Genetic variation makes evolution possible

Microevolution

A change in allele frequencies in a population over generations.

Heterozygote Protection

A harmful recessive allele can persist for generations by propagation in heterozygous individuals (where its harmful effects can be masked by the more favorable dominant allele). Such "heterozygote protection" maintains a huge pool of alleles that might not be favored under present conditions, but that could be beneficial if the environment changes.

Gene Duplication is a Potential Source of Variation

A key potential source of variation is the duplication of genes due to errors in meiosis (such as unequal crossing over), slippage during DNA replication, or the activities of transposable elements. Duplications of large chromosome segments, like other chromosomal aberrations, are often harmful, but the duplication of smaller pieces of DNA may not be.

Organisms reflect many generations of past selection, and hence their phenotypes tend to be suited for life in their environments.

As a result, most new mutations that alter a phenotype are at least slightly harmful.

Chromosomal Changes May Not Affect Phenotype

Chromosomal changes that delete, disrupt, or rearrange many loci are usually harmful. However, when such large-scale changes leave genes intact, they may not affect the organisms' phenotype.

3 Mechanisms Contribute to Shuffling

Crossing over, independent assortment of chromosomes, and fertilization. The combined effects of these three mechanisms ensure that sexual reproduction rearranges existing alleles into fresh combinations each generation, providing much of the genetic variation that makes evolution possible.

Genetic Variation

Differences among individuals in the composition of their genes or other DNA sequences.

Neutral Variation

Differences in DNA sequence that do not confer a selective advantage of disadvantage.

Crossing Over, Independent Assortment, and Fertilization

During meiosis, homologous chromosomes, one inherited from each parent, trade some of their alleles by crossing over. These homologous chromosomes and the allele they carry are then distributed at random into gametes. Then, because myriad possible mating combinations exist in a population, fertilization typically brings together gametes that have different genetic backgrounds.

Gene Duplications with Small Effects Can Persist

Gene duplications that do not have severe effects can persist over generations, allowing mutations to accumulate. The result is an expanded genome with new genes that may take on new functions.

How much do genes and other DNA sequences vary form one individual to another?

Genetic variation at the whole-gene level (gene variability) can be quantified as the average percentage of loci that are heterozygous.

HIV has an RNA Genome (example)

HIV also has an RNA genome, which has a much higher mutation rate than a typical DNA genome because of the lack of RNA repair mechanisms in host cells. For this reason, single-drug treatments are unlikely to be affective against HIV: Mutant forms of the virus that are resistant to a particular drug would tend to proliferate in relatively short order.

HIV Generation Time (example)

HIV has a generation time of about two days (that is, it takes two days for a newly formed virus to produce the next generation of viruses).

Determination from Two or More Genes

In contrast, other phenotypic differences vary in gradations along a continuum. Such variation usually results from the influence of two or more genes on a single phenotypic character. Many phenotypic characters are influenced by multiple genes, including coat color in horses, seed number in maize (corn), and height in humans.

Passing of Mutations in Multicellular Organisms

In multicellular organisms, only mutations in cell lines that produce gametes can be passed to offspring. In plants and fungi, this is not as limiting as it may sound, since many different cell lines can produce gametes. But in most animals, the majority of mutations occur in somatic cells and are not passed to offspring.

Sexual Reproduction Results in Combinations of Alleles

In organisms that reproduce sexually, most of the genetic variation in a population results from the unique combination of alleles that each individual receives from its parents. Of course, at the nucleotide level, all the differences among these alleles have originated from past mutations. Sexual reproduction then shuffles existing alleles and deals them at random to produce individual genotypes.

Rarely, Chromosomal Changes are Beneficial

In rare cases, chromosomal rearrangement may be beneficial. For example, the translocation of part of one chromosome to a different chromosome could link genes in a way that produces a positive effect.

Diploid Organisms Can Hide Recessive Alleles

In some cases, natural selection quickly removes such harmful alleles. In diploid organisms, however, harmful alleles that are recessive can be hidden from selection.

Little variation at the molecular level results in phenotypic variation

Many nucleotide variations occur within introns, noncoding segments of DNA lying between exons, the regions retained in mRNA after RNA processing. And of the variations that occur within exons, most do not cause a change in the amino acid sequence of the protein encoded by the gene.

Mutations can Quickly Generate Genetic Variation in Prokaryotes

Mutation rates tend to be low in plants and animals, averaging about one mutation in ever 100,000 genes per generation, and they are often even lower in prokaryotes. But prokaryotes have many more generations per unit of time, so mutations can quickly generate genetic variation in their populations. The same is true for viruses.

One common misconception is that individual organisms evolve.

Natural selection acts on individuals: Each organism's traits affect its survival and reproductive success compared with those of other individuals. But the evolutionary impact of natural selection is only apparent in how a population of organisms changes over time.

3 Mechanisms of Allele Frequency Change

Natural selection, genetic drift (chance events that alter allele frequencies), and gene flow (the transfer of alleles between populations). Each of these mechanisms has distinctive effects on the genetic composition of populations.

Mutation

New alleles can arise by mutation, a change in the nucleotide sequence of an organism's DNA. A change of as little as one base in a gene--a "point mutation"--can have a significant impact on phenotype, as in sickle cell disease.

Only Natural Selection Leads to Adaptation

Only natural selection consistently improves the degree to which organisms are well suited for life in their environment (adaptation).

Evolution Cannot Occur Without Variation

Only the genetically determined part of the phenotypic variation can have evolutionary consequences. As such, genetic variation provides the raw material for evolutionary change: Without genetic variation, evolution cannot occur.

Some Phenotypic Variation Does Not Result in Genetic Differences Among Individuals

Phenotype is the product of an inherited genotype and many environmental influences. Example --> Body builders alter their phenotypes dramatically but do not pass their huge muscles on to the next generation.

Determination of a Single Gene Locus

Some heritable phenotypic differences occur on an "either-or" basis, such as the flower colors of Mendel's pea plants: each plant had flowers that were either purple or white. Characters that vary in this way are typically determined by a single gene locus, with different alleles producing distinct phenotypes.

Increases in Gene Number have led to Evolution

Such increases in gene number appear to have played a major role in evolution. For example, the remote ancestors of mammals had a single gene for detecting odors that has since been duplicated many times. As a result, humans today have about 380 functional olfactory receptor genes

Genetic Variants

The genetic variation on which evolution depends originates when mutation, gene duplication, or other processes produce new alleles and new genes. Genetic variants can be produced rapidly in organisms with short generation times. Sexual reproduction can also result in genetic variation as existing genes are arranged in different ways.

Drug "Cocktails" (example)

The most effective AIDS treatments to date have been drug "cocktails" that combine several medications. This approach has worked well because it is less likely that a set of mutations that together confer resistance to all the drugs will occur in a short time period.

Redundancy in the Genetic Code

The redundancy in the genetic code is another source of natural variation: Even a point mutation in a gene that encodes a protein will have no effect on the protein's function if the amino acid composition is not changed. And even where there is a change in the amino acid, it may not affect the protein's shape and function.