Ch. 16:Evolution: Evolution of Populations

Section 16-1: Genes & VariationI. Terms to KnowA. Population- a group of individuals belonging to the

same species in a given areaB. Species- group of populations whose individuals can

interbreed and produce fertile offspringC. Population Genetics - study of kinds of number of

genes in a populationsD. Evolution- generation to generation change in a

population’s allele frequency

II. Gene Pools - all genes, including all the different alleles, that are present in a population.

A. Relative Frequency (of an allele) - the # of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.• Relative frequency is often expressed as a %.• In genetic terms, evolution is the change in the

relative frequency of alleles in a population.

In a total of 50 alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles? •If relative frequency of B allele decreased in the gene pool, what would happen to the relative frequency of the other allele? Increase or Decrease?

III. Sources of Genetic VariationA. Mutations- Changes in an organisms DNA due

to error in replication, radiation, or chemicals1. Some can affect an organisms fitness(ability to

survive & reproduce) others have no effect2. Heritable3. Can result in novel alleles (both good and bad)4. Ultimately change the gene pool

B. Gene Shuffling1. Occurs during the production of gametes.2. 23 pairs of chromosomes can produce 8.4 million

combinations of genes3. Crossing over increases the number of

genotypes that can appear in offspring4. Sexual reproduction results in many different

phenotypes, but it does not change the relative frequency of alleles in a population.

IV. Single-Gene & Polygenic Traits*The number of phenotypes produced for a given trait

depends on how many genes control the trait.

A. Single gene trait - controlled by single gene 1. Single with 2 alleles – only 2 possible phenotypes2. Ex Widow’s peak (allele is dominant over the

allele for a straight hairline but is less frequent)3. Compare by Bar graph.

B. Polygenic Traits - controlled by 2 or more genes

1. Each gene has 2 or more alleles – many possible genotypes and even more phenotypes.

2. Ex. Height in Humans. It is represented by symmetrical bell shaped curve.

Section 16-2: Evolution as Genetic Change

• Natural selection does not act on genes.

• It acts on phenotype and decides which phenotype is suitable to survive & reproduce.

I. Natural selection on single-gene traits A. Leads to changes in allele frequencies = evolution.

Figure shows a population of brown lizards in which mutation produces red & black forms. If population lives in the dark soil then how does a color affect the fitness of the lizards?

What do you predict the lizard population will look like by generation 50? Explain

II. Natural Selection on Polygenic Traits - action of multiple alleles on traits such as height produces a range of phenotypes that often fit a bell curve.

A. Directional Selection - individuals at one end of the curve have higher fitness than in the middle or at the other end.

Ex.- increase in the average size of the beaks of finches who compete for food.

Dotted - original distribution of beak sizes Solid - changed distribution of beak sizes.Peak shifts as average beak size increases.

B. Stabilizing Selection – individuals near the center of the curve have higher fitness than individuals at either end of the curve.

The smaller babies are less healthy, the larger have difficulty being born therefore both are less fit than the average. The average weighing babies are more likely to survive than smaller or larger.

C. Disruptive Selection - individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

1. The selection acts against an intermediate type & can cause the single curve to split into two (creates two phenotypes).

The average-sized seeds become less common than larger and smaller seeds. The bird population splits into two subgroups eating larger & smaller seeds.

III. Genetic Drift- Random change in allele frequency in a population due to chance.

A. Most likely to occur in small population or when a small group of organisms colonize a new habitat.

B. In small populations, an allele can become more or less common simply by chance.

1. Individuals that carry a particular allele may leave more descendants than others, just by chance.

2. Founder Effect - Allele frequencies change as a result of the migration of a small subgroup of a population

Founder Effect

Bottleneck Effect

IV. Hardy-Weinberg Theorem

• allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.A. Populations that meet these requirements are

said to be in Genetic Equilibrium and are not evolving.

B. This is useful for comparison. If frequencies of alleles deviate from values predicted by the theorem then the population is evolving.

Following Five conditions are required to maintain genetic equilibrium:

1.The population is large2.There is no migration3.No mutations4.Random Mating5.No natural selection

If the above conditions are not met, genetic equilibrium will be disrupted and the population will evolve.

Section 16-3: Process of Speciation

I. Speciation: the development of new speciesA. When two populations become

reproductively isolated, new species evolve.

B. What isolates a species? Barriers

Following are Reproductive Isolation - two populations cannot interbreed and produce fertile offspring.

A. Geographic Isolation: 2 populations are separated by geographic barriers such as rivers, mountains.

1. It does not form new species when it enables to separate populations.

2. Ex: Birds begin foraging on the ground while the others forage in the tree tops.

Geographic Isolation

Figure 24.8

A. formosus

Atlantic Ocean

A. nuttingi

Isthmus of Panama

Pacific Ocean

A. panamensis A. millsae

B. Behavioral Isolation: 2 populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.

1. the song of 2 frog/bird species is different; the plumage or dance of 2 species is different.

C. Temporal Isolation: 2 or more species reproduce at different times

1. 2 species that breed at different times of day, different seasons, or different years cannot mix their gametes.

2. nocturnal vs. day; seed release during spring vs. summer