Chapter 23: Gene Pools Evolve of Populations

Population

A group of individuals of the same species living in the same place at the same time. This is the smallest unit of evolution. Members of a population are capable of meeting and mating. Natural selection acts on individuals. However, evolution is defined only in terms of changes in a population over time.

Natural Selection

A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits. Natural selection acts on the gene pool by changing allele frequency within the population. Traits that enhance survival and reproduction will be represented wit increasing frequency in the gene pools.

Genetic drift can cause harmful alleles to become fixed

Alleles that are neither harmful nor beneficial can be lost or become fixed (reach a frequency of 100%) by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occurs, the population's survival can be threatened (as in greater prairie chickens).

p

dominant allele frequency

Effects of Genetic Drift

-Genetic drift is significant in small populations. -Genetic drift can cause allele frequencies to change at random. -Genetic drift can lead to a loss of genetic variation within populations. -Genetic drift can cause harmful alleles to become fixed.

Why natural selection cannot fashion perfect organisms

-Selection can act only on existing variations. -Evolution is limited by historical constraints. -Adaptations are often compromises. -Chance, natural selection, and the environment.

Conditions for Hardy-Weinberg Equilibrium

1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow Describes a population that is not evolving. In real populations, allele frequencies often do change over time. Such changes occur when one or more of the conditions for the equilibrium are not met.

Bottleneck Effect

A change in allele frequency following a dramatic reduction in the size of a population. Could be due to natural disasters or purposeful killing by humans.

Genetic Drift

A change in the allele frequency of a population as a result of chance events rather than natural selection. Ex: by chance alone, alleles encoding darker color have become more common over several generations.

Sexual Selection

A form of natural selection that depends on an individual's ability to obtain a mate. Can lead to changes in a gene pool over successive generations. Females may choose males for their traits. Intersexual selection or mate choice. Males may compete with each other for access to mates. Intrasexual selection.

No Natural Selection

Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success.

Chromosomal Changes

Altering gene number or position.

Genetic drift can cause allele frequencies to change at random

Because of genetic drift, an allele may increase in frequency one year, then decrease the next; the change from year to year is not predictable. Thus, unlike natural selection, which in a given environment consistently favors some alleles over other, genetic drift causes allele frequencies to change at random over time.

Differential Reproduction

Better adapted individuals are more likely to reproduce, and fertile offspring will make up a greater proportion of the next generation.

Genetic drift can lead to a loss of genetic variation within populations

By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.

No Gene Flow

By moving alleles into or out of populations, gene flow can alter allele frequencies.

Mechanisms of Variation

Changes to the genetic makeup of a population can arise via two mechanisms: mutations and sexual recombination.

Gene Pools

Combined genetic information of all the members of a particular population. Consists of all versions of all the genes carried by all the individuals in a population. Shows the evolution of populations. Natural selection acts on individuals. However, evolution is defined only in terms of changes in a population over time.

Chance, natural selection, and the environment

Chance events can affect the subsequent evolutionary history of populations.

Genetic drift is significant in small populations

Chance events can cause an allele to be disproportionately over or under represented in the next generation. Although chance events occur in populations of all sizes, they tend to cater allele frequencies substantially only in small populations.

Microevolution

Change in allele frequencies in a population over generations. Generation-to-generation change in the gene pool. Taken over many generations; can result in the gradual adaptation of species to the local environment.

Sexual Reproduction

Crossing over, recombination increases genetic variability

Hardy-Weinberg Equation

Describes the genetic makeup we expect for a population that is not evolving at a particular locus. If the observed genetic makeup of the population differs from expectations under Hardy-Weinberg, it suggests that the population may be evolving.

Differential Adaptiveness

Differences may affect how well an organism is adapted to environment.

Sexual Recombination

During the formation of sperm and eggs, chromosomes can exchange pieces of DNA, shuffling genes.

Adaptations are often compromises

Each organism must do many different things. A seal spends part of its time on rocks; it could probably walk better if it had legs instead of flippers, but then it would not swim nearly as well.

Evolution is limited by historical constraints

Each species has a legacy of descent with modification from ancestral forms. Evolution does not scrap the ancestral anatomy and build each new complex structure from scratch.

Environment

Every non genetic influence. Some phenotypic variation does not result from genetic differences among individuals, but rather from environmental influences. Only genetically determined variation can have evolutionary consequences.

Deviations from Hardy-Weinberg

Evolutionary change. Change in allele frequencies may result in change in phenotypic frequencies = evolution. Mutation, gene flow, genetic drift, natural selection, sexual selection.

Relative Fitness

Fitness differences between different variants of character.

Disruptive Selection

Form of natural selection in which a single curve splits into two; occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle. Favors individuals at both extremes of the phenotypic range. Ex: Rock pocket mice

Directional Selection

Form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve. Favors individuals at one extreme end of the phenotypic range. Ex: Long horns

Mutation

Formation of new alleles

Founder Effect

Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original population.

Random Mating

If individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing of gametes does not occur and genotype frequencies change.

Extremely Large Population Size

In small populations, allele frequencies fluctuate by chance over time (a process called genetic drift).

Inheritance

Many differences are inheritable differences.

Nucleotide Variability

Measured by comparing the DNA sequences of two or more individuals. Rarely results in phenotypic variation. Most differences occur in noncoding regions (introns). Variation that occur in coding regions (exons) can change the amino acid sequence of the encoded protein.

Variation

Members of populations must differ from each other.

Gene Flow

Movement of alleles into or out of a population due to the migration of individuals to or from the population. Introduces or eliminates alleles. Ex: The arrival of beetles with a spotted color pattern has Brough new alleles into the population. This can also affect adaptation to local environments. Ex: variation in banding pattern in Lake Erie water snakes represents adaptation to mainland and island habitats. On the mainland, most snakes are banded. On the islands, most snakes aren't banded. However, migration of banded snakes from mainland to island populations maintains alleles for the banded pattern in island populations.

Mechanisms that Change Gene Pools

Mutation, gene flow, sexual reproduction, genetic drift, natural selection, bottleneck and founder effect, and sexual selection.

Selection can act only on existing variations

Natural selection favors only the fittest phenotypes among those currently in the population, which may not be the ideal traits.

Stabilizing Selection

Natural selection that favors intermediate variants by acting against extreme phenotypes. Ex: Malaria and sickle cell mutation.

What mechanisms can cause evolution of populations?

Natural selection, genetic drift, and gene flow.

Causes of Microevolution

Natural selection, genetic drift, gene flow

Mutations

Random changes to DNA which can create new genes.

Evolution by Natural Selection

Requires four things: 1. Variation 2. Inheritance 3. Differential adaptiveness 4. Differential reproduction

Hardy-Weinberg Equilibrium

The condition describing a non-evolving population (one that is in genetic equilibrium). In a population where gametes contribute to the next generation randomly and Mendelian inheritance occurs, allele and genotype frequencies remain constant from generation to generation.

Fitness

The contribution that an individual makes in the gene pool of the next generation in comparison to the contributions form other individuals. The fittest individual is not always the strongest. There are many sorts of adaptations that can improve fitness.

No Mutations

The gene pool is modified if mutations occur or if entire genes are deleted or duplicated.

Genetic Variation

The variety of different types of genes in a species or population. Can arise through mutation. Most common mode is through sexual reproduction as it ensures that genes are randomly mixed. The presence of genetic variation in a population does not guarantee that the population will evolve. For evolution to occur, one or more factors must be at work.

2pq

heterozygous frequency

p^2

homozygous dominant allele frequency

q^2

homozygous recessive allele frequency

q

recessive allele frequency