Evidences Supporting the Theory of Evolution

Evidence for evolution has come from many different observations. These include fossils, embryology, comparative anatomy and morphology, biochemical similarities, selective breeding, and natural selection seen in living populations.

Together, these observations show that life forms changed over time and that new forms appeared at different stages in Earth’s history.

• Fossils are remains of hard parts of life forms found in rocks.
• Rocks form sediments, and the arrangement of these sediments in Earth’s crust shows layers formed over a long geological history.
• Different-aged sedimentary layers contain fossils of different life forms that died during the formation of those layers.
• Some fossils resemble modern organisms, while others represent extinct organisms such as dinosaurs.

A study of fossils in different sedimentary layers indicates the geological period in which those organisms lived. This study also shows that life forms varied over time and that certain forms were restricted to particular geological time spans.

Therefore, new forms of life arose at different times in the history of Earth. This evidence is called palaeontological evidence.

A family tree of dinosaurs and their living modern day counterpart organisms, like crocodiles and birds

Embryological support for evolution was proposed on the basis of common features seen during the embryonic stages of vertebrates.

• For example, embryos of all vertebrates, including humans, develop a row of vestigial gill slits just behind the head. 
• These gill slits are functional only in fish and are not found as functional organs in other adult vertebrates.
• This observation was proposed by Ernst Haeckel as support for evolution.
• However, this proposal was later disproved after careful study by Karl Ernst von Baer. He noted that embryos do not pass through the adult stages of other animals.

Comparative anatomy and morphology show similarities and differences among present-day organisms and those that existed earlier. These similarities can be used to infer whether organisms share a common ancestor.

• A major example is seen in the forelimbs of whales, bats, cheetahs, and humans.
• Although these forelimbs perform different functions, they have the same basic arrangement of bones: humerus, radius, ulna, carpals, metacarpals, and phalanges.
• In these animals, the same basic structure developed in different directions due to adaptations to different needs. This is called divergent evolution. Such organs are called homologous organs.
• Homology indicates common ancestry. Other examples include vertebrate hearts and brains.

• In plants, homology is seen in different modified structures.
• Thorns of Bougainvillaea and tendrils of Cucurbita are examples of homologous organs.
• These structures have a common origin but perform different functions.

• Analogy refers to a condition opposite to homology.
• Wings of butterfly and birds look alike and perform the same function, but they are not anatomically similar structures. Such structures are called analogous organs.
• Analogous organs are the result of convergent evolution.
• In convergent evolution, different structures evolve for the same function and therefore show similarity.
• Other examples include the eye of an octopus and mammals.
• Flippers of penguins and dolphins are also analogous.
• Sweet potato and potato form another example because sweet potato is a root modification, whereas potato is a stem modification.
• Similar habitats may lead to the selection of similar adaptive features in different groups of organisms for the same function.

• Similarities in proteins and genes that perform a given function across diverse organisms provide clues to common ancestry.
• These biochemical similarities support the shared ancestry suggested by structural similarities.

• Humans have bred selected plants and animals for agriculture, horticulture, sport, and security.
• Many wild animals and crops have been domesticated.
• Intensive breeding programmes have created breeds that differ from one another, such as different dog breeds, though they still belong to the same group.
• This suggests that if humans can create new breeds within hundreds of years, nature could produce larger changes over millions of years.

• Another observation supporting evolution by natural selection came from England.
• In collections made in the 1850s, before industrialisation, more white-winged moths than dark-winged moths were seen on trees.

Figure showing white - winged moth and dark - winged moth (melanised) on a tree trunk (a) In unpolluted area (b) In polluted area

• In collections from the same area after industrialisation, around 1920, more dark-winged moths were found, and the earlier proportion was reversed.
• The explanation was that predators can easily spot a moth against a contrasting background.
• After industrialisation, tree trunks became dark due to industrial smoke and soot.
• Under these conditions, white-winged moths were more easily seen and did not survive well, while dark-winged moths survived better.
• Before industrialisation, tree trunks were covered by thick growth of almost white-coloured lichens.
• In that background, white-winged moths survived better, while dark-coloured moths were picked out by predators.
• Moths that could camouflage themselves in the background survived.
• In rural areas where industrialisation did not occur, the number of melanic moths remained low.
• This showed that in a mixed population, those organisms that are better adapted survive and increase in population size.
• No variant is completely wiped out.

• Excess use of herbicides and pesticides has resulted in the selection of resistant varieties within a much shorter time scale.
• A similar effect is seen in microbes against which antibiotics are used, and in eukaryotic organisms or cells against which drugs are used.
• Resistant organisms or cells may appear within months or years rather than centuries.
• These are examples of evolution by anthropogenic action.

• Evolution is not a directed process in the sense of determinism.
• It is a stochastic process based on chance events in nature and chance mutation in organisms.