Modern Evolutionary Theory
Introduction
Modern evolutionary theory explains how living organisms change over generations and how the diversity of life develops over time. It builds upon Darwin’s theory of natural selection and Mendel’s work on genetics, while incorporating later discoveries in DNA, mutation, population genetics, and other biological processes. Understanding modern evolutionary theory is important because it provides a scientific framework for explaining adaptation, variation, the origin of new species, and the evolutionary history of humans and other organisms.
Darwin’s theory of evolution by natural selection and Mendel’s pioneering work on genetics laid the foundation for evolutionary theory, but many advances have occurred since the late 1800s, which are outlined in this section.
Key Concepts in Modern Evolutionary Theory
Key concepts in evolutionary theory include mutation, natural selection, sexual selection, gene flow, genetic drift, and adaptive radiation.
Mutation
Mutations, which are errors in the replication of DNA, are the ultimate source of variation within populations. They can be neutral, beneficial, or harmful. We often hear about the bad mutations that are passed on via sperm or egg, since they can cause diseases, deformities, and death. DNA is complex and errors occur all the time. Your hair and fingernails grow, for example, by replication of cells. If you have a mutation in those cells, it is no concern. If, however, there is a mutation in the sperm or egg, it can be a big deal.
Mutations usually occur simply as copying errors. They can also occur, however, from exposure to radiation, chemicals, and viruses. All human variability ultimately derives from mutations.
Normal cell → DNA change occurs → Mutation forms → DNA is copied → Mutation is copied too → Cell divides → Daughter cells carry the mutation
If it occurs in a germ cell → DNA change → Mutation → Sperm/egg carries mutation → Fertilization → Fertilized egg carries mutation → Cells divide → Child inherits mutation → Mutation can be passed to the next generation
Types of Mutation
Mutations can be classified in several ways, but the most common classification is based on how they affect DNA or chromosomes.
Important terms - nucleotide - is basically unit of DNA/RNA
alleles - gene that helps determine a specific trait
Point Mutation: A change in a single nucleotide/base of DNA.
Example: A substitution in the HBB gene can cause sickle-cell disease.
Insertion: One or more nucleotides are added to a DNA sequence. This can cause a frameshift mutation if the number added is not a multiple of three.
Deletion: One or more nucleotides are removed from a DNA sequence. Like insertion, it can cause a frameshift mutation.
Duplication: A segment of DNA is copied more than once, resulting in an extra copy of a gene or DNA region.
Inversion: A segment of DNA is reversed within the chromosome.
Translocation: A segment of one chromosome moves to another chromosome.
Based on their effect
Beneficial mutation: Provides an advantage under particular environmental conditions.
Example: Certain mutations can provide resistance to diseases.
Neutral mutation: Has little or no effect on an organism’s survival or reproduction.
Harmful mutation: Reduces fitness or may contribute to genetic disorders or disease.
As anthropologist Dr. Eben Kirksey (2021) states in his book The Mutant Project: Inside the Global Race to Genetically Modify Humans, “strictly speaking, we are all mutants” (p. 7). He elaborates further:
At a molecular level, each of us is unique. Each of us starts life with forty to eighty new mutations that were not found in our parents.… During the course of a normal human life we also accumulate mutations in our bodies, even in our brains. By the time we reach age sixty a single skin cell will contain between 4,000 and 40,000 mutations … These genetic changes are the result of mistakes made each time our DNA is copied during cell division, or when cells are damaged by radiation, ultraviolet rays, or toxic chemicals. Generally, mutations aren’t good or bad, just different. (p. 7)
Indeed, it is because of mutations that our ancestors had variability from which bipedalism and larger brains emerged. It may have been a mutation, for example, that allowed one person to walk upright for a few meters farther than the average person could walk. Under specific conditions, that ability may have been favorable and may have led to that individual having more offspring, who also had the favorable trait. It is likely that the mutations that were selected for since the split of our common ancestor with chimpanzees and bonobos number in the millions.
In recent years, many people have been engaged in the study of mutations among viruses. Viruses that cause the flu in humans, for instance, regularly mutate, which is why new flu vaccines are created every year. Similarly, the virus that causes COVID-19 has undergone several mutations that have led to the development of multiple new variant strains.
Gene Flow and Genetic Drift
Gene flow and genetic drift are important concepts.
Gene flow is when genes move between populations that are members of the same species but that do not normally mate with one another. Because of separation, new alleles may have formed in one population that may then be passed on to another. Regarding human evolution, for example, it was almost certainly the case that people moving out of Africa in the distant past mated with pre-existing populations in the Middle East and Asia. Gene flow would have occurred.
Population A → Individuals move to Population B → They reproduce → Their genes enter Population B → Allele frequencies change → Population B becomes genetically more similar to Population A
Genetic drift, on the other hand, is a random factor in evolution. It is when changes in allele frequency occur by chance; for instance, when a small group leaves its parent population and begins a new population elsewhere. The smaller the group, the larger the changes may be in allele frequency.
Population → Random event occurs → Some individuals survive/reproduce more than others by chance → Their alleles become more common → Other alleles become less common → Allele frequencies change by chance → Genetic variation may decrease
Adaptive Radiation
Adaptive radiation occurs when a species rapidly adapts to an ecological niche, often expanding its population quickly and diversifying into multiple species.
A good example of adaptive radiation was when mammals adapted to new ecological niches following the extinction of dinosaurs, adaptive radiation of Galapagos finches More recently, adaptive radiation likely occurred when humans first left Africa, quickly expanding across Asia and perhaps diversifying into different species.
One ancestral species → Population becomes separated into different environments → Different selection pressures → Different adaptations develop → Populations become more different → New species form → Many different species arise from one ancestor.
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Darwin's finches on the Galápagos Islands demonstrate this process, splitting from one common South American ancestor into roughly 14–18 distinct species with specialized beak shapes to fit unique food sourceNatural Selection and Sexual Selection
Natural selection, as proposed by Darwin, remains an important concept in evolution. In regard to human evolution, for example, it is likely that it was natural selection that led to significant loss of body hair and changes in skin color.
Population has variation → Some individuals have traits that help them survive/reproduce → They survive and reproduce more → Their genes are passed to more offspring → Helpful trait becomes more common → Population changes over generations
Types of Natural Selection
There are three main types of natural selection:
Directional Selection: Favors one extreme phenotype, causing the population to shift toward that trait.Example: During droughts, birds with larger, stronger beaks may survive better because they can crack harder seeds.
Stabilizing Selection: Favors the average phenotype and selects against extreme traits.Example: In human babies, average birth weight tends to have higher survival than extremely low or extremely high birth weight.
Disruptive Selection: Favors both extreme phenotypes while selecting against the intermediate phenotype. Example: In a bird population, birds with very small or very large beaks may survive better than birds with medium-sized beaks when only small and large seeds are available.
Even Darwin recognized that there were likely other selection processes as well, such as sexual selection, which essentially means personal mate selection. For instance, a variety of male animals create colorful displays and performances to entice the female of their species to have sex (consider the mating displays of fireflies or the architectural feats of the bower bird). Similar kinds of selection among humans, with mate selection having nothing to do with the potential for increased survival of the species, have likely been going on for millions of years. These mate choices may have been based on physical features, such as facial symmetry, or cultural factors, such as the ability to heal others.
Gradualism and Punctuated Equilibrium
Gradualism and punctuated equilibrium are models pertaining to the speed at which evolution occurs. Gradualism suggests a slow, steady change, with a new species eventually emerging. Punctuated equilibrium suggests slow, steady change occasionally interrupted by short periods of significant change. Punctuated equilibrium explains lack of transitional forms in the fossil record. Both models can be used to explain human evolution at various times. The transition from Australopithecus to Homo habilis more than two million years ago, for example, has left few fossils that can be described as transitional, so punctuated equilibrium best explains the transition. On the other hand, Homo erectus fossils show evidence of well over one million years of fairly slow, gradual change.
Fig: PUNCTUATEDEQUILIBRIUM VERSUS GRADUALISM.
Species and the Definition of a Species
There are many working definitions of species. The definition a species is a population of individuals that can mate and produce fertile offspring in the wild. For instance, a horse and a donkey are different species. When they mate, they can produce a mule, but mules are almost always sterile.
Hybrids are known to occur in controlled conditions and increasingly in the wild as natural habitats decrease. In regard to human evolution, when anthropologists refer to different species of humans, the assumption is usually that they would not have been able to mate and produce fertile offspring. In recent years, DNA research is increasingly showing that different kinds of humans, such as Neandertals and modern humans, were indeed mating and producing fertile offspring, leading to the conclusion they were the same species.
Speciation
Speciation is the process by which new species emerge. It can happen in numerous ways. Sometimes new species emerge from geographic isolation. It was likely the development of the Congo River in Africa 1.5 to 2 million years ago that separated the common ancestor of chimpanzees and bonobos, leading to those two separate species. Sometimes it is a single population of a species that evolves into another species, coexisting for a time with its ancestral species before the ancestral species becomes extinct. This is probably the more common situation in human biological evolution.
The evolution of species is rarely a simple, linear event with all populations of a species evolving into another; nor is it always useful to consider evolution as a tree with branches representing new species. In some cases, such as with humans, it may be best to consider evolution through a model known as the “braided stream” analogy, in which various populations of a species sometimes branch off and evolve independently for a time before re-joining the main part of the stream again, bringing new evolutionary changes with them.
One population → Population becomes separated → Gene flow stops → Different mutations + natural selection + genetic drift occur → Populations become genetically different → They can no longer successfully interbreed → New species forms
The major types of speciation are:
Allopatric Speciation: Occurs when populations become separated by a geographical barrier, such as mountains, rivers, or physical distance. Over time, the isolated populations may accumulate genetic differences and become separate species.
Example: The formation of different species of Darwin’s finches on the Galápagos Islands.
Sympatric Speciation: Occurs when new species arise within the same geographical area, without physical separation. It may result from genetic changes, differences in habitat use, or reproductive isolation.
Example: Polyploidy ( extra set of chromosome ) in plants can produce individuals that are reproductively isolated from the original population.
Parapatric Speciation: Occurs when neighboring populations occupy partially overlapping or adjacent geographical ranges and experience different environmental conditions. Limited gene flow combined with natural selection can eventually lead to reproductive isolation.
Example: Grass species growing in areas with different soil conditions, such as normal and metal-rich soils.
Peripatric Speciation: Occurs when a small population becomes isolated at the edge of a larger population’s geographic range. Genetic drift and natural selection can cause rapid divergence in the smaller population.
Example: Island populations that become isolated from mainland populations may eventually develop into distinct species.
Extinction
The concept of extinction is important. Sometimes an entire species evolves into another species and the originating species therefore becomes extinct. In other cases, a species becomes extinct for other reasons, such as environmental change to which it cannot successfully adapt since it does not have enough variability from which to choose. This is likely what happened in the case of the dinosaurs 65 million years ago and in the extinction of mammoths and mastodons about 10,000 years ago.
In other situations, species may become extinct because of the introduction of a new species that outcompetes them for resources. Extinction is normal, but occasionally extinction rates are extremely high. It is commonly accepted that there have been five periods of mass extinctions, when approximately half the species on earth became extinct. All previous mass extinctions occurred before the emergence of humans. Many believe we are on the verge of a sixth mass extinction, except this time humans appear to be the cause. Some biologists claim the rate of animal extinctions occurring in recent and contemporary times is 1,000 or more times above normal.
Genome, Genomics, Epigenetics, and Genetic Engineering
Terms that have appeared in recent years include genome, genomics, epigenetics, and genetic engineering. Genetics tends to refer to the study of individual genes and their role in inheritance. Genome refers to the entire genetic makeup of an individual or species, including all its DNA and genes, and genomics is the study of genomes.
Epigenetics is a new area of research. We realize that genes are the primary way traits are inherited but that other factors, such as chemical reactions due to life events or stressors, may also have a role. Epigenetics thus refers to the study of how factors other than DNA or genes may influence the occurrence of specific traits. In other words, epigenetics can change a phenotype without a corresponding change in the genotype. Recent research indicates there are a variety of reasons that can lead to this inheritance, including the environment, diet, age, and disease. For example, a pregnant mother’s exposure to pollution can increase her child’s susceptibility to asthma.
Genetic engineering involves the deliberate alteration of genes, often to increase the productivity of plants and animals used by humans for food. Traditionally, food producers would select specific plants and animals to breed based on phenotypes, but in recent years they have begun to use genetic engineering, selecting for, and modifying, genotypes. Consideration is now also being given to the potential genetic engineering of humans and how that may transform humanity as we know it (Kirksey, 2021).
The Modern Evolutionary Synthesis
The combination of the basics of natural selection and genetics, developed in the nineteenth century, with our greater understanding of processes such as mutation, gene flow, genetic drift, and speciation are often termed the modern evolutionary synthesis, or the extended evolutionary synthesis.
While some researchers consider modern evolutionary theory and extended evolutionary synthesis to be one and the same, others prefer to reserve the use of the term extended evolutionary synthesis to refer to a more comprehensive set of theoretical concepts and recent findings that include the potential role of epigenetics and of other such factors that may lead to evolution. potential genetic engineering of humans and how that may transform humanity as we know it (Kirksey, 2021).
The combination of the basics of natural selection and genetics, developed in the nineteenth century, with our greater understanding of processes such as mutation, gene flow, genetic drift, and speciation are often termed the modern evolutionary synthesis, or the extended evolutionary synthesis. While some researchers consider modern evolutionary theory and extended evolutionary synthesis to be one and the same, others prefer to reserve the use of the term extended evolutionary synthesis to refer to a more comprehensive set of theoretical concepts and recent findings that include the potential role of epigenetics and of other such factors that may lead to evolution.
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