19.1A: Defining Population Evolution

Learning Objectives

• Describe how the forces of genetic drift, genetic hitchhiking, gene flow, and mutation can lead to differences in population variation

The Evolution of Populations

According to evolutionary theory, every organism from humans to beetles to plants to bacteria share a common ancestor. Millions of years of evolutionary pressure caused some organisms to died while others survived, leaving earth with the diverse life forms we have today. Within this diversity is unity; for example, all organisms are composed of cells and use DNA. The theory of evolution gives us a unifying theory to explain the similarities and differences within life’s organisms and processes.

Figure \(\PageIndex{1}\): Evolution on earth: Evolution has resulted in living things that may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution.

Genetic Variation in Populations

A population is a group of individuals that can all interbreed, often distinguished as a species. Because these individuals can share genes and pass on combinations of genes to the next generation, the collection of these genes is called a gene pool. The process of evolution occurs only in populations and not in individuals. A single individual cannot evolve alone; evolution is the process of changing the gene frequencies within a gene pool. Five forces can cause genetic variation and evolution in a population: mutations, natural selection, genetic drift, genetic hitchhiking, and gene flow.

Mutations

Why do some organisms survive while others die? These surviving organisms generally possess traits or characteristics that bestow benefits that help them survive (e.g., better camouflage, faster swimming, or more efficient digestion). Each of these characteristics is the result of a mutation, or a change in the genetic code. Mutations occur spontaneously, but not all mutations are heritable; they are passed down to offspring only if the mutations occur in the gametes. These heritable mutations are responsible for the rise of new traits in a population.

Natural Selection

Just as mutations cause new traits in a population, natural selection acts on the frequency of those traits. Because there are more organisms than resources, all organisms are in a constant struggle for existence. In natural selection, those individuals with superior traits will be able to produce more offspring. The more offspring an organism can produce, the higher its fitness. As novel traits and behaviors arise from mutation, natural selection perpetuates the traits that confer a benefit.

Figure \(\PageIndex{1}\): Mutation and natural selection: As mutations create variation, natural selection affects the frequency of that trait in a population. Mutations that confer a benefit (such as running faster or digesting food more efficiently) can help that organism survive and reproduce, carrying the mutation to the next generation.

Genetic Drift

When selective forces are absent or relatively weak, gene frequencies tend to “drift” due to random events. This drift halts when the variation of the gene becomes “fixed” by either disappearing from the population or replacing the other variations completely. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations.

Figure \(\PageIndex{1}\): Genetic drift and gene fixation: In this simulation, there is fixation in the blue gene variation within five generations. Images these dots are beetles and some of them are destroyed by a wildfire. As the surviving population changes over time, some traits (red) may be completely eliminated from the population, leaving only the beetles with other traits (blue).

Genetic Hitchhiking

When recombination occurs during sexual reproduction, genes are usually shuffled so that each parent gives its offspring a random assortment of its genetic variation. However, genes that are close together on the same chromosome are often assorted together. Therefore, the frequency of a gene may increase in a population through genetic hitchhiking if its proximal genes confer a benefit.

Gene Flow

Gene flow is the exchange of genes between populations or between species.If the gene pools between two populations are different, the exchange of genes can introduce variation that is advantageous or disadvantageous to one of the populations. If advantageous, this gene variation may replace all the other variations until the entire population exhibits that trait.

Key Points

• The theory of evolution gives us a unifying theory to explain the similarities and differences within life’s organisms and processes.
• Populations (or gene pools ) evolve as gene frequencies change; individual organisms cannot evolve.
• Variation in populations is determined by the genes present in the population’s gene pool, which may be directly altered by mutation.
• Natural selection is the gradual process that increases the frequency of advantageous inherited traits (allowing it to survive and reproduce) and decreases the frequency of detrimental inherited traits within a population.
• A population’s genetic makeup can also be affected by random chance events like genetic drift, or when genes are inherited together in genetic hitchhiking.

Key Terms

• gene flow: the transfer of alleles or genes from one population to another
• genetic hitchhiking: a phenomenon in which a gene increases in a population because it lies near genes on the same chromosome that are advantageous to an organism
• genetic drift: an overall shift of allele distribution in an isolated population, due to random fluctuations in the frequencies of individual alleles of the genes
• fitness: an individual’s ability to propagate its genes
• natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce
• mutation: any heritable change of the base-pair sequence of genetic material