Evolution of Organisms

After finishing high school in the early 1950s, I ended up studying for a bachelor’s degree in agriculture. One of the interesting subjects was about the breeding of plants and animals, at the core of which was the science of heredity. We learnt about the laws of heredity based on the experiments of Gregor Mendel on pea plants (Pisum sativum) and of Thomas Morgan on fruit flies (Drosophila melanogaster). But no one ever mentioned a word about the origins of life on earth and its evolution. We never got to learn about evolutionary biology. We didn’t hear the names of Charles Darwin or Alfred Wallace, proponents of an idea that has revolutionized our understanding of the biological world. Part of the explanation was cultural: the idea of evolution of species was contrary to the idea of divine creation. Frankly, like many others of my background, I had compartmentalized my life until I retired from work and started to ask questions about the role of religion and science. I took a 100-level university course in evolutionary biology. That opened the floodgates to all sorts of related questions in philosophy, history, anthropology, and biology including evolution and genetic engineering.

When we look at the diversity of living organisms — there are over ten million species of plants and animals on earth — we see the work of a “blind watchmaker” and not of an “intelligent designer”. The story of creation told in the Scriptures is well known — you must believe it and not reason it out. A divine being (God) miraculously created life from nothing (ex nihilo) in a few days some thousands of years ago and it has remained unchanged. Though attempts have been made to defend the idea of an intelligent designer — perhaps William Paley in England first proposed it in eighteenth century — who created the living organisms as separate and unchanging entities. This story and others like it are full of unanswered and unanswerable mysteries that run counter to evidence and logic. In contrast, the explanation in science about life and its evolution by natural selection is far more compelling since it is based on good evidence (verifiable facts). Scientific propositions are subject to revision in the light of new facts. Scientists must keep their minds open: there is no place for dogma.

Scientists have traced the beginnings of life on earth to single-cell organisms emerging some 3.8 billion years ago. (Their explanation is however still a bit speculative.) These micro-organisms managed to reproduce in a clonal fashion, otherwise life would have ended in one generation. (Scientists are on a firmer ground here.) And if the parents had continued to produce their exact copy in each generation, the world would be populated today only by these single-cell organisms. The reproductive process, however, created variations by error and chance. These variations gradually transformed into simple multi-cellular organisms. Some of these organisms had advantage in surviving the environment, giving them the opportunity to live and change. After about a billion years of mutations and competition, there was a massive leap forward with sexual reproduction: it introduced a new way of creating diversity. The genetic information in the male sperm and the female egg on mating was shuffled and passed on to the offspring. This process of diversification and mutations led to the evolutionary process of speciation: some fish evolved into amphibians, some amphibians into reptiles, some reptiles into mammals, and then some of those mammals changed into us (Homo sapiens).

Charles Darwin and Alfred Wallace separately hypothesized the branching out process of new species by natural selection: survival and reproduction of those best capable of adaptation to the environment. But it was Darwin who described the details of the evolutionary process in his voluminous book, On the Origin of Species by Means of Natural Selection, in 1859. The “descent with modification”, or changes in the genetic make-up of the species, has four basic characteristics. (1) Evolution is gradual — it takes several generations to produce change, e.g., birds from reptiles. (2) Every species goes back to a common ancestor. (3) Speciation means that different groups can’t inter-breed or exchange genes. (4) Natural selection is the competitive ability of the individual organism to survive and reproduce in its environment. (There could be pressures other than natural selection for evolutionary change, like random errors or changes in the proportion of genes.) Evolution by natural selection rests on three conditions: that the living beings must reproduce; they must have a hereditary system; and the hereditary system must exhibit variability.

The idea of natural selection is embedded in the fact that populations exhibit variations and, when these variations are caused by genetic changes, they are inherited from one generation to the next. Some of these variants will affect characteristics that make some individuals more successful in producing offspring. This means that the offspring with these variants will make up a larger proportion of the population in the next generation. This process produces organisms well adapted to their habitats. There are all sorts of constraints or pressures in the process of evolution — competition for food or mates, diseases, and random mutations — to ensure that some individuals do better and reproduce more than others. Whereas the process of evolution by natural selection is glacial, humans have been breeding by artificial selection plants and animals for ages to develop desirable traits for their benefit. Humans started to domesticate wild plants and animals by artificial selection after they settled down in different parts of the world between 8,000 and 12,000 years ago. But it was all a game of trial and error without the knowledge of precise mechanism by which heredity worked.

Darwin’s theory of evolution by competition and mutation in nature has been proved to be true many times over with the help of knowledge in geology, paleontology, chemistry, and genetics. Darwin didn’t know about Mendel’s work on heredity — it lay in obscurity until about the end of nineteenth century. And Mendel didn’t know the exact mechanism by which heredity worked. It was only in the twentieth century that the workings of chromosomes, genes, and the deoxyribonucleic acid (DNA) were discovered. In the nucleus of almost every cell, pairs of chromosomes carry bundle of genes with the DNA (code of instructions) to make proteins by which an organism is built. In 1953 James Watson and Francis Crick discovered the double helix on which the code of instructions is laid out. The double helix does two things that the hereditary material must do to underpin the survival and perpetuation of life. It encodes the information needed by cells and organisms to maintain, grow, and reproduce themselves. And it replicates itself reliably and precisely so that each new cell and organism receives a complete set of genetic instructions.

The evidence for the evolutionary process by natural selection is everywhere and it is scientifically verifiable: the bodies of plants and animals carry their evolutionary history. First, look at the vestigial organs: they were once useful in ancestors but are of no use now (e.g., wings in ostrich, pelvis and leg bones in whales, and the appendix and tail bone in humans). Second, there is atavism or re-expression of genes that were functional in ancestors then went dormant for generations. Third, there are numerous imperfections (bad designs) because evolution modifies the existing organism and doesn’t work from scratch (e.g., reproductive organs in some species and the circuitous path of the laryngeal nerve in humans.) Fourth, the genomes of some species show vestigial (dead) genes (e.g., the gene for GLO in humans), but are active in other species. Finally, in the development of human embryo, for example, the fish-like and furry fetus gives evidence of evolution.

Theodosius Dobzhansky, an eminent biologist, had this to say about the theory of evolution: “Nothing in biology makes sense except in the light of evolution.” The diversity of organisms on our planet can only be accounted for by cumulative selection of favourable genetic variations over innumerable generations. (“Darwin’s” finches in the Galapagos Islands prove the truthfulness of the theory of evolution.) The evidence from the rocks and fossils and from the genomes of species is overwhelming for the interconnectedness of organisms; for common ancestry (humans and chimpanzees had a common ancestor some 5–7 million years ago); for genetic variation among species; for cases of imperfect adaptations; and for natural selection in the wild. Evolution by natural selection marches on, though human intervention (e.g., medicine and genetic engineering) creates new pressures for its path in the future.


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Mahmood Hasan Khan

Mahmood Hasan Khan

Retired professor of Economics, Simon Fraser University, Canada