to specify when speciation has occurred – it can only be retrospective. He argues that there is nothing intrinsic to individuals that could tell us that they would be founders of a new species. What is important is what happens to subsequent generations.

Species Sorting

Species sorting (or species selection) is a term used to describe differential speciation or extinction rates of species within a larger group. Niles Eldredge describes species selection as ‘a major determinant of the fate of adaptations in evolutionary time. It is a molder and shaper, not of organismic adaptive features, but of the patterns of both the persistence and disappearance of those adaptations through evolutionary time.’

Accepting the existence of species as discrete entities, it is clear that extinction rates and speciation rates are very different in different lineages. It follows that there must be factors which bias these rates. These biases would not be expected to decide which adaptations appear, but would influence their ultimate fate – adaptations disappear when species become extinct, faster speciation rates could lead to faster adaptation.

Eldredge sees a paradox here: ‘that adaptive change seems so completely bound up in speciation events; even though we know there should be no necessary correlation between reproductive and adaptive change in evolution.’ However the paradox disappears when we consider that we only ever get to see successful species. Speciation does not cause adaptive change, it depends on whether adaptive change takes place. Consider the fact that most fledgling species do not survive long enough to get established. However, fledgling species that are ecologically different from the parent species have a much greater chance of survival. Thus successful speciation is often linked to rapid adaptive change. There must be some bias which affects the survival rates of fledgling species, which is one aspect of species sorting.


In 1979 Stephen Jay Gould and Richard Lewontin published their paper The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. In it they criticised the fixed mind-set of the ‘adaptationist programme’ for its ‘faith in the power of natural selection as an optimizing agent.’ Earlier in the course we defined an adaptation as a physical characteristic or behavioural pattern that has evolved due to selection pressures. Gould and Lewontin argued that some features appear from ‘architectural necessity’ and introduced the term spandrel to describe an attribute that is not an adaptation. The central points they were making are

  • Organisms are necessarily compromises of design – there are many obstacles or constraints to what natural selection can achieve:

  • Sufficient genetic variability may not be available.

  • ‘The dead hand of history’.

  • There are constraints on what aspects of organismic design can be successfully changed - The intricacies of a building a complex plant or animal limit the ability of selection to alter design fundamentally.

  • Some physical attributes are corollaries of other attributes (e.g. large horns in Titanotheres may have evolved as a side effect of selection for increased body-size).

  • These constraints are very important in governing the direction of evolutionary history.

In the early 1980s Elisabeth Vrba extended the argument when she argued that functions are often co-opted. A feature that was selected for one function (a true adaptation) could later be co-opted for another function. It would then be incorrect to describe that feature as an adaptation (e.g. African black herons use their wings to create shade under which unsuspecting fish will shelter, placing it directly under the heron’s beak). Gould and Vrba published a paper entitled Exaptation – a missing term in the science of form in 1982. In it they argued that a useful physical characteristic should only be called an adaptation if natural selection to perform its present function, otherwise it would be better described as an exaptation.

Daniel Dennett counters that, by this argument, every adaptation is some sort of exaptation; ‘no form is eternal’ – every adaptation must have developed from a pre-existing structure which had some other use (or no use).

Niles Eldredge describes the adaptationist view as one which sees selection as constantly seeking to hone adaptations, and accuses its exponents of ‘a systematic and wilful refusal to examine alternative reasons organisms have come to look and behave the way they do.’

Stephen Jay Gould discusses constraints in some detail in Eight Little Piggies: A Dog’s Life in Galton’s Polyhedron. He argues that artificial selection cannot successfully introduce any trait into a breed – breeders are constrained by the gene pool they are working with. The same must also apply to natural selection – thus constraints are ‘active participants in the pathways of evolutionary change.’

To illustrate, two metaphors are introduced:

  • The organism is pictured as a billiard ball on a smooth table, natural selection is the cue. Movement of the ball represents evolutionary change. Thus the cue (selection) dictates the direction and speed of motion (which evolutionary changes take place and how quickly).

  • The organism is pictured as a rough stone, or a polyhedron. Such a shape would not have an infinite number of possibilities in which way it would lie if pushed. As Gould puts it; ‘The polyhedral stone will not move at all unless natural selection pushes hard. But the polyhedron’s response to selection is restricted by its internal structure; it can only move to a limited number of definite places.’

He goes on to discuss domesticated animals – in particular dogs. He argues that

  • Domestication required a pre-existing behavioural structure – The ancestor of the domestic dog, the wolf, had a ‘predisposition to human companionship’

  • Considering the diversity of breeds of dogs it might seem that any variation on the basic could be achieved – However, this is false.

Gould considers that there are two important aspects of allometry (differences in shape associated with variation in size) which reveal some of the constraints in dogs:

  • Ontogeny; Changes in shape that occur during the growth of individuals from foetus to adult.

  • Interspecific scaling; Differences in shape among adults of varying sizes within the family under consideration.

Gould draws attention to a study of domestic breeds by Robert K. Wayne who found that measures of skull length show little variation in:

  1. the ontogenetic and interspecific scaling patterns
  2. shape at different sizes
  3. among breeds or species at any common size

Wayne also showed that there is great variety of width elements in dogs.

Replicator Power

The Extended Phenotype

All DNA is made up of the same building blocks (the bases adenine, thymine, guanine and cytosine). A particular gene is selected over others because of its phenotypic effects, that is the way it manifests itself either as a physical attribute or as a behavioural trait.

Central Theorem of the Extended Phenotype

An animal’s behaviour tends to maximise the survival of genes ‘for’ that behaviour, whether or not those genes happen to be in the body of the particular animal performing it.’ (Dawkins, 1989)

To summarise Dawkins’ selfish gene/extended phenotype argument:

  • The fundamental unit of life is the replicator.
  • The first replicators came about by chance.
  • Once replicators exist they will propagate, i.e. produce copies of themselves.
  • Errors in copying (mutations) introduce variety in the replicator population.

  • Those varieties which are most successful at self-replication will come to dominate the population.

  • As increasingly elaborate ways of replicating evolve, replicators survival depends on their consequences on the world.

  • For any particular replicator in the population, the biggest influence on survival chances are other replicators and their consequences.

  • There came a point in the history of life on Earth when mutually beneficial replicators (DNA) combined to form discrete vehicles (cells, many-celled bodies).

  • Those vehicles that evolved with a bottlenecked lifecycle were particularly successful.

  • Natural selection does not act at the level of individual organisms (vehicles) but at the level of the genes (replicators).

  • Genes can have phenotypic effects outside of the individual bodies they reside in.


  • The tension between the gene and the individual organism as candidates for the central role in natural selection.

  • Organisms do not replicate themselves, they act as vehicles for the replication of their genes.

  • The individual organism and group of organisms can both be vehicles, but neither of them are replicators.

  • To be an effective gene vehicle an entity must have an impartial exit channel into the next generation.

  • The unity of purpose and coherence of an individual organism is incomparable to that of a group.


© C J M Hewett, 2003