Grade 12 Biology: Evolution

Evolution

• Evolution is a change in genetic composition of a population over successive generations, which may be caused by meiosis, hybridization, natural selection, or mutation.
• This leads to a sequence of events by which the population diverges from other populations of the same species and may lead to the origin of a new species.

Theories of the Origin of Life

• There are five main theories of the origin of life on Earth.
• Autotrophs are organisms that produce organic molecules from inorganic material and are the base for all food chains.
• Prokaryotes are the first organisms that appeared 4 billion years ago and are organisms consisting of a prokaryotic cell (a cell without a nucleus).

Types of Prokaryotes

• Archaebacterium: consists of thermophilic sulfobacteria, methanobacteria, halophilic bacteria, and are the first bacteria to develop on Earth.
• Eubacterium: ordinary bacteria and cyanobacteria (blue-green algae) and are any bacteria that are not archaebacterium.
• Eukaryote: proctists, fungi, plants, and animals, and are all organisms other than bacteria.

Evolution of Early Life Forms

• One great change that affected the evolution of early life forms was the shift from the reducing atmosphere to an atmosphere containing oxygen, which took place about 2.4 billion years ago.

Neo-Darwinism Theory

• It is the revised version of Darwin's theory and combines Darwin's original theory, genetic theory, and animal behavior.
• Gene pool of a population is the sum of all alleles (genes) available in the population.
• Postulates of Neo-Darwinism:
  • Genetic variability: genetic variation exists within a population due to mutations, genetic recombination, and other processes.
  • Natural selection: individuals with advantageous traits are more likely to survive and reproduce, passing on their beneficial traits to the next generation.
  • Reproductive isolation: new species can arise when populations become reproductively isolated from each other, preventing gene flow between them.

Evidence for Evolution

Comparative Anatomy

• Analogous structures: structures having the same function but different anatomy.
• Example: the wings of a bird, butterfly, and mosquito.

Comparative Embryology

• Studies the way in which the embryos of vertebrates develop before they hatch or are born.
• Early in development, all vertebrate embryos have gill slits and tails.
• The embryonic tail does not develop into a tail in all species.

Homologous Structures

• Example: the bones in human hands, flippers of the whale, and wings of the bats.

Comparative Biochemistry

• Various chemicals have been studied in order to find evidence of evolutionary relationships.
• Organisms that share very similar molecules and biochemical pathways are closely related evolutionarily.
• Examples: DNA, proteins such as cytochrome c and haemoglobin.
• Haemoglobin is a molecule found in red blood cells that carries oxygen and has four polypeptide chains.
• The haemoglobin molecule is similar in all animals that possess it, but there are differences.