1. How Does Natural Selection Change The Genetic Makeup Of A Population Over Time?

Theme 1: What Makes Us Unique?

1.3 The Genetic Basis of Evolution

Evolution is defined equally a change in traits in a population over time. Small changes in the frequencies of specific traits from 1 generation to the next are typically referred to as micro-evolution. Bigger changes- such as one species diverging into two over many, many generations, are typically referred to every bit macro-evolution. In this course nosotros will examine several mechanisms by small changes can happen within populations over generations (micro-evolution) as well every bit look at how these changes accumulate to create the diverse species we come across today (macro-evolution). We will focus on human evolution- agreement how humans relate to other species and what this ways nigh our characteristics, also as how humans continue to evolve. In add-on to this text, your lab manual provides a basic overview of mechanisms of evolution with examples.

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Evolution is the Source of New Species

All species of living organisms evolved at some point from a common ancestor. Although it may seem that living things today stay much the same from generation to generation, that is not the case: development is ongoing. Evolution is the process through which the characteristics of species change and through which new species ascend.

The theory of evolution is a unifying theory of biology, significant information technology is a framework inside which biologists ask questions about the living world. Its power is that it provides management for predictions nearly living things that are borne out in experiment after experiment. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that "nothing makes sense in biology except in the light of evolution." ^[1] He meant that the principle that all life has evolved and diversified from a common ancestor is the foundation from which we empathise all other questions in biological science. This chapter will explain some of the mechanisms for evolutionary modify and the kinds of questions that biologists can and have answered using evolutionary theory.

Natural Selection is a Machinery of Development

The theory of evolution by natural pick describes a mechanism for species change over time. That species change had been suggested and debated well before Darwin. The view that species were static and unchanging was grounded in the writings of Plato, yet there were also ancient Greeks that expressed evolutionary ideas.

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Charles Darwin and Natural Selection

Natural selection as a machinery for evolution was independently conceived of and described by two naturalists, Charles Darwin and Alfred Russell Wallace, in the mid-nineteenth century. Chiefly, each spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the earth onH.M.S. Beagle, visiting Due south America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin's journey, like Wallace's later journeys in the Malay Archipelago, included stops at several isle bondage, the last being the Galápagos Islands (westward of Ecuador). On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species that each had a unique beak shape ( Figure ane ). He observed both that these finches closely resembled some other finch species on the mainland of South America and that the grouping of species in the Galápagos formed a graded series of beak sizes and shapes, with very small differences betwixt the nearly similar. Darwin imagined that the isle species might be all species modified from one original mainland species. In 1860, he wrote, "Seeing this gradation and diversity of structure in 1 modest, intimately related group of birds, ane might actually fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.^[2]

Illustration shows four different species of finch from the Galápagos Islands. Beak shape ranges from broad and thick to narrow and thin. — **Figure 1.** Darwin observed that beak shape varies amongst finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire dissimilar nutrient sources. This analogy shows the beak shapes for four species of footing finch: one. Geospiza magnirostris (the large ground finch), 2. G. fortis (the medium ground finch), 3. G. parvula (the small tree finch), and four. Certhidea olivacea (the green-warbler finch).

Wallace and Darwin both observed similar patterns in other organisms and independently conceived a mechanism to explain how and why such changes could accept place. Darwin called this mechanism natural selection.Natural pick, Darwin argued, was an inevitable outcome of three principles that operated in nature. Commencement, the characteristics of organisms are inherited, or passed from parent to offspring. Second, more offspring are produced than are able to survive; in other words, resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, in that location is a contest for those resources in each generation. Both Darwin and Wallace'southward understanding of this principle came from reading an essay by the economist Thomas Malthus, who discussed this principle in relation to human populations. Third, offspring vary amongst each other in regard to their characteristics and those variations are inherited. Out of these three principles, Darwin and Wallace reasoned that offspring with inherited characteristics that allow them to best compete for express resource will survive and take more offspring than those individuals with variations that are less able to compete. Considering characteristics are inherited, these traits will exist improve represented in the next generation. This will lead to change in populations over generations in a process that Darwin called "descent with modification."

Papers past Darwin and Wallace ( Figure 2 ) presenting the idea of natural option were read together in 1858 before the Linnaean Society in London. The following year Darwin'southward book, On the Origin of Species,was published, which outlined in considerable detail his arguments for development by natural selection.

Pictures of Charles Darwin and Alfred Wallace are shown. — **Effigy 2. (a)** Charles Darwin and **(b)** Alfred Wallace wrote scientific papers on natural pick that were presented together before the Linnean Society in 1858.

Demonstrations of evolution by natural selection can be time consuming. One of the best demonstrations has been in the very birds that helped to inspire the theory, the Galápagos finches. Peter and Rosemary Grant and their colleagues accept studied Galápagos finch populations every twelvemonth since 1976 and have provided important demonstrations of the operation of natural selection. The Grants plant changes from i generation to the next in the beak shapes of the medium basis finches on the Galápagos isle of Daphne Major. The medium basis finch feeds on seeds. The birds take inherited variation in the nib shape with some individuals having broad, deep bills and others having thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on pocket-sized, soft seeds. During 1977, a drought menses altered vegetation on the island. After this period, the number of seeds declined dramatically: the decline in small, soft seeds was greater than the reject in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year. The year following the drought when the Grants measured beak sizes in the much-reduced population, they establish that the average pecker size was larger ( Figure three ). This was articulate bear witness for natural pick (differences in survival) of neb size caused by the availability of seeds. The Grants had studied the inheritance of nib sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, and then the option would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated option on and evolution of beak size in this species in response to irresolute conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare.

Two graphs show the number of birds on the y axis and bill depth in millimeter on the x axis. The graph on the left has data for the year 1976 with a total of 751 birds measured. The mean beak depth is about 9.5 millimeters. The graph on the right has data for the year 1978, after a drought caused the death of many birds. The total number of surviving birds measured for this data was 90, and the mean beak depth is about 10 millimeters. — **Figure 3.** A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the minor-beaked finches to die. This acquired an increase in the finches' boilerplate beak size between 1976 and 1978.

Variation and Adaptation

Natural selection can only take place if at that place is variation, or differences, amongst individuals in a population. Importantly, these differences must have some genetic basis; otherwise, selection will not lead to modify in the next generation. This is critical because variation amid individuals can be caused by non-genetic reasons, such every bit an individual being taller because of better diet rather than different genes.

Genetic variety in a population comes from ii primary sources: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new genetic variation in any population. An individual that has a mutated factor might take a different trait than other individuals in the population. Withal, this is non always the case. A mutation can have one of three outcomes on the organisms' appearance (or phenotype):

A mutation may affect an organism's traits in a way that gives information technology reduced fettle—lower likelihood of survival, resulting in fewer offspring.
A mutation may produce a trait with a beneficial effect on fettle.
Many mutations, called neutral mutations, will have no result on fitness.

Sexual reproduction can likewise generate novel combinations of traits that may accept positive or negative effects on the survival of offspring. For example, your Deoxyribonucleic acid is organized into 23 pairs of chromosomes– i member of each pair is from your mother, and one from your father. Since you inherit merely half of your mother'due south chromosomes and only half of your father's chromosomes, and the verbal chromosomes yous become from each is determined by adventure, yous are a unique combination of your parents, with traits slightly different from either of parent. This re-combination of Dna at each generation gives sexually reproducing organisms like us some guaranteed variation in our populations.

A heritable trait that aids the survival and reproduction of an organism in its present environment is called an adaptation. An accommodation is a "match" of the organism to the environment. Adaptation to an environs comes about when a alter in the range of genetic variation occurs over fourth dimension that increases or maintains the lucifer of the population with its environs. The variations in finch beaks shifted from generation to generation providing adaptation to food availability.

Whether or non a trait is favorable depends on the environment at the time. The same traits do not ever accept the aforementioned relative benefit or disadvantage considering environmental weather can change. For example, finches with large bills were benefited in one climate, while small bills were a disadvantage; in a different climate, the relationship reversed.

Patterns of Evolution

The evolution of species has resulted in enormous variation in form and function. When two species evolve in different directions from a common signal, it is called divergent evolution. Such divergent evolution tin can be seen in the forms of the reproductive organs of flowering plants, which share the same basic anatomies; however, they can wait very different every bit a event of option in different physical environments, and adaptation to different kinds of pollinators ( Figure four ).

Photo A shows a stalk with several clusters of small purple flowers with long, delicate petals. Photo B shows a daisy-like flower with purple petals and a large central structure with many spikes, resembling a sea urchin. — **Effigy 4.** Flowering plants evolved from a common ancestor. Discover that the (a) dumbo blazing star and (b) royal coneflower vary in appearance, yet both share a similar basic morphology. (credit a, b: modification of piece of work past Cory Zanker)

In other cases, like phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both accept structures nosotros refer to as wings, which are adaptations to flying. The wings of bats and insects, however, evolved from very different original structures. When similar structures arise through development independently in different species it is chosen convergent development. The wings of bats and insects are chosen analogous structures; they are similar in part and appearance, just do non share an origin in a mutual antecedent. Instead they evolved independently in the 2 lineages. The wings of a hummingbird and an ostrich are homologous structures, meaning they share similarities (despite their differences resulting from evolutionary divergence). The wings of hummingbirds and ostriches did non evolve independently in the hummingbird lineage and the ostrich lineage—they descended from a common antecedent with wings.

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The Modern Synthesis

The mechanisms of inheritance, genetics, were not understood at the time Darwin and Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to comprehending many aspects of evolution. Darwin and Wallace were unaware of the genetics piece of work past Austrian monk Gregor Mendel, which was published in 1866, not long after publication of On the Origin of Species. Mendel's work, which described the genetic basis of inheritance, was rediscovered in the early on twentieth century. and integrated in what became known as the modern synthesis—the coherent agreement of the relationship between natural option and genetics. The modern synthesis describes how evolutionary pressures, such as natural selection, can affect a population's genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory likewise connects this gradual change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, chosen macroevolution.

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Population Genetics

Until now, nosotros take defined evolution as a change in the characteristics of a population of organisms, merely backside that change in characteristics is genetic change. In population genetic terms, development is defined equally a change in the frequency of specific gene in a population. Using the ABO claret system as an example, the frequency of the cistron that codes for A claret poly peptide,I ^A, is the number of copies of that cistron divided by the total number of all A, B or O claret protein coding genes in the population. For instance, a study in Jordan institute a frequency ofI ^A to be 26.1 percent.^[3] TheI ^B,I ⁰ claret coding genes made up thirteen.four percent and threescore.five percent of the blood protein coding genes respectively, and all of the frequencies add up to 100 percent. A change in this frequency over time would constitute evolution in the population.

1 manner the frequency of a particular factor in a population can change is natural selection. If the cistron confers a trait that allows an individual to have more than offspring that survive and reproduce, that gene, by virtue of beingness inherited by those offspring, volition be in greater frequency in the next generation. Since gene frequencies ever add upwards to 100 percent, an increment in the frequency of one gene always ways a corresponding decrease in 1 or more of the other genes. Highly beneficial genes may, over a very few generations, get "fixed" in this manner, significant that every private of the population volition bear the cistron. Similarly, detrimental genes may be swiftly eliminated from the gene pool, the sum of all the genes in a population. Part of the study of population genetics is tracking how selective forces change the frequencies of certain genes in a population over fourth dimension, which can give scientists clues regarding the selective forces that may be operating on a given population. The studies of changes in wing coloration in the peppered moth from mottled white to dark in response to soot-covered tree trunks and and then back to mottled white when factories stopped producing so much soot is a classic case of studying evolution in natural populations ( Figure five ).

A graph shows two moths, one light and one dark in color. The population line shifts from the light phenotype on the left to the dark one on the right in response to a darker natural environment. The text next to the graph reads: Light-colored peppered moths are better camouflaged against a pristine environment; likewise, dark-colored peppered moths are better camouflaged against a sooty environment. Thus, as the Industrial Revolution progressed in nineteenth-century England, the color of the moth population shifted from light to dark. — **Figure 5.** As the Industrial Revolution acquired trees to darken from soot, darker colored peppered moths were better inconspicuous than the lighter colored ones, which acquired there to be more than of the darker colored moths in the population.

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Section Summary

Evolution by natural selection arises from three atmospheric condition: individuals within a species vary, some of those variations are heritable, and organisms have more offspring than resource tin can back up. The consequence is that individuals with relatively advantageous variations volition be more likely to survive and have higher reproductive rates than those individuals with different traits. The advantageous traits will be passed on to offspring in greater proportion. Thus, the trait will accept higher representation in the next and subsequent generations leading to genetic change in the population.

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Glossary

adaptation: a heritable trait or behavior in an organism that aids in its survival in its present environment

coordinating structure: a construction that is like because of evolution in response to similar option pressures resulting in convergent evolution, not similar because of descent from a common ancestor

convergent evolution: an development that results in similar forms on different species

divergent evolution: an evolution that results in different forms in 2 species with a common ancestor

factor puddle: all of the alleles carried past all of the individuals in the population

homologous structure

a construction that is similar because of descent from a common ancestor

macroevolution: a broader scale of evolutionary changes seen over paleontological time

microevolution: the changes in a population's genetic structure (i.e., allele frequency)

modern synthesis

the overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today

natural pick: the greater relative survival and reproduction of individuals in a population that take favorable heritable traits, leading to evolutionary change

population genetics: the written report of how selective forces alter the allele frequencies in a population over fourth dimension

variation: the diverseness of traits in a population