4. Evolution of Population (V)

What is a population?

A group of organisms of the same species that can mate with each other.

What is the smallest unit of evolution?

The population.

What is an allele?

A particular form, a specific version of a gene.

Genotype is made up of what in the shell color example?

Two alleles for a gene (e.g., shell color).

What are the two chromosomal mechanisms that can create new genes?

Chromosomal rearrangement and gene duplication.

What can chromosomal rearrangement produce?

A new gene region at the chromosome tip that could yield a new protein; it could be beneficial, neutral, or harmful.

What is gene duplication?

Duplication of a gene during replication, creating two copies; mutations on the new copy can create a new phenotype.

What processes during meiosis generate genetic variation in gametes?

Crossing over during meiosis and independent assortment during metaphase I.

What is random fertilization?

Random union of egg and sperm; any combination can occur, increasing offspring genetic diversity.

What is Hardy-Weinberg equilibrium?

A population will not evolve if certain conditions are met and allele frequencies stay constant across generations.

What are the five conditions for Hardy-Weinberg equilibrium?

No mutations, random mating, no natural selection, very large population size, no gene flow.

Why is Hardy-Weinberg equilibrium useful in studying evolution?

If a population is evolving, we can identify which condition is not met to understand the cause of evolution.

What is mutation’s role in evolution?

Mutations are the ultimate source of variation; they introduce new alleles, but a single mutation alone usually doesn’t greatly change allele frequencies.

What is gene flow?

Movement of alleles between populations via migrating individuals or gametes; can introduce new alleles and alter allele frequencies.

What is non-random mating?

Mating that is not random; includes assortative mating where individuals mate with similar phenotypes, affecting heterozygosity.

What is assortative mating?

Mating with like phenotypes; increases homozygosity and decreases heterozygosity.

What is genetic drift?

Random changes in allele frequencies due to chance; not caused by natural selection.

What is the founder effect?

A few individuals from a main population establish a new population; new population has different allele frequencies and reduced variation.

What is the bottleneck effect?

A random environmental event drastically reduces population size; survivors have different allele frequencies and reduced variation.

What is the small population effect?

In a small population, allele frequencies can fluctuate randomly due to chance; less likely in a large population.

What is fixation?

An allele reaches 100% frequency in a population and becomes the only allele at that locus.

What is a classic example of incomplete dominance discussed in the notes?

Red (RR) and White (WW) alleles produce Pink (RW) in heterozygotes; pink is an intermediate phenotype.

How does random fertilization contribute to genetic variation in populations?

Random fertilization creates many different zygote genotypes, increasing variation across offspring.

What is natural selection?

The process by which allele frequencies in a population change due to differential survival and reproduction driven by environmental pressures; individuals with advantageous heritable traits are more likely to reproduce and pass those alleles on.

What are the three conditions for natural selection?

Variation in heritable traits; overproduction and competition; and differential reproductive success (fitness) that affects which individuals reproduce more.

What does 'fitness' mean in natural selection?

Relative reproductive success; the number of offspring an individual leaves compared with others in the population.

What are the three modes of natural selection?

Directional, disruptive, and stabilizing.

What is directional selection?

Favors one extreme phenotype and shifts the population toward that extreme; often reduces genetic variation. Examples include darker fur in desert mice on dark ground and the rise of antibiotic resistance under antibiotic use.

What is disruptive selection?

Favors extreme phenotypes at both ends and selects against the intermediate, producing a bimodal distribution. Examples include populations with very light or very dark fur rather than intermediate color and birds with two distinct beak sizes.

What is stabilizing selection?

Favors the intermediate phenotype and removes extremes, reducing genetic variation. Example: an intermediate clutch size (number of eggs) being favored.

What is heterozygote advantage?

A form of balancing selection where heterozygotes have higher fitness in a given environment, maintaining two alleles in the population (e.g., sickle cell trait provides malaria resistance in some regions).

How does sickle cell disease illustrate heterozygote advantage?

Heterozygotes (one normal Hb allele and one sickle allele) have some malaria resistance without typically severe disease, increasing fitness in malaria-endemic areas.

What is frequency-dependent selection?

The fitness of a phenotype depends on how common it is in the population; often, common phenotypes are selected against, helping maintain variation (e.g., left- and right-mouthed scale-eating fish).

What is sexual selection?

A mode of natural selection where one sex (usually females) chooses mates based on certain traits, leading to features like a peacock's tail and often resulting in sexual dimorphism.

What is sexual dimorphism?

Marked differences between males and females in appearance or behavior, typically arising from sexual selection.

Can natural selection act together with gene flow?

Yes; natural selection and gene flow can operate simultaneously and influence allele frequencies, as seen when populations exchange genes and experience different selective pressures.

Why can't natural selection produce perfect organisms?

Because there are limits to optimization due to trade-offs, historical constraints, and changing environments; selection improves fitness in a given context but does not create perfection.