How does evolutionary change occur in populations

For example, in a sample population of humans, the frequency of the I A allele might be 0. If we also know that the frequency of the I B allele in this population is 0. A change in any of these allele frequencies over time would constitute evolution in the population. When allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies, the phenomenon is called genetic drift.

The smaller a population, the more susceptible it is to mechanisms such as genetic drift as alleles are more likely to become fixed at 0 absent or 1 universally present.

Random events that alter allele frequencies will have a much larger effect when the gene pool is small. Genetic drift and natural selection usually occur simultaneously in populations, but the cause of the frequency change is often impossible to determine. Natural selection also affects allele frequency.

If an allele confers a phenotype that enables an individual to better survive or have more offspring, the frequency of that allele will increase. Because many of those offspring will also carry the beneficial allele and, therefore, the phenotype, they will have more offspring of their own that also carry the allele. Over time, the allele will spread throughout the population and may become fixed: every individual in the population carries the allele. If an allele is dominant but detrimental, it may be swiftly eliminated from the gene pool when the individual with the allele does not reproduce.

However, a detrimental recessive allele can linger for generations in a population, hidden by the dominant allele in heterozygotes. In such cases, the only individuals to be eliminated from the population are those unlucky enough to inherit two copies of such an allele. The founder effect occurs when part of a population becomes isolated and establishes a separate gene pool with its own allele frequencies. When a small number of individuals become the basis of a new population, this new population can be very different genetically from the original population if the founders are not representative of the original.

Therefore, many different populations, with very different and uniform gene pools, can all originate from the same, larger population. Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population. The Founder Effect : Here are three possible outcomes of the founder effect, each with gene pools separate from the original populations.

The Hardy-Weinberg principle can be used to estimate the frequency of alleles and genotypes in a population. Ultimately, the Hardy-Weinberg principle models a population without evolution under the following conditions:. Although no real-world population can satisfy all of these conditions, the principle still offers a useful model for population analysis.

According to the Hardy-Weinberg principle, the variable p often represents the frequency of a particular allele, usually a dominant one. For example, assume that p represents the frequency of the dominant allele, Y, for yellow pea pods.

The variable q represents the frequency of the recessive allele, y, for green pea pods. If p and q are the only two possible alleles for this characteristic, then the sum of the frequencies must add up to 1, or percent. If the frequency of the Y allele in the population is 0. From the Hardy-Weinberg principle and the known allele frequencies, we can also infer the frequencies of the genotypes. Since each individual carries two alleles per gene Y or y , we can predict the frequencies of these genotypes with a chi square.

If two alleles are drawn at random from the gene pool, we can determine the probability of each genotype. Population Genomics. Semelparity and Iteroparity.

Geographic Mosaics of Coevolution. Comparative Genomics. Cybertaxonomy and Ecology. Ecological Opportunity: Trigger of Adaptive Radiation.

Evidence for Meat-Eating by Early Humans. Resource Partitioning and Why It Matters. The Evolution of Aging. Forbes Dept. Krimmel Dept. Citation: Forbes, A. Nature Education Knowledge 3 10 Evolution describes changes in inherited traits of populations through successive generations.

To fully understand the science of ecology, one must first be able to grasp evolutionary concepts. Aa Aa Aa. What Evolution Is Not. Figure 1: The "drunkard's walk" as an explanatory metaphor for patterns of increasing complexity in evolution. A drunken man leaving a bar at the end of the night starts with the locked door to his back and is equally likely to stagger to the left or to the right.

Microevolution and Macroevolution. Pesticides are applied between 0 km and 20 km from the coast during summer months. References and Recommended Reading Dobzhansky, T. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Topic rooms within Evolution Close.

No topic rooms are there. Lead Editor: Nick Bisceglia Evolution. Or Browse Visually. There is evidence that fish, birds, mice, and primates including humans select mates with different HLA genotypes than themselves. We humans also tend to mate more often with individuals who resemble us phenotypically positive phenotypic assortment. These shifts can occur without changing the proportion of each allele in the population, also called the allele frequency.

Watch this Ted Ed video to review these concepts with an easy way to remember them but note that this video does not refer to genetic drift by name, but emphasizes that changes in allele frequencies due to chance alone is more likely to occur in small populations :. Biological Principles. Skip to content. Home Evolution Strong Inference What is life?

What is evolution? Evolution by Natural Selection Other Mechanisms of Evolution Population Genetics: the Hardy-Weinberg Principle Speciation Phylogenetic Trees Earth History and History of Life on Earth Origin of Life on Earth Molecules and Metabolism Chemical context for biology: origin of life and chemical evolution Biological molecules Membranes and Transport Cells Energy and enzymes Respiration, chemiosmosis and oxidative phosphorylation Oxidative pathways: electrons from food to electron carriers Fermentation, mitochondria and regulation Why are plants green, and how did chlorophyll take over the world?

This results in a drastic reduction of the total genetic diversity of the original gene pool. The small surviving population is considerably be farther from the original one in its genetic makeup.

Founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals that are cleaved from a larger population.

This new population does not have the genetic diversity of the previous one. Because the community is very small and also geographical or socially isolated, some genetic traits are becoming more prevalent in the population.