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Microevolution refers to the small-scale changes that occur within a species over a short amount time. These changes are usually a result of processes like mutation, selection, gene flow, and genetic drift. Microevolution can lead to slight changes in traits, but the organisms remain the same species.

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The phenotypic (physical or outward characteristics) differences that you observed among the meerkats (fur color, size, height) are due to differences in genes that code for those traits. Certain differences may offer a competitive advantage compared to the rest of the population. A particular phenotype may allow individuals to survive longer and reproduce more efficiently, both of which increase the total number of offspring produced. So, over time the phenotype becomes more prevalent.

This gradual favoring of advantageous traits within a population is called natural selection, and it directly affects the population's gene pool. A gene pool is the collection of alleles found in all the individuals of a population.

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You can use the total number of alleles, the number of dominant alleles, and the number of recessive alleles to find the allele frequency in a population. Allele frequency is the proportion of one allele, compared with all the alleles for that trait, in the gene pool. To find the frequency of a particular allele, divide the number of times the allele is present by the total number of alleles in the population.

Allele frequency can also be expressed as a percentage by multiplying the frequency by 100. The frequencies of all the different alleles in a population should equal 1.0, or 100 percent.

Allele frequency is used to track genetic variation in populations and detect changes in alleles. Imagine that periodic fires blacken the ground in the habitat shown in the Field Mice Alleles diagram. The black mice may be better camouflaged, providing more protection against predators. If they survive and reproduce more effectively than brown mice, the frequency of the b allele may increase over time relative to the B allele frequency.

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Different combinations of alleles in a gene pool can be formed when organisms mate and have offspring. Alleles are different forms, or versions, of genes. For example, mice with either one or two copies of the dominant B allele have brown fur, while mice with two recessive b alleles have black fur.

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If penguin beak lengths and their frequencies are graphed, the result is a bell-shaped curve like the one shown in the Normal Distribution graph. The shape of the curve shows that the beak length of the majority of the individuals is close to the mean length. Mean (also called average) beak length is determined by adding the beak lengths of all the individuals and then dividing the sum by the number of individuals. The graph also shows that there are not many individuals with extreme traits (very short or very long beaks).

A normal distribution shows an arrangement of data in which most of the values fall in the middle of the data set, represented by the mean. The curve that results is bell-shaped and symmetrical. The frequency is highest near the mean value and decreases toward each extreme end of the range. This means that for a population showing normal distribution, the alleles for the mean phenotype are more advantageous than the alleles associated with either extreme phenotype.

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In stabilizing selection, average traits are favored over extreme ones. For example, human birth weight - babies that are too small or too large have lower survival rates, while average-sized babies tend to be healthiest.

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When one extreme trait is favored over others, that's directional selection. Remember those photos of peppered moths during the Industrial Revolution? As tree trunks got darker from pollution, darker moths had better survival rates than lighter ones.

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In disruptive selection, both extreme phenotypes are favored, while intermediate forms are selected against. Sometimes both extremes of a trait are better than the middle! This is disruptive selection. Think about African seed crackers (birds) - those with very small or very large beaks survive better than birds with medium-sized beaks because they can eat different types of seeds.

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Gene flow is physical movement of alleles from one population to another.

When organisms move between populations and bring their genes with them, that's gene flow! This helps increase genetic diversity!

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Genetic drift is a change in allele frequencies due to chance alone, occurring most commonly in small populations.

Sometimes changes in gene frequencies happen by chance - that's genetic drift. In small populations, these random changes can have a big effect on which traits become common or rare.

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When a population suddenly gets much smaller (like after a natural disaster), many genes might be lost - that's the bottleneck effect.

The northern elephant seal suffered from the bottleneck effect. This is genetic drift resulting from an event that drastically reduces the size of a population. Through genetic drift, some alleles can be completely lost from the gene pool and others can be fixed in the population, resulting in lower genetic diversity.

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When a small group starts a new population somewhere else, they only bring a sample of genes from their original population. This is the founder effect.

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Sexual selection is selection in which certain traits enhance mating success; traits are therefore passed on to offspring.

Ever wonder why peacocks have those amazing tail feathers? That's sexual selection at work. When animals choose mates based on certain traits, those traits become more common in the population.

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Natural selection is the process by which organisms that are better adapted to their environment survive and reproduce more successfully than others.