Genes, Variations & Evolution

Gene Pools Population – collection of individuals of the same species in a given area and is the smallest biological unit that can evolve (individuals can’t evolve) Ex. All the students in SHS is an example of a population All members of a population can interbreed and they share a common group of genes Gene Pool – combined genetic information of all the members of a particular population / total collection of alleles Gene pool contains two or more forms of a certain gene for each trait Ex. Brown or black fur in mice

Relative Frequency Relative Frequency – the number of times an allele occurs in a gene pool compared with the number of times the other allele(s) occur When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale. Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

Sources of Genetic Variation Darwin couldn’t explain how variations occurred – he didn’t know about genetics We know now that mutations and genetic shuffling that results from sexual reproduction are sources of variations

Mutations Change in a sequence of DNA that occurs due to mistakes in DNA replication or from radiation/chemicals in the environment Produce changes in an organism’s phenotype (physical characteristics) – can affect fitness (ability to survive and reproduce in the environment)

Gene Shuffling Chromosomes “move around” the cell during meiosis (formation of sperm and egg cells) Crossing over of the chromosomes occurs during meiosis creating different genetic combinations Sexual reproduction is a major source of variation within a population (even more so than mutations)

Single-Gene and Polygenic Traits Number of phenotypes produced for a given trait depends on how many genes control that trait Variable traits in a population may be polygenic, resulting from the combined effects of several genes, or determined by a single gene. Polygenic traits tend to produce phenotypes that vary more or less continuously. Single-gene traits tend to produce only a few distinct phenotypes

Single-Gene Trait Trait controlled by one gene with two alleles (Ww) – variation leads to only TWO phenotypes Ex. Widow’s Peak or no Widow’s Peak 100 80 60 40 20 Frequency of Phenotype (%) Widow’s peak No widow’s peak Phenotype

Frequency of Phenotype Polygenic Traits Traits controlled by two or more genes. Each gene of a polygenic trait has two or more alleles = many phenotypes possible. Ex. Height in humans – short, tall, and everything inbetween. When the average is graphed, a “bell-curve” is created for that trait Frequency of Phenotype Phenotype (height)

Natural Selection & Genetics ****Natural Selection acts directly on phenotypes**** - affects which individuals having different phenotypes will survive and which will not Natural selection does change relative frequencies of alleles in a population over time Any factor that causes alleles to be added or removed from a population will change relative frequencies of alleles. If an individual dies without reproducing, its genes are removed from the population. If an individual reproduces a lot, that individual’s genes increase in the gene pool

Natural Selection & Genetics (continued) Evolution is any change in the relative frequencies of alleles in a population’s gene pool ****Evolution acts on populations, NOT individuals

Natural Selection on Single-Gene Traits Can lead to changes in allele frequencies and thus to evolution Ex. Lizard color – brown color is normal, whereas red and black are mutations. These lizards live on dark soil. The brown and black lizards are hard for birds to see, but the red is easy for birds to see. Predict what will happen to the 3 variations of color…

Analyzing Gene Pools A gene pool consists of all the alleles in a population at any one time and is a reservoir from which the next generation draws its alleles. Alleles in a gene pool occur in certain frequencies.

Analyzing Gene Pools (continued) Alleles can be symbolized by p for the relative frequency of the dominant allele in the population (ex. R), q for the frequency of the recessive allele in the population (ex. r), and p + q = 1. Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.

Analyzing Gene Pools (continued) Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving). The Hardy-Weinberg formula p2 + 2pq + q2 = 1 can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles. p2 = frequency of homozygote for one allele (ex. RR) 2pq = frequency of heterozygotes (ex. Rr) q2 = frequency of other homozygote (ex. rr)

Analyzing Gene Pools (continued) Alleles can be symbolized by p for the relative frequency of the dominant allele in the population (ex. R), q for the frequency of the recessive allele in the population (ex. r), and p + q = 1. Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.