What is true of microevolution

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial.

Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring. Natural selection is a driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments. But natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it does not create anything from scratch.

Natural selection is also limited because it works at the level of individuals, not alleles, and some alleles are linked due to their physical proximity in the genome, making them more likely to be passed on together linkage disequilibrium. Any given individual may carry some beneficial alleles and some unfavorable alleles. As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; likewise, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.

Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency due to the fact that going from the less beneficial to the more beneficial trait would require going through a less beneficial phenotype.

Think back to the mice that live at the beach. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of grass. The dark-colored mice may be, overall, more fit than the light-colored mice, and at first glance, one might expect the light-colored mice be selected for a darker coloration.

But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice—they cannot blend in with either the sand or the grass and are more likely to be eaten by predators. As a result, the light-colored mice would not be selected for a dark coloration because those individuals that began moving in that direction began being selected for a darker coat would be less fit than those that stayed light.

Finally, it is important to understand that not all evolution is adaptive. Evolution has no purpose—it is not changing a population into a preconceived ideal. It is simply the sum of the various forces described in this chapter and how they influence the genetic and phenotypic variance of a population. Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution.

Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population.

In contrast, diversifying selection results in increased genetic variance by selecting for two or more distinct phenotypes. Other types of selection include frequency-dependent selection, in which individuals with either common positive frequency-dependent selection or rare negative frequency-dependent selection phenotypes are selected for.

Finally, sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two. The distribution of phenotypes in this litter of kittens illustrates population variation. Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms.

Populations with two or more variations of particular characteristics are called polymorphic. Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against.

Acquired traits, for the most part, are not heritable. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child. Ultimately, heritability tells us how much phenotypic variation in a population is ulimately due to genetic differences as opposed to acquired differences.

The diversity of alleles and genotypes within a population is called genetic variance. This also helps reduce the risks associated with inbreeding , the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances.

The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average.

That is, this would occur if this particular selection pressure , or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure.

By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time when the receptive female walked by or because the other one happened to be in the wrong place at the wrong time when a fox was hunting.

Click for a larger image. Genetic drift in a population can lead to the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele B are dominant over rabbits with the white coat color allele b.

In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of. Only half of the individuals reproduce, resulting in a second generation with p and q values of. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost. Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance.

A chance event or catastrophe can reduce the genetic variability within a population. Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population.

Known as the bottleneck effect , it results in a large portion of the genome suddenly being wiped out Figure 3. In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.

Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind.

In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations.

These are subtle changes that can occur in very short periods of time, and may not be visible to a casual observer. Mathematically, we can determine whether microevolution is occuring by assessing whether a population is in Hardy-Weinberg Equilibrium. The five "forces" that can cause shifts in gene frequency microevolution are:. Microevolution Microevolution is defined as changes in the frequency of a gene in a population.

Gene Flow Migration : when there is mixing of genes from previously isolated populations that have diverged, this can rapidly change gene frequencies in the newly merged population. Mutation : when an advantageous mutation spontaneously arises in an organism, this mutated gene can increase in frequency over generations if it conveys an advantage over those who do not have it.

By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. The Amish population in the U. The population has grown to almost , individuals who rarely interact with people outside the Amish community. One of the founders carried a recessive allele for a rare condition called Ellis-van Creveld syndrome - a type of dwarfism that results in extra fingers and short limbs as seen in this image.

Today the Amish population has far more cases of this syndrome than any other population in the world. Mutation Mutation creates new genetic variation in a gene pool. Gene Flow Gene flow occurs when individuals move into or out of a population. Natural Selection Natural selection occurs when there are differences in fitness among members of a population.

Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is a sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different average heights and body shapes.

Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive, and this keeps birth weight within a relatively narrow range.

Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. Larger beaks were selected for during drought, so beak size increased over time. Natural selection may affect the distribution of a polygenic trait. The top panel shows the disruptive selection in the oyster shell shades.

The lightest and darkest shades are more prevalent. The middle panel shows the stabilizing selection. Most lizards have median-sized tails. The bottom bottom panel shows the directional selection of the giraffe's neck size. Feature: Human Biology in the News Recently reported research may help solve one of the most important and long-lasting mysteries of human biology.

Review Why are populations, rather than individuals, the units of evolution? What is a gene pool? List and define the four forces of evolution. Why is mutation needed for evolution to occur, even though it usually has little effect on allele frequencies? What is the founder effect? Give an example. Identify three types of natural selection for polygenic traits. Explain why genetic drift is most likely to occur in a very small population.

In some species, females prefer to mate with males that have certain genetically determined characteristics, such as bright coloration or a large, showy tail.

How will this alter allele frequency in a population? Which of the following may cause genetic drift? A natural disaster A large population where members mate with each other and also with new migrants that come into the population. An island with no birds that becomes populated by a small number of a species of bird. Both A and C True or False. Allele frequencies can change within an organism. True or False. Most populations on Earth are in Hardy-Weinberg equilibrium.