Darwinism- The Natural Selection for “Adaptations and Origin of New Species”
Principle 1: Changes (variations) in organisms are “Selected” by Nature continuously, either “to live” or “to remove” and make “new fittest species” on the earth for successful survival.
Principle 2: “Darwinism” is “gradualism” (new species formed gradually, but not suddenly).
@ Natural selection promotes the fittest organisms ” to live” and establishes new species on the earth(e.g.all the diversified organisms living on the earth, today).
@Natutral Selection stresses the unfitted organisms “to remove” and delete the species from the earth- extinction (e.g. Dinosaurs of the Mesozoic era).
Introduction:
Every organism living on the Earth occupies a suitable habitat called “Ecosystem” and is desirous to continue its race and reproduce a possible number of offspring during its lifetime, named “Overproduction”. Yet, organisms in every generation are influenced, impacted and regulated by the dynamic “Physico-Chemical and Biological environments” (simply called NATURE) and maintain the “equilibrium state (carrying capacity)”. The environment of limited resources such as availability of food to take, space to live etc., provokes the “Struggle for existence” with the “Competition” and struggle with unstable climatic and edaphic (soil and its minerals and water) environmental conditions. Winners are achievers who struggle with some necessary changes in their bodies and become eligible to survive with fitness in their lives. Thus, according to the changes that take place in the environment, organisms also try hard to change, modify or adapt to live in that particular ecosystem successfully. These changes may be obviously stimulated, triggered and directed by the evolutionary drivers of nature such as mutations, Sexual selection or selective mating, Genetic Drift, Gene flow (Immigration or Emigration), Genetic Recombinations (of meiotic division), and more. According to Charles Darwin whatever changes appear in the organisms are called “Variations”. Variations are variable capacities of organisms to live in the existing environment. These variations may be more or less beneficial or sometimes harmful, even may be neutral. Exactly, now “Natural Selection”, the leader of evolutionary drivers enter and determines the future of the variations by the selection process, the organisms either “to live fruitfully” or “to be eliminated” from nature. The selection and adaptation processes are continuous by the “Inheritance of acquired Characters” by every generation leaving the offspring with variations. These variations may not be visible to the naked eye in one or a few generations, but the gradual accumulation (gradualism) of the aggregated changes establish the large cumulative changes called new big adaptation (s) and form the new species called “Speciation”. Darwin researched extensively on finch birds that migrated from South America to the Galapagos Islands of different climatic conditions and evolved as different species, which were explored as “adaptive Radiation” (figure 1.1).
Characteristics of population and the process of Natural Selection:
I. Overproduction: How many seeds are being produced by a tamarind tree in a year? How many eggs are laid by a butterfly before ending its life? How many zygotes are being released by a fish in its lifetime? Seeds are in thousands, the eggs laid by butterflies may be in hundreds in a year and the zygotes released from a fish may be in lacks, throughout life! This elevated level of reproductive desire and capacity of organisms is referred to as “Overproduction”. Do all offspring take birth in the next generation survive? If all the seeds of all the tamarind trees on this globe are germinated and live, within a few years, our planet is filled with only tamarind trees! Similarly, if all the eggs of all the butterflies are hatched and produce larvae, our earth is occupied by only butterfly species! Consequently, all the water bodies are filled with only fish populations! Why is this elevated level of reproduction found in organisms? If all could survive, does their environment provide infinite food and shelter to these organisms? Impossible to do this! Every ecosystem has limited resources to facilitate the organisms. Limitless growth leads to the mass extinction of a species! However, this overproduction is automatically regulated in nature.
II. Struggle for existence: What are the struggles of populations to adapt and live in nature?
As per the carrying capacity of a particular ecosystem, the population sizes of a species are regulated by competition. Individuals in the population may face three types of competitors.
1. Struggle within the species – Intraspecific competition: Competition among the members of a population or species for food, shelter and mating.
2. Struggle with other species – Interspecific competition: Competition among the members of the different populations or species for food (cattle fights with goats and sheep populations, as their food is grass), shelter (ants, termites and snakes in another competition, as all try to live in mounds). No mating competition, as genetic variations occur between the species (Figure 1.2).
3. Struggle with the environment – Physico-chemical environment: Physical factors such as availability of temperature, light, humidity, gravity etc., and Chemical factors such as availability of minerals, acidity, and alkalinity etc., are dynamic and change over a period of time. according to the changes in the physicochemical world, organisms adapt to adjust or habituate alternate sources to fulfil their needs.
Due to the above three ways of competition and struggle, only a few organisms, those who are strong enough (fittest) and won the fight could live and are treated as “fittest Organisms”.
III. Variations and Survival of the Fittest: What are different variations and how are they selected by the “Natural Selection”?
Variations in the organisms are developed due to competition and struggle with the environment. These variations may be either beneficial, or harmful, and may be neutral sometimes. Organisms with beneficial variations fit in that competitive and changed environment to lead a successful life, called the “survival of the fittest”.
IV. Natural Selection by gradualism: Who is eligible to live in Nature? How do organisms get this eligibility?
Nature (environment) or a particular ecosystem can select the specific fittest organisms to live and those who have unsuitable and unfavourable variations are selected and eliminated from the environment. This is a continuous process and organisms undergo gradual changes, instead of sudden origin.
V. Inheritance of Acquired Characters: Could the achieved characters be transferred?
This is actually proposed by “Lamarck”, and accepted by Darwin. The fittest organisms with better variations can transfer their characteristics to the next generations, which go through the above processes, the overproduction, struggle for existence and produce variations in every generation.
VI. Speciation- The Origin of Species”: How do new species are formed on the earth?
In every generation, due to the struggle for existence, variations are produced in populations. These variations aggregate generation by generation for many years and the cumulative small changes once become a big change(s) and “Origin” as a new species and “isolated” from the pre-existing populations.
VII. Adaptive Radiation- Darwin Finches, the Divergent and Convergent Evolution
Adaptive radiation demonstrates how species can diversify and fill various ecological niches when “faced with new opportunities” in their environment. It provides valuable insights into the mechanisms of evolution and speciation. Adaptive radiation is a process of “rapid” diversification of species from “a pre-existing common ancestor” into a variety of ecological niches or adaptive zones. It typically occurs when a population or a small group of organisms encounters new, diverse, and unoccupied habitats, leading to the evolution of multiple species, each adapted to exploit a specific ecological opportunity.
Characteristics of Adaptive Radiation:
- Diverse Habitats: Adaptive radiation often occurs in environments with a wide range of ecological niches or resources that are unexploited or underutilized by existing species. These niches may include different food sources, habitats, or ways of life.
- Rapid Speciation: During adaptive radiation, multiple species evolve relatively quickly from a common ancestor. This rapid speciation is driven by natural selection favouring traits that enhance an organism’s ability to exploit specific ecological opportunities.
- Divergent or unparallel Evolution: As new species evolve within the adaptive radiation tend to radiate and diverge in multiple directions as different from each other in terms of habits, morphology, behaviour, and ecological roles although their origin is common. This divergence allows them to coexist without directly competing for the same resources, the type of food, habitat, climate etc.
- Convergent or Parallel Evolution: Different lineages within adaptive radiation may independently evolve similar adaptations in response to similar environmental challenges such as the same food, habitat and climates. This can result in convergent evolution, where “unrelated species” develop similar traits or characteristics.
Examples of Adaptive Radiation:
- Darwin’s Finches: One of the most famous examples of adaptive radiation is the diversification of finch species on the Galápagos Islands, which inspired Charles Darwin’s theory of evolution. These finches adapted to various ecological niches on different islands, with beak shapes and sizes that matched their primary food sources, such as seeds, insects, or nectar.
- Hawaiian Honeycreepers: The Hawaiian Islands have been a hotspot for adaptive radiation in bird species. Honeycreepers, a diverse group of birds, evolved into various forms, each adapted to different habitats and food sources, including nectar-feeding, seed-eating, and insect-eating species.
- African Cichlid Fish: The African Great Lakes, such as Lake Victoria and Lake Malawi, are home to numerous species of cichlid fish that have undergone adaptive radiation. These fish have evolved a wide range of body shapes, colour patterns, and feeding strategies, driven by the availability of various niches and food sources within the lakes.
- Anolis Lizards in the Caribbean: Anolis lizards in the Caribbean have experienced adaptive radiation on different islands. They have evolved distinct body sizes, limb lengths, and behaviours to exploit specific microhabitats, such as tree branches, twigs, or leaf litter.
- Mammals in the African Savanna: The African savanna is inhabited by a diverse array of herbivores, including antelopes, zebras, and giraffes. Each of these groups has evolved adaptations related to their feeding habits, locomotion, and social structures, resulting in a wide range of species.
# The Industrial Melanism – Example of the Natural Selection.
This example illustrates how changes in the environment can lead to shifts in the prevalence of certain traits within a population over a period of time by “Natural Selection.
Between 1760 – 1840, the period was the Industrial Revolution. During that time, there were two types of coloured peppered moths (Biston betularia) that were being lived, one was light and another black (figure 1.3). Before the industrial revolution, in many areas in Europe, the trees had light-coloured bark (natural) and a huge number of light-coloured moths were well-camouflaged and lived in that area in huge numbers, due to protection from predatory birds, while the black moths were few in numbers, because of the high predation pressure. During the Industrial Revolution, the widespread use of coal led to increased air pollution. This pollution, in turn, darkened the tree barks in industrial areas due to soot and other pollutants. As the tree bark darkened, the light-coloured moths became more conspicuous to predators against the dark background. In the polluted industrial areas, the dark-coloured melanic moths had a survival advantage over their light-coloured counterparts because they were better camouflaged against the now-darkened tree bark. As a result, they were less likely to be eaten by birds. Increase in Melanic Moths populations over several generations, The frequency of the dark melanic moths in the population increased dramatically due to the selective advantage conferred by their colouration. This change in the moth population’s colouration was driven by natural selection, as individuals with the advantageous trait (melanism) were more likely to survive and reproduce.
Reduced pollution reversed melanism: In the mid-20th century, efforts to reduce air pollution, such as the Clean Air Acts, led to a decrease in industrial pollution and a return to lighter-coloured tree bark. As a result, the advantage of melanism (Black coloured) in peppered moths decreased, and the frequency of light-coloured moths began to rise again.
The example of industrial melanism in peppered moths provides a clear demonstration of how environmental changes can exert selective pressures on populations, leading to shifts in the prevalence of specific traits within a relatively short time frame. It is often cited as one of the most well-documented cases of natural selection in response to human-induced environmental changes.
Drawbacks and limitations of Darwin’s Natural Selection:
While Charles Darwin’s theory of natural selection is a foundational concept in biology and has provided valuable insights into the process of evolution, it is not without its limitations and drawbacks. Some of the limitations and criticisms associated with Darwin’s theory of natural selection include:
- Incomplete explanation of the genetic basis of evolution: Darwin’s theory provided a mechanism for how evolution occurs (natural selection), but it did not explain the underlying genetic basis of heredity and variation. The understanding of genetics and the role of DNA in inheritance came after Darwin’s time, and the modern synthesis of genetics and evolution (known as neo-Darwinism) combined both concepts.
- Didn’t reveal the presence of Intermediate forms on Earth: If most adaptive and fittest organisms can survive, why do some intermediate forms live on the Earth, today? This ambiguous phenomenon was not explored by Natural Selection.
- No clear explanation about the ill-adapted organisms in Nature: Are ill-adapted organisms deleted compulsory from Nature? Darwinism talks about the inheritance of only favourable characteristics which is also not always true. some of the poorly adapted organisms are also found in every community and population.
- Lack of Explanation for the Origin of Variation: While Darwin recognized the importance of variation within populations, he did not have a complete explanation for the sources of variation. Mendelian genetics and later discoveries in molecular genetics provided a more comprehensive understanding of genetic variation.
- Missing Transitional Fossils (missing links): At the time of Darwin, the fossil record was incomplete, and the lack of transitional fossils was seen as a challenge to his theory. Since then, many transitional fossils have been discovered, providing substantial support for the theory of evolution.
- Inheritance of Acquired Traits: Darwin’s theory did not account for the inheritance of acquired characteristics, a concept proposed by Jean-Baptiste Lamarck. This idea was later discredited, and the modern understanding of genetics supports the inheritance of genetic information rather than acquired traits.
- Didn’t answer the rate of Evolution: Darwin’s theory did not provide a specific rate or timeline for evolutionary changes. The pace of evolution can vary widely among different species and environments, and neo-Darwinian theory has since incorporated molecular and genetic data to help explain this variation.
- Didn’t explore Cooperation and Altruism among organisms: Darwin’s theory primarily focused on competition and the survival of the fittest. It did not readily explain behaviours involving cooperation, altruism, or social interactions among organisms. The evolution of such behaviours has since been explored through the concept of inclusive fitness and kin selection.
- No clarity on complex Adaptations: Some critics argued that certain complex adaptations, such as the human eye, could not have evolved gradually through natural selection. However, subsequent research has provided plausible explanations for the stepwise evolution of complex structures.
- Not approach the Cultural Evolution: Darwin’s theory primarily addressed biological evolution, while cultural evolution, which encompasses the development of human societies, languages, and technologies, involves mechanisms beyond natural selection and genetic inheritance.
Natural Selection vs. Mutational Theory:
“Mutations constantly create and propagate genetic diversity, While natural selection, directs the process of changes and “decides” which mutation is useful and which is not”.
Natural Selection and the Mutational Theory are two distinct but closely related concepts in the field of evolutionary biology. They both play crucial roles in explaining how species evolve over time, but they focus on different aspects of the evolutionary process.
1. Natural Selection: It is the process by which certain traits or characteristics become more or less common in a population over generations based on their impact on an organism’s ability to survive and reproduce in its environment.
Key Principles:
- Variation: Within any population, there is variation in traits or characteristics due to environmental triggering. This variation can be inherited from one generation to the next.
- Heritability: Some of the variations are heritable, meaning they can be passed on to offspring genetically.
- Differential Survival and Reproduction: Individuals with advantageous traits that better suit them to their environment are more likely to survive and reproduce. This leads to the increased frequency of these advantageous traits in the population over time.
- Adaptation: Natural selection results in the adaptation of species to their environments, as traits that enhance survival and reproduction become more common.
Example: A classic example is the evolution of the long neck in giraffes. Over generations, giraffes with longer necks were better able to reach leaves high in trees for food, which increased their chances of survival and reproduction. As a result, the population evolved to have longer necks.
2. Mutational Theory: The mutational theory of evolution focuses on the role of mutations in the process of evolution. Mutations are random, heritable changes in an organism’s DNA sequence. This theory emphasizes that mutations are the ultimate source of genetic variation upon which natural selection acts.
Key Principles:
- Mutation as the Source of Variation: Mutations introduce new genetic variations in populations. These mutations can be beneficial, harmful, or neutral in their effects.
- Role of Chance: The occurrence of mutations is largely a chance process, meaning that the types of mutations that arise are not directed or purposeful.
- Genetic Drift: Genetic drift, which refers to the random change in the frequency of gene variants in a population, can also play a role in the spread of mutations in small populations.
Example: Suppose a population of bacteria contains individuals with a gene that provides resistance to a particular antibiotic. This resistance arises due to a mutation. When the antibiotic is introduced, bacteria with the resistance gene are more likely to survive and reproduce, leading to an increase in the frequency of the resistance gene in the population. Here, mutation (the development of antibiotic resistance) is the initial source of variation, and natural selection acts on this variation.
Differences between Natural Selection and Mutational Theory:
- Focus: Natural selection focuses on the process by which certain traits become more or less common in a population due to their impact on survival and reproduction, while the mutational theory emphasizes the role of mutations as the ultimate source of genetic variation.
- Causation: Natural selection is a non-random process driven by the environment, where traits that confer a survival or reproductive advantage become more common. Mutations, on the other hand, are random events that introduce genetic variation without a specific purpose.
- Timing: Natural selection acts on existing genetic variation within a population, whereas mutations occur randomly and can generate new genetic variation.
Conclusion: natural selection and mutational theory are complementary concepts that together help explain how species evolve over time. Natural selection acts on genetic variation produced by mutations and other processes, favouring traits that enhance an organism’s fitness in its environment, while mutations provide the raw material for evolution by introducing new genetic variation into populations.
Darwinism vs. Lamarckism:
Darwinism and Lamarckism are two contrasting theories of evolution that were proposed by Charles Darwin and Jean-Baptiste Lamarck, respectively. These theories offer different explanations for how species change over time, and they have been significant in the history of evolutionary biology. Here are the key differences and comparisons between Darwinism and Lamarckism. While Darwinism is widely accepted and supported by modern biology, Lamarckism has been largely rejected due to its lack of empirical evidence and its inconsistency with the principles of modern genetics.
Differences and Comparisons:
- Mechanism of Evolution: The fundamental difference between the two theories is the mechanism of evolution. Darwinism is based on natural selection acting on genetic variation, while Lamarckism relies on the inheritance of acquired traits.
- Heritability: Darwinism emphasizes the heritability of pre-existing genetic traits, while Lamarckism suggests that acquired traits can be inherited, which is not supported by modern genetics.
- Empirical Support: Darwinism is supported by extensive empirical evidence from various fields, including genetics, palaeontology, and ecology. Lamarckism lacks empirical support and is largely discredited in modern biology.
- Timescale: Darwinism operates on long evolutionary timescales, whereas Lamarckism implies rapid changes within a single generation, which is inconsistent with observed patterns of evolution.