10.3 - Gene Pools & Speciation

Gene Pool

Gene pool: all of the genes and their different alleles that are present in an interbreeding population

• Species are a group of organisms that can reproduce fertile offspring

Genes can be reproductively or geographically isolated from one another, creating multiple gene pools for one species

Genetic equilibrium: all of the population has an equal chance of contributing to the gene pool

Gene (allele) frequency

• Evolution requires a change in gene frequency

when all genotypes in a population are known, we can count the number of each type of allele

• Allele frequency can be between the values 0-1

  • 1: all members of the population are homozygous, showing the same phenotype

Evolution

Evolution: cumulative change in heritable characteristics of a population over time

• Requires a change in allele frequencies in populations

  • Single gene locus or multiple gene loci

For basic mechanisms of evolutionary change to operate, there must be genetic variation within the population.

Sources of genetic variation:

• Gene flow: movement of genes from one population to another.

• Sexual reproduction: new allele combinations, reshuffling of gamete formation and random fertilization

• Mutations: permanent alteration in the DNA base sequence of a genome, which can accumulate

Selection pressure

Selection pressure: environmental factors that act on certain phenotypes

• Directly impacts natural selection and acts as a driving force for evolution

  • Biotic (living) or abiotic (nonliving) factors reducing chances of survival only allows the fittest to survive, causing a change in the allele frequency after a few generations

Directional vs stabilizing vs disruptive selection

Stabilizing: selection pressures act to remove extreme varieties

Ex: average birth weight in human babies is preferred over low or high birth weight

Disruptive: removes intermediate varieties, favoring the extremes

• Sometimes advantageous to have two opposing varieties of a phenotype rather than one

Directional: selection of one extreme over the other, usually because it is better adapted

Ex: peppered moths

Speciation

Formation of a new species by the dividing of a new population

• Reproductive isolation can be temporal (very similar species but their way of life doesn’t allow reproduction), behavioral, or geographic

• Can be slow or abrupt

Reproductive isolation: inability of two organisms of two different species to mate and produce fertile offspring

Calculating allele frequency

All individuals have at least 2 alleles for each gene

• Total number of alleles is twice that of the number of individuals in the population

• Allele frequency total is 1.0

  • Allele denoting dominant trait is p and recessive is q

p² + 2pq + q²

• Allele frequency stays constant when random mating occurs, population is large, all phenotypes survive equally well

• Populations staying constant are in Hardy-Weinberg principle

Polyploidy & Speciation

Polyploidy can be caused by the total non-disjunction (homologous chromosomes fail to separate) of chromosomes during mitosis or meiosis

• More than normal set of chromosomes

Once an organism becomes a polyploid, they are immediately isolated from the original species