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A number of processes affect evolution in populations, some directly by changing the gene frequencies, some indirectly by increasing the variation in the population, which is then affected by the other processes which change the gene frequencies directly.

Processes which Produce/Increase Genetic Variation with Populations

Those processes which do not change gene frequencies in the population are:

  • Crossing-over. The exchange of genetic information between members of a homologous pair of chromosomes at meiosis. Results in new combinations of alleles on chromosomes. 

  • Recombination. Mendel's Law of Independent Assortment. Results in new combinations of chromosomes in every gamete (egg or sperm). 

  • Mating. Random combination of parents for most alleles. Results in new combinations of chromosomes in every child. Every child is a unique combination with 50% of genes coming from each parent. 

Those processes which do change gene frequencies in the population (i.e. they cause evolution directly) are:

  • Mutation. A change in genetic code known as a mutation consists of changes in the hereditary instructions contained within a gene. A mutation, the ultimate source of all genetic variation, must occur in reproductive cells to cause genetic change in offspring. The process of mutation involves random genetic change primarily due to chemical factors and ionizing radiation

  1. Advantageous. Makes a better allele (rare).

  2. Neutral. Does not affect functioning of allele (common).

  3. Disadvantageous. Disrupts functioning of allele (most common). To have significance in the long run, the mutation must occur in an egg or a sperm, and so be inherited by next generation. Mutations are the ultimate source of all new information, and new alleles.

  • Gene Flow (immigration). Gene flow, the movement of genes from one population to another, describes the process of the loss or gain of genes in a population due to the emigration or immigration of fertile individuals, or the transfer of gametes, between populations. This process introduces new genes into a population resulting in populations becoming more similar to one another.

Processes which reduce genetic variation with populations (cause evolution directly)

  • Natural Selection. Natural selection refers to the differential survival and reproduction of organisms as a result of their biological characteristics.

  1. "Garden variety" natural selection - selective reduction in genetic variation, causes changes in gene frequencies through time as individuals carrying alleles not well-adapted to that particular environment reproduce less successfully than individuals carrying alleles well-suited to that particular environment. Reduces variation by reducing or removing all the alleles carried by poorly adapted individuals. Results in population changing to become better and better adapted to their environment, as long as the environment remains the same. If the environment changes, the game begins anew. Differential net Reproductive success is measured by the number of offspring who survive to reproduce. Even small differences in reproductive success can lead to major changes in the gene frequencies over time. [Under certain circumstances, natural selection can lead to an increase in variation within a population, as when an adaptive mutant allele appears and is selected for, but this is the only way natural selection can increase variation within a population.

  2. Artificial Selection. Where natural humans are the agents; the source of all domesticated plants and animals, including such wonders as the modern strawberry and Chihuahuas.

  3. Sexual Selection. Natural selection with female choice of males for mating partners (based on genetically derived traits) as the agents; the source of most sexually dimorphic features such as lions' manes, antlers in deer, canines in baboons, and the like. 

  • Random Genetic Drift. The process of genetic drift, dealing with chance preservation or extinction of particular genes, may be defined as a variation in the gene pool, a change in allele frequency, in a small population due to chance. Random reduction in genetic variation, causes changes in gene frequencies through time as individuals die or fail to reproduce as successfully as others because of non-genetic factors: the operation of chance factors in evolution. Most important in small populations.

  1. Random death not related to genetic makeup -- bad luck.

  2. Not successful at reproduction due to non-genetic factors -- become a nun, priest, don't like kids, radiation makes you infertile, gonorrhea, syphilis, schistosomiasis, and the like,. make you infertile.

  3. Random factors in the production of gametes -- some alleles are not passed on by chance, e.g., if both my children inherit my O rather than my A allele for blood type.

  • Founder's Effect. A small group of people originated the population, did not carry all the variation present in parent population, e.g., group stranded on desert isle, Pitcairn's Island.

These evolutionary processes operate continuously, both individually and in various combinations, to change the gene frequencies in a population over time -- that is, to cause evolution. Evolution occurs by any and all of these processes. Charles Darwin's contribution was in recognizing the important role of natural selection in Evolution, but evolution also occurs through these other processes, which may be as important, or even more important, than natural selection in many instances.


Page last edited: 01/25/06 06:55 PM


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