MECHANISMS OF EVOLUTION

LEARNING OUTCOMES

  • Define evolution
  • Outline the four mechanisms of evolution
  • Define mutation
  • Explain how mutations can be beneficial or deleterious
  • Define gene flow
  • Define population
  • Explain how gene flow works
  • Define genetic drift
  • Explain how genetic drift works
  • Describe the founder effect and bottleneck effect
  • Define natural selection
  • Explain how natural selection works
  • Understand how natural selection gave rise to Darwin’s finches

There are four mechanisms that make evolution work: mutation, gene flow, genetic drift and natural selection.

1. MUTATION

A mutation is a random change in the DNA sequence within a gene or chromosome of a living organism. They can also be described as copying errors within the DNA.

Mutations can be spontaneous (random), but they can also be triggered by other many internal and external factors, such as toxins, chemical substances or radiation.

Image by  BioNinja

Image by BioNinja

 
 
Image by  Amoeba Sisters

Many mutations are neutral, i.e. they can neither harm nor benefit the organism, but they can also be deleterious or beneficial.

  • Deleterious mutations can affect the characteristics of the organism and in turn, reduce their fitness and increase their susceptibility to several illnesses, such as cancer.
  • Beneficial mutations can lead to the reproductive success and adaptability of an organism to its environment. These beneficial mutations can be spread and fixed in the population due to natural selection if they help individuals in the population to reach sexual maturity and to successfully reproduce.

 For example, a mutation may allow organisms in a population to produce enzymes that will allow them to eat certain food materials. Over time, these types of individuals survive, while those that don’t have the mutations are more likely to die (natural selection). This is therefore a beneficial mutation.

However, for the mutation to passed down to offspring and help bring about evolution, it has to affect the DNA in the gametes (egg cell or sperm cell).

So in summary, creatures pass on their traits (in the form of DNA) to their offspring via the heredity process. Random recombination and mutations result in offspring with random mixes of traits, which in turn, mix these randomly, and pass them on to their offspring. This allows genetic diversity in the species.

Having a good mutation will help an organism to survive and have a better chance of survival. However, mutation is only the starting place for evolution, providing raw material for the other three processes: gene flow, genetic drift and natural selection.

 
Image by  Amoeba Sisters

2. GENE FLOW

Another mechanism of evolution is gene flow (also called migration).

Gene flow is any movement of individuals, and/or the genetic material they carry, from one population to another (migration).

A population is all the organisms of the same species, which live in a particular geographical area, and have the capability of interbreeding.

 
Image by  BioNinja

Image by BioNinja

When the migrating individuals interbreed with the new population, they contribute their genes to the gene pool of that local population, therefore giving gene flow to this population.

For example, some individuals from a population of brown beetles might migrate and join a population of green beetles. That would make genes for brown coloration more frequent in this green beetle population than they were before the brown beetles migrated into it.

If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. Gene flow also tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.

3. GENETIC DRIFT

Genetic drift is another mechanism of evolution.

Genetic drift is where allele frequencies of a population change over generations by chance. It occurs in all populations of different sizes, but its effects are strongest in small populations. It may result in the loss or rise of some alleles (including beneficial ones)

Genetic drift can have major effects when a population is sharply reduced in size by a natural disaster (bottleneck effect) or when a small group splits off from the main population to start a colony in another region (founder effect).

 
Image by  BioNinja

Image by BioNinja

 
Image by  Buzzle.com

Image by Buzzle.com

 

4. NATURAL SELECTION

So we’ve already three of the four mechanisms of evolution – mutation, gene flow and genetic drift. All these mechanisms help a population evolve and change its genetic makeup over generations.

But there's one mechanism of evolution that's a bit more famous than the others - natural selection.

What makes natural selection so special? Out of all the mechanisms of evolution, it's the only one that can consistently make populations adapted, or better-suited for their environment, over time.

 
 
Image by  BioNinja

Image by BioNinja

 

So what is natural selection?

Natural selection (also known as “survival of the fittest”) is the process by which species adapt to their environment. 

Natural selection leads to evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate than other individuals in a population, and pass on these genetic characteristics to their offspring. 

Selection is a process that each individual is subjected to. Every creature has a unique mix of traits and characteristics. This mix may or may not help them to survive in their environment. Anyone with an unsuitable mix may lead them to death and/ or prevent them from producing offspring. However those with the right mix survive, and have the chance to pass on these traits that helped them to their offspring.

Environmental influences have an effect on all living creatures, and these are known as selection factors. These selection factors can include:

  • Predators
  • Parasites
  • Animals of the same species
  • Toxins
  • Changes in habitat
  • Climate
 

Fitness is a measure of how well organisms survive and reproduce during this process of natural selection, with emphasis on "reproduce".

Survival is one important component of fitness. In order to leave any offspring at all in the next generation, an organism has to survive to reproductive age (puberty), where they can then pass their traits down to their offspring.

However, survival is not the only part of the fitness equation. Fitness also depends on the ability of the organism to attract a mate and reproduce to make fertile offspring. An organism that survived for many years, but never successfully attracted a mate or had offspring, would have very low (zero) fitness. Whereas an organism that survived to puberty and had many offspring would have very high fitness.

This is why diversity and the production of offspring is so important. By creatures producing as many offspring as possible, they increase the likelihood that at least one of their offspring can pass the natural selection process, and therefore maximise the survival of their species.

In summary, organisms with heritable traits that help them survive and reproduce in a particular environment tend to produce more offspring than their peers. If this continues over generations, these heritable traits that aid survival and reproduction will become more and more common in the population. The population will not only evolve (change in its genetic makeup and inherited traits), but it will also evolve in such a way that it becomes adapted (better-suited) to its environment (e.g. like a camel that is suited to surviving in the desert). Over long periods of time, these successful traits can lead to the development of a new species.

AN EXAMPLE OF NATURAL SELECTION: DARWIN'S FINCHES

A good example of natural selection is Darwin’s finches, a type of bird living on a remote island. They are some of the most famous animals in the world of science, and are named after their discoverer, Charles Darwin. Below is a video on Darwin's finches, and how natural selection caused different species to arise from an initial group of stranded mainland finches:

 

 

Video by FuseSchool