Theories of Evolution

THEORIES OF EVOLUTION

Lamarckism

  • Lamarckism, named after the French naturalist Jean-Baptiste Lamarck, is an early theory of evolution.
  • Lamarck proposed that the evolution of life forms occurred through the inheritance of acquired characteristics. 

1. Use and Disuse of Organs:

  • Lamarck suggested that organisms could adapt to their environment by using or not using specific body parts.
  • For example, he used the elongation of giraffe necks as an illustration. Giraffes stretched their necks to reach leaves on tall trees, and over time, this acquired long-neck characteristic was passed on to their offspring.

 

 

2. Inheritance of Acquired Traits:

  • Lamarck argued that the changes an organism acquired during its lifetime would be inherited by its descendants.
  • This implied that the environment could directly influence an organism's characteristics and evolution.

 Problems with Lamarckism:

1. Lack of Scientific Evidence:

Lamarck's theory lacked scientific evidence to support the idea that acquired traits could be inherited. In reality, acquired traits during an individual's lifetime do not alter its genetic makeup. 

2. Contradiction with Genetic Inheritance:

Lamarckism contradicts the principles of genetic inheritance, which were later elucidated by Mendel's work. Genetic inheritance is based on the transmission of specific genes, not acquired characteristics. 

Mutation Theory of Evolution

  • The Mutation Theory of Evolution, proposed in the early 20th century, introduced a significant shift in our understanding of how new species and variations arise.
  • The Mutation Theory of Evolution, introduced by Hugo de Vries, emphasizes the role of sudden and large-scale mutations, including the concept of saltation, in driving the evolution of species. 

1. Origin of Variation:

  • Mutation theory suggests that the origin of variation in species is primarily due to mutations.
  • Mutations are sudden, large-scale genetic changes that can occur spontaneously and randomly in an organism's DNA.

 2. Role of Mutations:

  • Unlike Darwin's concept of gradual and minor variations, mutation theory emphasizes that it's these sudden, drastic mutations that drive the evolution of species.
  • Mutations can lead to new traits, which may provide a selective advantage in specific environments. 

The Concept of Saltation:

  • Within the framework of mutation theory, a specific concept known as "saltation" is introduced.
  • Saltation refers to the idea that some mutations can lead to a single-step, large-scale change in a population.
  • This concept suggests that evolution can occur more rapidly due to such significant genetic transformations. 

Comparison with Darwinian Evolution:

1. Speed of Evolution:

  •  Darwin proposed that evolution occurred gradually over long periods of time through the accumulation of small, heritable variations.
  •  In contrast, mutation theory, particularly through saltation, suggests that evolution can happen more rapidly due to significant genetic changes in a single step.

2. Nature of Variations:

  • In Darwinian evolution, variations were seen as small and directional, influenced by natural selection.
  • Mutation theory, on the other hand, highlights the random and directionless nature of mutations, with the potential for dramatic changes.

 3. Speciation:

  • Mutation theory suggests that mutations, especially saltation, can lead to speciation (the formation of new species) more directly, while Darwinian evolution emphasizes gradual changes in species over time.

 Evidence and Clarity:

  • Over time, population genetics and molecular biology have shed more light on the role of mutations in evolution.
  • While mutations can indeed bring about substantial genetic changes, they often need to be followed by natural selection to determine the fate of a new trait within a population.

 Modern Synthetic Theory of Evolution

  • The Modern Synthetic Theory of Evolution, also known as Neo-Darwinian Synthesis, represents the integration of Darwinian evolution with Mendelian genetics.
  • It was developed by prominent evolutionary biologists like T. Dobzhansky, J.B.S. Haldane, R.A. Fisher, Sewall Wright, G.L. Stebbins, and Ernst Mayr.

 1. Genetic Basis of Evolution:

  • This theory emphasizes the genetic foundation of evolution, defining it as "the changes occurring in allele frequencies within populations."

2. Genetic Variation:

  • Genetic variations within a population are the raw materials for evolution. These variations arise from mutations and genetic recombination.

 3. Reproductive and Geographical Isolation:

  • The process of speciation involves reproductive isolation, where populations become reproductively isolated from one another. Geographic isolation can lead to this.

 4. Natural Selection:

  • Natural selection acts on genetic variations, favoring individuals with advantageous traits for survival and reproduction.

 Factors Influencing Allele Frequencies:

1. Genetic Recombination:

  • Genetic recombination occurs during sexual reproduction through processes like crossing over during meiosis, leading to the formation of new allele combinations.

 2. Mutation:

  • Mutations are inheritable changes in genes, including chromosomal mutations. They provide a source of genetic diversity, and advantageous mutations can be selected over time.

 3. Genetic Drift and Gene Flow:

  • Genetic drift is the random change in allele frequencies due to chance events, more significant in small populations.
  • Gene flow results from the migration of individuals between populations, altering allele frequencies.

 4. Natural Selection:

  • Organisms better adapted to their environment are selected by natural forces, causing changes in gene frequencies over generations.

 5. Isolation:

  • Reproductive isolation is crucial in speciation, preventing interbreeding between related organisms. Geographic isolation often contributes to this process.

 6. Hybridization:

  • Hybridization between different species increases genetic variability within populations.

Hardy-Weinberg Principle

  • The Hardy-Weinberg Principle, formulated by G. H. Hardy and Wilhelm Weinberg, is a fundamental concept in population genetics.
  • It provides a mathematical framework for understanding how allele frequencies remain stable within a population over generations.

 1. Genetic Equilibrium: The Hardy-Weinberg principle states that allele frequencies within a population remain stable and constant from one generation to the next, under certain conditions. This state is referred to as genetic equilibrium.

 2. Gene Pool: The gene pool of a population represents the total genes and their alleles. In the context of the Hardy-Weinberg principle, it remains constant unless specific evolutionary factors come into play.

 3. Allelic Frequencies: To analyze allele frequencies, geneticists often use symbols like 'p' and 'q.' These symbols represent the frequency of specific alleles within a population. For example, 'p' might represent the frequency of allele A, and 'q' might represent the frequency of allele a.

 4. Mathematical Representation: In a diploid population, you can calculate the frequencies of different genotypes based on these allele frequencies:

- The frequency of individuals with two copies of allele A (AA) is represented as p² (p squared).

- The frequency of individuals with two copies of allele a (aa) is represented as q² (q squared).

- The frequency of individuals with one copy of allele A and one copy of allele a (Aa) is represented as 2pq (2 times p times q).

 5. Binomial Expansion: The equation p² + 2pq + q² = 1 represents the Hardy-Weinberg equilibrium. It's essentially a binomial expansion of (p + q)², where p + q equals 1 since these two variables account for all the alleles in the population.

 

 

 

6. Detecting Evolutionary Changes: By comparing observed allele frequencies with the expected values based on the Hardy-Weinberg equilibrium, geneticists can assess whether the population is evolving. Deviations from the expected values suggest that evolutionary forces like genetic drift, selection, mutation, or gene flow are influencing the population's genetic makeup.

Factor Affecting Hardy-Weinberg Equilibrium

1. Gene Migration or Gene Flow:

  • When a section of a population migrates to a new area, gene frequencies change in both the original and new populations.
  •  New genes and alleles may be introduced into the new population, while they are lost from the original population.
  • Repeated gene migration events result in gene flow between populations.

 2. Genetic Drift:

  • Genetic drift refers to the random change in allele frequencies within a population.
  • Sometimes, these changes are so significant that they can lead to the formation of a new species.
  • The original drifted population becomes founders, and this effect is known as the founder effect.

 

3. Mutation:

  • Mutations introduce new genetic variation into a population. While most mutations are neutral or harmful, some can be advantageous.
  • Pre-existing advantageous mutations, when selected, can result in the observation of new phenotypes over several generations.

 4. Genetic Recombination:

  • Genetic recombination during gametogenesis generates genetic diversity within a population.
  • Recombination results in new combinations of alleles, contributing to the gene pool's variability.

 5. Natural Selection:

  • Natural selection is a critical mechanism of evolution. It favors individuals with heritable variations that enhance their survival and reproductive success.
  • Natural selection can lead to stabilizing selection (where more individuals acquire the mean character value), directional change (where more individuals acquire values different from the mean), or disruptive selection (where more individuals acquire peripheral character values at both ends of the distribution curve).

 6. Diagrammatic representations of the operation of natural selection on different traits can help illustrate how selection influences the distribution of traits within a population. There are three primary modes of natural selection: stabilizing selection, directional selection, and disruptive selection. Let's explain each mode along with its diagrammatic representation:

 i. Stabilizing Selection:

Description: Stabilizing selection is a mode of natural selection that favors the intermediate, or average, phenotype in a population. It operates to reduce the extremes of a trait, leading to a narrower and more concentrated range of trait values.

 Diagrammatic Representation:

- In the diagram for stabilizing selection, you would see a bell-shaped curve or normal distribution, where the peak of the curve represents the most advantageous trait value. The tails of the curve, representing extreme trait values, become less frequent over time.

- The curve gradually narrows, indicating a reduction in variation, as individuals with intermediate trait values have a higher fitness and produce more offspring.

ii. Directional Selection:

Description: Directional selection occurs when a specific trait value becomes more advantageous, causing a shift in the distribution of the trait over time. One extreme of the trait range becomes favored.

 Diagrammatic Representation:

- In the directional selection diagram, you will see a shift in the entire curve to one side, either to the left or right. The shift represents the favored trait value.

- The curve is no longer centered; instead, it is skewed towards the advantageous trait value, which increases in frequency over generations.

iii. Disruptive Selection:

Description: Disruptive selection operates to favor extreme trait values while reducing the number of individuals with intermediate trait values. It often occurs in environments with multiple distinct niches.

 Diagrammatic Representation:

- The diagram for disruptive selection shows a bimodal or "W"-shaped curve, indicating two peaks, each representing a distinct trait value.

- The two peaks diverge, becoming more pronounced, as individuals with extreme trait values are favored. Intermediate trait values become less common.