EVOLUTION Other Mechanisms of Evolutionary Change

Evolution Basics Reminders  Evolution = The change in the genetic composition of a population over time  Natural Selection = differential survival of individuals with favorable genotypes based on environmental conditions

Mechanisms of Evolution I  Differential reproduction +/or survival of genotypes (i.e. natural selection as opposed to artificial selection practiced by plant breeders)  Some genotypes cause individuals to die prior to reproduction or not produce viable offspring (Muscular dystrophy, Tay Sachs)  Other genotypes might cause an individual to withstand an infection better, survive & reproduce (Sickle cell disorder in malarial infested areas)

Mechanisms of Evolution II  Mutation: creates genetic variation but does not change gene frequencies very fast  Mutations of a particular gene occur on the order of 1 x or -5  Hb C (Hemoglobin C) in Central Africa  One base change in Hemoglobin DNA  6 th amino acid in polypeptide changes from hydrophyllic to hydrophobic  protein folds differently and changes shape  change shape and function of red blood cells

Figure Mapping malaria and the sickle-cell allele Heterozygote for Hb C is resistant to infection by Plasmodium falciparum – red blood cells don’t support its growth

Mechanisms of Evolution III  Migration: can cause an influx of a new gene that might spread throughout the population because it is novel or gives a selective advantage  Movement of alleles from one population to another is called gene flow  Generally increases genetic diversity of receiving population

Gene Flow

Mechanisms of Evolution IV  Random Genetic Drift: due to random accumulation of genetic change because of small population size, inbreeding, genetic bottlenecks  Small populations are more likely to be effected by chance  Due to chance (who reproduces and who does not) some alleles may increase, others may decrease, some may even be eliminated altogether  That is genetic drift

Figure 23.4 Genetic drift

Bottleneck effect

 Population bottlenecks occur when a population’s size is reduced for at least one generation.  Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a lot.  Even if the bottleneck doesn’t last for very many generations.  This is illustrated by the bags of marbles shown above, where, in generation 2, an unusually small draw creates a bottleneck.

Founder Effect  A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have:  reduced genetic variation from the original population.  a non-random sample of the genes in the original population.  For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency.  This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

So why does this all matter 1. Because it changes gene frequency 2. The population changes genetic diversity – if less then can be maladaptive in the face of environmental challenges 3. Lethal homozygous recessive traits may be more common