B15 Genetics and evolution


B15.1 History of genetics

Mendel cross-bred smooth, wrinkled, green, and yellow peas together, and counted the offspring. He found out that characteristics were inherited in clear, predictable patterns. He suggested there were separate units of inherited material and realised some characteristics were dominant over others and that they never mixed together. 

By the late 19th century, chromosomes had been seen through microscopes. Mendel's experiments were repeated. In the early 19th century, it was suggested units of inheritance were carried onto chromosomes.

In the 1950s, scientists showed DNA is the material of inheritance. The structure of DNA was looked at through X-rays, leading to the idea of a double-helix structure. The mechanism of gene function was worked out.

This all led to the development of gene theory - that the effects of genes was the coding of proteins. Scientists are now beginning to understand how the environment can affect proteins made by a gene.

B15.2 Theories of evolution

Lamarck suggested individual animals adapted and evolved to their environment and that animals evolved from primitive worms, but changed by the inheritance of acquired characteristics. He also proposed that how organisms behaved affected their body features - if something was used a lot over a lifetime, this would grow and develop; if it wasn't used, it'd be lost. Any useful changes occurring to the organism would be inherited.

People did not like this theory because they didn't like to hear they evolved from worms and also, changes in body weren't inherited.

Darwin, noticing the variety of life forms and collecting many specimens, proposed that all organisms evolved from simpler life forms by natural selection. His main ideas were that in a species, a wide range of variation for a characteristic was displayed. Organisms inheriting the characteristics most suited to their environment were more likely to survive and breed to pass on these characteristics.

When Darwin suggested this theory, nobody knew about genes.

B15.3 Accepting Darwin's ideas

Darwin took a long time to publish his theory because he needed a lot of evidence, which was the variety of animals he had seen (which were different due to natural selection). He also carried out breeding experiments with pigeons to show how features could be artificially selected. Darwin also studied the habitats of different barnacles, giving him more evidence of natural selection.

Darwin built a network of pigeon breeders, scientists, and friends to gather evidence.

Darwin's theory was only gradually accepted since it challenged the view that God had made all animals and plants, and many scientists felt there wasn't enough evidence, There was also no way to explain variety or inheritance.

B15.4 Evolution and speciation

Alfred Wallace published joint writings with Darwin in 1858, prompting Darwin to publish The Origin of Species in 1859. Wallace worked worldwide to gather evidence for evolutionary theories, like warning colouration on animals, and developed clear theories on the role of geographic isolation in the formation of species - speciation.

A population will have natural genetic variation (a wide range


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