Chapter 9 : The Evidence for Evolution

9.1 What is evolution?

Evolution just means change. For example language evolves over time (try reading Shakespeare!). Biological evolution means a change in the characteristics of a population over several generations. The changes that are important are the ones that are inherited in genes.

A species is a group of individuals that can interbreed to produce fertile offspring. A simple example of evolution is the change in the population of head lice, which have gradually become resistant to the pesticide permethrin. Fig 9.1. Natural selection means that individuals with particular genes have an advantage, they will have more offsping, and so those genes will tend to increase in the population in future generations.

A scientific theory has been supported by several lines of evidence, and has been tested many times by experiments. For example the theory of gravity explains how planets and smaller objects move, and atomic theory explains how atoms and molecules interact.
Evolutionary theory says that all species on Earth today are descended from a single common ancestor, and the different species were produced by millions of years of evolutionary change. Fig 9.2.

9.2 Charles Darwin

Before Darwins time, several people had suggested that species change over time. For example Lamarck around 1810 wrote that species evolve because they pass on traits they acquire during their life. In other words if you work out your children should be born with big muscles. It did not take long to prove this idea was wrong (this was before anyone had figured out anything about genes).

Darwin spent 5 years sailing around the world, collecting plants and animals mainly from South America. Fig 9.4. On the Galapagos islands, hundreds of miles off the coast of Ecuador, he noticed that the birds and tortoises on each island were different. Fig 9.5 and 9.6. He realized that each population had evolved to the different conditions on each island. Around the same time Alfred Wallace, who was working in the islands of southern Asia had come to a similar conclusion. Darwin and Wallace published a summary of their ideas on evolution in 1858.

In 1859 Darwin published his book On the Origin of Species by means of Natural Selection, which explained how species slowly change and also explained the common ancestry of all species. Natural selection means that nature selects which individuals survive. Most plants and animals produce a lot of offspring, and typically many of them die young. The few that survive are the ones born with the best genes for their environment.

9.3 Evidence for common descent.

Prior to the 1850s most ideas of the origin of species were taken directly from the Bible. Creationists believe that all species were individually created by God in 6 days and they have not changed in kind since then. The Bible implies that the world is less than 10,000 years old. Darwin and Wallaces idea of evolution was obviously different from this idea of creationism. Some of the evidence for evolution and common descent includes:
  1. Classification: animals with similar characteristics are grouped together. For example monkeys, apes and humans are all in the class primates as they have eyes in the front of the face, and hands that can grasp a branch. This implies that these species are related. Fig 9.11.

  2. Comparative anatomy means comparing the structure of the body, or parts of the body, in different animals. For example the bones of the front limbs of many mammals show homology: they are equivalent bones that have the same basic pattern. Fig 9.12. A simple example of that is that most mammals and reptiles have 5 fingers or toes.

  3. Vestigial organs are organs that have no function but are inherited from a distant ancestor where they did function. For example flightless birds still have wings, and humans have tail bones and "goose bumps". Fig 9.13.

  4. Embryo development: mammals, birds and reptiles go through a very similar stage in the embryo during which they produce a tail and gill slits (even in humans). Fig 9.15. This implies that they all descended from an ancestor that had a tail and gills.

  5. Biochemistry: all organisms contain similar molecules (proteins, fats, carbohydrates, DNA). The DNA of closely related species is very similar: human and chimp DNA is about 99% the same. Fig 9.17. Even mammals such as mice have over 95% of the same genes that we do (which is why studies of new drugs start with experiments on mice and rats).

  6. Biogeography: different parts of the world have different species. Kangaroos and their relatives are only found in Australia because they evolved there. America has cactus plants, Africa has euphorbs. Fig 9.20.

  7. Fossil evidence. Fossils are the remains of organisms that have been left in soil or rock. Radiometric dating uses the radioactive decay of chemical elements to show the age of a fossil. Fig 9.24. Humans are bipedal (walking upright on two feet), whereas chimps and gorillas walk on all fours. Fig 9.22 The earliest human fossils are all from Africa, and date back about 6 million years. The genus Australopithecus had several species of early human, roughly 2 - 4 million years ago. Fig 9.23 & 9.25. Our species Homo sapiens is much more recent, evolving roughly 150,000 years ago. Fig 9.27. There are many other examples in the fossil record of the evolution and common descent of a wide variety of animals. Fig 9.28.

9.4 Are alternatives to Evolution equally valid?

The static model which says that the earth is less than 10,000 years old and species have not changed is based on creationism, and can be rejected as a scientific idea as it does not fit the facts of either radiometric dating or the fossil and other evidence outlined above.

The transformation model says that species arise separately but then evolve over time to adapt to their environment. Fig 9.7. This can be rejected because it does not fit the evidence of anatomical or biochemical similarities between different species.

The model of separate types says that species do evolve and produce new species, but the different types of organism had separate ancestors. This does not explain the similarity in DNA between a wide variety of organisms. It also does not fit in with the fact that common organelles such as mitochondria that are found in both plants and animals.

The common descent model of evolution is the best scientific explanation of how modern organisms came about. The evidence for this theory is strong, and comes from a wide variety of different subjects. It helps us to understand human genes, disease and underscores the unity of life on earth.

Last edited September 2008, by David Byres, dbyres@fccj.edu