BIO.B.2.3 Adaptations and Evolution
Topic Summary for Evolution by Natural Selection:
Natural selection on a single-gene trait can lead to changes in allele frequencies and changes in phenotype frequencies. For polygenic traits, populations often exhibit a range of phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with the average (in the case of beak size, the extremes may be tiny and large beaks). Natural selection on polygenic traits can cause shifts to the bell curve depending upon which phenotype is selected for.
Directional selection takes place when individuals at one end of the bell curve have higher fitness than those near the middle or at the other end of the curve. For example, when large seeds are plentiful, large-beaked birds in a population may be selected for.
Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than individuals at either end.
Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
In small populations, alleles can become more or less common simply by chance. This kind of change in allele frequency is called genetic drift.
The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.
The founder effect is a change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat.
If allele frequencies in a population do not change, the population is in genetic equilibrium. Evolution is not taking place.
The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. These factors include: non-random mating, small population size, immigration or emigration, mutations, and natural selection.
Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring. For example, many species exhibit non-random mating patterns. Sexual selection, or the process in which an individual chooses its mate based on heritable traits (such as size or strength), is a common practice for many organisms.
Topic Summary for Evolution by Natural Selection:
Natural selection on a single-gene trait can lead to changes in allele frequencies and changes in phenotype frequencies. For polygenic traits, populations often exhibit a range of phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with the average (in the case of beak size, the extremes may be tiny and large beaks). Natural selection on polygenic traits can cause shifts to the bell curve depending upon which phenotype is selected for.
Directional selection takes place when individuals at one end of the bell curve have higher fitness than those near the middle or at the other end of the curve. For example, when large seeds are plentiful, large-beaked birds in a population may be selected for.
Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than individuals at either end.
Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
In small populations, alleles can become more or less common simply by chance. This kind of change in allele frequency is called genetic drift.
The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.
The founder effect is a change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat.
If allele frequencies in a population do not change, the population is in genetic equilibrium. Evolution is not taking place.
The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. These factors include: non-random mating, small population size, immigration or emigration, mutations, and natural selection.
Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring. For example, many species exhibit non-random mating patterns. Sexual selection, or the process in which an individual chooses its mate based on heritable traits (such as size or strength), is a common practice for many organisms.