Unit 6: Genetics and Evolution: Population Genetics

Population genetics

discipline that focuses on the analysis of the main forces of evolution

MIcroevolution

explains how populations change from one generation to another

Macroevolution

describes how new species are created.

Speciation

the formation of new species through the dividing of a single lineage into two or several genetically independent lineages

Species

refer to a number of related organisms that share common traits and are capable of interbreeding.

Biological species concept

refers to a group of organisms able to interbreed and that is reproductively isolated (their members are not capable of interbreeding with organism outside the group)

Reproductive isolation

a biological property of organisms belonging to the same species that prevent them from interbreeding with organisms from other species.

Ecological species concept

The view that emphasizes the role of natural selection

Genotype

refers to the genetic constitution of an organism. In other words, the genotype refers to the genetic makeup; the total number of genes that an individual carries.

Phenotype

refers to the observable physical appearance of an organism

Alleles

refer to the alternate forms of the same genes.

Homozygous

When an individual carries two of the same kinds of alleles

Heterozygous

when an individual carries copies of two different alleles

Dominant

individuals with only one A allele have the same phenotype, yellow peas, that individuals with two A alleles.

Recessive

when it has no effect on phenotype in heterozygotes. In the previous example, the a allele is recessive because it does not show on phenotype in heterozygotes (that is individuals with only one a allele).

Polygenic

they are affected by two or more genes.

Pleiotropic genes

there are some genes that have multiple effects, i.e. they affect more that one physical trait

Gene pool

genetic material of a population

Hardy-Weinberg law of equilibrium

According to this principle, in the absence of evolution, the frequencies of the genotypes will remain stable because no evolutionary change takes place

Mutation

refers to the chance alteration of genetic material producing a new variation.

Spontaneous mutations

have an unknown cause and they are related to a variety of sources, including DNA copying errors, spontaneous lesions, and transposable genetic elements.

Transposable elements

are mobile segments of DNA that copy themselves into new areas of the chromosomes. Most spontaneous mutations are the result of mistakes in DNA replication that involve a base substitution (that is, they involve incorrect base pairing).

Point mutations

mistakes in DNA replication that involve a base substitution (that is, they involve incorrect base pairing).

2 ways point mutations occur

1) A base substitution in which a nitrogen base is subsisted by another base

2) A base insertion in which a nitrogen base is incorrectly inserted into the DNA template.

Synonymous point mutations

in which the substituted nitrogen base creates an altered triplet in the DNA but the alteration does not change the encoded amino acid (and, therefore, the protein created is the same)

Nonsynonymous point mutations

in which a mutation results in a different amino acid and, therefore, in a different protein. A mutation of this kind is at the heart of the sickle-cell disease or sickle-cell anemia.

Induced mutations

are related to environmental agents called mutagens, such as X-rays or UV rays and chemical agents such as mustard gas.

Stabilizing selection

a kind of natural selection that favours the average version of a trait.

Disruptive selection

in which organisms at both extreme ends of a trait’s range have a better chance of surviving and reproducing that do individuals possessing the average version of a trait. 

Genetic drift

refers to changes in gene frequency caused by random fluctuations. 

Founder effect

occurs when a population splits up into two or more small groups, one of which is small and is reproductively isolated.

Gene flow (also called gene migration or admixture)

fundamental factor that brings variation to the gene pool of a population, that is the transfer of genes across populations.

Allopatric speciation

Some isolating mechanisms are the geographic or environmental barriers that isolate members of a population and, throughout time, favour the appearance of new species.

Parapatric selection

holds that selection alone is not enough to produce a new species, but new species can be formed if selection is combined with partial genetic information. 

Sympatric speciation

suggests that strong selection favouring different phenotypes can lead to speciation even when there is no geographic separation and, therefore, initially there is extensive gene flow among individuals in the population.

Darwinian gradualism

Following Darwin’s theory, most scientists originally considered that speciation happened as a slow rate

Punctuated equilibria

a model according to which evolution is a process that conjugates long periods of stability punctuated by periods of rapid change.