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: • When a population is small, chance fluctuations can cause changes in allele frequencies • When mating is nonrandom, individuals preferred as mates will pass on their alleles in greater numbers then those who are not preferred • 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 • When individuals migrate they remove their alleles from on population and add them to the other • 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 drift • Example: 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 effect • Occurs when a severe event drastically reduces the number of individuals in a population, resulting in significant genetic drift
Founder effect • Results when a few individuals from a large population leave to establish a new population (genetic drift) • Allele frequencies will be different from parent population • This 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 1720 • One member had a rare recessive allele causing short limbs • Today 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 Galapagos • They observed a population of finches that would visit the island every year • One year they witnessed 3 males and 2 females remaining on the island to breed • They produced 17 young birds which became the founders of the new population on the island • They have remained ever since and upon further investigation this population is now genetically different from the original population
Gene flow • The movement of alleles from one population to another==migration • Example: prairie dogs live in large populations that do not allow new members in • However in late summer, male pups are permitted into adjacent populations, affecting the gene pool • Genetic information can also be shared if the individuals don’t move permanently, instead only breeding and leaving • This 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 pool • What effects can mutations have? • How frequently do they occur?
Mutation • They can be neutral, harmful or beneficial • Because they are random changes to the genetic code they are more likely to be neutral or harmful • Neutral mutation: one that has no immediate effect on an individual’s fitness, usually silent in the non-coding portion of the DNA • Note: 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 mitosis • Beneficial 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 evolution • Point mutations: a single change in a DNA base pair • They are neutral when occurring in the non-coding regions • If it occurs in the coding portion, it could be lethal or may have no significant effect. Rarely will it will result in a beneficial mutation • Insertions/deletions: these almost always produce non-functioning genes when in coding areas==harmful • Do not play an important role in evolution since they are usually never beneficial
Types of mutations • Gene duplication: leads to the production of an extra copy of a gene locus, usually the result of unequal crossing over during meiosis • Important because it is the source of new genes • Initially 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 bacteria • 1/gamete in species with large genomes • However usually result in unobservable traits==death of the gamete before birth
Reference Pgs 550-554 Have a Good WEEKEND!