MUTATION AND VARIATION

Evolution depends on variation.

FIGURE 1 Tho strawberry poison dart frog (Dendrobates pumilio) has conspicuous coloration that warns predators it is toxic. Why this speces is so variable, however, isnot understood. Central questions in evolutionary biology include what maintains variation, and how variation is shaped by selection and other evolutionary factors.
Genetic code is shared by virtually all life on earth
FIGURE 2

FIGURE3 Synonymous changes to a DNA sequence do not alter the amino acids in a protein, but nonsynonymous changes do. At top is the DNA sequenco for three codons of the A allelo of the / P hemoglobin gene in humans. Beeneath eacch codon is an abbreviation for tho amino acid it codes for. The sixth codon, GAG, codes for.glutamic acid (Glu). A change in that Codon from GAG to GAA does not alter the amino acid, so this is a synonymous change. A change from GAG to GTG howewer replaces the glutamic acid with valine (Val), so this is a nonsynonymous change. The change to the CTG codon produces the S allele. which results in The sickle-cell condition.

Like the genetic code, the cellular machinery for transcribing DNA and translating mRNA.k almost universal across life on Earth. The DNA or mRNA from a sea urchin is translated into a protein if it is injected Into a bacterlum. This universality Is the basis of genetic engineering.FIGURE 4 Allele frequencies and genotype frequencies. In this population, the frequency of the A1A1 homozygote genotype is 1/2 the frequency of the A1A2 heterozygote genotype is 1/4, and the frequency of the A2A2 homozygote genotype is 1/4. Ten of the 16 copies of the gene are the A1 alele, so its allele frequency is p=10/16=0625.

Segregation is the selection of one of the two copies of a locus when a gamete is made during meiosis. It does not occur in organisms that do not have meiosis. The mixing of genes caused by segregation changes the proportions of genotypes in a population.

• Hardy-Weinberg equilibrium, which tells us the relative proportions of genotypes in a population when segregation is the only factor that changes genotype frequencies.
• The key conditions for the Hardy-Weinberg equilibrium are:

- An infinite population size
- No natural selection
- No mutation
- No movement between populations
- Random mating.

• ➢ When an allele at one locus is found together in a population more often than expected by chance with an allele at a second locus, we say the loci are in Linkage disequlibrium.
• ➢ Epistatis is the situation in which the offect of an allele at one locus depends on the allele at a second locus. If some combinations of alleles have high fitness, selection will generate linkage disequilibrium between them.
• ❖ Horizontal gene fransler (HGT) is the movement of DNA between different individuals without help from sexual reproduction.
• Mutation are the ultimate source of genetic variation in all organisms. Without these errors, there would be no variation, no evolution, and no life.

FIGURE 6 Five types of structural mutations that alter chromosomes.

FIGURE 7 The peloria mutant of toadflax (Linaria vulgaris) results from an epigenetic mutation. One of the five petals normally has a long nectar-bearing spur, but in the peloria mutant all five petals have this form, transforming the flower’s symmetry from bilateral to radial. The mutation can result from either a change in the DNA’s sequence or its methylation pattern. (Courtesy of R. Grant-Downton.)

FİGURE 8 The direction of coiling in the snail Lymnaea peregra is determined by the genotype of an individual’s mother, not its own genotype [23]. The shell can coil either to the left (L) or to the right (R). The locus that affects coiling has two alleles, D and d. (A) All offspring of a dd female with an L shell also have L shells. (B) All offspring of a DD female with an R shell also have R shells. Although the offspring genotypes in the two panels are the same, their shell phenotypes differ, and are determined by their mother’s genotype. This is an example of a maternal effect.
SUMMARY

• Genetic variation is produced when the genes of two parents are mixed during sexual reproduction by segregation and recombination. In asexual species, genes are mixed only by recombination, and less often than in sexual species.
• The Hardy-Weinberg equilibrium occurs under idealized conditions in which no evolutionary forces are acting. At this equilibrium, the three genotypes at a locus with two alleles are in the ratios p2 :: 2p(1 – p) :: (1 – p)2 . Deviations from those proportions can be used to detect selection and other evolutionary factors.
• Linkage disequilibrium occurs when alleles at two loci occur together more often than expected by chance. It is eroded by recombination, and increased by selection and other evolutionary forces.
• Horizontal gene transfer (HGT) is the movement of genes between organisms by mechanisms that do not involve meiosis, and is particularly important to prokaryotes. It can move genes between individuals of the same species and of different species. HGT is important to human health because it is the major pathway by which bacteria evolve resistance to antibiotics.
• Mutation, which is an error that occurs when DNA or RNA is replicated, is critical to evolution because it is the ultimate source of genetic variation. Mutations can affect anywhere from a single base to a large piece of chromosome. Mutation rates vary greatly among species.
• Mutations in coding regions are called synonymous if they do not change the protein, and are called nonsynonymous if they do. This distinction has important evolutionary implications.
• Many mutations have no measurable effect on survival or reproduction. Those that do are typically deleterious. A small fraction are advantageous, and their spread leads to adaptive evolution.
• Mutations that affect one trait virtually always have pleiotropic effects, meaning that they also affect other traits.
• In species with separate somas and germ lines, a mutation can be inherited if it alters a gene in a cell in the germ line. Mutations to somatic cells leave no descendants to the next generation.
• Experiments show that mutation is random with respect to what will increase fitness.
• There are several mechanisms of nongenetic inheritance. One is cultural inheritance, an essential part of human civilization.