Introduction to evolutionary theory

Darwin (1859: On the Origin of Species)

• Life on Earth has gradually changed over time, rather than having been created in one moment and remained static from then on
• All species of life have descended over time from common ancestors
• This branching pattern of evolution resulted from a process he called natural selection.

Others (Plato, Erasmus Darwin), before Darwin, had also proposed that life on Earth had been changing across time. Darwin was the first person to propose a viable mechanism by which evolution could take place - natural selection.

• Natural selection is based upon biological variation and competition over limited resources
• Differences in certain biologically heritable traits lead to some individuals being better adapted to their environment and more able to win out in the competition over limited resources
• Those who win the resources have a better chance of surviving and reproducing. If these traits are heritable, they will increase in frequency and the population will change over time.
• Random mutations arise in the genome of an individual organism, and offspring can inherit such mutations
• Over time, this process can result in populations that specialise for particular ecological niches.

• Blending inheritance explained that inherited traits were determined randomly, from a range of traits found in the parents. For example, the height of a person with one short parent and one tall parent was thought to always be of some interim value between the two parents' heights.
• Biological traits are blended together like colour pigments in paint
• Blending theory is incompatible with natural selection - blending means that all variability will eventually be eliminated. Nature needs something to select amongst; if everything is the same there is nothing to select
• In each family, the potential for variation would tend to dramatically narrow with each generation, and so it would go for the entire population with every trait. If blending inheritance were true, in the height example, all members of a species would eventually converge upon a single value for height for all members
• In addition to this, blending failed to explain how traits that seemingly disappeared for several generations often reasserted themselves down the line, unaltered

• Mendel's pea plant experiments conducted between 1856 and 1863 established many of the rules of heredity, now referred to as the laws of Mendelian inheritance
• Mendel worked with 7 characteristics of pea plants: plant height, pod shape and colour, seed shape and colour, and flower position and colour
• Using colour as an example, Mendel showed that when a yellow pea and a green pea were cross-bred their offspring were always yellow. However, in the next generation, the green peas reappeared at a ratio of 1:3. To explain this phenomenon, Mendel coined the terms "recessive" and "dominant" in reference to certain traits
• Mendelian genetics preserves inherited variability. Without variability, natural selection would have nothing to select amongst

• "Survival of the fittest" was coined by Herbert Spencer, and originated from Darwinian evolutionary theory as a way of describing the mechanism of natural selection
• Biological fitness is not just a matter of how physically fit or healthy an individual is, or of personal survival
• The biological concept of fitness is defined a reproductive success. In Darwinian terms, the phrase is best understood as "survival of the form that will leave the most copies of itself in successive generations"

• Behavioural genetics is the field of scientific research that pertains to using genetic methods in order to investigate the nature and origins of individual differences in behaviour
• Modern-day behavioural genetics begain with Francis Galton who primarily studied the heritability of human abilities and mental characteristics. His work in behavioural genetics was undermined by another one of his contributions - the founding of the eugenics movement
• Dog breeders have been selecting behavioural traits in addition to physical features for years. Scott & Fuller (1965) provided experimental evidence of inherited behavioural differences between breeds of dogs

• Strategically comparing different (but often closely related) species in order to (1) identify unique features in a species and (2) discern evolutionary trends
• Comparative analysis of the human hand: the human hand has facilitated our technological achievements. All primates have grasping dextrous hands, but we have the most opposable thumb and we are the only primate with fleshy pads in our finger tips (Napier, 1993). Most comparisons with humans involve primates, because we are a type of primate
• Evolution of the Styloid Process (Ward et al, 2014): the styloid process is a protrusion of bone on the finger bone that attaches to the wrist. It allows for greater grip and strength. It is not present in modern chimpanzees nor an early hominid, Australopithecus, but it has been detected in Homo erectus. It facilitates tool use
• Carl Vone Linne (1758) classified human beings as a kind of primate
• Darwin (1871) argued that not only are humans most anatomically similar to monkeys and apes, but we also share common ancestors with living primates