generation n

this genome dies off this genome multiplies

this genome survives

crossover position

generation n + 1

Selection keeps the size of the population constant but increases the fitness of the next generation. Genomes with a higher fitness (darker shading) proliferate and genomes with lighter shading die off.

Crossover is a way of

combining two genomes. A crossover position determines where the genomes break and are recombined.

mutation

Mutation makes an occasional random change to a random position in a genome. This allows features to appear that may not have been in the original population.

Figure 13.2 The basic operators in genetic algorithms are selection, crossover, and mutation.

SIMPLE OVERVIEW OF GENETICS

Life depends on proteins, which consist of sequences of 20 basic units called amino acids. The chromosomes in the nucleus of a cell are strands of DNA (deoxyribonucleic acid) that carry the blueprints for the proteins needed by the cell. The 23 chromosomes in each human cell together are the genome for that individual. In general, the genomes of different individuals in a species are very similar to each other; however, there are some individual differences.

The DNA in the genome encodes these blueprints for the amino acids sequences using strands of nucleotides. These nucleotides constitute the four letters of the genetic alphabet:

♦ A, adenine

♦ C, cytosine

♦ G, guanine

♦ T, thymine

Triplets of nucleotides represent the 20 amino acids. For instance, the amino acid called methionine corresponds to the triplet ATG. Another amino acid, lysine, has two spellings : AAA and AAG. So, if a strand of DNA contains the following letters:

ATGAAGATGCGA

then it decodes into a protein containing four amino acids: methionine, ATG; lysine, AAG; methionine, ATG; followed by arginine, CGA (see figure). This description intentionally glosses over the details of the actual biochemical mechanism that turns the blueprints into proteins, but it provides a high-level outline of the mapping from genetic information in DNA to the building blocks of proteins.

A biological example of encoding is the mapping from nucleotides in DNA to amino acids in protein.

In this simplified model, the process of evolution works as follows. The proteins produced by the representations in the DNA express themselves as features of the living organism, such as blue eyes, five fingers, the structure of the brain, a long trunk, and so on. Genes can express themselves in damaging ways, causing the resulting organism to die. Healthy organisms survive to produce offspring and pass their DNA to the next generation. In higher-level animals, the DNA is actually combined with the DNA from another survivor during sexual replication, using a technique called crossover. Sometimes, mistakes are made in passing genes from one generation to the next-these are mutations. The combination of all these processes over the course of many generations results in organisms highly adapted to their environment: the process of evolution.

SIMPLE OVERVIEW OF GENETICS (continued)

DNA with Amino Acids

Nucleotides for Proteins

Adenine

Thymine

Guanine

Adenine

Adenine

Guanine

Adenine

Thymine

Guanine

C ytosine

Guanine

Methionine

Lysine

Methionine

Arginine