Biology - Natural Selection and the Hardy-Weinburg Equilibrium

Natural Selection

• The process by which individuals with favorable, inherited traits leave more offspring than individuals with other traits.

• Individuals with traits not well-suited for their environment will die/leave few offspring.

• Individuals with traits well-suited for their environment will survive and reproduce more successfully.

Gene

Short segment of DNA that contains the instructions for a single trait

Alleles

Alternate forms of a gene. For each gene, there are two or more.

Population

Interbreeding group of organisms that belong to the same species, are located in the same area, and produce fertile offspring. The smallest unit in which evolution occurs.

Three sources for genetic variation:

1. Mutations

2. Recombination (production of gametes; passing of traits through meiosis)

3. Random pairing of gametes (sex cells)

Mutation

Random change that occurs in a gene when the sequence of nitrogen bases is changed in the DNA. These changes are passed to future offspring.

Recombination

The shuffling of genes in a diploid organism. Most heritable traits are due to the gene shuffling that occurs during meiosis.

The Principle of Independent Assortment

Genes for separate traits segregate independently during the formation of gametes, creating 8.4 million possible genetic variations.

Crossing over

The process in which homologous chromosomes exchange portions of their chromatids during meiosis, increasing the number of genetic variations that can appear in an offspring.

Gene pool

The sum total of all the alleles of all the genes of all the individuals in the population.

Allele frequency

The number of times that one allele occurs in a gene pool compared with the number of times that other alleles for the same gene occur

Why is it important to know the allele frequencies in a population?

It allows us to determine of a population is changing, and therefore, evolving.

Evolution

Any change in the relative frequency of alleles in a population; the result of changes in the gene pool.

Hardy-Weinburg Principle

States that allele frequencies in a population tend to remain the same from one generation to the next unless acted on by an outside force. If allele frequencies do not change, evolution does not occur.

Five conditions for Hardy-Weinburg equilibrium:

1. Large population (chance variation does not change genotype frequencies)

2. No migration

3. No mutation; alleles remain the same.

4. Mating must be random

5. No natural selection, no differences in fitness

Why use the Hardy-Weinburg principle?

• Allows scientists to detect changes in gene pools over time

• Provides a mechanism by which evolution can be viewed by observing allele frequencies in a given population

Hardy-Weinberg Equation

p² + pq + q² = 1

Evolution

The change in allele frequencies of a population over time

5 factors that cause changes in allele frequencies

1. Mutations

2. Gene flow

3. Genetic drift

4. Non-random mating

5. Natural selection

Gene Flow

The movement of alleles in or out of a population as a result of immigration (organisms moving into a population) or emigration (organisms moving out of a population)

Genetic Drift

• A change in the gene pool that occurs as a result of change

• In small populations, this skews allele frequencies

Non-random mating

Choosing a mate based on specific circumstances or traits, such as close proximity or mating dance

Directional Selection

Natural selection that occurs when conditions favor individuals with one extreme phenotype

Disruptive Selection

Natural selection that occurs when conditions favor individuals at both phenotypic extremes, at the expense of the average

Stabilizing Selection

Acts against both extreme phenotypes and favors the average form of a trait

• reduces variation in the population

Speciation

• The formation of a new species

• Begins with the separation of gene pools of 2 populations

Species

A group of organisms that breed with one another and produce fertile offspring

Geographical Isolation

Physical separation of members of a population

• natural selection and genetic drift act upon the populations, causing them to diverge and become incompatible for mating

Reproductive Isolation

Groups of organism are genetically isolated and can no longer produce fertile offspring

Divided into:

• Premating isolation

• Postmating isolation

Premating Isolation

Prevents fertilization from occurring by either:

• Preventing different species from mating

• Preventing attempted mating from being successful

• Hindering fertilization if mating is a success

Habitat Isolation

Two species live in different habitats within the same area, and may rarely encounter each other

Behavioral Isolation

Two populations are capable of interbreeding, but have different mating rituals

Temporal Isolation

Two or more species reproduce at different times

Mechanical Isolation

Closely related species attempt to mate but fail because they are anatomically incompatible

Gametic Isolation

Gametes of two different species meet, but rarely fuse to form a zygote

Postmating Mechanisms

Prevents the production of fertile offspring if matings occur between two different species

Types of Speciation

1. Allopatric speciation

2. Sympatric speciation

Allopatric speciation

Occurs when species arise as a result of geographical isolation

Sympatric speciation

Occurs when species become reproductively isolated

May occur if gene flow is reduced by:

• Polyploidy (extra chromosomes in a cell)

• Habitat Differentiation (a change in habitat or resources)

• Sexual Selection (choosing a mate based on specific characteristics)