In this lesson we will learn how evidence for the theory of evolution is provided by a range of scientific disciplines.
By the end of this lesson you will be able to:
- Describe how the fossil record shows changes in species over time.
- Illustrate how anatomical similarities and differences between species reflect their evolutionary relationships.
- Discuss how the distribution of fossils and living organisms relates to their evolutionary origins.
- Explain how studies of DNA support the idea of a single common ancestor and provide a mechanism for novel forms.
The Evidence for Evolution
- Evidence for evolution can be divided into several categories:
- The fossil record – a timeline of different species.
- Comparative anatomy – similarities and differences between species.
- Biogeography – the distribution of past and present species.
- Molecular biology – the genetic composition of different species.
- These different scientific disciplines provide corroborating evidence supporting the process of evolution by natural selection.
(Images: tomakuma, Pixabay; Charles Darwin, Wikimedia Commons; OpenClipart-Vectors, Pixabay; PaintingValley)
The Fossil Record
- A fossil is the preserved remains or imprint of a long-dead organism.
- Fossils include bones, shells, petrified remains and impressions left in stone.
- They range in age from tens of thousands of years old to billions of years old.
- The fossil record refers to the total collection of all known fossils.
- There are various methods that can be used to estimate the age of fossils.
- This enables scientists to assemble fossils into a timeline.
(Images: Rauantiques, Wikimedia Commons; stux, Pixabay)
Extant and Extinct Species
- Fossils of extant (currently existing) species make up a very small portion of the fossil record – most fossils are of species that are now extinct.
- Also, fossils of existing species generally extend back a relatively short way in history, whereas fossils of extinct species extend much further back into the past.
- This shows that currently existing species represent a small proportion of all species that have existed, and only a short period of the time that life has existed on Earth.
(Image: polyfish, Pixabay)
Change Over Time
- The fossil record shows numerous examples of how species have changed over time.
- These include ancestral species, with only slight similarities to currently existing species.
- For example, African and Indian elephants are members of a group of organisms known as Proboscideans, which have descended from a small trunkless ancestor, dating back approximately 60 million years.
(Image: H. N. Hutchinson, Wikimedia Commons)
- Transitional fossils are fossils of species that possess traits found in two different groups of organisms.
- They are important as they show the divergence of forms from a common ancestor.
- For example, Archaeopteryx was a small bird-like dinosaur which had features of both reptiles and birds.
(Images: Vesta, Wikimedia Commons; Ballista, Wikimedia Commons)
- Comparative anatomy is the comparison of body structures of different species.
- It supports the theory of evolution as it shows that there is a continuum of relatedness between species, based on their anatomical structures.
- Closely related species share many similarities, whereas distantly related species share less similarities, but still show relatedness.
(Image: Jakovche, Wikimedia Commons)
- Homologous structures are body parts with similar physical characteristics between species, due to their common evolutionary origin – they are controlled by a common sets of genes.
- Although homologous structures share a common underlying anatomy, they do not always appear similar and often have evolved different functions.
- For example, the hands of humans and chimpanzees share many obvious similarities; less obvious is that they are related to dog paws, whale flippers and bat wings. These body parts are all examples of pentadactyl limbs – limbs with five fingers or toes – whose structures and functions have diverged as a result of adaptive radiation.
- Homologous structures are an example of divergent evolution – the differentiation of related structures due to adaptation to different environments.
- Not all structures that appear similar between species are a result of common ancestry.
- Analogous structures are body parts with similar physical characteristics between species, due to adaptation to similar environments, rather than a common evolutionary origin – they are controlled by different sets of genes.
- For example, dolphins and sharks share many similar structures, such as flippers and pectoral fins, but these have evolved separately. Dolphins are mammals, whereas sharks are cartilaginous fish – two different evolutionary lineages.
- Analogous structures are an example of convergent evolution – the emergence of unrelated structures with similar form or function due to adaptation to similar environments.
- Vestigial structures are body parts with no apparent function, but which remain in organisms as they are no longer under the effect of selection.
- Vestigial structures give important clues about the evolutionary origins of organisms.
- For example, being mammals, dolphins and whales evolved from ancestors with four limbs. Upon adapting to aquatic life, the forelimbs evolved into flippers and the hindlimbs effectively disappeared. They still possess vestigial pelvic bones (where the hindlimbs would have been attached), which provide clues to their evolutionary origins.
- Other examples of vestigial structures include tail bones in humans and apes, wings on flightless birds, and eyes in moles and other sightless animals.
(Image: Meyers Konversionlexikon, Wikimedia Commons)
- Comparative embryology is the comparative anatomy of developing embryos.
- Comparative embryology shows that there are more similarities between organisms during early stages of development.
- It also reveals certain structures that appear in embryonic stages of organisms but disappear before full development.
- Similar to vestigial structures, these similarities and temporary embryonic structures give clues about the evolutionary origins of organisms.
- For example, all vertebrate embryos appear very similar in early stages of development. They all exhibit gill slits (pharyngeal pouches), which later disappear in species that have evolved lungs, but indicate their aquatic ancestral origins.
(Image: A. J. Dadson, Wikimedia Commons)
- Biogeography is the study of the geographic distribution of species, both past and present.
- The distribution of past and present species, when viewed alongside the theory of plate tectonics, provides support for the theory of evolution.
- (Plate tectonics describes the slow movement of tectonic plates in the earth’s crust. It suggests that all of the earth’s landmasses were once connected but have separated and moved to their current locations over long periods of time.)
Distribution of Fossils
- Fossils of organisms that existed before the breakup of the supercontinents are found across continents that later separated.
- For example, species that previously existed on the supercontinent Gondwana, and whose fossils are found on continents derived from Gondwana, include Glossopteris (plant), Cynognathus (land reptile), Lystrosaurus (land reptile) and Mesosaurus (freshwater reptile).
(Image: Osvaldocangaspadilla, Wikimedia Commons)
Distribution of Present-Day Species
- Similar to the distribution of fossils, groups of currently existing organisms that evolved before the breakup of the supercontinents are found across continents that later separated.
- For example, the plant family Proteaceae formed on Gondwana and is now distributed across all continents that were once part of this super continent.
(Images: Angiosperm Phylogeny Website; Pixabay)
- Conversely, groups of currently existing organisms that evolved after the breakup of the supercontinents are limited to certain continents.
- For example, the plant genus Banksia, which is a subgroup of the Proteaceae family, is only found in Australia and New Guinea (which were recently joined).
(Images: Australian National Herbarium; Pixabay)
- The longer groups of organisms have been separated, the more diverse and unique they tend to be, as they have had more time to evolve.
- For example, the Australian continent has a wide diversity of unique plant and animal species, due to being separate for a long period of time.
- Molecular biology is the branch of biology that studies life processes at the molecular level, particularly the macromolecules (large molecules) DNA, RNA and proteins.
- The ability to determine DNA sequences of different organisms has provided additional support for the theory of evolution.
Universality of DNA and the Genetic Code
- The structure of DNA is the same in all living things, including bacteria, fungi, plants and animals.
- Additionally, the ‘genetic code‘ which determines how substances, such as proteins, are synthesised based on the instructions encoded in DNA, is also the same for all organisms.
- This commonality of structure and function supports the idea of a single common ancestor for all life on Earth.
(Image: Madprime, Wikimedia Commons)
DNA Sequencing and the Relatedness of Species
- DNA sequencing is a laboratory process that determines the nucleotide sequence of DNA molecules.
- DNA sequencing of different organisms shows that the more closely related organisms are, the more similar their DNA sequences.
- In other words, similarities between groups of species based on anatomical structures is reflected in similarities between DNA sequences. In fact, DNA sequencing has replaced comparative anatomy in determining relatedness between species, as it is more precise and reveals more detail.
Gene Duplication as a Mechanism for Evolution
- DNA sequencing has also revealed that the majority of DNA is non-coding and has no apparent function, but is still passed on between generations, in a manner much like vestigial structures.
- DNA sequence analysis has also revealed that much of this extraneous DNA contains duplicated variants of genes that no longer code for functional products.
- This discovery of gene duplication provides a greater understanding of how new forms arise in species. The idea that a gene can change to an improved form by a single mutation is very unlikely, but the process of gene duplication makes the idea of advantageous mutations much more plausible as it involves multiple steps – if the original form of a gene is retained while duplicates are produced, there is no loss of function for the organism, while at the same time the duplicate genes are able to undergo further random change until a superior form is produced.
(Image: Smedlib, Wikimedia Commons)
- Evidence for evolution can be divided into several categories:
- The fossil record.
- Comparative anatomy.
- Molecular biology.
- The fossil record reveals how species have changed over time, and includes ancestral species and transitional forms.
- Comparative anatomy shows that there is a continuum of relatedness between species, based on their anatomical structures.
- Homologous structures are body parts with a common evolutionary origin, that have diversified due to adaptation to different environments.
- They are examples of divergent evolution.
- Analogous structures are body parts with separate evolutionary origins, that have a similar form or function due to adaptation to similar environments.
- They are examples of convergent evolution.
- Vestigial structures are body parts which no longer have a function but persist as ‘remnants’ of evolution.
- Comparative embryology shows that there are more similarities between organisms in early stages of development, including structures that disappear before full development in some species.
- Biogeography shows that organisms that existed before the breakup of the supercontinents are found across continents that later separated, while organisms that evolved after the breakup of the supercontinents are limited to certain continents.
- It also shows that the longer groups of organisms have been separated, the more diverse and unique they tend to be as they have had more time to evolve.
- Molecular biology has revealed that the structure of DNA and the genetic code are the same for all organisms, thus suggesting a single common ancestor for all life on Earth.
- DNA sequencing shows that the more closely related organisms are, the more similar their DNA sequences.
- It also reveals gene duplication within organisms as a mechanism for evolutionary change.
(Image: Flying Puffin, Wikimedia Commons)
(Header image: midobun2014, Adobe Stock)