Genetic Drift, Mutation and Selection
The three major forces of evolution are genetic drift, mutation and selection. These forces cause changes in genotypes and phenotypes over time, and also determine the amount and kind of variation seen in a population.
Genetic Drift
Genetic drift is the random changes in the frequency of alleles (One member of a pair or series of genes that occupy a specific position on a specific chromosome.) in a gene pool, usually of small population. Random genetic drift is a stochastic process (by definition). One aspect of genetic drift is the random nature of transmitting alleles from one generation to the next given that only a fraction of all possible zygotes become mature adults.
In the population of five worms below, each worm gives rise to exactly one worm in the next generation. There are five alleles (skin colors) at generation 0 and the same five alleles at generation 4.
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The model above starts with a diverse population (5 worms, 5 alleles).
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With no diversity in generation 0 and no forces of evolution acting on the population, the model above begins and ends with all worms in the population having the same allele.
In the above examples, the populations of worms are not evolving. Neither the genotypes nor phenotypes are changiing. For the changes to occur, there must be mutation, selection or random genetic drift.
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When genetic drift is introduced into the model above, these are the results.
In generation 2, the pink worm produces 1 offspring, the 3 green worms produced none, and the dark blue worm produced 4.
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In a population model with genetic drift, alleles will eventually become "fixed". When an allele is fixed, all members of the population have that allele. In the model above,the dark blue allele fixed after 4 generations.
Mutations
A mutation is a permanent change in the DNA sequence of a gene.
Example:
A butterfly may produce offspring with a new mutation caused by ultraviolet light from the sun. In most cases, this mutation is not good, since obviously there was no 'purpose' for such change at the molecular level. However, sometimes a mutation may change the butterfly's color, making it harder for predators to see it; this is an advantage and the chances of this butterfly surviving and producing its own offspring are a little better, and over time the number of butterflies with this mutation may form a large percentage of the species.
A mutation event is how the allele sequence changes.
Two things must happen:
1)A change in the molecular structure of DNA
2)Failure of editing enzymes to correct the change; copying into new DNA
Mutant strain: A population of descendents of the individual in which the original mutation event occurred. The “mutation” is now inherited by the regular reassortment and recombination mechansisms, as are other alleles.
Rate of mutation: How often a given map position mutates. In practice, this is hard to measure.
Frequency of mutation: What percent of alleles contain a sequence defined to be “mutant,” in a given population at a given point in time. This is easy to measure.
Mutation events are rare. How to detect them?
1)Observation of large numbers of progeny.
2)Positive selection. For traits which confer survival advantage: Subject them
to the selective environment. Example: plate bacteria on agar containing an
antibiotic.
3)Negative selection. For traits which prevent survival, under a given
condition. Example: Replica plate bacteria colonies on agar lacking a
nutrient which the “wild type” strain can make with its own enzymes.
Note that different types of mutations have different effects on the gene product and on the resulting phenotype as shown in the table below.
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Natural Selection
Natural selecion is the process in nature by which, according to Darwin's theory of evolution, only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics in increasing numbers to succeeding generations while those less adapted tend to be eliminated.
Darwin's grand idea of evolution by natural selection is relatively simple but often misunderstood. To find out how it works, imagine a population of beetles:
There is variation in traits.
For example, some beetles are green and some are brown.
There is differential reproduction.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
End result:
The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
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