Evolution Random Change

Evolution In terms of genetics, it is any change in allele frequencies within a population.The H-W, provided conditions that evolution would not occur, thus the following are key for evolution to occur:I. When a population is small, chance fluctuations can cause changes in

allele frequenciesII. When mating is nonrandom, individuals preferred as mates will pass

on their alleles in greater numbers then those who are not preferredIII. When genetic mutations occur, new alleles can be created or old

ones changed into new ones, effectively changing allele frequencies in both new and original alleles

IV. When individuals migrate they remove their alleles from on population and add them to the other

V. When natural selection occurs, individuals with certain alleles have better reproductive success than others, thus increasing the frequency of their alleles in the next generation

Genetic driftExample: 2 % of cricket frogs carry a certain allele, C. If the population was large, say 10, 000, you would expect 200 frogs to carry the allele. If severe weather conditions caused 50% of them to die, then you would expect 100 of 500 surviving frogs to carry the allele. But in this case the species is endangered and there are only 100 frogs. In this case only 2 carry the C allele. If 50% of the frogs died then there would be a good chance that both of those frogs would die (eliminating the C allele forever) or both could survive doubling the frequency of the C allele (2/50 is 4%). This may be an extreme example (much more pronounced in small populations) but it demonstrates genetic drift, which is a change in the genetic makeup of a population due to chance

Genetic drift in stoneflies

Fixation of alleles results in completely homozygous individuals, reducing genetic diversity

Bottleneck effectOccurs when a severe event drastically reduces the number of individuals in a population, resulting in significant genetic drift

Founder effectResults when a few individuals from a large population leave to establish a new population (genetic drift)Allele frequencies will be different from parent populationThis is common in nature, for example-seeds carried away by birds or wind==in self pollinating species an entire population can establish from a single seed

Founder effect examples

Members of the Amish population living in Pennsylvania are descendents of about 30 people from Switzerland who emigrated in 1720One member had a rare recessive allele causing short limbsToday the frequency of that allele is 7% vs. 0.1% in most other Amish populations

Founder effect examples

In 1982, two scientists were working on Daphne Major of the GalapagosThey observed a population of finches that would visit the island every yearOne year they witnessed 3 males and 2 females remaining on the island to breedThey produced 17 young birds which became the founders of the new population on the islandThey have remained ever since and upon further investigation this population is now genetically different from the original population

Gene flowThe movement of alleles from one population to another==migrationExample: prairie dogs live in large populations that do not allow new members inHowever in late summer, male pups are permitted into adjacent populations, affecting the gene poolGenetic information can also be shared if the individuals don’t move permanently, instead only breeding and leavingThis is different from genetic drift, as it tends to reduce genetic differences between populations

Mutation Mutations are the only new source of genetic material and alleles Only concerned with mutations in a gamete since these can be passed on and enter the gene poolWhat effects can mutations have?How frequently do they occur?

Mutation They can be neutral, harmful or beneficialBecause they are random changes to the genetic code they are more likely to be neutral or harmfulNeutral mutation: one that has no immediate effect on an individual’s fitness, usually silent in the non-coding portion of the DNANote: fitness refers to the reproductive success of an individual

Mutation Harmful mutation: reduces fitness, occurring when a cell loses the ability to produce proteins or when chromosomal changes adversely affect meiosis and/or mitosisBeneficial mutation: occurs when a cell gains the ability to produce a new or improved protein, increasing fitness

Types of mutations Different types of mutations vary in their ability to affect phenotypes and thus evolutionPoint mutations: a single change in a DNA base pairThey are neutral when occurring in the non-coding regionsIf it occurs in the coding portion, it could be lethal or may have no significant effect. Rarely will it will result in a beneficial mutationInsertions/deletions: these almost always produce non-functioning genes when in coding areas==harmfulDo not play an important role in evolution since they are usually never beneficial

Types of mutationsGene duplication: leads to the production of an extra copy of a gene locus, usually the result of unequal crossing over during meiosisImportant because it is the source of new genesInitially just a redundancy in the genome, but over time it has the ability to mutate and maybe gain a new function This can result in gene “families” having similar structures, located close together, but have altered functions==histones

How common are mutations?

1/10, 000 in small genomes, such as in bacteria1/gamete in species with large genomesHowever usually result in unobservable traits==death of the gamete before birth

Reference Pgs 550-554

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