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Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life — but exactly how does it work?

Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:

  • Descent and the genetic differences that are heritable and passed on to the next generation;
  • Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change;
  • The importance of genetic variation;
  • The random nature of genetic drift and the effects of a reduction in genetic variation;
  • How variation, differential reproduction, and heredity result in evolution by natural selection; and
  • How different species can affect each other’s evolution through coevolution.

The components of natural selection: variation, differential reproduction, and heredity

Descent with modification

We’ve defined evolution as descent with modification from a common ancestor, but exactly what has been modified? Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are heritable and can be passed on to the next generation — which is what really matters in evolution: long term change.

Compare these two examples of change in beetle populations. Which one is an example of evolution?

1. Beetles on a diet
Imagine a year or two of drought in which there are few plants that these beetles can eat.
First generation of starving beetles
All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in the population are a little smaller than the preceding generation of beetles. Second generation of starving beetles
2. Beetles of a different color
Most of the beetles in the population (say 90%) have the genes for bright green coloration and a few of them (10%) have a gene that makes them more brown.
First Generation
Some number of generations later, things have changed: brown beetles are more common than they used to be and make up 70% of the population. Second Generation

Which example illustrates descent with modification — a change in gene frequ

ency over time?

The difference in weight in example 1 came about because of environmental influences — the low food supply — not because of a change in the frequency of genes. Therefore, example 1 is not evolution. Because the small body size in this population was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply.

The changing color in example 2 is definitely evolution: these two generations of the same population are genetically different. But how did it happen?

Mechanisms of change

Each of these four processes is a basic mechanism of evolutionary change.

A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make genes for brown coloration more frequent in the population than they were before the mutation.
Some individuals from a population of brown beetles might have joined a population of green beetles. That would make genes for brown coloration more frequent in the green beetle population than they were before the brown beetles migrated into it.
Genetic drift
Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation — but just by chance. These chance changes from generation to generation are known as genetic drift.
Genetic drift
Natural selection
Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration on to their offspring. So in the next generation, brown beetles are more common than in the previous generation.
Natural Selection

Download this series of graphics from the Image library.

All of these mechanisms can cause changes in the frequencies of genes in populations, and so all of them are mechanisms of evolutionary change. However, natural selection and genetic drift cannot operate unless there is genetic variation — that is, unless some individuals are genetically different from others. If the population of beetles were 100% green, selection and drift would not have any effect because their genetic make-up could not change.

So, what are the sources of genetic variation?


Genetic variation

Without genetic variation, some of the basic mechanisms of evolutionary change cannot operate.

There are three primary sources of genetic variation, which we will learn more about:

  1. Mutations are changes in the DNA. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.
  2. Gene flow is any movement of genes from one population to another and is an important source of genetic variation.
  3. Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation.
Genetic shuffling
Genetic shuffling is a source of variation.



Mutation is a change in DNA, the hereditary material of life. An organism’s DNA affects how it looks, how it behaves, and its physiology — all aspects of its life. So a change in an organism’s DNA can cause changes in all aspects of its life.

Mutations are random
Mutations can be beneficial, neutral, or harmful for the organism, but mutations do not “try” to supply what the organism “needs.” In this respect, mutations are random — whether a particular mutation happens or not is unrelated to how useful that mutation would be.

Not all mutations matter to evolution
Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, not all mutations matter for evolution. Somatic mutations occur in non-reproductive cells and won’t be passed onto offspring.

For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. The seeds of this apple do not carry the mutation.

The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations.

A single germ line mutation can have a range of effects:

  1. No change occurs in phenotype
    Some mutations don’t have any noticeable effect on the phenotype of an organism. This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but ends up not affecting the amino acid sequence of the protein.
  2. Small change occurs in phenotype
    Cat with curled-ear mutation

    A single mutation caused this cat’s ears to curl backwards slightly.

  3. Big change occurs in phenotype
    Some really important phenotypic changes, like DDT resistance in insects are sometimes caused by single mutations. A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals — and it doesn’t get more negative than that.

There are some sorts of changes that a single mutation, or even a lot of mutations, could not cause. Neither mutations nor wishful thinking will make pigs have wings; only pop culture could have created Teenage Mutant Ninja Turtles — mutations could not have done it.

The causes of mutations

Mutations happen for several reasons.

  1. DNA fails to copy accurately
    Most of the mutations that we think matter to evolution are “naturally-occurring.” For example, when a cell divides, it makes a copy of its DNA — and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation.

    Following cell division, the copied DNA is imperfect

    To download this image, right-click (Windows) or control-click (Mac) on the image and select “Save image.”
  2. External influences can create mutations
    Radioactive signMutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural — even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutation.

    Gene flow

    Gene flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another.

    Gene flow in beetle populations

    Sex and genetic shuffling

    Shuffling of
	  gene combinations

    Sex can introduce new gene combinations into a population and is an important source of genetic variation.You probably know from experience that siblings are not genetically identical to their parents or to each other (except, of course, for identical twins). That’s because when organisms reproduce sexually, some genetic “shuffling” occurs, bringing together new combinations of genes. For example, you might have bushy eyebrows and a big nose since your mom had genes associated with bushy eyebrows and your dad had genes associated with a big nose. These combinations can be good, bad, or neutral. If your spouse is wild about the bushy eyebrows/big nose combination, you were lucky and hit on a winning combination!

    This shuffling is important for evolution because it can introduce new combinations of genes every generation. However, it can also break up “good” combinations of genes.


    Development is the process through which an embryo becomes an adult organism and eventually dies. Through development, an organism’s genotype is expressed as a phenotype, exposing genes to the action of natural selection.

    Studies of development are important to evolutionary biology for several reasons:

    Explaining major evolutionary change
    Changes in the genes controlling development can have major effects on the morphology of the adult organism. Because these effects are so significant, scientists suspect that changes in developmental genes have helped bring about large-scale evolutionary transformations. Developmental changes may help explain, for example, how some hoofed mammals evolved into ocean-dwellers, how water plants invaded the land, and how small, armored invertebrates evolved wings.

    Mutated fly with two pairs of wings Mutated fly with legs instead of antennae
    Mutations in the genes that control fruit fly development can cause major morphology changes, such as two pairs of wings instead of one. Another developmental gene mutation can cause fruit flies to have legs where the antennae normally are, as shown in the fly on the right.

    Learning about evolutionary history
    An organism’s development may contain clues about its history that biologists can use to build evolutionary trees.

    Developmental stages of embryos
    Characters displayed by embryos such as these may help untangle patterns of relationship among the lineages.

    Limiting evolutionary change
    Developmental processes may constrain evolution, preventing certain characters from evolving in certain lineages. For example, development may help explain why there are no truly six-fingered tetrapods.

    Artificial selection

    Long before Darwin and Wallace, farmers and breeders were using the idea of selection to cause major changes in the features of their plants and animals over the course of decades. Farmers and breeders allowed only the plants and animals with desirable characteristics to reproduce, causing the evolution of farm stock. This process is called artificial selection because people (instead of nature) select which organisms get to reproduce.

    As shown below, farmers have cultivated numerous popular crops from the wild mustard, by artificially selecting for certain attributes.

    Artificial selection in the mustard family

    These common vegetables were cultivated from forms of wild mustard. This is evolution through artificial selection.

    Misconceptions about natural selection

    Because natural selection can produce amazing adaptations, it’s tempting to think of it as an all-powerful force, urging organisms on, constantly pushing them in the direction of progress — but this is not what natural selection is like at all.

    First, natural selection is not all-powerful; it does not produce perfection. If your genes are “good enough,” you’ll get some offspring into the next generation — you don’t have to be perfect. This should be pretty clear just by looking at the populations around us: people may have genes for genetic diseases, plants may not have the genes to survive a drought, a predator may not be quite fast enough to catch her prey every time she is hungry. No population or organism is perfectly adapted.

    Second, it’s more accurate to think of natural selection as a process rather than as a guiding hand. Natural selection is the simple result of variation, differential reproduction, and heredity — it is mindless and mechanistic. It has no goals; it’s not striving to produce “progress” or a balanced ecosystem.

    Formula for natural selection

    Evolution does not work this way
    Evolution does not work this way.

    This is why “need,” “try,” and “want” are not very accurate words when it comes to explaining evolution. The population or individual does not “want” or “try” to evolve, and natural selection cannot try to supply what an organism “needs.” Natural selection just selects among whatever variations exist in the population. The result is evolution.

    At the opposite end of the scale, natural selection is sometimes interpreted as a random process. This is also a misconception. The genetic variation that occurs in a population because of mutation is random — but selection acts on that variation in a very non-random way: genetic variants that aid survival and reproduction are much more likely to become common than variants that don’t. Natural selection is NOT random!


Motility in Alimentary Canal Food is moved through digestive tracts by cilia or by specialized musculature, and often by both. Movement is usually by cilia in acoelomate and pseudocoelomate metazoa that lack the mesodermally derived gut musculature of true coelomates. Cilia move intestinal fl uids and materials also in some eucoelomates, such as most molluscs, in which the coelom is weakly developed. In animals with well-developed coeloms, the gut is usually lined with two opposing layers of smooth muscle: a longitudinal layer, in which smooth muscle fibers run parallel with the length of the gut, and a circular layer, in which muscle fi bers embrace the circumference of the gut . A characteristic gut movement is   segmentation,   the alternate constriction of rings of smooth muscle of the intestine that constantly divide and squeeze contents back and forth …

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During meiosis II, the chromosomes condense and become attached to a new spindle apparatus  (prophase II).  They then move to positions in the  equatorial plane of the cell  (metaphase II),  and their centromeres split to allow the constituent sister chromatids to move to opposite poles  (anaphase II),  a phenomenon called  chromatid disjunction. 
During  telophase II,  the separated chromatids—now called chromosomes—gather at the poles and daughter nuclei form around them. Each daughter nucleus contains  a  haploid set of chromosomes. Mechanistically, meiosis II is therefore much like mitosis. However, its products are haploid, and unlike the products of mitosis, the cells that emerge from meiosis II are not genetically identical. Pair of homologous chromosomes Homologue 1 Centromeres One chromatid Chiasma Two chiasmata Homologue 2 Synapsis and crossing over Tetrad Recombinant chromatids
 One reason these cells differ is that homologous chromosomes pair and disjoin from each other during meiosis I. Within each pair of chromosomes, one homologue was inherited from the organism’s mother, and the other was inherited from its father. During meiosis I, the maternally and paternally inherited homologues come together and synapse. They are positioned on the meiotic spindle and become oriented randomly with respect to the spindle’s poles. Then they disjoin. For each pair of chromosomes, half the daughter cells produced by the first meiotic division receive the maternally inherited homologue, and the other half receive the paternally inherited homologue. Thus, from the end of the first meiotic division, the products of meiosis are destined to be different. These differences are compounded by the number of chromosome pairs that disjoin during meiosis I. Each of the pairs disjoins independently. Thus, if there are 23 pairs of chromosomes, as there are in humans, meiosis I can produce 223  chromosomally different daughter cells—that is, more than 8 million possibilities. To test your understanding of this concept go to Solve It: How Many Chromosome Combinations in Sperm?


Let’s consider the general features of RNA. Although both RNA and DNA are nucleic acids, RNA differs from DNA in several important ways: 

1. RNA is usually a single-stranded nucleotide chain, not a double helix like DNA. A consequence is that RNA is more flexible and can form a much greater variety of complex three-dimensional molecular shapes than can double-stranded DNA. An RNA strand can bend in such a way that some of its own bases pair with each other. Such intranwkcular base pairing is an important determinate of RNA shape. 

2. RNA has ribose sugar in its nucleotides, rather than the deoxyribose found in DNA. As the names suggest, the two sugars differ in the presence or absence of just one oxygen atom. The RNA sugar contains a hydroxyl group (OH) bound to the 2′-carbon atom, whereas the DNA sugar has only a hydrogen atom bound to the 2′-carbon atom. As you will see later in this chapter, the presence of the hydroxyl group at the 2′-carbon atom facilitates the action of RNA in many important cellular processes. Like an individual DNA strand, a strand of RNA is formed of a sugar-phosphate backbone, with a base covalently linked at the 1′ position on each ribose. The sugar-phosphate linkages are made at the 5′ and 3′ positions of the sugar, just as in DNA; so an RNA chain will have a 5′ end and a 3′ end. 

3. RNA nucleotides (called ribonucleotide,) contain the bases adenine, guanine, and cytosine, but the pyrimidine base uracil (abbreviated U) is present instead of thymine. Uracil forms hydrogen bonds with adenine just as thymine does. In addition, uracil is capable of base pairing with G. The bases U and G form base pairs only during RNA folding and not during transcription. The two hydrogen bonds that can form between U and G are weaker than the two that form between U and A. The ability of U to pair with both A and G is a major reason why RNA can form extensive and complicated structures, many of which are important in biological processes.

4. RNA—like protein, but unlike DNA—can catalyze biological reactions. The name ribozyme was coined for the RNA molecules that function like protein enzymes. 


Changes in chromosome structure, called rearrangements, encompass several major classes of events. A chromosome segment can be lost, constituting a dele-tion, or doubled, to form a duplication. The orientation of a segment within the chromosome can be reversed, constituting an inversion. Or a segment can be moved to a different chromosome, constituting a translocation. DNA breakage is a major cause of each of these events. Both DNA strands must break at two differ-ent locations, followed by a rejoining of the broken ends to produce a new chromo-somal arrangement ( Chromosomal rearrangements by breakage can be induced artificially by using ionizing radiation. This kind of radia-don, particularly X rays and gamma rays, is highly energetic and causes numerous double-stranded breaks in DNA. 

To understand how chromosomal rearrangements are produced by breakage, several points should be kept in mind: 

1. Each chromosome is a single double-stranded DNA molecule. 

2. The first event in the production of a chromosomal rearrangement is the generation of two or more double-stranded breaks in the chromosomes of a cell (see Figure 16-19, top row at left). 

3. Double-stranded breaks are potentially lethal, unless they are repaired.

4. Repair systems in the cell correct the double-stranded breaks by joining broken ends back together (see Chapter 15 for a detailed discussion of DNA repair). 

5. If the two ends of the same break are rejoined, the original DNA order is restored. If the ends of two different breaks are joined together, however, one result is one or another type of chromosomal rearrangement. 

6. The only chromosomal rearrangements that survive meiosis are those that produce DNA molecules that have one centromere and two telomeres. If a rearrangement produces a chromosome that lacks a centromere, such an acentric chromosome will not be dragged to either pole at anaphase of mitosis or meiosis and will not be incorporated into either progeny nucleus. 
Therefore acentric chromosomes are not inherited. If a rearrangement produces a chromosome with two centromeres (a dicentric), it will often be pulled simultaneously to opposite poles at anaphase, forming an anaphase bridge. Anaphase-bridge chromosomes typically will not be incorporated into either progeny cell. If a chromosome break produces a chromosome lacking a telomere, that chromosome cannot replicate 

Gene regulation. 

Despite their simplicity of form, bacteria have in common with the larger and more complex members of other kingdoms the fundamental task of regulating the expression of their genes. One of the main reasons is that they are nutritional opportunists. Consider how bacteria obtain the many important compounds, such as sugars, amino acids, and nucleotides, needed for metabolism. Bacteria swim in a sea of potential nutrients. They can either acquire the compounds that they need from the environment or synthesize them by enzymatic pathways. Synthesizing the necessary enzymes for these pathways expends energy and cellular resources; so, given the choice, bacteria will take compounds from the environment instead. To be economical, they will synthesize the enzymes necessary to produce these compounds only when there is no other option—in other words, when these com-pounds are unavailable in their local environment. Bacteria have evolved regulatory systems that couple the expression of gene products to sensor systems that detect the relevant compound in a bacterium’s local environment. The regulation of enzymes taking part in sugar metabolism provides an example. Sugar molecules can be oxidized to provide energy or they can be used as building blocks for a great range of organic compounds. However, there are many different types of sugar that bacteria could use, including lactose, glucose, galactose, and xylose. A different import protein is required to allow each of these sugars to enter the cell. Further, a different set of enzymes is required to process each of the sugars. If a cell were to simultaneously synthesize all the enzymes that it might possibly need, the cell would expend much more energy and materials to produce the enzymes than it could ever derive from breaking down prospective carbon sources. The cell has devised mechanisms to shut down (re-press) the transcription of all genes encoding enzymes that are not needed at a given time and to turn on (activate) those genes encoding enzymes that are needed. For example, if only lactose is in the environment, the cell will shut down the transcription of the genes encoding enzymes needed for the import and metabolism of glucose, galactose, xylose, and other sugars. Conversely, the cell will initiate the transcription of the genes encoding enzymes needed for the import and metabolism of lactose. In sum, cells need mechanisms that fulfill two criteria: 

1. They must be able to recognize environmental conditions in which they should activate or repress the transcription of the relevant genes. 

2. They must be able to toggle on or off, like a switch, the transcription of each specific gene or group of genes. Let’s preview the current model for prokaryotic transcriptional regulation and then use a well-understood example—the regulation of the genes in the metabo-lism of the sugar lactose—to examine it in detail. In particular, we will focus on how this regulatory system was dissected with the use of the tools of classical genetics and molecular biology. 


The followings are characteristics of SUBPHYLUM VERTEBRATA.

  1.  Chief diagnostic features of chordates—notochord, dorsal tubular nerve cord, pharyngeal pouches, endostyle  or thyroid gland,  and  postanal tail—all present at some stage of the life cycle
  2.  Integument  of two divisions, an outer epidermis of stratifi ed epithelium from ectoderm and an inner dermis of connective tissue from mesoderm; many modifi cations of skin, such as glands, scales, feathers, claws, horns, and hair 
  3. Distinctive cartilage or bone  endoskeleton  consisting of vertebral column (except in hagfishes, which lack vertebrae) and a head skeleton (cranium and pharyngeal skeleton) derived largely from  neural crest cells Muscular pharynx;  
  4. In fishes pharyngeal pouches open to the outside as slits and bear gills; in tetrapods pharyngeal pouches are sources of several glands Complex,  
  5. W-shaped muscle segments or  myomeres  to provide movement Complete,  muscularized digestive tract  with distinct liver and pancreas 
  6. Circulatory system consisting of a  ventral heart  of multiple chambers; closed blood vessel system of arteries, veins, and capillaries; blood containing  erythrocytes  with hemoglobin;  paired aortic arches connecting ventral and dorsal aortas and giving off branches to the gills among aquatic vertebrates; in terrestrial forms; aortic arches modifi ed into pulmonary and systemic circuits Well-developed  coelom  divided into a pericardial cavity and a pleuroperitoneal cavity
  7.  Excretory system consisting of  paired, glomerular kidneys provided with ducts to drain waste to the cloaca Highly differentiated  tripartite brain;  
  8. 10 or 12 pairs of cranial nerves;  a pair of spinal nerves for each primitive myotome;  paired special sense organs  derived from epidermal placodes Endocrine system  of ductless glands scattered throughout the body 
  9. Nearly always separate sexes; each sex containing gonads with ducts that discharge their products either into the cloaca or into special openings near the anus 
  10. Most vertebrates with two pairs of appendages supported by limb girdles and appendicular skeleton
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