UNIT-4-Population-Genetics

The main causes of evolution include:

1. Mutation - Changes in DNA sequences that can introduce new traits.
2. Gene Flow - The transfer of genetic material between populations, which can increase genetic diversity.
3. Genetic Drift - Random changes in allele frequencies, especially in small populations.
4. Natural Selection - The process where organisms better adapted to their environment tend to survive and produce more offspring.
5. Non-random Breeding - Mating patterns that can affect allele frequencies in a population.

Natural Selection acts on individuals, but it is the populations that evolve over time.

Microevolution refers to a change in allele frequencies in a population over generations.

The two main processes that produce genetic variation in gene pools are mutation and sexual reproduction.

Camouflage is a crucial survival adaptation that allows caterpillars to blend into their environment, reducing predation risk. This trait can arise from genetic variation, where different individuals possess varying abilities to mimic their surroundings. Over time, those with better camouflage are more likely to survive and reproduce, passing on their advantageous traits to future generations.

Average heterozygosity measures the average percent of loci that are heterozygous in a population. It is determined by assessing the amount of heterozygosity at the gene and molecular levels.

Geographic variation refers to differences between gene pools of separate populations or population subgroups. It can manifest as a cline, which is a graded change in a trait along a geographic axis.

Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful.

The mutation rates in animals and plants are generally low.

Sexual reproduction shuffles existing alleles into new combinations, which is crucial for creating genetic diversity. This recombination of alleles is more significant than mutation in generating the genetic differences necessary for adaptation.

It tests whether a sexually reproducing population is evolving.

A population is a localized group of individuals (a species in an area) capable of interbreeding and producing fertile offspring.

A gene pool consists of all the alleles for all loci in a population.

A locus is fixed if all individuals in a population are homozygous for the same allele.

The symbols p and q represent the frequencies of two alleles at a locus in a population.

The frequency of all alleles in a population will add up to 1, expressed as p + q = 1.

The Hardy-Weinberg principle describes an ideal population that is not evolving.

The closer a population is to the criteria of the Hardy-Weinberg principle, the more stable the population is likely to be.

The formula for calculating genotype frequencies is:

p2 + 2pq + q2 = 1

where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype.

1. No mutations: Genetic changes do not occur.
2. Random mating: Individuals pair by chance, not according to their genotypes or phenotypes.
3. No natural selection: All individuals have equal chances of survival and reproduction.
4. Extremely large population: The population size is sufficiently large to prevent genetic drift.
5. No gene flow: No migration of individuals into or out of the population.

The conditions for a locus to be in Hardy-Weinberg equilibrium include:

1. Low mutation rate of the gene.
2. Random mating with respect to the allele (no preference for carriers).
3. Natural selection acting only on rare homozygous individuals who do not follow dietary restrictions.
4. A large population size to minimize genetic drift.
5. No migration affecting allele frequencies, as similar frequencies exist in other populations.

The frequency of normal alleles is p = 0.99 (calculated as 1 - q, where q = 0.01).

The frequency of heterozygotes/carriers is approximately 2% or 0.0198, calculated using the formula 2pq, where p = 0.99 and q = 0.01.

1. Natural selection - non-random process that favors certain alleles over others.

2. Genetic drift - random changes in allele frequencies, often significant in small populations.

3. Gene flow - random movement of alleles between populations through migration.

Natural selection is the process where differential success in reproduction leads to certain alleles being passed to the next generation in greater proportions by the more fit individuals. This results in changes in allele frequencies over time, favoring advantageous traits.

Genetic drift describes the random fluctuations in allele frequencies from one generation to the next. It is more pronounced in smaller populations, where the chance of deviation from expected results is greater, leading to potential loss of alleles and reduced genetic variation.

The smaller the population size, the greater the chance of deviation from predicted allele frequencies due to genetic drift. This can lead to significant changes in allele frequencies and a reduction in genetic variation over time.

Genetic drift tends to reduce genetic variation within a population through the loss of alleles. As allele frequencies fluctuate randomly, some alleles may become fixed while others are lost, decreasing overall genetic diversity.

Genetic drift can lead to significant changes in allele frequencies over generations, often resulting in the loss of genetic diversity. In the example provided:

1. Generation 1: CR allele frequency = 0.7, CW allele frequency = 0.3
2. Generation 2: CR allele frequency decreased to 0.5, CW allele frequency increased to 0.5
3. Generation 3: CR allele frequency increased to 1.0, leading to the complete loss of the CW allele (frequency = 0.0).

Genetic drift contributes to the loss of genetic diversity by causing allele frequencies to fluctuate randomly from one generation to the next. This can result in:

• Fixation of alleles: An allele may become fixed (frequency = 1.0) while others are lost (frequency = 0.0).
• Reduction in heterozygosity: As alleles are lost, the genetic variation within the population decreases, leading to a more homogenous population.

In the example, the transition from multiple flower colors to only red flowers illustrates this loss of diversity.

The founder effect occurs when a few individuals become isolated from a larger population, leading to different allele frequencies in the small founder population compared to the larger parent population.

The bottleneck effect is a sudden reduction in population size caused by environmental changes, such as a natural disaster. This leads to a gene pool that may not represent the original population's genetic diversity.

The bottleneck effect results in a gene pool that may no longer reflect the genetic diversity of the original population, potentially leading to reduced genetic variation.

If a population remains small after a bottleneck effect, it may be further affected by genetic drift, which can lead to random changes in allele frequencies over generations.

The bottleneck effect occurs when a significant reduction in population size leads to a loss of genetic diversity. This happens when a population undergoes a drastic event that reduces its numbers, resulting in only a small, non-representative sample of the original population surviving. The surviving population may have different allele frequencies compared to the original population, which can affect future generations.

Genetic drift is significant in small populations because it can lead to random changes in allele frequencies, which can have a more pronounced effect compared to larger populations.

Genetic drift causes allele frequencies to change at random, meaning that the changes are not influenced by natural selection or other evolutionary pressures.

Genetic drift can lead to a loss of genetic variation within populations, as certain alleles may become fixed or lost entirely due to random sampling effects.

Genetic drift can cause harmful alleles to become fixed in a population, which can negatively impact the overall fitness of that population.

Gene flow is the movement of alleles among populations, which can occur through the movement of fertile individuals or gametes.

Gene flow tends to reduce differences between populations over time.

Gene flow is more likely than mutation to directly alter allele frequencies in populations.

Natural selection is the only mechanism that consistently causes adaptive evolution.

Natural selection brings about adaptive evolution by acting on an organism's phenotype.

Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

These phrases imply direct competition among individuals, whereas reproductive success is influenced by many subtle factors beyond just competition.

Selection favors certain genotypes by acting on the phenotypes of certain organisms, influencing their reproductive success.

Directional selection favors individuals at one end of the phenotypic range, leading to a shift in the population's traits in a specific direction.

Disruptive selection favors individuals at both extremes of the phenotypic range, which can lead to the emergence of two or more contrasting phenotypes within a population.

Stabilizing selection favors intermediate variants and acts against extreme phenotypes, promoting the maintenance of a particular trait within a population.

Directional selection favors one extreme trait while selecting against the other extreme. This results in a shift in the trait distribution towards the favored extreme.

Stabilizing selection reduces variation by favoring moderate traits and selecting against both extremes, leading to a more peaked distribution around the intermediate trait.

Disruptive selection results in the selection against the intermediate trait, favoring both extremes. This can lead to a population with two distinct peaks in trait distribution.

Directional selection is characterized by a shift in the population curve towards one extreme phenotype, as shown by the evolved population curve moving to the right of the original population curve. This indicates that individuals with a specific trait (darker fur color in this case) are favored by selection pressure.

Diversifying selection leads to the evolution of two distinct phenotypes, resulting in a bimodal distribution (two peaks) in the population curve. In contrast, stabilizing selection results in a narrower and taller population curve, favoring the average phenotype and selecting against extremes.

The evolved population curve in stabilizing selection indicates that extreme phenotypes are selected against, leading to a concentration of individuals around the average phenotype, which is represented by a taller and narrower curve compared to the original population.

Natural Selection increases the frequencies of alleles that enhance survival and reproduction.

Adaptive evolution refers to the match between an organism and its environment.

Adaptive evolution is a continuous process because environments change over time, necessitating ongoing adaptations in organisms to survive and thrive in their surroundings.

Genetic drift and gene flow are random processes that do not consistently lead to adaptive evolution. They can either increase or decrease the match between an organism and its environment, making their effects unpredictable in terms of adaptation.

Sexual selection is a form of natural selection that focuses on mating success, influencing the evolution of traits that enhance an individual's chances of attracting mates.

Sexual dimorphism refers to the marked differences between the sexes in secondary sexual characteristics, often resulting from sexual selection.

Male showiness, driven by mate choice, can increase a male's chances of attracting a female but may also decrease his chances of survival due to increased visibility to predators.

The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for.

Frequency-dependent selection is a process where the fitness of a phenotype declines if it becomes too common in the population. This means that selection favors whichever phenotype is less common.

Scale-eating cichlids exhibit frequency-dependent selection where prey become more alert to the more common phenotype, allowing less common phenotypes to have a predatory advantage. This results in fluctuations in the frequency of left-jawed individuals over time.

Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage. Examples include:

• Variation in noncoding regions of DNA
• Variation in proteins that have little effect on protein function or reproductive fitness.

Natural selection can only act on existing variations within a population, meaning it cannot create new traits or characteristics from scratch.

Evolution is influenced by historical constraints, meaning that the traits and adaptations of organisms are shaped by their evolutionary history, which limits the potential for perfect adaptations.

Adaptations are often seen as compromises, balancing different selective pressures rather than being perfect solutions to environmental challenges.

Natural selection is influenced by the interplay of chance, natural selection, and the environment, which can lead to unpredictable evolutionary outcomes.