Genes and Variation

• In the 1930’s, evolutionary biologists began to connect Mendel’s work to Darwin’s theory.

• With the discovery of DNA’s structure in 1953 by Watson and Crick, the gene became the focus of evolution. [Scientists began speaking about evolution in genetic terms.]

• In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population.

Points to make:1. Gene pool all genes (alleles) that are

present in a population.2. Frequency the number of times that an

allele occurs in a gene pool.• Gene pools are important to evolutionary

theory, because evolution changes populations over time.

3. Evolution is studied and understood in populations.

Why do allele frequencies change?

1. Mutations in the DNA • Mutations are rare and many are harmful.

• Is it likely that mutations are solely responsible for shifting allele frequencies?

• No…

2. Migration – Gene Flow

• Immigration of new individuals with new/different alleles.

3. Genetic Drift

• A change in allele frequency caused by a chance event. (e.g. massive death – survival is unrelated to genotype – contradicts Darwin’s “survival of the fittest”)

4. Natural Selection

• Differential survival and reproduction of traits controlled by genes.

Natural Selection by the numbers…• Gene frequency in a population of 100 organisms: 25 AA /

50 Aa / 25 aa (Generation 1)• (100 A’s)/200 total alleles = 50% A• (100 a’s)/200 total alleles = 50% a • Nature selects against A and causes a 20% of those expressing

A to die. Resulting in…• 20 AA / 40 Aa / 25 aa (Generation 2)• (80 A’s)/170 total alleles = 47% A• (90 a’s)/170 total alleles = 53% a• Generation 3 16 AA / 32 Aa / 25 aa• (64 A’s)/146 total alleles = 43% A• (57 a’s)/146 total alleles = 56% a

Are there any conditions under which evolution will not occur?

• To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place.

• If allele frequencies do not change, the population will not evolve. This is called genetic equilibrium.

• What has to occur for a population to reach genetic equilibrium? (Hardy-Weinberg equilibrium)

Five conditions required to maintain genetic equilibrium (no evolution)

• RANDOM MATING – All the members of the population must have an equal opportunity to produce offspring. Nonrandom mating would cause certain genes to be selected over others.

• LARGE POPULATION SIZE - Genetic Drift has little effect on large populations.

• NO GENE FLOW – No movement into or out of the population.

• NO MUTATIONS• NO NATURAL SELECTION

Mutations+ Gene Flow+ Genetic Drift+ Natural SelectionEvolution (Microevolution)

Moving beyond microevolution…• Mutation, chance events, and natural

selection changes the allele frequency within a population…

• …but how (when) do you get a new species?• That depends on how you define the term

‘species’…• …’species’ two organisms that can

interbreed and produce fertile offspring.

Same species?• Yes…Why?• They all

interbreed with each other.

• Theridion grallator.

What helps new species form? (Speciation)

Isolation • Two types of isolation…

Geographic Isolation

• Two populations are separated by geographic barriers such as rivers, mountains, oceans…

• The Kaibab (left) squirrel evolved from the Abert (right)

squirrel when the Colorado river split the species (Abert) into two separate populations, thus two separate gene pools.

• Natural selection worked separately on each group and led to the formation of two distinct species of squirrels.

Behavioral Isolation• When two populations are capable of interbreeding but

don’t since they have differences in reproductive strategies that involve behavior. (Eastern/Western Meadowlark)

• They use different songs to attract mates.

Peter and Rosemary Grant

•Mutation/Gene Flow/Genetic Drift/Natural Selection

+ 3.8 billion yearsEvolution (Macroevolution)