17.1 Genes and Variation

Genetics Joins Evolutionary Theory Darwin developed his theory of evolution without knowing how heritable traits passed from one generation to the next or where heritable variation came from. Researchers discovered that heritable traits are controlled by genes. Changes in genes and chromosomes generate variation. For example, all of these children received their genes from the same parents, but they all look different.

Genotype and Phenotype in Evolution Natural selection acts directly on phenotype, not genotype.   Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

Populations and Gene Pools A population is a group of individuals of the same species that mate and produce offspring.   A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population. Researchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

For example, this diagram shows the gene pool for fur color in a population of mice.  

Populations and Gene Pools Evolution is any change in the relative frequency of alleles in the gene pool of a population over time.   Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations. Populations, rather than individuals, evolve.

Three sources of genetic variation are: mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

Mutations Mutations are any change in the DNA sequence. Mutations that produce changes in phenotype may or may not affect fitness. Some mutations may be lethal or may lower fitness; others may be beneficial.   Mutations matter in evolution only if they can be passed from generation to generation. The mutation must occur in the germ line cells that produce either eggs or sperm.

Genetic Recombination in Sexual Reproduction Most heritable differences are due to genetic recombination during sexual reproduction. This occurs during Meiosis when each chromosome in a pair moves independently. Genetics recombination also occurs during crossing-over in meiosis.  

Lateral Gene Transfer Lateral gene transfer occurs when organisms pass genes from one individual to another that is not its offspring.   It can occur between organisms of the same species or organisms of different species. Lateral gene transfer can increase genetic variation in a species that picks up the “new” genes.

Single-Gene and Polygenic Traits What determines the number of phenotypes for a given trait?  

Single-Gene Traits The number of phenotypes produced for a trait depends on how many genes control the trait. A single-gene trait is a trait controlled by only one gene. Single-gene traits may have just two or three distinct phenotypes.   The most common form of the allele can be dominant or recessive.

Polygenic Traits Polygenic traits are traits controlled by two or more genes.   Each gene of a polygenic trait often has two or more alleles. A single polygenic trait often has many possible genotypes and even more different phenotypes.

Polygenic Traits Human height, which varies from very short to very tall, is an example of a polygenic trait.   The bell-shaped curve in the graph is typical of polygenic traits.

17.2 Evolution as Genetic Change in Populations

Insect populations often contain a few individuals that are resistant to a particular pesticide. Those insects pass on their resistance to their offspring and soon the pesticide-resistant offspring dominate the population. The relationship between natural selection and genetics explains how pesticide resistance develops.

How Natural Selection Works Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.   Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection. Evolutionary fitness is the success in passing genes to the next generation. Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution.   For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution.   For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

Natural Selection on Single-Gene Traits If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.  

Natural Selection on Single-Gene Traits Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

Natural Selection on Polygenic Traits Polygenic traits have a range of phenotypes that often form a bell curve.   The fitness of individuals may vary from one end of the curve to the other. Natural selection can affect the range of phenotypes and hence the shape of the bell curve.

Directional Selection Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others. For example, if only large seeds were available, birds with larger beaks would have an easier time feeding and would be more successful in surviving and passing on genes.

Stabilizing Selection Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph. For example, very small and very large babies are less likely to survive than average-sized individuals. The fitness of these smaller or larger babies is therefore lower than that of more average-sized individuals.

Disruptive Selection Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes. For example, in an area where medium-sized seeds are less common, birds with unusually small or large beaks would have higher fitness. Therefore, the population might split into two groups—one with smaller beaks and one with larger beaks.

Genetic Drift Genetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.

Genetic Bottlenecks The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.   For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.

The Founder Effect  The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population. Two groups from a large, diverse population could produce new populations that differ from the original group.

17.3 The Process of Speciation

How does one species become two? Factors such as natural selection and genetic drift can change the relative frequencies of alleles in a population, but this alone does not lead to development of a new species.   How does one species become two?

Isolating Mechanisms Speciation is the formation of a new species. A species is a population whose members can interbreed and produce fertile offspring.

Isolating Mechanisms Reproductive isolation occurs when a population splits into two groups and the two populations no longer interbreed. When populations become reproductively isolated, they can evolve into two separate species.

Behavioral Isolation Behavioral isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors. The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They have different mating songs.

Geographic Isolation Geographic isolation occurs when two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. Separate gene pools formed, and genetic changes in one group were not passed on to the other. Kaibab Squirrel Abert’s Squirrel

Temporal Isolation Temporal isolation happens when two or more species reproduce at different times.   Cicadas breed every 17 years Cicadas breed every 13 years