C HAPTER 16 E VOLUTION AND P OPULATIONS

G ENES AND V ARIATION Fitness, adaptation, species, and evolutionary change are now defined in genetic terms. Gene pool: all the genes (and its alleles) present in a population. Relative Frequency: the number of times an allele appears in a gene pool compared with the number of times other alleles for that same gene occur. In genetic terms, evolution is any change in the relative frequency of alleles in a population.

S OURCES OF G ENETIC V ARIATION Mutations - are random - occur in the DNA - most are harmless but some can be lethal Gene Shuffling - occurs during sexual reproduction - is responsible for most differences - each chromosome has a pair of homologous chromosomes that may sort independently during meiosis; 23  8.4 million different gene combos - crossing over Sexual reproduction produces different phenotypes, but it does not change the relative frequency. The probability remains the same.

S INGLE G ENE T RAITS The number of phenotypes produced for a given trait depends on how many genes control the trait. A single-gene trait is a trait that is controlled by a single gene with two alleles. ex: widow’s peak vs. straight hairline Just because a trait is dominant does not mean that more people have it. A recessive allele may contribute to an organism’s fitness, therefore leading to a population with more organisms having the recessive allele.

P OLYGENIC T RAITS Polygenic traits are controlled by two or more genes. Each gene often has two or more alleles. As a result, one polygenic trait may have many possible genotypes and phenotypes. ex: height in humans Refer to pg. 396 in your textbook showing a bell- shaped curve (or normal distribution).

E VOLUTION AS G ENETIC C HANGE Natural selection never directly acts on genes. It acts on the entire organism- not a single gene. It is the organism that survives and reproduces or dies without reproducing. Natural selection can only affect which individuals survive and reproduce and which do not. If an organism dies without reproducing, then it does not contribute its genetics to the populations gene pool; if it reproduces before it dies, then it contributes its genetics to the population’s gene pool. If an individual produces many offspring, then its alleles stay in the gene pool and contribute to the relative frequency of the allele.

N ATURAL S ELECTION ON S INGLE -G ENE T RAITS Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. An example is the story of the peppered moth… Break for activity…. HHMI DVD on mice…

N ATURAL S ELECTION ON P OLYGENIC T RAITS When traits are controlled by more than one gene, the effects of natural selection are more complex. Remember, the distribution of a polygenic trait can be seen on a typical bell curve. There won’t be much difference in fitness between individuals who are next to each other on the curve, but the individuals at either end of the curve will be very different. Natural selection can affect the distribution of phenotypes in any of three ways: 1) directional selection, 2) stabilizing selection, or 3) disruptive selection.

D IRECTIONAL S ELECTION Individuals at one end of the curve have more fitness than individuals in the middle or at the other end of the curve. The range of phenotypes shifts as some individuals fail to survive and reproduce while others succeed. See page 398 for the example. Population after selection Original population Directional Selection

S TABILIZING S ELECTION Selection Against Both Extremes Population after selection Original population Individuals near the center of the curve have more fitness than the individuals at either end of the cure.

D ISRUPTIVE S ELECTION Selection against the mean Population after selection Original population Individuals at the upper and lower ends of the curve have higher fitness than the individuals near the middle.

G ENETIC D RIFT Natural selection is not the only source of evolutionary change. In small populations, an allele can become more or less common simply by chance. Probability laws are applied to genetics to make genetic predictions. But the smaller the population, the more unlikely it becomes that the laws of probability can be applied. Genetic Drift: In small populations, individuals can carry a particular allele that may leave more descendants than other individuals, just by chance. Over time, a series of occurrences of this type can cause an allele to become common in a population.

G ENETIC D RIFT Sample of original population Founding Population A Founding Population B Descendants

F OUNDER A FFECT It is a situation in which allele frequencies change as a result of the migration of a small subgroup of a population. Therefore, two small groups from a large, diverse population could produce new populations that differ from the original group. island Founder Effect: a few individuals from a population start a new population with a different allele than the original population. Founder Effect also referred to as the Bottle Neck Effect.

E VOLUTION V ERSUS G ENETIC E QUILIBRIUM Hardy-Weinberg Principle: allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. Genetic equilibrium is when allele frequencies remain constant and do not change; populations do not evolve. 5 conditions required to maintain genetic equilibrium from generation to generation are: 1. random mating 2. population must be large 3. there can be no movement in or out of a population 4. no mutations 5. no natural selection

T HE P ROCESS OF S PECIATION Natural selection and chance occurrences can change allele frequencies, but how do entirely new species form? Species: a group of organisms that breed with one another and produce fertile offspring. In order for a species to evolve into a new species, the gene pools of two populations must become separated. As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

I SOLATING M ECHANISMS ( TYPES OF REPRODUCTIVE ISOLATION ) Behavioral isolation Geographic isolation Temporal isolation

B EHAVIORAL I SOLATION Occurs when two populations are capable of interbreeding, but have differences in courtship rituals or other reproductive strategies that involve behavior. Ex: Eastern Meadowlark vs. Western Meadowlark They have overlapping habitats. Won’t mate with each other because they use different songs to attract mates. Notice these birds are singing different mating songs even though they are the same species. This prevents them from hooking up.

G EOGRAPHIC I SOLATION Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. Ex: the Albert squirrel in the Southwest. Its population got separated when the Colorado River split the area. The Albert and Kaibab squirrels have many similarities, but fur color differs. 1. Original beetle population 2. River arises, effectively splitting the population. 3. After many generations, each population evolves genetic differences. 4. After the river dries up, genetic differences prevent interbreeding.

T EMPORAL I SOLATION Two or more species reproduce at different times. Ex: three species of orchid are found in the same rain forest. They release pollen only on a single day. Since they each release pollen on different days, they cannot pollinate each other.

S PECIATION IN D ARWIN ’ S F INCHES Read pgs