Evolution: Types and Theories

• Evolution is a process of gradual developmental modification that establishes diversity and complexity in the living world.
• It indicates that complex plants and animals have evolved from simpler life forms.

Types of Evolution

• J.R. Savage (1960) identified sequential and divergent evolution.
• Microevolution is the evolution of sub-species or geographic races.
• Macroevolution is the evolution of species and genera into higher categories.
• Megaevolution is for large-scale evolution occurring at the level of families, orders, classes, and phyla.

Theories of Evolution

• Lamarckism (Theory of Inheritance of Acquired Characters)
• Darwinism (Theory of Natural Selection)
• Mutation Theory of De Vries
• Neo-Darwinism (Modern concept or Synthetic theory of evolution)

Postulates of Lamarckism

• New needs arise.
• Use and disuse of organs occurs.
• Inheritance of acquired characters is a key principle.
• Speciation occurs.
• Living organisms or their component parts tend to increase in size.
• Production of new organ results from a new need.
• The use of an organ makes it more developed, while disuse results in degeneration.
• Acquired characteristics are inheritable and accumulate over time, leading to new species.

Evidences in favor of Lamarckism

• Phylogenetic studies show evolution from simple to complex forms.
• The Giraffe evolved a long neck

Examples

• Snakes evolved limbless bodies from limbed ancestors through the disuse of limbs.
• Aquatic birds like ducks and geese developed from terrestrial ancestors, with reduction of wings due to continued disuse.
• Flightless birds like the ostrich developed from flying ancestors due to the disuse of wings.

Criticism of Lamarckism

• August Weismann proposed the "Theory of Continuity of Germplasm" in 1892 A.D.
• This theory states that environmental factors only affect somatic cells, not germ cells.
• Weismann suggested that germplasm contains special particles ("ids") that control the development of parental characters in offspring.
• Weismann's experiment involved mutilating the tails of mice for around 22 generations; tailless mice were never born.

Neo-Lamarckism

• Neo-Lamarckism revives Lamarckism with genetic findings confirming that the environment affects form, structure, color, size, and that these characters are inheritable.
• Germ cells may be formed from somatic cells, indicating similar nature of chromosomes and gene makeup in two cell lines.
• Environmental effects can impact germ cells through somatic cells.
• Environment directly affects germ cells.
• Germ cells are not immune from environmental effects.
• Germ cells can carry somatic changes to the next progeny.
• Germ cells may be directly affected by environmental factors.

Darwinism

• Darwinism is also known as the Theory of Natural Selection
• Charles Darwin (1809-1882 A.D.)
• Main postulates are: Geometric increase, limited food and space, struggle for existence, variations, natural selection (survival of the fittest), inheritance of useful variations, and speciation.
• All species have a high potential to increase, but only a few survive.
• Organisms compete with each other to survive due to environmental pressures.
• Individuals in a population vary greatly in their characteristics.
• Individuals who best adapt to the environment are the ones to survive.
• Natural selection means that offspring inherit favorable traits, and unfavorable traits are reduced.
• Speciation is the process in which new genetically distinct species evolve, generally through genetic isolation from the main population.
• Parallels exist between natural and artificial selection
• There is a correlation between position of nectaries in the flowers and length of the proboscis of the pollinating insect.
• Lamarck's theory states that the environment induces the organisms to change and leads to a perfection drive

Mutation Theory

• The Mutation Theory of Evolution was proposed by Hugo de Vries (1848-1935 A.D.)
• Evolution occurs by mutations, and individuals with mutations are called mutants.
• Elementary species are produced in large number to increase chances of selection by nature.
• Mutations recur, increasing the chances of their selection by nature.
• Mutations occur in all directions, causing gain or loss of characters.
• An example is the appearance of a short-legged sheep variety (Ancon) from long-legged parents in 1791 A.D.

Neo-Darwinism

• Also known as modern concept or synthetic theory of evolution.

• Genetic variability is a source of variability in a gene pool through:

  • Mutations
  • Recombination of genes through crossing over
  • Hybridization
  • Physical and chemical mutagens
  • Genetic drift: Elimination of genes by extreme reduction in population.

• Natural selection in Neo-Darwinism operates through differential reproduction and comparative reproductive success, not just "survival of the fittest."

• Reproductive isolation reduces the chances of interbreeding between related groups of living organisms.

Mechanisms of Evolutionary Change

• Natural Selection
• Genetic Drift
• Mutation
• Gene Flow

Natural Selection

• Natural selection leads to evolutionary change where individuals with certain traits have a higher survival and reproductive rate.
• Cumulative effects of natural selection rise to populations evolved to succeed in specific environments, based on differential reproductive success (fitness) of individuals.

Genetic Drift

• Genetic drift is a random event that influences allele frequency in a population as a result of sampling error from generation to generation.
• Alleles may be lost within a generation due to genetic drift, even if they are beneficial.
• Allele is defined as any one of two or more genes that may occur alternatively at a given site (locus) on a chromosome.

Mutation

• Mutation is a change in the DNA sequence within a gene or chromosome.
• Mutations can be neutral, deleterious, or beneficial.
• According to their magnitude, mutations include:
  • Gene mutations
  • Chromosome mutations
  • Genome mutations
• A gene mutation is a change in the sequence of nucleotides of the genetic material of an organism.
• A chromosome mutation is a change in the structure or arrangement of the chromosomes
• Genome mutations are alterations in the number of chromosomes in the genome.
• Aneuploidy refers to losses and/or gains of individual chromosomes.
• Euploidy refers to variations in complete sets of chromosomes.

Gene Flow

• Gene flow (gene migration) is the transfer of genes from one population to another, changing the frequency and/or range of alleles.

Isolation

• Temporal Isolation: Populations do not interbreed because they reproduce at different times.
• Mechanical Isolation: Different species are isolated by their mechanics.
• Behavioral Isolation: Allopatric species do not mate because of different courtship behavior.
• Reproductive Isolation: Animals are close enough to interact but cannot interbreed.
• Ecological Isolation: Organisms share a range but do not interact due to different foods or habitats.
• Geographic Isolation: A population is split by a physical barrier.

Evidences of Evolution

• Direct Evidences:
  • Evidences from paleontology.
• Indirect Evidences:
  • Evidences from comparative anatomy, taxonomy, embryology, physiology and biochemistry, cytology, genetics, and biogeographical distribution.

Evidences from Comparative Anatomy

• Analogy is the similarity of function with different origins.
• Homology involves structures with common origin modified for different functions.

Connecting Links

• Viruses (Living and Non-living)
• Euglena (Plants and Animals)
• Proterospongia (Protozoans and Porifera)
• Neoplina (Annelida and Mollusca)
• Peripatus (Annelida and Arthropoda)
• Balanoglossus (Non-chordates and Chordates)
• Lung Fish (Fishes and Amphibians)
• Archeopteryx (Birds and Reptiles)
• Egg-laying Mammals (Reptiles and Mammals)

Comparative Embryology

• Vertebrate embryos have gill slits and tails
• All vertebrate embryos except fish lose their gill slits, and some lose their tail
• In humans, the tail is reduced to the tail bone.

Comparing DNA

• DNA sequences are the strongest evidence for evolution from a common ancestor.