Variation and Evolution

Variation and Evolution

1. Variation

   • Variation - differences in the characteristics of individuals
   • Variation can be due to differences in multiple things
     • Genes individuals have inherited
     • Conditions in which individuals have developed (environment)
     • Combination of both genetic and environmental causes
   • Sexual reproduction produces different combinations of alleles
     • Only mutations create new alleles
   • Most mutations do not affect phenotype
     • Very rarely, a mutation may produce a phenotype that gives an organism a great survival advantage

2. Natural Selection

   • Evolution - gradual change in the inherited characteristics of a population over time, may lead to formation of a new species
   • Put forward by Charles Darwin
   • Most scientists support this theory
   • States that all species have evolved from simple life forms that first developed more than 3 billion years ago
   • During his travels, Darwin observed: organisms produce large number of offspring, populations stay about the same size, organisms are all slightly different (variation), characteristics can be inherited
   • He made these conclusions: there's a struggle for existence, more organisms are born than can survive, the ones that survive and breed are the ones best-suited to the environment, they pass characteristics onto their offspring, over time characteristics change
   • 1858 - Alfred Russel Wallace suggest similar theory to Darwin, made Darwin realise he could publish his work
   • 1859 - On the Origin of Species was published by Darwin
   • Took a long time for Darwin's theory to be accepted
     • It challenged the idea that God made all the organisms that live on Earth
     • Not enough evidence so didn't convince many scientists
     • Mechanism of inheritance and variation was not known until 50 years after the theory was published
   • Jean-Baptiste Lamarck made had another theory
     • Based on the idea that changes that occur in an organism during its lifetime can be inherited
     • In most cases, this type of inheritance cannot occur

3. Evidence for Evolution

   • Fossils provide some evidence for evolution
     • Remains of organisms from hundreds of thousands of years ago that are found in rocks
     • Formed in various ways: from hard parts of animals that do not decay easily, from parts that have not decayed because one or more conditions were absent, when parts of organisms are replaced by other materials in decay, preserved traces of organisms
     • Scientists use them to look how organisms have gradually changed over long periods of time
     • Gaps in fossil records: many early life forms were soft-bodied (left very few traces behind), traces that were left might have been destroyed by geological activity
   • Development of antibiotic-resistant strains of bacteria
     • Can evolve rapidly as they reproduce at a fast rate, mutations occur when they reproduce, some mutated bacteria might be resistant to antibiotic and are not killed, these bacteria survive and reproduce, so a resistant strain develops
   • Still a lot of debate among scientists over the theory of evolution and the origins of life