Genetics: 19th and 20th Centuries
Although Charles Darwin is credited with discovering the first observations of natural selection, he never explained how or why the process happens. Other scholars tackled these problems.
Gregor Mendel (1822-1884)
Darwin recognized the importance of individual variation in process of natural selection, but could not explain how individual differences were transmitted from one generation to another.
Although none of main scientists in the 19th-century debate about evolution knew it, the key experiments necessary to understand how genetic inheritance really worked had already been performed by an obscure monk, Gregor Mendel, who lived near Brno, in the Czech Republic.
Between 1856 and 1863, Mendel performed many breeding experiments using the common edible garden pea plants. He meticulously recorded his observations and isolated a number of traits in order to confirm his results.
In 1866, Mendel published a report where he described many features of the mode of inheritance which Darwin was seeking. He proposed the existence of three fundamental principles of inheritance: Segregation; Independent Assortment; Dominance and Recessiveness.
Because the basic rules of inheritance Mendel discovered apply to humans as well as to peas, his work is of prime relevance for paleoanthropology and human evolution.
Nevertheless Mendel's work was beyond the thinking of the time; its significance was overlooked and unrecognized until the beginning of the 20th century.
Mendel observed that his peas had seven easily observable characteristics, with only two forms, or variants, for each trait:
|Seed interior color||yellow||green|
|Seed coat color||gray||white|
|Position of flowers on stem||along stem||end of stem|
|Length of stem||long||short|
After crossing plants, Mendel noted and carefully recorded the number of plants in each generation with a given trait. He believed that the ratio of plant varieties in a generation of offspring would yield clues about inheritance, and he continually tested his ideas by performing more experiments.
From his controlled experiments and the large sample of numerous breeding experiments, Mendel proposed the existence of three fundamental principles of inheritance:
- Independent Assortment
- Dominance and Recessiveness
Mendel began crossing different varieties of purebred plants that differed with regard to a specific trait. For example, pea color.
In the experiment:
- The first generation (parental, Fo) of plants were either green or yellow. As they matured, the first hybrid offspring generation was not intermediate in color, as blending theories of inheritance (Darwin) would have predicted. To the contrary, they were all yellow.
- Next, Mendel allowed these plants to self-fertilize and produce a second generation of plants (generation F1). But this time, only 3/4 of offspring plants were yellow, and the remaining 1/4 were green.
These results suggested an important fact:
- Different expressions of a trait were controlled by discrete units, which occurred in pairs, and that offspring inherited one unit from each parent.
This is Mendel's first principle of inheritance: principle of segregation.
Mendel also made crosses in which two traits were considered simultaneously to determine whether there was a relationship between them. For example: Plant height and seed color.
Results of experiments: No relationship between the two traits were found; nothing to dictate that a tall plant must have yellow (or green) seeds; therefore, expression of one trait is not influenced by the expression of the other trait.
Based on these results, Mendel stated his second principle of inheritance: the principle of independent assortment. This principle says that the genes that code for different traits assort independently of each other.
Dominance and Recessiveness
Mendel also recognized that the trait that was absent in the first generation of offspring plants had not actually disappeared at all - it had remained, but was masked and could not be expressed.
To describe the trait that seemed to be lost, Mendel used the term recessive; the trait that was expressed was said to be dominant.
Thus the important principle of dominance and recessiveness was formulated; and it remains today an essential concept in the field of genetics.
Implications of Mendel's research
Mendel thought his findings were important, so he published them in 1866.
Scientists, especially botanists studying inheritance, in the late 19th century, should have understood the importance of Mendel's experiments. But instead, they dismissed Mendel's work, perhaps because it contradicted their own results or because he was an obscure monk.
Soon after the publication of his work, Mendel was elected abbot of his monastery and was forced to give up his experiments.
His ideas did not resurface until the turn of the 20th century, when several botanists independently replicated Mendel's experiments and rediscovered the laws of inheritance.
The role of cell division in inheritence
Mitosis and Meiosis
By the time Mendel's experiments were rediscovered in 1900, some facts were well known:
- virtually all living organisms are built out of cells;
- all the cells in complex organisms arise from a single cell through the process of cell division.
In order for plants and animals to grow and maintain good health, body cells of an organism must divide and produce new cells. Cell division is the process that results in the production of new cells.
Two types of cell division have been identified:
- Mitosis: a process when chromosomes (and genes) replicate, forming a second pair that duplicates the original pair of chromosomes in the nucleus. Thus, mitosis produces new cells (daughter cells) that have exactly the same number of chromosome pairs and genes, as did the parent cell
- Meiosis: while mitosis produces new cells (which contain a pair of homologous chromosomes), meiosis leads to development of new individuals, known as gametes (which contain only one copy of each chromosome). With this process, each new cell (containing only one copy of each chromosome) is said to be haploid: when new individual is conceived, a haploid sperm from father unites with a haploid egg from the mother to produce a diploid zygote. The zygote is a single cell that divides mitotically over and over again to produce the millions and millions of cells that make up an individual's body.
Mendel and chromosomes
Mendel stated in 1866 that an organism's observed traits are determined by "particules" (later named genes by the American geneticist T.H. Morgan) acquired from each of the parents. This statement was only understood by further research.
Between the time of Mendel's initial discovery of the nature of inheritance and its rediscovery at the turn of the century, a crucial feature of cellular anatomy was discovered: the chromosome.
- Chromosomes are small, linear bodies contained in every cell and replicated during cell division.
In 1902, a graduate student from Columbia University, (Walter Sutton) made the connection between chromosomes and properties of inheritance discovered by Mendel's principles:
- genes reside on chromosomes because individuals inherit one copy of each chromosome from each parent
- therefore an organism's observed traits are determined by genes from both parents
- these propositions are consistent with the observation that mitosis transmits a copy of both chromosomes to every daughter cell, so every cell contains copies of both the maternal and paternal chromosomes.
In first half of the 20th century, geneticists made substantial progress in:
- describing the cellular events that took place during mitosis and meiosis
- understanding the chemistry of reproduction.
By the middle of the 20th century it was known that chromosomes contain two structurally complex molecules: protein and DNA (deoxyribonucleic acid). It was also determined that the particle of heredity postulated by Mendel was DNA, not protein - though exactly how DNA might contain and convey the information essential to life was still a mystery.
In the early 1950s, several biologists (led by Francis Crick and James Watson), at Cambridge University, made a discovery that revolutionized biology: they deduced the structure of DNA.
Through this discovery, we now know how DNA stores information and how this information controls the chemistry of life, and this knowledge explains why heredity leads to the patterns Mendel describes in pea plants, and why there are sometimes new variations.
Cells are basic units of life in all living organisms. Complex multicellular forms (plants, insects, birds, humans, ...) are composed of billions of cells, all functioning in complex ways to promote the survival of the individual.
Complex molecule with an unusual shape: like two strands (called nucleotides) of a rope (composed of alternating sequences of phosphate and sugar molecules) twisted around one another (double helix). Chemical bases that connect two strands constitute code that contains information to direct production of proteins.
It is at this level that development of certain traits occurs; . . Yet, since the DNA in a single chromosome is millions of bases long, there is room for a nearly infinite variety of messages.
DNA molecules have the unique property of being able to produce exact copies of themselves: as long as no errors are made in the replication process, new organisms will contain genetic material exactly like that in ancestral organisms.
A Gene is a short segment of the DNA molecule that directs the development of observable or identifiable traits. Thus genetics is the study of how traits are transmitted from one generation to the next.
Each chromosome contains a single DNA molecule, roughly two meters long that is folded up to fit in the nucleus. Chromosomes are nothing more than long strands of DNA combined with protein to produce structures that can actually be seen under a conventional light microscope
Each kind of organism has characteristic number of chromosomes, which are usually found in pairs. For example, human cells contain 23 pairs.
In addition to preserving a message faithfully, hereditary material must be replicable. Without the ability to make copies of itself, the genetic message that directs the activities of living cells could not be spread to offspring, and natural selection would be impossible.
Cells multiply by dividing in such a way that each new cell receives a full complement of genetic material. For new cells to receive the essential amount of DNA, it is first necessary for the DNA to replicate:
- Specific enzymes break the bonds between bases in the DNA molecule, leaving the two previously joined strands exposed
- When process is completed, there are two double-stranded DNA molecules exactly like the original one.
One of most important functions of DNA is that it directs protein synthesis within the cell. Proteins are complex, three-dimensional molecules that function through their ability to bind to other molecules.
Proteins function in myriad ways:
- Collagen is most common protein in body and major component of all connective tissues
- Enzymes are also proteins; their function is to initiate and enhance chemical reactions
- Hormones are another class.
Proteins are not only major constituents of all body tissues, but also direct and perform physiological and cellular functions. Therefore critical that protein synthesis occur accurately, for, if it does not, physiological development and metabolic activities can be disrupted or even prevented.
Evolutionary significance of cellular processes
Meiosis is a highly important evolutionary innovation, since it increases variation in populations at a faster rate than mutation alone can do in asexually reproducing species.
Individual members of sexually reproducing species are not genetically identical clones of other individuals. Therefore each individual represents a unique combination of genes that has never occurred before and will never occur again.
Genetic diversity is therefore considerably enhanced by meiosis. If all individuals in a population are genetically identical over time, the natural selection and evolution cannot occur. Therefore, sexual reproduction and meiosis are of major evolutionary importance because they contribute to the role of natural selection in populations.
Synthesizing the knowledge
Darwin believed that evolution proceeded by the gradual accumulation of small changes. But Mendel and the biologists who elucidated the structure of the genetic system around the turn of the century proved that inheritance was fundamentally discontinuous.
Yet turn-of-the-century geneticists argued that this fact could not be reconciled with Darwin's idea that adaptation occurs through the accumulation of small variations.
- If generation of parent plants are tall and short, then there will be no intermediate in the generation of offspring and size of peas cannot change in small steps. In a population of short plants, tall ones must be created all at once by mutation, not gradually lengthened over time by selection.
These arguments convinced most biologists of the time, and consequently Darwinism was in decline during the early part of the 20th century.
In the early 1930s, a team of British and American biologists showed how Mendelian genetics could be used to explain continuous variation. Their insights led to the resolution of two main objections to Darwin's theory:
- the absence of a theory of inheritance
- the problem of accounting for how variation is maintained in populations
When their theory was combined with Darwin's theory of natural selection and with modern biological studies, a powerful explanation of organic evolution emerged. This body of theory and the supporting empirical evidence is now called the modern synthesis.
Darwin knew nothing about genetics, and his theory of adaptation by natural selection was framed as a "struggle for existence": there is variation of observed traits that affects survival and reproduction, and this variation if heritable.
Also, the blending model of inheritance appealed to 19th century thinkers, because it explained the fact that for most continuously varying characters, offspring are intermediate between their parents.
- When yellow and blue parents are crossed to produce a green offspring, the blending model posits that the hereditary material has mixed, so that when two green individuals mate they produce only green offspring.
According to Mendelian genetics, however, the effects of genes are blended in their expression to produce a green phenotype, but the genes themselves remain unchanged. Thus, when two green parents mate, they can produce blue, yellow and green offspring.
Sexual reproduction produces no blending in the genes themselves, despite the fact that offspring may appear to be intermediate between their parents. This is because genetic transmission involves faithful copying of the genes themselves and reassembling them in different combinations in zygotes.
The only blending that occurs takes place at the level of the expression of genes in phenotypes (ex. Beak depth, pea color). The genes themselves remain distinct physical entities.
Yet, these facts do not completely solve the problem of the maintenance of variation. Indeed, even is selection tends to deplete variation, there would still be variation of traits due to environmental effects. In fact, without genetic variation there can be no further adaptation.
Genes are copied with amazing fidelity, and their messages are protected from random degradation by a number of molecular repair mechanisms.
However, every once in a while, a mistake in copying is made that goes unrepaired. These mistakes damage the DNA and alter the message that it carries.
These changes are called mutations, and they add variation to a population by continuously introducing new genes, some of which may produce novel traits that selection can assemble into adaptations. Although rates of mutation are very slow, this process plays an important role in generating variation.
More importantly, this process provides the solution to one of Darwin's dilemma: the problem of accounting for how variation is maintained in populations.
Twentieth century research has shown that there are two pools of genetic variation: hidden and expressed. Mutation adds new genetic variation, and selection removes it from the pool of expressed variation. Segregation and recombination shuffle variation back and forth between the two pools with each generation.
In other words: if individuals with a variety of genotypes are equally likely to survive and reproduce, a considerable amount of variation is protected (or hidden) from selection; and because of this process, a very low mutation rate can maintain variation despite the depleting action of selection.
Human evolution and adaptation are intimately linked to life processes that involve cells, replication and decoding of genetic information, and transmission of this information between generations. Because physical anthropologists are concerned with human evolution, adaptation, and variation, they must have a thorough understanding of the factors that lie at very root of these phenomena. Because it is genetics that ultimately links or influences many of the various subdisciplines of biological anthropology.