This is a literature course on the book Evolution by Douglas J. Futuyma & Mark Kirkpatrick (Fourth Edition) during the first half of 2023. This write-up covers chapters 1 to 3 and is authored by Louis Addo (a Ph.D. student at KAU)
The historical background of Evolution Biology
Evolution in biology refers to the shift in the heritable traits of biological populations over successive generations. Charles Robert Darwin (English naturalist and biologist, Figure 1. left) and Alfred Russel Wallace (an English naturalist, explorer, geographer, anthropologist, biologist and illustrator, Figure 1. right) separately developed the theory of evolution by natural selection in the middle of the 19th century, and it was extensively outlined in Darwin’s book On the Origin of Species. The hypotheses of evolution postulated that all living things share a common ancestor, and that natural selection acting on genetic variations is responsible for the changes in their populations over time. The theory of evolution became a scientific fact supported by other scientists in paleontology, genetics, and biochemistry. Prior to the postulation of the theory of evolution, the dominant worldview was that each species was uniquely created by God and had set characteristics. However, this worldview was challenged by the Enlightenment movement leading to the emergence of science. The foundation for evolutionary thought was laid by astronomers and geologists who postulated theories about the creation of stars, planets, and the earth including the changes the earth has gone through as well as its many extinct species. Unique to Darwin’s theory were its five components explaining concepts “descent with modification” and “natural selection”.
The five distinct components of Darwin’s theory of evolution are natural selection which means that species characteristics change over time to meet changes in their environment, evolution which proposes that organisms change over time, gradualism which means that the differences between even fundamentally different organisms have evolved through intermediate forms rather than by large leaps, common descent which suggests that species diverged from shared progenitors and that species might be seen as one enormous family tree that represents ancestry, and finally population change which means that evolution occurs by changes in the proportions (frequencies) of different variant kinds of individuals within a population.
Most scientists acknowledged the historical truth of evolution through descent with modification from common ancestors by the 1870s, but natural selection, which drives evolution, was not widely accepted until around 60 years after The Origin of Species was published. Many ideas, including neo-Lamarckian, orthogenetic, and mutationist theories, were put out during this period.
In general, the idea of evolution is a scientific truth that explains how ancestors give rise to various descendants and how species change over time. Biologists generally agree with the theory, and while work is still being done in some areas, the fundamental ideas of evolutionary theory are well-supported.
The “Tree of Life” and phylogenetics
The concept of the Tree of Life was first proposed in Charles Darwin’s book On the Origin of Species and proposes that all species, alive and extinct, descended from a single ancestral form of life that existed billions of years ago. The history of the events by which species or other taxa have arisen from common ancestors is called phylogeny, which is often represented by a phylogenetic tree that shows the genealogical relationships among the taxa. Anagenesis, which is the evolutionary modification of a lineage’s (species’) physical characteristics, and cladogenesis, which is the division of a lineage into two or more descendant lineages, are the two main processes that contribute to the development of higher taxa. After cladogenesis, anagenesis causes each descendant lineage to diverge even farther from the others. The branching order and the length of the branch in the phylogenetic tree can show which species are more closely and less distantly related to one another.
Phylogenetic studies reveal the evolutionary relationships between organisms and gene sequences, often showing the pattern of separate and divergent lineages. However, sometimes branches of a phylogenetic tree rejoin, forming a network rather than just a branching tree. Hybrid speciation is one example of this, which is especially common in plants, where some species evolve from hybrid crosses between two different ancestors. Another example is horizontal gene transfer (HGT) where genes are passed among organisms, enabling them to adapt to changing circumstances.
Phylogenetic analysis is an effective method for determining how different traits in organisms have evolved through time. This approach shows that species have evolved from similar creatures since it is based on homologous traits that have descended from them. The majority of an organism’s traits are changed from traits that already existed in its ancestors and do not develop independently. Although they often have comparable genetic and developmental bases, homologous physical characteristics between species can vary more than the finished products. For instance, in the 1970s, scientists analyzed the protein amino acid sequences of pairs of animals that split from their common progenitors at various points in time. They created a molecular clock by plotting the matching DNA sequence discrepancies against predicted divergence periods and finding that the rise in differences increased linearly with time (Futuyma & Kirkpatrick 41). Assuming the genes in these lineages had developed at the same pace as those in the animals with fossil records, this permitted calculation of the period of divergence even for lineages lacking a fossil record. Yet, there is no one molecular clock and rates of sequence evolution vary among different types of organisms.
The analysis of the evolution of different organismal traits with phylogenetic analysis offers overwhelming proof of evolution. It is feasible to evaluate homologous traits that are descended from common ancestors since characteristics of organisms nearly invariably evolve from pre-existing aspects of their predecessors. However, a character may be shared by several species but not necessarily in the same character state. It can be challenging to determine if two species’ traits are homologous, but embryological research and anatomical correspondence of position and structure, are the two most often used methods for making this determination are often successful methods.
The concepts of Natural Selection and Adaptation
Changes in the environmental adaptation requirements of organisms trigger biological changes in their behavior and physical features to ensure their existence in the altered environment. Thus, altered environments can cause natural selection on the genetic variation in many characteristics of a species or population. For instance, soapberry beetles adapted to new food sources by changing the length of their beaks, and several insect and plant species have developed a resistance to heavy metals and chemical pesticides. Fish overfishing has also affected behavior and accelerated the onset of sexual maturity. The process by which individuals with advantageous traits have a better chance of surviving and reproducing than those without, resulting in the preservation of preferred variations and the rejection of harmful ones, is referred to as natural selection. Darwin first introduced this idea in “On the Origin of Species.” When various biological entities consistently vary in fitness, natural selection takes place. It is important to understand that evolution, which may also be brought about by other mechanisms like genetic drift, is not the same as natural selection. The environmental conditions that force natural selection on a species are determined by its traits. Certain species can create ecological niches for themselves by eliminating elements of their environment so that these no longer force natural selection. Natural selection can take place at several scales, including genes, cell types, individual organisms, populations, and species.
Selfish genes, which multiply in the genome whether they are advantageous to the organism or not, are an illustration of gene-level selection. Gene-level selection can conflict with individual selection and cause harm to organisms. Selection among individuals occurs at a higher level than selection among genes. Characteristics develop through individual selection; altruism can lower individual fitness but may develop through social selection; cooperation develops through kin selection. By varying the percentage of species throughout time, species selection modifies the diversity of biological traits. It has an impact on organism disparity but not adaptations. For instance, more asexual populations experience extinction than sexual groups.
Adaptation in biology refers to both the process by which organisms evolve over generations to improve survival and reproduction, and to a characteristic that evolved by natural selection. A trait must be developed and give greater fitness than the ancestral condition in order to qualify as an adaptation. A feature that unintentionally fulfills a new purpose is known as a preadaptation. For instance, parrots can eat fruits and seeds with the help of their strong, pointed beaks but, in case a new resource is presented, they can also use it to feed on that resource. Exaptation is the process of adapting a characteristic for use unrelated to the one for which it was originally selected. In birds that initially evolved feathers to keep warm, an example of an adaptation would be the use of feathers for mating displays or flying. Pre-adaptation is another name for an exaptation. Exaptations and preadaptations are common in the early stages of the evolution of new adaptations. To determine if a certain characteristic is an actual adaptation, scientists examine the available data. Species traits are not always adaptations or random characteristics but can be flawed and limited, such as in the case of mammals that are unable to evolve beneficial modifications in the number of vertebrae. The vast diversity of life is the result of natural selection, as varied habitats and other factors may force selection on different traits within a species.
The next blog will cover chapters 4 and 5 and will be authored by Samuel Shry.
References
Futuyma, D. J., & Kirkpatrick, M. (2017). Evolutionary. Evolution (Fourth ed.). pp. 3-76. Sunderland, Massachusetts: Sinauer Associates, Inc.