NOTES
1. Dawkins presents ‘The Genius of Charles Darwin’, op. ct.
2. Oxford Dictionaries Online.
3. Ibid.
4. Litchfield, Emma Darwin
5. Oxford Dictionaries Online.
6. Baron-Cohen, Simon, Zero Degrees of Empathy, p.87.
7. Oxford Dictionaries Online.
8. Newberg and Walman, How God Changes Your Brain, pp.55–6.
Chapter 39
Genetic Science Vindicates Darwin and Provides an Explanation for Variation
Darwin would have been gratified to know that the science of genetics not only corroborates the truth of his theory of evolution, but also provides infinitely greater possibilities for understanding the precise manner in which organisms are genetically related to one another, and sheds light on how they have evolved.
The cell is the smallest structural and functional unit of an organism, and typically consists of cytoplasm and a nucleus enclosed in a membrane. (However, not all cells contain nuclei, for example, bacteria and human red blood cells). Cells contain threadlike structures called chromosomes, the number of which varies according to species. Human cells, for example, of which there are billions, contain forty-four chromosomes, arranged in pairs (each pair consisting of one chromosome inherited from the father and one from the mother), plus a pair of ‘sex chromosomes’ (two X chromosomes in females, one X and one Y in males). Between them the chromosomes contain the entire information – blueprint – for the organism, and also a mechanism whereby that organism can reproduce. This is explained as follows.
Chromosomes contain deoxyribonucleic acid (DNA) in the form of a long, doublestranded molecule in the shape of a double helix, which has been likened to a ladder, twisted into a helical shape. The total length of DNA in each cell is approximately one metre, and the ability of the two DNA strands to separate enables new bases to attach to each, and hence another double strand to form – a process which occurs in cell division. The fact that DNA is to be found in the nuclei of all the cells of all living organisms is in accordance with all forms of life being interrelated and having evolved from a common ancestor.
DNA has a backbone structure of sugar and phosphate molecules, to which are attached nitrogen-containing bases, four in number – adenine, cytosine, guanine and thymine. It is the bonds between these pairs which holds the double helix together, cytosine always bonding to guanine, and adenine to thymine. Each human being has in excess of three billion of these so-called base-pairs.
A gene, of which each human being possesses about 20,000–25,000, is a length of chromosome containing a distinct sequence of nucleotides, and upon the order of this sequence – or code – depends what type of protein is produced. The gene achieves this by forming ribonucleic acid (RNA) which transfers the coded message from the nuclear DNA to the cell’s cytoplasm, where amino acids are pieced together accordingly. In a given cell, the vast majority of genes are switched off, and only those which relate to the function of that particular cell are operational.
Different genes code for different proteins, which are the structural materials from which the cell is built, and for enzymes (which are also proteins), which regulate the chemical reactions which occur within the cell, and therefore its activity. Although some traits are coded for predominantly by a single gene, for the overwhelming majority of traits, the participation of a number of genes is required.
Genes are located in pairs, opposite to one another on the chromosome pair – one inherited from the father (as is the chromosome) and one from the mother (likewise). The two genes – which are called alleles – may be identical, or one may be a variant of the other, in which case only the so-called dominant gene is expressed (and not its recessive partner).
The Human Genome Project
A genome is defined as the complete set of genes or genetic material present in a cell or organism.1 The Human Genome Project was begun in 1989 with the aim of identifying all of the approximately 20,000 to 25,000 genes contained within human DNA, and determining the sequences of its 3 billion base-pairs. This was successfully achieved fourteen years later, in 2003.
The genes of animals and plants
The cells of every animal and every plant contain similar quantities of DNA, though the number of chromosomes and genes which they contain varies greatly. For example:
Chromosomes
Genes
Mouse
25,000
40
Fruit fly
13,600
10
Arabidopsis thaliana (a member of of the mustard family of plants)
25,500
5
E. coli (a bacterium)
3,200
1
However, how similar or different one organism is to another is dependent upon how much of their respective DNA sequence – or ‘code’ – they share in common. For example
We humans share 99 per cent of the same DNA sequence as chimpanzees, from whom we split 6 million years ago, 90 per cent with mice (100 million years), and even 31 per cent with yeast (1.5 billion years).2
Not for nothing has man sometimes been referred to as the ‘fifth ape’!
A DNA-based evolutionary tree of life
The discovery of DNA and the mapping of the genomes of various animals and plants will one day enable a comprehensive DNA-based tree of life to be constructed.
Variation explained
All his working life Darwin grappled, unsuccessfully, with the question of how variation in nature comes about. Was it something that was determined a), by chance b), by the external environment or c), at the volition of the individual?
It was Austrian priest and botanist Gregor J. Mendel (1822–84), who had provided an early explanation of variation. In the garden of the Augustinian Abbey of St Thomas in Brno (then the capital city of Moravia, now the Czech Republic), of which he became abbot, Mendel conducted experiments in order to discover how the characteristic features of plants were inherited. First, he obtained tall plants which bred true (i.e. produced only tall plants when self-pollinated or cross-pollinated with other tall plants) and short plants which bred true. When he crossed true-breeding tall plants with true-breeding short plants, he discovered that all the offspring produced were tall. He concluded that plants receive one character from each of its parents, tallness being a dominant characteristic and shortness being a recessive, or hidden, characteristic, which only reappeared in subsequent generations. But the answer as to how variation operated on a molecular level, would only come through advances in science, and this would not be until long after Darwin’s death.
Asexual reproduction (mitosis)
Asexual reproduction involves ‘a type of cell division that results in the formation of two “daughter cells”, each having the same number and kind of chromosomes as the parent cell’.3 It is the primary form of reproduction for single-celled organisms, such as bacteria, and also for many plants and fungi. Here, variation may occur as the result of mutations brought about when mistakes happen in the course of the cell copying its DNA in preparation for cell division.
Sexual reproduction (‘meiosis’)
Sexual reproduction is the primary method of reproduction for the vast majority of animals and plants. In the case of human beings, when gametes (spermatozoa and ova) are created, this involves the production of four (diploid) cells containing only twenty-three single – i.e. unpaired – chromosomes from a standard (haploid) parent cell containing twenty-three pairs of chromosomes. In this process, there is an exchange of genes between those derived from the father and those derived from the mother. The newly produced chromosomes, and therefore the four daughter cells are each unique, being genetically different from one other, and from the parent cell. Furthermore, when two gametes (sperm and ovum) come together to form a new diploid cell (or zygote, which then undergoes cell division to create the embryo) there is a further exchange of genes (alleles) between the two newly-formed chromosome pairs.<
br />
Organisms which reproduce by meiosis therefore contain an enormous mixture of genes, deriving not only from their immediate parents, but also from their ancestors, in the ways described above. From this, it can be seen not only that variation is an inevitable consequence of sexual reproduction, but that the possibilities for such variation are endless.
Gender variation is itself an example of variation. If an egg (ovum) is fertilized by a sperm containing a Y chromosome, the resulting zygote will be XY or male. On the other hand, if an egg is fertilized by a sperm containing an X chromosome, the resulting zygote will be XX – or female.
But does the above account provide the only explanation for variation?
A newly discovered, epigenetic mechanism for variation
Tim Spector, Professor of Genetic Epidemiology at King’s College, London has drawn attention to the fact that there is another mechanism by which variation may arise. He points to the work of Pilar Cubas and others of the John Innes Centre for Plant Science and Microbiology, Norwich, UK who discovered an alternative variety of the wild flower common toadflax. This variation, however, was not the result of a change in the structure of the plant’s DNA (as is the case with mutations), but to a phenomenon called methylation. In the toadflax variation
a key gene (called Lcyc) is extensively methylated and in the normal plant it is not.
What methylation means is that at certain sites (usually cytosine bases) of the gene’s DNA, small chemical methyl groups floating around the cell attach themselves to it … . This has the effect of stopping the gene producing a protein.
In other words, methylation stops the gene from functioning, by switching it of and thereby preventing it from expressing itself. It was also shown that the methylated gene could be passed on to subsequent generations.4
Spector points to aridopsis, a small, flowering plant related to the cabbage, as an example of how a gene may be switched off by this mechanism.
In response to prolonged cold (as in winter), the Flowering Locus C gene which normally prevents flowering is methylated and deactivated, allowing this variety to flower in the spring. This trait is then passed on to the next generation [when the plant will once again flower in the spring], even if there is no cold winter.5
Although the example of a plant has been cited above, it is likely that the phenomenon of methylation of a gene or genes, and the inheritance of such epigenetically altered genes (epigenetic – resulting from external rather than genetic influences),6 together with the characteristics that they code for, is to be found in all forms of animal and plant life. Spector cites several examples of the role which epigenetics may play in various aspects of life:
i. Mindset (the established set of attitudes held by someone)7 and personality
we now know that both genes and their related mindsets, which we thought to be hard-wired, can be modified and reset along with the traits and personalities that define us as individuals.8
ii. Belief systems
There is a suggestion, says Spector, that ‘even our patterns of beliefs could be altered epigenetically – with faith genes switched on and off.’9
iii. Maltreatment in infancy
Studies involving rodents, says Spector
show that maltreatment in the first week of life causes certain genes to be switched off epigenetically. The glucocorticoid receptor gene controlling the stress response is the best example, where a methyl group is attached and so the gene cannot be expressed. This leads to a cascade of changes in many other genes related to emotion and stress, and can last a lifetime.
In respect of human beings, when previously maltreated children become parents
because of their abnormal methylation [they] often fail to bond with their own children because their empathy or bonding genes are not working normally. This leads to an increasing cycle of dysfunctional parenting or abuse.
Finally
this methylated gene will be present in the sperm or egg of the child [i.e. who was once maltreated, but is now an adult] and then usually passed on to the next generation when they reproduce.10
iv. Mother/child bonding
Similarly, ‘dangerous alterations in gene methylation’ may occur when the ‘bond of interaction’ between mother and child ‘goes wrong’.11
v. Stress
Spector also suggests that epigenetics can ‘explain how stress translates into health problems’ as a result of methylation ‘epigenetically deactivating’ the ‘immune genes [i.e. the genes which code for immunity]’, leading to a weakening of the immune system in those who are experiencing stress.12
vi. Cancer
In cases of cancer, says Spector, it is generally the case that the DNA of cancer cells is
under-methylated – enabling many genes that are normally suppressed literally to run wild. [On the other hand] a few DNA areas show the opposite: they are hyper-methylated and the genes suppressed. These genes are the body’s built-in protection system; the tumour-suppressor genes that keep the DNA under control.13
vii. Pain
It is also the case that certain ‘pain genes’ can be switched on or off epigenetically.14 (The difference in the degree to which this occurs may explain why some people’s pain threshold is higher or lower than others.)
Because methylated genes may be passed on down the generations, says Spector, both the environment and the behaviour of our ‘parents and grandparents may influence us in a number of ways – modifying our growth, altering our brain development, and affecting our risk of diabetes and heart disease’.15 Therefore, says Spector, in respect of the
inheritance of acquired characteristics’ [or so-called] ‘soft inheritance’, proposed by Lamarck and accepted as possible by Darwin … despite the ridicule this received for most of the last 150 years, we now know that it can occur.16
Spector also points out that the inheritance of characteristics acquired in an epigenetic manner
is the parallel, faster route by which we human beings adapt to our surroundings, and also explains many of the emerging ideas of how we are moulded into individuals.17
In other words, variation which is brought about epigenetically is more advantageous to the organism concerned, and occurs at a more rapid rate than would otherwise be the case.
The most important lesson that we’ve learnt is that you can change your genes, your destiny and that of your children and grandchildren. It really does matter what you do to your body, and importantly what your grandparents did to theirs many years ago.18
The discovery of this new, epigenetically driven mechanism for variation (which in no way invalidates Darwin’s theory of evolution by natural selection) is exciting because it opens up an entirely new field of research into how diseases – both physical and mental – are caused, and into possible therapies.
NOTES
1. Oxford Dictionaries Online.
2. Spector, Identically Different: Why you can Change your Genes, p.11.
3. Ibid.
4. Cubas, Vincent and Coen. ‘An Epigenetic Mutation Responsible for Natural Variation in Floral Symmetry’. Nature, 401: pp.157–61, in Spector, op. cit., pp.34–6.
5. Spector, op cit., p. 36.
6. Oxford Dictionaries Online.
7. Ibid.
8. Spector, op. cit., p.65.
9. Ibid, p.107.
10. Ibid, p.148.
11. Ibid, p.151.
12. Ibid, p.163.
13. Ibid, p.197.
14. Ibid, p.259.
15. Ibid, p.164.
16. Ibid, p.293.
17. Ibid, pp.39–40.
18. Ibid, p.293.
Chapter 40
Darwin and Downe’s Church of St Mary the Virgin
When the Darwins arrived at Downe in the autumn of 1842 the vicar of its Parish Church of St Mary the Virgin was the Reverend James Drummond.
There was no Unitarian chapel in the vicinity, and the family attended the local Anglican church, St Mary’s, each Sunday. All the c
hildren were baptized and confirmed in the Church of England. The whole family took the sacrament, although Emma [mindful of her Unitarian beliefs] used to make the children turn around and face the back on occasions when the rest of the congregation recited the Athanasian Creed. Around 1850, Darwin himself stopped attending services. He would accompany the family to church and would often wait outside, chatting with the village constable, or would stroll around the village until the service ended. Yet despite absenting himself from worship, Darwin was actively engaged in church affairs. He took a lead in local charities, supervised church and school finances, and worked to uphold the status of the church in the community.1
Darwin was a man of principle, and to attend church would be to betray his principles, to be a hypocrite. So why did he support the Church in other ways? Perhaps for sentimental reasons, and to keep on good terms with the vicar and his flock, but primarily to propitiate his wife Emma.
In assuming what he saw as his ‘pastoral responsibilities in the village … Darwin worked closely with the Anglican incumbent John Brodie Innes [Drummond’s successor], who became Perpetual Curate of Downe in 1846’. This, despite the fact that Innes, ‘A Tory and High Churchman … [High Church – of, or adhering to, a tradition within the Anglican Church emphasizing ritual, priestly authority, sacraments, and historical continuity with Catholic Christianity2] preached eternal torment …’ – i.e. eternal punishment in Hell for sinners and unbelievers; a doctrine Darwin had previously described as ‘damnable’.
But their political and doctrinal differences were glossed over, it would seem, in their shared sense of duty toward the community, especially its poor. They collaborated on the running of village charities, a Coal and Clothing Club [a local charitable institution which supplied parishioners with coal and clothes in exchange for regular contributions], and a Friendly [local savings and insurance] Club. Darwin served as treasurer of both organizations, and would read out the accounts to members, who assembled on his lawn for regular meetings.3
Charles Darwin Page 33