Biology 102 - General Biology

Evolution I

Darwin and Wallace: The Evidence and Reasoning

In 1859 Charles Darwin, along with Wallace, proposed the theory of evolution through natural selection to explain the origin of the present day species found on the earth. Darwin and Wallace proposed that all living things are descended from a common ancestor. The theory of evolution proposed that species are not fixed entities but have come about through a process of gradual (or sometimes abrupt) changes from preexisting ancestral and different species. In other words, everything and everyone have shared a common ancestor somewhere in their history.

Darwin lived in rural England and had plenty of opportunity to watch local farmers practice the art of animal breeding. He recognized these new breeds as evolutionary changes and realized the variants arose from heritable variations and not from exposure to a different environment. However, Darwin knew nothing of the science of genetics. Mendel was developing his rules of inheritance in 1864 and these were not rediscovered until 1900.

Darwin traveled to the Galapagos Islands, off the coast of South America, where he observed the finches that lived on the islands. He proposed that the ancestral finches who came to the islands, found no competitors or predators and that through adaptive radiation they came to occupy the variety of ecological niches on the islands. The geographic isolation prevented breeding between those in different areas and subspecies of the original finches arose. He observed a variety of ground finches which obtain food on the ground or in low shrubs, tree finches who live primarily on insects, and subgroups within those two. It was through his keen observations and his broad knowledge in a variety of fields including geology that he was led to propose his theory of evolution.

We now know that inherited variation comes about through mutation, random assortment of chromosomes and genes in meiosis, sexual reproduction where two parents contribute (different) genes to the offspring, and out breeding between different populations of the same species. Both natural and domestic selection can act on inherited variations. We can readily see the effects of domestic selection by dog breeders in the wide variety of dogs we have today. Genetic differences within the original species of dogs have been exploited by dog breeders to produce the wide variations in present day dogs. Despite the great differences among the breeds of dogs, they still belong to the same species and can still interbreed. Due to size discrepancies, however, there is sexual isolation between some breeds like Chihuahuas and Great Danes.

The inheritance of acquired characteristics has been (and continues to be) a common misconception. In fact, there are many people today who believe that a person can "mark" their unborn child, that a pregnant woman who looks at the moon will cause her child to have a birth defect or that a drunken father will cause his child to be mentally retarded. A man named Weissman did an experiment with mice to disprove the idea that acquired characteristics could be inherited. He cut off the tails of mice and then bred them for generation after generation. However, each generation produced mice with tails for as long as he did the experiment.

In the mid 1900's, the Russian government set out to grow wheat in the colder regions of their country. Their communist ideology of "equality" was not compatible with their interpretation of the new science of genetics. Therefore, they neglected to select seedlings which had been selected to grow in colder regions and to continue to select from those plants ones which withstand increasingly colder temperatures. Instead they insisted on planting ordinary wheat on the farms in the colder regions. What happened was that very soon they had to buy wheat from the United States to save their people from starvation.

Another example of how politics and science may not mix well was Hitler and his Nazi followers. They implemented a plan of eugenics to "cleanse" the earth of "undesirables." Not surprisingly, they proposed to breed only a selected group of "Aryans" like them. They decreed them to be genetically superior. As a consequence of their misuse of the new science of genetics, they embarked on genocide and exterminated six million Jewish people along with "cripples" and gypsies.

Darwin was familiar with the theory proposed by a contemporary economist (clergyman), Malthus. The Malthusian theory was that the human population was increasing at a faster rate than its means of subsistence and that unless checked by moral restraint or by disease, famine, war or other disaster, widespread poverty and chaos would inevitably result. The world Malthus foresaw did not come about when he thought it would because of the Industrial Revolution at that time. (It might yet happen, however!)

The Struggle: Survival of the Fittest

Health, Sexual Attraction, Number of Surviving Offspring

Darwin saw the Malthusian theory in a broader context. He knew that all species have a high reproductive potential. From bacteria to hippos each species is capable of filling the earth with its kind. He also knew that except for minor fluctuations, the population of any given species remained fairly constant from year to year. He reasoned, therefore, that all creatures face a continual struggle for existence.

Furthermore, Darwin knew there is inherited variation among the individuals of any species. He concluded that those individuals whose variations best fit them for their environment would be most likely to survive. This idea of the survival of the fittest is what we call Darwin's theory of natural selection. Darwin proposed this as the mechanism that accomplishes in nature what man's selective breeding accomplishes in domestication.

The measures of fitness include survival due to adaptive traits both physiological and behavioral. An example of fitness is the differential survival of the black and white moth variants in the forests outside big cities. Up until the Industrial Revolution the trees had light-colored lichens growing on the bark. The white moths were not as visible when resting on the tree trunks to predators as were the darker moths in the population. However, after the factories released toxic fumes and soot, the lichens were killed and the trees became darker. When this happened, the darker moths had the advantage of not being visible to their predators. Another example of differential survival is seen in the increase in antibiotic resistant bacteria due to the overuse and misuse of antibiotics. Incidentally, any trait which increases longevity beyond the age of reproduction would not be of selective value. And conversely, any trait which causes death prior to reproductive age or prevents reproduction is a genetic lethal and will be lost.

Sexual selection by a mate and the production of sufficient progeny to replace the parents are other adaptive traits necessary for the survival of a population. Attraction to and by a mate is essential and quite varied among species. "If you aren't pretty, you'd better be rich!" The production of viable offspring is another essential measure of fitness. You might have a lot of offspring and hope enough survive or you could have less offspring but take very good care of them.

In general, the effect of increased selection pressure is over specialization while the effect of relaxed selection pressure is that new and different genotypes thrive. When times are hard, the forces of natural selection operate with greatest efficiency. Hard times may occur with an increase in predators, parasites and/or competitors for food and space. These situations constitute an increase in selection pressure and only those genotypes that confer the best and quickest advantage in that environment will live to reproduce. This temporary success for species survival ends up resulting in less variation in the population. On the other hand, the reduction of predation or an increase in the food supply (e.g., Darwin's finches) allows atypical genotypes to compete on more nearly equal terms with their formerly better adapted relatives. The relaxed selection pressure enables the population to experiment with new, even less efficient, genotypes. Genetic variability flourishes.

Species are groupings of individuals which can interbreed. The formation of new species involves isolation and reunion. It comes about when sub populations of a species are separated geographically (geographical isolation) or are prevented from engaging in sexual reproduction (sexual isolation) and upon later reunion, they can no longer interbreed. Darwin's finches illustrate the essentials of the process of species formation. Upon arrival at the new unoccupied territory of Galapagos Islands there was reduced selection pressure. This situation allowed increased variability from the process of sexual reproduction, working with genes no longer being selected against. The atypical finches were no longer selected against and were free to explore the environment. By their choice of different ecological niches, the sub populations no longer interbred. When the populations had been separated long enough, differences occurred that prevented mating or the production of fertile offspring. At that time, the populations could be considered geographic races or sub species. If competition between the two groups occurred then further divergence of their traits occurred. With the finches on the Galapagos Islands, this process has occurred more than once, producing a number of diverse species from a single ancestral one. This is referred to as adaptive radiation.

Convergent evolution

Another interesting phenomenon is called convergent evolution whereby different species of different ancestry come to resemble one another closely because of adaptation to similar environments. The marsupials of Australia resemble placental mammals in both appearance and habits. For example, the flying squirrel, the woodchuck and many other placental mammals have marsupial counterparts.

EXAMPLES OF CONVERGENT EVOLUTION

Homologous and analogous organs

Homologous structures are those that develop from similar embryological origins. Analogous organs are those that are adapted to the same purpose. Some organs are both homologous and analogous.

Evidence for evolution: Before and now

The evidence for evolution comes from a variety of sources, only some of which were known in Darwin's day. For example, the science of biochemistry did not develop until well into the 20^th century, however, the biochemical evidence of a common ancestor is perhaps the most compelling of all. As you have already learned, DNA is the common genetic material of all organisms and the genetic code is universal. This means that the DNA and RNA codons code for the same amino acids in all organisms, from the simplest to the most complex, from bacteria to humans. Furthermore, all the biomolecules are the same in all organisms. Similar genes, such as those for the cytochromes, can be found in all living organisms from bacteria to humans. This does not mean that at the origin of life on this planet many cellular and biochemical experiments were not attempted. But it does mean that due to natural selection and the survival of the fittest, only one type of cell system survived and its biochemistry persists to this day.