Selection of Phenotypes and Breeding

Consider the work of a professional dog breeder. Lets say the person is trying to obtain a breed that is best suited for hunting of water fowl. This work can only succeed if:

• phenotypic heritability in morphological and behavioral features is coded by <genes> (e.g. webbed toes, willingness to swim at least partially controlled by genes)
• phenotypic variability is present (some have it, some don't)
• differential survival/reproductive success of phenotypic variants is produced (you pick the ones who do)

Video Clip: Directional selection for short-stemmed dandelions. Only those that escape the mower will reproduce. Click on the image for a download.© 2000 lobsterman

Animal breeding is a slow process, however, a combination of strong selection and a high degree of heritability can change the relative abundance of genes in a population up to 10 % per generation.

Selection for new breeds may be.

• stabilizing (in a constant environment): e.g. birthweights of human babies
• directional (improving adaptation in a direction appropriate to environmental changes): e.g. short-stemmed dandelions
• disruptive (select for extremes): differentiation of gametes

Unintended Selection

• Antibiotic resistance

Natural Selection

• More offspring are produced than than finite resources can support
• individuals face a constant struggle for existence
• individuals in a population vary in their phenotypes
• some of this variation is heritable
• successful variants are most likely to survive and reproduce
• their genotypes will be relatively overrepresented in the next generation

Density-independent and density-dependent growth models: Exponential Model: a species can potentially increase in numbers according to a geometric series -- Thomas Robert Malthus (1766-1834) Logistic Model: the rate of population increase may be limited, i.e. it may depend on population density -- Pierre Verhulst (1838). <Carrying Capacity> (K): an environment's maximum persistently supportable load (Catton 1986).

<Natural selection> is the process by which environmental effects lead to varying degrees of reproductive success among individuals of a population of organisms with different hereditary characters, or traits. The characters that inhibit reproductive success decrease in frequency from generation to generation. It is the process whereby certain genes (alleles) gain greater representation in the following generations compared to other alleles. <Adaptations>: the complement of traits that increases the fitness of the owner. An individual's <Fitness> or Reproductive Success is the relative probability that an animal of a particular genotype and phenotype will manage to contribute its genes to the next generation

Aside from Natural Selection, changes in gene frequencies within a population may also arise from a variety of other sources including:

• Mutations: changes to individual nucleotide bases along the DNA to largescale rearrangements of chromosomes
• Gene flow: Emmigration, Immigration
• Genetic drift: Changes in the genetics of the population as a result of chance. Bottlenecks and Founder Effect: Ellis-van Creveld Syndrome is an autosomal recessive skeletal dysplasia that results in short-limbed disproportionate dwarfism. The syndrome most commonly occurs in the Amish population of Lancaster, Pa. The incidence is approximately 5 per 1000 live births and 2 per 1000 living persons. No case had been described in the Amish of Ohio, Indiana, or other Amish areas at the time of an extensive search performed by McKusick, et al. The genealogy of the disorder in the Amish of Lancaster, Pa. can be traced back to the immigrants Samuel King and his wife

Example: H.B.D. Kettlewell's work on Industrial Melanism

Reading Assignment

• Wikibooks: Animal Behavior - Chapter 4

Food for Thought

How could nature ever select for something as unpractical as a peacock's tail or a moose's antlers?

Links of Interest

• Without Miracles - Universal Selection Theory and the Second Darwinian Revolution
• The History of the Black Death and Alliance for the Prudent Use of Antibiotics