Evolution

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Evolution

Historical background

The modern synthesis

Evidence for evolution

Evolutionary mechanisms

Species diversity and speciation

Phylogenetic systematics: Reconstructing evolutionary history

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Evolution is the natural process by which all living things have descended with modifications from shared ancestors, shaped largely by natural selection. It involves change in the genetic constitution of populations over time due to the systematic filtering by natural selection of randomly occurring changes in DNA. These changes are passed on from parents to their offspring.

Evolution does not involve individual organisms. Individuals develop, but they do not evolve. The study of evolution encompasses two major facets: deducing the history of descent among living things (phylogenetic systematics) and investigating evolutionary

mechanisms as revealed by laboratory and field studies of living organisms and their fossilized remains. Evolutionary change occurs as a result of beneficial mutation, migration, genetic drift and natural selection, and it is ultimately a passive process, devoid of any purpose or goal. As a scientific theory, it is an interconnected series of hypotheses, corroborated by a large body of evidence. Thus, biologists accept the historical reality of evolution, even though many details that are unclear continue to be investigated.

Historical background

The birth of modern evolutionary theory can be traced to the mid-nineteenth century and the publication of Charles Darwin’s On the Origin of Species by Means of Natural Selection in 1859. The book is considered by some to be the most influential in all of biology and by others an unsubstantiated theory. It was not, however, a new concept. Even in the late eighteenth century, the French scientist Maupertuis, the philosopher Diderot, and others entertained the notion that new life-forms might arise by natural means, including spontaneous generation. Their definition of evolution more closely resembles our modern concept of development, since the course of evolution is unpredictable. Interestingly, Darwin did not use the word “evolution” in The Origin of Species, except in the form “evolved,” the last word of the book. He preferred the bulkier but more precise phrase “descent with modification.”

French naturalist Jean-Baptiste Lamarck (1744–1829) was the first to clearly articulate the theory that species could change over time into new species, a process later called speciation. In his 1809 book Philosophie zoologique, he argued that living things progress inevitably toward greater perfection and complexity, through a process guided by the natural environment. A changing environment would alter the needs of living creatures, which would respond by using certain organs or body parts more or less. The use or disuse of these parts would consequently alter their size or shape, a change that would be inherited by the creatures offspring. Lamarck, although not the first, discussed this notion of “acquired characters,” which his contemporaries—including Darwin, to some extent—accepted. It is unfortunate that Lamarck is famous today primarily because he was wrong in this belief, in that it implies that bodily changes acquired during an individuals lifetime can affect the inherited material (now known as DNA) and be inherited. Although contemporary biologists have learned that changes to cell chemistry associated with DNA can, to some extent, be passed on from one generation to the next—a phenomenon called epigenetic change—Lamarck’s belief that this effect could be the primary driving engine of evolution was incorrect. Darwin accepted the inheritence of acquired characters, but argued (correctly) that natural selection was the main source of evolutionary novelty, not Lamarckian change.

Charles Darwin (1809–1882) and his contemporary Alfred Russell Wallace (1823–1913) are credited with independently providing the first plausible theory for a mechanism to explain evolutionary change, which they called natural selection. However, Wallace did not present so great a synthesis, or consider all the ramifications of evolution, as Darwin did in The Origin of Species and his later works. One major difference between the two men was that Wallace did not believe that natural selection could have produced the human brain; Darwin rejected his colleague’s contention that human intellect could only have been created directly by a higher power.

In The Origin of Species, Darwin concluded that some individuals in a species are better equipped to find food, survive disease, and escape predators than others. He reasoned that if the differences between the survivors and the doomed are passed from parents to their offspring, these differences will be inherited and gradually will characterize the whole population. Darwin knew that the greatest weakness in his theory was his lack of knowledge about the mechanics of inheritance; his attempt at a theory of inheritance, known as pangenesis, was later refuted. The nature of genes and DNA were unknown in Darwin’s time. However, he did clarify the distinction between genotype and phenotype. The genotype is a set of genetic instructions for creating an organism; the phenotype is the observable traits manifested by those instructions. The issue that explains the mechanism of inheritance, however, went unsolved for several decades later.

The modern synthesis

By the 1920s, Mendel’s theory of heredity had been rediscovered, but the early Mendelians, such as William Bateson and Hugo de Vries, opposed Darwin’s theory of gradual evolutionary change. These geneticists favored the position that evolution proceeds in large jumps, or macro-mutations, rather than the gradual, incremental changes proposed by Darwin. The dispute between the Mendelians and the Darwinians was reconciled by important theoretical work published in the early 1930s by independent population geneticists Ronald A. Fisher (1890–1962), J.B.S. Haldane (1892–1964), and Sewall Wright (1889–1988). Their classic work showed that natural selection was compatible with the laws of Mendelian inheritance.

This reconciliation gave rise to a period of prolific research in genetics as the basis of evolution. Theodosius Dobzhansky (1900–1975), a Russian emigrant to the United States, began his classic investigations of evolution in fruit fly populations, and in 1937 published a book entitled Genetics and the Origin of Species, which has been among the most influential books in modern genetics. Others, like E. B. Ford (1901–1988) and H. B. D. Kettlewell (1901–1979), helped pioneer the subject Ford called “ecological genetics.” This was a fundamental concept because it brought together two formerly separate areas of investigation: ecological variation (the influence of different environmental conditions) and genetic variation (differences in genetic make-up) in natural populations.

Evidence for evolution

The evidence for evolution is vast and of many kinds. Scientists continue to wrangle happily over the details of how evolution happened, but the fact that it did happen (and is still happening) is as well-established as the nature of the solar system or any other major component of the scientific world-story.

Darwin recognized that some of the best evidence of evolution is hidden within the bodies of living creatures. He reasoned that, if organisms have a history, then their history can be deciphered from their remnants. In fact, virtually all living creatures possess vestigial or rudimentary features that were once functional traits of their ancestors. Fetal whales, still in their mothers’ womb, produce teeth like all vertebrates, only to reabsorb them in preparation for a life filtering plankton from their ocean habitat. Snakes, whose vertebrate ancestors ceased walking on four legs millions of years ago, still possess vestigial hind limbs, with reduced hip and thigh bones.

In some cases the same structures may be adapted for new uses. The cat’s paw, the dolphin’s flipper, the bat’s wing, and a human hand all contain counterparts of the same five bones forming the digits (fingers in humans). There is no known environmental or functional reason why there should be five digits. In theory, there could just as easily be four, or seven. The point is that the ancestor to all tetrapods (vertebrates with four legs) had five digits, and thus all living tetrapods have that number, although in a modified form. Such traits, when they reflect shared ancestry, are known as homologous structures or characters. Traits that have functional but not truly ancestral similarity, such as the wings of insects and the wings of birds, are known as analogous characteristics.

Further evidence of evolution involves the most basic homology of all, the genetic code, or the molecular “language” that is shared by all living things. Although the genes of each kind of organism may be represented by different “words,” coding for each species unique structures, the molecular alphabet forming these words is the same for all living things on Earth. This is interpreted as evidence that the genetic code arose in a way that a single organism was the ancestor to all organisms that exist today.

Finally, there is the evidence from the fossil records that the remains of invertebrates, plants, and animals appear in the rocky layers of Earth’s crust in the same order that their anatomical complexity suggests they should, with the more primitive organisms in the older layers and the more complex organisms in the more recent deposits. No one has ever found a flowering plant or a mammal in deposits from the Devonian age, for instance, because those organisms did not appear on Earth until much later.

This is the merest sketch of the evidence for evolution. In an important sense, all of modern biology is evidence for evolution, because evolution is the organizing principle of modern biology, the concept by which biologists make sense of millions of diverse phenomena. Evolution is used to understand the behavior of disease germs, insects developing resistance to insecticides, the origins of genetic disease, DNA, and literally everything else studied by the life sciencese. In the famous words of Theodosius Dobzhansky, evolutionary biologist (and Orthodox Christian), “Nothing in biology makes sense except in the light of evolution.”

Evolutionary mechanisms

To an evolutionary biologist, evolution is a change in the proportion of genes present in an existing population. Each individual in a breeding population possesses a unique genotype, the set of paired genetic alternatives that determines its physical attributes, or phenotype. According to a principle known as the Hardy-Weinberg equilibrium, if certain conditions prevail, the frequency of each genotype in the population will be constant over generations, unless evolution has occurred to cause a shift in the gene frequencies. What factors can cause the proportion of genotypes in a population to change?

Natural selection is one example. An organisms environment—including its habitat, diseases, predators, and others of its own kind—present numerous challenges to its survival and to its ability to reproduce. Individual organisms of the same species have slightly different phenotypes (observable attributes), and some of these individuals may be slightly better equipped to survive the hazards of existence than others. The survivors produce proportionally more offspring than do the others of their species, and so their traits will be better represented in subsequent generations. These traits lead to an outnumbered or skewed representation of the offspring compared to less successful individuals (in terms of reproductive capacities) with alternative genetic complements. The genotype that is better adapted to the prevailing conditions will spread throughout the population—at least until conditions change and new genotypes are favored.

Another means by which gene frequencies may change is genetic mutation. When organisms reproduce, a copy of the DNA from each parent is transmitted to the offspring. Normally, the parental DNA is copied exactly; but occasionally, errors occur during replication in their sex cells, the egg and the sperm. Many of these errors occur in non-coding regions of the DNA, and so may have no known effect on the offspring’s phenotype; others may be lethal. In some cases, however, an offspring with a slightly modified genetic makeup survives. Mutation is thought to be an important source of new variation, particularly in rare cases where the mutation confers a selective survival advantage or a gain of function. After all, not all mutations are disadvantageous. In fact, nature’s method for ensuring the survival of a species is to rely on gene mutations that might enhance the function of the protein it encodes in such a way that there is a survival advantage. The determination of this advantage does not only rely on the type of mutation but the environment circumstances that prevail.

Evolution may also occur by genetic drift, a random process in which gene frequencies fluctuate due to chance alone. For example, if a certain genetic background is over-represented in one generation, perhaps because the region happened to be colonized by a small, homogeneous population, these genetic complements are more likely to remain high in the future generations. Genetic drift has more potent effects in small, isolated populations where some characteristic are not necessarily the result of natural selection and therefore do not represent a selective survival advantage.

Finally, the proportions of genotypes in a population may change as a result of migration, resulting in gene flow, or the movement of individuals (and their sex cells) into and out of the population.

Species diversity and speciation

Biologists have estimated that there are as many as 50 million species living on Earth today. Remarkably, it is believed that this figure is only 1% of the species that have ever lived. The great diversity of organisms, living and extinct, is the result of speciation, the splitting and divergence of biological lineages to create new species.

The term “species” is derived from the Latin word for kind, and species are considered to be the fundamental natural units of life which can and will evolve. Species have been defined in various ways over the years, but the most widely used definition at present is the biological species concept. Most often associated with the name of Ernst Mayr (1904-), it defines a species as a group of interbreeding populations that is reproductively separate from other such groups.

The crucial event for the origin of new species is reproductive separation. How do barriers to reproduction emerge? Perhaps a homogeneous population becomes separated into two distinct populations due to environmental factors. For example, when a river divides the species into two geographically distinct populations. Once they are separated, one or more evolutionary mechanisms may act on each in different ways over time, with the result that the two will diverge so they can no longer interbreed. As another example, perhaps a genetic variant arises in a population of interbreeding individuals by mutation or immigration; if the new variant spreads, bearers of the new variant may begin to breed preferentially with other bearers, preferring the new type. Along the way, further ecological, behavioral, and phenotypic changes may occur. Over many years, the organisms can diverge into two distinct species based on a dually but mutually exclusive preferential mating selection process.

Phylogenetic systematics: Reconstructing evolutionary history

The process of classifying and reconstructing the evolutionary history, or phylogeny, of organisms is known as phylogenetic systematics. Its goal is to group species in ways that reflect a common ancestry. The members of each group, or taxon, share uniquely derived characteristics that have arisen only once. For instance, the taxon Amniota includes amphibians, reptiles, birds and mammals, all of which arose from a single common ancestor that possessed an amnion in the egg stage. Classifying species involves only one apect of phylogenetic systematics. Understanding the evolutionary interrelationships of organisms by investigating the mechanisms leading to diversification of life and the changes that take place over time encompasses phylogenetic systematics. Systematics exceeds taxonomy or naming groups within species by attempting to develop new theories to describe the possible mechanisms of evolution. Historical remnants leave residual clues that allow phylogeneticists to piece together using hypotheses and models to describe history and how organisms evolve.

Systematists gather as much evidence as they can concerning the physical, developmental, ecological, and behavioral traits of the species they wish to group, and the results of their analysis are one (or more) branching “tree” diagrams, representing the hypothetical relationships of these taxa. Analysis of the genetic material itself has become an increasingly valuable tool in deducing phylogenetic relationships. The approach is to determine the nucleotide sequence

KEY TERMS

Genetic drift— Random change in gene frequencies in a population.

Genotype— The full set of paired genetic elements carried by each individual, representing the its genetic blueprint.

Hardy-Weinberg equilibrium— The principle that, if certain conditions prevail, the relative proportions of each genotype in a breeding population will be constant across generations.

Macromutation— Mutation having a pronounced phenotypic effect, one that produces an individual that is well outside the norm for the species as it existed previously.

Mutation— Alteration in the physical structure of the DNA, resulting in a genetic change that can be inherited.

Natural selection— The process of differential reproduction, in which some phenotypes are better suited to life in the current environment.

Nucleotide— Molecular unit that is the building block of DNA.

Phenotype— The outward manifestation of the genotype, including an organism’s morphological, physiological, and many behavioral attributes.

Phylogeny— Branching tree diagram representing the evolutionary history of an organism, and its relationship to others via common ancestry.

Speciation— The divergence of evolutionary lineages, and creation of new species.

Systematics— The classification of organisms according to their evolutionary relationships, and shared genetic and phenotypic characteristics.

Taxon (taxa)— Group of related organisms at one of several levels, such as the family Hominidae, the genus Homo, or the species Homo sapiens.

Tetrapod— The group of vertebrates having four legs, including amphibians, reptiles, mammals and birds.

of one or more genes in the species and use this to contribute to determining the appropriate phylogeny. Comparison of the differences in these DNA sequences provides an estimate of the time elapsed since the divergence of the two lineages. With the ability to sequence an entire organisms genome, the remnants of various species DNA, extinct (depending on the quality of the DNA) and living, can be compared and contrasted. This has exciting implications in the analysis of evolutionary modeling and applications to phylogenetic systematics.