In this lesson we will learn about the processes involved in evolution by natural selection.
By the end of this lesson you will be able to:
- Outline the key stages in the evolutionary process.
- Explain why genetic variation and reproductive isolation are required for the evolution of new species.
- Discuss the role of mutation, natural selection and genetic drift in evolution.
- Describe speciation and adaptive radiation.
- Interpret a phylogenetic tree.
- Evolution can be described as the gradual, cumulative genetic changes in populations of organisms over time, that can result in different forms and new species.
- Charles Darwin was a British scientist who published “On the Origin of Species By Means of Natural Selection” in 1859, as a result of extensive travels and observations of species around the world.
- He concluded that all life on Earth evolved from a common ancestor as a result of a process called natural selection.
- Evolution was not a new concept, but Darwin popularised the idea by describing a mechanism for how evolution occurs.
- This is now known as the theory of evolution by natural selection.
- Darwin’s theory of evolution not only explains the evolutionary process, it provides a unifying theory for all of biology.
- In the words of the evolutionary biologist Theodosius Dobzhansky, “nothing in biology makes sense except in the light of evolution”.
(Images: Public Domain)
Overview of the Process of Evolution
- Evolution is a process that includes several key stages:
- The creation of genetic variation in populations due to mutation.
- Reproductive isolation of populations.
- Changes in the genetic composition of populations due to natural selection, genetic drift and new mutations.
- Speciation – the formation of new species.
Genetic Variation in Populations
- The process of evolution begins with genetic variation within populations.
- Without genetic variation, there can be no evolution.
- The source of all genetic variation is mutation as it is the only way new alleles are created.
- The gene pool of a population is the sum of all genetic variation that exists within that population.
- In particular, it refers to the total number of alleles for all genes.
- Genetic variation in populations can be enhanced by:
- Sexual reproduction, which creates different combinations of alleles (genotypes), resulting in different phenotypes.
- Gene flow (migration) between populations, which can add new alleles to a gene pool.
Reproductive Isolation of Populations
- Reproductive isolation refers to the separation of populations, resulting in the prevention of gene flow between them.
- For example, rising sea levels may separate two populations of land-dwelling animals.
- The genetic separation of populations means that their genetic compositions can change independently of each other.
Natural Selection, Genetic Drift and New Mutations
- Over time, the genetic composition of the separated populations will change, due to three main factors:
- Natural selection – the favouring of certain alleles over others, as populations respond to changing environments.
- For example, separated populations of birds may adapt different beak shapes as a response to different food sources.
- Genetic drift – random changes in allele frequencies due to chance.
- The influence of genetic drift is strongest when there is an absence of selection pressures.
- Mutation – the formation of new alleles.
- This occurs at a much slower rate than natural selection and genetic drift.
- Over time, the combined effects of selection, genetic drift and mutation can result in the separated populations becoming so genetically different, that if they did come together again, they would be unable to interbreed.
- This gradual differentiation of two isolated populations into separate species is known as speciation.
- Continued occurrences of speciation can result in multiple new species evolving from a common ancestral species, each with their own set of adaptations to their different environments.
- This is known as adaptive radiation.
- The central idea of evolution is that all life on Earth evolved from a single common ancestor. In other words, evolution is one continuous occurrence of adaptive radiation.
- Humans are therefore distant cousins, not just of chimpanzees and gorillas, but all species.
- A phylogenetic tree or evolutionary tree is a branched diagram showing the evolutionary relationships between different species or groups of species (phylogenetic groups).
- They have a variety of forms, but all show the diversification of currently existing species from ancestral species along an approximate time scale.
- Branching points on a phylogenetic tree represent speciation events, showing the divergence of species from a common ancestor.
- In the diagram above:
- Species 1 is a common ancestor of species A and B.
- Species 2 is a common ancestor of species D and E.
- Species 3 is a common ancestor of species A, B and C.
- Species 4 is a common ancestor of species A, B, C, D and E.
- The more recently two groups have diverged from a common ancestor, the more closely related they are.
- In the diagram above:
- Species B is the closest relative of species A.
- Species E is the closest relative of species D.
- Species C is more closely related to species B than species D.
(Image: M ralser, Wikimedia Commons)
Some Misconceptions About Evolution
- Humans did not evolve from monkeys – monkeys and humans share a close common ancestor, which was neither human nor monkey.
- Evolution does not create genetic diversity – it occurs because of it.
- Evolution is not a guided or conscious process towards an ideal – it is a response to a set of environmental conditions that exist in a particular place at a particular time.
- Adaptation and evolution do not occur at the individual level – they occur at the level of populations and species.
(Image: © University of California Museum of Paleontology. Used with permission.)
- Evolution is the gradual genetic change of populations over time, that results in different forms and new species.
- Charles Darwin developed the theory of evolution by natural selection.
- He concluded that all life on Earth evolved from a common ancestor as a result of natural selection.
- Darwin’s theory provides a lens through which all biological processes can be viewed.
- The evolutionary process includes several key stages:
- Genetic variation.
- Genetic variation is created by mutation – the source of all new alleles – and enhanced in populations by sexual reproduction and gene flow.
- Reproductive isolation.
- Reproductive isolation is the separation of populations, which results in independent genetic changes.
- Natural selection, genetic drift and new mutations.
- Natural selection is the favouring of more suitable alleles over others.
- Genetic drift is the random change in allele frequencies due to chance.
- New mutations arise very slowly over time.
- Speciation is the gradual differentiation of isolated populations into separate species.
- Adaptive radiation is the formation of multiple new species from a common ancestor.