Speciation, Understanding of Genetics & Evidence for Evolution | Biology for Grade 10 PDF Download

Darwin & Wallace

• Alfred Russel Wallace was a scientist who, after conducting his own travels around the world and gathering much evidence, independently developed his own theory of evolution based on the process of natural selection
• He published scientific papers on this theory with Darwin in 1858 (Darwin published his book, On the Origin of Species, the following year
• Wallace is best known for:
  • His work studying the warning colouration of species (particularly butterflies) and how this must be an example of a beneficial characteristic that had evolved by natural selection, as the warning colouration helps to deter predators
  • Developing the theory of speciation

Speciation

• Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation
• Speciation is a process that results in the formation of a new species
• When populations of the same species become so different that they are unable to interbreed and produce fertile offspring, they are considered different species and speciation has occurred
• Speciation can occur as a result of a combination of isolation (when populations of the same species become separated) and natural selection:
  • Populations of the same species can become isolated from one another due to the formation of a physical barrier (eg. a new river or mountain range) – this is known as geographic isolation
  • The environment will be different on either side of this physical barrier (eg. different climates or different food available)
  • The environmental differences on either side will provide different selection pressures and natural selection will cause a different set of characteristics to become more common in the two isolated populations
  • Over many generations, individuals from the two populations will have become so distinct (genetically, behaviorally, physically) that they will no longer be able to interbreed and produce fertile offspring
  • The two populations are now separate species
    
    The process of speciation

Mendel's Work on Genetics

• Gregor Mendel was an Austrian monk
• He was trained in mathematics and natural history at the University of Vienna
• In the mid-19th century, Mendel carried out breeding experiments on plants
• He studied how characteristics were passed on between generations of plants
• For example, he conducted studies with pea plants and looked at how the height characteristic was inherited
  
• One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged
• Using the example above, Mendel showed that height in pea plants was the result of separately inherited ‘hereditary units’ passed down from each parent plant to the offspring plants – this particular experiment showed that the ‘unit’ for tall plants (T) was dominant over the ‘unit’ for short plants (t)
• His work eventually provided the foundation for modern genetics
• The importance of Mendel’s discovery was not recognised until after his death:
  • His studies were totally new to science in the 19th century
  • There was no knowledge of the mechanisms behind his findings (DNA, genes and chromosomes had not been discovered yet)

Chromosome Behaviour

• In the late 19th century and early 20th century, the behaviour of chromosomes during cell division was observed
• Scientists realised that chromosomes behaved in a very similar way to Mendel’s ‘hereditary units’
• Scientists believed Mendel’s ‘units’ must be located on chromosomes
• We now know that this is true and call these ‘units’ genes

Discovery of DNA Structure

• In the mid-20th century (1953 to be precise) the structure of DNA was determined and the mechanism of gene function worked out
• This scientific work by many scientists led to the gene theory being developed

Evolution in Bacteria

• The theory of evolution by natural selection is now widely accepted and many sources of data are now available to support the theory of evolution
• One very clear piece of evidence for evolution is antibiotic resistance in bacteria
• An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria
• Antibiotics are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease
• Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span
• Like all other organisms, within a population, there will be variation caused by mutations
• A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)
• When the population is treated with this antibiotic, the resistant bacteria do not die
• This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead
• Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
• Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment

Evidence for Darwin's Theory

• Darwin’s theory of evolution by natural selection is now widely accepted
• Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes
• Our understanding of genetics has made clear the mechanism by which natural selection can occur
• There is further evidence for evolution in the fossil record and our knowledge of how resistance to antibiotics evolves in bacteria