Miller & Levine Biology - 17.1 Genes and Variation Flashcards
Genetics and Evolution
Evolution is defined in genetic terms as any change in the relative frequency of alleles in the gene pool of a population over time.
Scientific researchers discovered that heritable traits are controlled by genes.
Variation is generated by changes in genes and chromosomes.
Gregor Mendel's ideas regarding inheritance were the "missing information" needed for evolutionary theory, but he died before the scientific world recognized his work.
Genotype and Phenotype in Evolution
Genotype: An organism's genotype is the particular combination of alleles it carries.
Phenotype: Includes all physical, physiological, and behavioral characteristics of an organism.
An individual’s phenotype is produced by its genotype together with environmental conditions.
Natural Selection's Role:
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment than others.
Individuals with advantageous phenotypes produce more offspring and pass more copies of their genes to the next generation.
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Populations and Gene Pools
Gene Pool: A population's gene pool consists of the alleles present in that population.
Evolution involves any change in the relative frequency of alleles in the gene pool of a population over time.
Natural selection operates on individuals, but the resulting changes in allele frequencies show up in populations.
Therefore, populations evolve, rather than individuals.
Example: A diagram of a mouse population illustrates the gene pool specifically for fur color.
Sources of Genetic Variation
There are three primary sources of genetic variation:
Mutation:
Mutations that produce changes in phenotype may or may not affect fitness.
Some mutations may be lethal or lower fitness; others may be beneficial.
Mutations matter in evolution only if they can be passed from generation to generation.
To be heritable, the mutation must occur in the germ line cells (sex cells) that produce either eggs or sperm.
Genetic Recombination in Sexual Reproduction:
(While the transcript text for this section repeats mutation details, it is identified as a distinct source of variation occurring during the production of offspring).
Lateral Gene Transfer:
Occurs when organisms pass genes from one individual to another that is not its offspring.
It can occur between organisms of the same species or organisms of different species.
This process can increase genetic variation in a species that picks up the "new" genes.
Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control that trait.
Single-Gene Traits:
These are traits controlled by a single gene.
Example: Dark bands on the shells of certain snail species.
In this snail population, the allele for shells without bands is dominant, yet the population may show a greater frequency of the "with bands" phenotype.
Polygenic Traits:
These are traits controlled by two or more genes.
Example: Human height, which varies from very short to very tall.
Graphical Representation: The distribution of polygenic traits typically results in a bell-shaped curve.