Chapter 16 Evolution of Populations

gene shuffling

2nd source of genetic variation. Causes most heritable differences during the production of gametes. Each chromosome of a homologous pair moves independently during meiosis. Crossing over occurs in meiosis too and further increases the number of different genotypes that can appear in offspring. Sexual reproduction produces many different combinations of genes, but in itself does not alter the relative frequencies of each type of allele in a population.

single gene trait

A trait is controlled by a single gene that has two alleles. Variation in this gene leads to only two distinct phenotypes.

Reproductive Isolation

Finches from different islands do not interbreed. They choose their mate carefully and part of the courtship behavior is to inspect the potential partners's beak very closely. Finches prefer to mate with birds that have the same-sized beak as they do. The gene pools of the two bird populations remain isolated from each other. The two populations have now become separate species.

Peter and Rosemary Grant

Great variation of heritable traits among the Galápagos finches. Many characteristics appeared in bell-shaped distributions typical of polygenic traits. Individual birds with different -sized beaks had different chances of survival during a drought. When food for the finches was scarce, individuals with the largest beaks were more likely to survive. Beak size also played a role in mating behavior, because big-beaked birds tend to mate with other big beaked birds. Average beak size in the finch population increased dramatically over time, ex. of directional selection operating on an anatomical trait. Natural selection takes place frequently-and sometimes very rapidly.

no mutations

If genes mutate from one form into another, new alleles may be introduced into the population, and allele frequencies will change.

genetic drift

In small populations, individuals that carry a particular allele may leave more descendants than other individuals , just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

no movement into or out of the population

Individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. In genetic terms, the population's gene pool must be kept together and kept separate from the gene pools of other populations.

Founders arrive

Many years ago, a few finches from the South American mainland flew or were blown to one of the Galápagos Islands. They managed to survive and reproduce.

5 conditions to maintain genetic equilibrium from generation to generation.

There must be random mating; the population must be very large; there can be no movement into or out of the population; no mutations, and no natural selection. If these conditions are not met, the genetic equilibrium will be disrupted, and the population will evolve.

large population

a large population size is also important in maintaining genetic equilibrium. That is because genetic drift has less effect on large populations.

random mating

all members of the population must have an equal opportunity to produce offspring. Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. In natural populations, however, mating is rarely completely random.

evolution in genetic terms

any change in the relative frequency of alleles in a population.

gene pool

consists of all genes, including all the different alleles, that are present in a population.

natural selection on single-gene traits

lead to changes in allele frequencies and thus evolution.

Separation of populations

some birds from the species A crossed to another island. These birds do not usually fly over open water, thus the finch population on the two islands were isolated from each other anon longer shared a common gene pool.

Temporal Isolation

two or more species reproduce at different times.

mutations

any change in a sequence of DNA. It can occur because of mistakes in replication of DNA or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism's phenotype (different DNA codon but codes for same amino acid). Many mutations do produce changes in phenotype. Some can affect an organism's fitness, or its ability to survive and reproduce in its environment. Other mutations may have no effect on fitness. it is one of two main sources of genetic variation.

no natural selection

All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. In other words, there can be no natural selection operating on the population.

Ecological competition

As two new species live together in the same environment they compete with each other for available seeds. During the dry season individuals that are most different from each other have the highest fitness. The more specialized birds have less competition for certain kinds of seeds, and the competition among individual finches is also reduced.

natural selection on polygenic traits

Can affect the distribution of phenotypes in any of 3 ways: directional selection, stabilizing selection, or disruptive selection.

founder effect

allele frequencies change as a result of the migration of a small subgroup of a population.

Hardy-Weinberg principle

allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

genetic equilibrium

allele frequencies remain constant, so the population will not evolve.

natural selection

never acts directly on genes because it is an entire organism-not a single gene-that either survives and reproduces or dies without reproducing. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. Evolution is any change over time in the relative frequencies of alleles in a population. It's populations, not individual organisms, that can evolve over time.

Speciation in the Galápagos finches

occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, reproductive isolation, and ecological competition.

behavioral isolation

one type of isolating mechanism when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.

genotype

organism's genetic makeup.

Changes in the gene pool

over time, populations on each island became adapted to their local environments. Over time, natural selection would have caused the population of finches on the island with big seeds to evolve larger brakes, forming a separate population, B.

phenotype

physical, behavioral, and biochemical characteristics.

relative frequency of an allele

the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. It is often expressed as a percentage.

polygenic traits

traits controlled by two or more genes Each gene of a polygenic trait often has two or more alleles. One polygenic trait can have many possible genotypes and phenotypes. Symmetrical bell shape=normal distribution.

geographic isolation

two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. Geography barriers do not guarantee the formation of new species. If two formerly separated populations can still interbreed, they remain a single species. Also, any potential geographic barrier may separate certain types of organisms but not others.

directional selection

when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

disruptive selection

when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Selection acts most strongly against individuals of an intermediate type. If the pressure of natural selection is strong enough and lasts long enough, this situation can cause the single curve to split into two distinct phenotypes.

stabilizing selection

when individuals near the center of the curve have higher fitness than individuals at either end of the curve. Keeps the center of the curve at its current position, but it narrows the overall graph.

reproductive isolation

when the members of two populations cannot interbreed and produce fertile offspring. As new species evolve, populations become reproductively isolated from each other. At that point, the populations have separate gene pools. They respond to natural selection or genetic drift as separate units. Reproductive isolation can develop by : behavioral isolation, geographic isolation, and temporal isolation.