The Mechanisms of Evolution

22The Mechanisms of Evolution

22 The Mechanisms of Evolution

• 22.1 What Facts Form the Base of Our Understanding of Evolution?

• 22.2 What Are the Mechanisms of Evolutionary Change?

• 22.3 What Evolutionary Mechanisms Result in Adaptation?

• 22.4 How Is Genetic Variation Maintained within Populations?

• 22.5 What Are the Constraints on Evolution?

• 22.6 How Have Humans Influenced Evolution?

22.1 What Facts Form the Base of Our Understanding of Evolution?

The young Charles Darwin was passionately interested in geology and natural science.

In 1831, he was recommended for a position on the H.M.S. Beagle, for a 5-year survey voyage around the world.

Figure 22.1 Darwin and the Voyage of the Beagle (Part 1)




Darwin often went ashore to study rocks and collect specimens, and make observations about the natural world.

In the Galapagos Islands he observed that species were similar to, but not the same as, species on the mainland of South America. He also realized that species varied from island to island.


Darwin postulated that species had reached the islands from the mainland, but then had undergone different changes on different islands.

Part of the puzzle was determining what could be a mechanism for such changes.


These observations, and many others, led Darwin to propose an explanatory theory for evolutionary change based on two propositions:

• Species change over time.

• The process that produces the change is natural selection.


Darwin continued to amass evidence to support his ideas until 1858, when he received a letter from another naturalist, Alfred Russel Wallace.

Wallace proposed a theory of natural selection almost identical to Darwin’s.

A paper with the work of both men was presented in 1858 to the Linnean Society of London.


Darwin published his book, The Origin of Species in 1859.

The book provided exhaustive evidence from many different fields to support evolution and natural selection.


Darwin and Wallace were both influenced by economist Thomas Malthus, who published An Essay on the Principle of Population in 1838.

Populations of all species have the potential for rapid increase.

But this does not occur in nature, so death rate must also be high.


Darwin observed that, though offspring tended to resemble their parents, they are not identical.

He suggested that slight variations among individuals affect the chances of surviving and producing offspring: natural selection.


Natural selection:

Differential contribution of offspring to the next generation by various genetic types belonging to the same population.


Darwin had observed variation and artificial selection of certain desirable traits in plants and animals by breeders.

Darwin himself bred pigeons.

Figure 22.2 Many Types of Pigeons Have Been Produced by Artificial Selection


Individuals do not evolve. Populations do.

Population: A group of individuals of the same species that live and interbreed in a particular geographic area.

Members of a population become adapted to the environment in which they live.


Adaptations: The processes by which useful characteristics evolve; and the characteristics themselves.

An organism is considered to be adapted to a particular environment when it can be demonstrated that a slightly different organism survives and reproduces less well in that environment.


For a population to evolve, its members must possess heritable genetic variation.

The phenotype is the physical expression of an organism’s genes.

Features of a phenotype are the characters (e.g., eye color), specific form of a character is a trait (e.g., blue).


A heritable trait is at least partly determined by genes.

Genetic makeup of an organism is the genotype.


Population genetics has three main goals:

• Explain the origin and maintenance of genetic variation

• Explain patterns and organization of genetic variation

• Understand mechanisms that cause changes in allele frequencies


Different forms of a gene are called alleles.

The gene pool is the sum of all copies of all alleles at all loci in a population.

Figure 22.3 A Gene Pool


Populations have genetic variation for many characters.

Artificial selection for different characters in a single species of wild mustard produced many crop plants.

Figure 22.4 Many Vegetables from One Species (Part 1)

Figure 22.4 Many Vegetables from One Species (Part 2)


In laboratory experiments with Drosophila, researchers selected for high or low numbers of body bristles from an initial population with intermediate numbers.

After 35 generations, numbers for both high-bristle and low-bristle fell outside the range of the original population.

Figure 22.5 Artificial Selection Reveals Genetic Variation


Locally interbreeding groups are called Mendelian populations.

Allele frequencies, or their proportion in the gene pool, are estimated by counting alleles in a sample of individuals.


Allele frequency:

If a locus has two alleles, A and a, there could be three genotypes: AA, Aa, and aa. The population is polymorphic at that locus.

population in the alleles of sum

population in the allele theof copies ofnumber p

Calculate genotype & allele frequencies:

• 100 monsters = AA

• 75 monsters = Aa

• 2 monsters = aa

Figure 22.6 Calculating Allele Frequencies


If p is the frequency of allele A, and

q is the frequency of allele a,

p + q = 1

q = 1 – p


Genotype frequencies may not be the same as allele frequencies.

Frequencies of different alleles at each locus and the frequencies of genotypes in a Mendelian population make up the genetic structure of the population.


If certain conditions are met, the genetic structure of a population

does not change over time.does not change over time.

If an allele is not advantageous, its frequency remains constant.

The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies do not change.


Conditions that must be met:

• Mating is random.

• Population size is infinite. Large populations aren’t affected by genetic drift.

• No gene flow—no migration into or out of the population.

• No mutation.

• Natural selection does not affect survival of any genotypes.


If these conditions hold:If these conditions hold:

Allele frequencies remain constant;

after one generation, genotype frequencies occur in these proportions:

Genotype AA Aa aa

Frequency p2 2pq q2

Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)

Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)


For generation 1, probability of two A alleles coming together is:

Probability of two a alleles:

3025.0)55.0( 22 ppp

2025.0)45.0( 22 qqq


There are two ways of producing a heterozygote:

The Hardy-Weinberg equation:

pqpqqp 2or,or

12 22 qpqp


Populations in nature never fit the conditions for Hardy-Weinberg equilibrium.

But, it is useful in predicting genotype frequencies from allele frequencies; and, because the model describes conditions that would result in no evolution, patterns of deviation from the model help identify specific mechanisms of evolution.

22.2 What Are the Mechanisms of Evolutionary Change?

Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent.

Known evolutionary mechanisms:

• Mutation• Gene flow• Genetic drift• Nonrandom mating• Natural selection

H-W Practice!H-W Practice!

IB QuestionsIB Questions


Mutation is the origin of genetic variation.

Mutation is any change in DNA; it appears to be random with respect to the adaptive needs of an organism.

Most mutations are harmful or neutral, but if conditions change, could become advantageous.


Mutations can also restore alleles that other processes remove.

Mutation rates are low—about one per locus in a million zygotes.

Creates a lot of variation because of the number of genes that can mutate, chromosome rearrangements that can change many genes simultaneously, and large numbers of individuals.


Because mutation rate is low, mutations in themselves result in only minor deviations from Hardy-Weinberg equilibrium.

If large deviations are found, it is appropriate to look for other mechanisms.


Gene flow is a result of the migration of individuals and movements of gametes between populations.

New alleles can be added to the gene pool, or allele frequencies changed.


Genetic drift results from random changes in allele frequencies.

In large populations, genetic drift can influence frequencies of alleles that don’t affect survival and reproduction.

If populations are reduced to a small number of individuals—a population bottleneck, genetic drift can reduce the genetic variation.

Figure 22.8 A Population Bottleneck


A population forced through a bottleneck is likely to lose much genetic variation.

Example: Greater prairie chickens in Illinois were reduced to about 50 birds in the 1990s; California fan palms are now restricted to a few oases in southern California.

Figure 22.9 Species with Low Genetic Variation (Part 1)

Figure 22.9 Species with Low Genetic Variation (Part 2)


Genetic drift also effects small populations that colonize a new region.

Colonizing population is unlikely to have all the alleles present in the whole population.

Founder effect—equivalent to a bottleneck.


Example of founder effect:

Populations of European fruit fly D. subobscura began in Chile, and then in Washington state.

Both populations grew and expanded their ranges.

These populations have very similar genetic structure, and much less variation than the European populations.

Figure 22.10 A Founder Effect


Nonrandom mating occurs when individuals choose mates with particular phenotypes.

If individuals choose the same genotype as themselves, homozygote frequencies will increase.


Nonrandom mating in primroses (Primula):

Two flower types—pin and thrum. Pollen from one type can fertilize only flowers of the other type.

Figure 22.11 Flower Structure Fosters Nonrandom Mating


Selfing, or self-fertilization, is a common form of nonrandom mating.

Selfing reduces the frequency of heterozygotes, and increases homozygotes, but does not change allele frequencies in the population.

22.3 What Evolutionary Mechanisms Result in Adaptation?

Adaptation occurs when some individuals in a population contribute more offspring to the next generation.

Allele frequencies change in a way that adapts individuals to the environment that influenced that reproductive success.

This is natural selection.


Natural selection acts on phenotype.

Fitness is the reproductive contribution of a phenotype to subsequent generations.

Changes in the relative success of different phenotypes in a population leads to change in allele frequencies.


Fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.


Most characters are influences by alleles at more than one locus.

Such characters often show quantitative variation instead of qualitative.

Example: The distribution of body size of individuals in a population is likely to resemble a bell-shaped curve.


Natural selection can act on characters with quantitative variation in three ways:

• Stabilizing selection preserves average phenotype.

• Directional selection favors individuals that vary in one direction.

• Disruptive selection favors individuals that vary in opposite directions from the average.

Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 1)




Stabilizing selection reduces variation in a population, but does not change the mean.

Rates of evolution are slow because natural selection is usually stabilizing.

Example: human birth weights

Figure 22.13 Human Birth Weight Is Influenced by Stabilizing Selection


Directional selection occurs when individuals at one extreme are more successful.

If directional selection operates over many generations, an evolutionary trend occurs.

Example: resistance to tetrodotoxin (TTX) in garter snakes

Figure 22.14 Resistance to TTX Is Associated with the Presence of Newts


Disruptive selection: Variation is created when individuals at either extreme are more successful than average individuals.

Example: bill size in black-bellied seed crackers

Figure 22.15 Disruptive Selection Results in a Bimodal Distribution


Sexual selection is a special type of natural selection, which acts on characters that determine reproductive success.

If an individual survives but does not reproduce, it makes no contribution to the next generation.

Sexual selection favors traits that increase the chances of reproduction.


Traits such as bright colors, long horns, and elaborate courtship displays, may improve ability to compete for mates (intrasexual selection);

or to be more attractive to the opposite sex (intersexual selection).

Such traits are costly, but reliably demonstrate the fitness of the bearer to the choosing sex.


This has been shown experimentally in long-tailed widowbirds.

Male tails were shortened or lengthened.

Both were able to successfully defend their territories, but males with lengthened tails attracted more females.

Long tails indicate the health and vigor of the male.

Figure 22.16 The Longer the Tail, the Better the Male (Part 1)

Figure 22.16 The Longer the Tail, the Better the Male (Part 2)


Sexual selection has also been shown in zebra finches.

Brightness of male’s bill is an indicator of health.

Color is due to carotenoids, which are also important in the immune system. A brighter bill indicates more carotenoids and greater overall health.

Figure 22.17 Bright Bills Signal Good Health (Part 1)



22.4 How Is Genetic Variation Maintained within Populations?

Many mutations do not affect the function of the resulting proteins.

An allele that does not affect fitness is a neutral allele. They tend to accumulate in a population.

Molecular techniques allow neutral alleles to be identified and used to estimate rates of evolution (see Chapter 24).


Sexual reproduction results in new combinations of genes through crossing over and independent assortment, and the combination of gametes.

Sexual recombination produces genetic variety that increases evolutionary potential.


But sexual reproduction has disadvantages:

• Recombination can break up adaptive combinations of genes.

• Reduces rate at which females pass genes to offspring.

• Dividing offspring into genders reduces the overall reproductive rate.


How did sexual reproduction arise?

Possible advantages:

• Sexual reproduction facilitates repair of damaged DNA. Damage on one chromosome can be repaired by copying intact sequence on the other chromosome.


• Permits elimination of deleterious mutations.

In asexually reproducing species, deleterious mutations can accumulate; only death of the lineage can eliminate them—Muller’s ratchet.


• Sexual recombination produces some individuals with many deleterious mutations, some with few. The individuals with few deleterious mutations are more likely to survive.


• The variety of genetic combinations possible in sexually reproducing species may be especially valuable in defense against pathogens and parasites.


• Sexual recombination does not affect the frequency of alleles, but generates new combinations of alleles on which natural selection can act.


Frequency-dependent selection: A polymorphism can be maintained when fitness depends on its frequency in the population.

Example: a scale-eating fish in Lake Tanganyika. “Left-mouthed” and “right-mouthed” individuals are both favored; the host fish can be attacked from either side.

Figure 22.18 A Stable Polymorphism


Environmental variation also helps to preserve genetic variation.

Example: Colias butterflies live in an environment with temperature extremes. The population is polymorphic for an enzyme that influences flight at different temperatures.

Heterozygotes are favored because they can fly over a larger temperature range.

Figure 22.19 A Heterozygote Mating Advantage (Part 1)

Figure 22.19 A Heterozygote Mating Advantage (Part 2)


Subpopulations in different geographic regions maintain genetic variation.

The subpopulations may be subjected to different environmental conditions and selective pressures.

Example: populations of white clover that produce cyanide as defense against herbivores. Plants that produce cyanide are more likely to be killed by frost.

Figure 22.20 Geographic Variation in a Defensive Chemical

22.5 What Are the Constraints on Evolution?

Lack of genetic variation can prevent evolution of potentially favorable traits.

If the allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be favored by natural selection.


Evolution must work within the boundaries of universal constraints such as:

• Cell size, constrained by surface area-to-volume ratios

• Protein folding, constrained by types of bonding that can occur

• Laws of thermodynamics that constrain energy transfers


Developmental processes also constrain evolution.

All evolutionary innovations are modifications of previously existing structures.


Example: two lineages of bottom-dwelling fishes

Skates and rays evolved from a common ancestor with sharks. They started with a flattened body plan, and can swim along the ocean floor.

Sole and flounder evolved from laterally flattened bony fishes. They can’t swim well, but lie still on the bottom. The eyes gradually shifted to one side.

Figure 22.21 Two Solutions to a Single Problem (Part 1)

Figure 22.21 Two Solutions to a Single Problem (Part 2)


Adaptations involve both fitness costs and benefits.

Benefit must outweigh cost if adaptation is to evolve—the trade-offs must be worthwhile.

Example: Garter snake resistance to TTX only occurs where poisonous newts are common.


Conspicuous features used by some males to compete with other males are a trade-off with reproductive success.

Species in which males have multiple mates are polygynous. Males are usually larger than females and often have weapons—horns, antlers, etc.

Dramatic differences between sexes are known as sexual dimorphism.


In polygynous species, males must defend their mates against other males.

Defensive structures require a lot of energy, but the male has a lot of reproductive success.


Short-term changes in allele frequencies can be observed and manipulated to demonstrate the processes by which evolution occurs.

But patterns of long-term evolutionary change can be influenced by infrequent events (e.g., meteorites) or very slow processes (e.g., continental drift). Other types of evidence are used to study these long-term changes.

22.6 How Have Humans Influenced Evolution?

As humans have changed their environments, selective forces that act on human populations have changed also.

Today, survival and reproductive success are related to genes that confer defense against diseases such as malaria and AIDS, and stresses of modern industrial life such as hypertension.


Humans have also influenced the evolution of other species:

Efforts to control populations of “pests” make humans agents of evolutionary change.

Sport hunters that seek large trophy animals are removing the biggest and healthiest animals from populations.

Figure 22.22 Trophy Hunting Selects for Smaller Males


Humans also move species around the globe, and modify species through breeding and biotechnology.

Humans are also changing the climate; and have caused the extinction of many species.

Photo 22.1 Western sandpipers (Calidris mauri), CA; all look the same to unpracticed eyes.

Photo 22.2 Fall gathering of ladybird beetles (Hippodamia convergens).

Photo 22.3 Brussel sprouts, emphasizing lateral buds.

Photo 22.4 Cauliflower, selected for flower clusters.

Photo 22.5 Broccoli, selected for stems and flowers.

Photo 22.6 Ornamental kale, emphasizing appearance of leaves.

Photo 22.7 Canola, or rape, selected for seed.

Photo 22.8 Northern elephant seals (Mirounga angustirostrus) experienced a population bottleneck.

Photo 22.9 Primrose (Primula sp.) flowers in cross section. Left: pin flower; Right: thrum flower.

Photo 22.10 Large blue mussels (Mytilus edulis) are better adapted to low salinity than young.

Photo 22.11 Buddhist monks at Hemis Gompa; Ladakh, northern India.

Photo 22.12 Ouighur women, men, and children gape at foreign tourists; Turfan, China.

Photo 22.13 Food vendors along the Irawaddy River, Burma.

Photo 22.14 Formal family photograph at Shinto wedding ceremony; Tokyo, Japan.

Photo 22.15 Members of the Yali tribe; Baliem Valley, Irian Jaya, Indonesia.

Photo 22.16 Headman and young women of Naitawoli Village, Fiji.

Photo 22.17 A Ja'aliyin mother and her children; Nile River, northern Sudan.

Photo 22.18 Village women preparing feast; Toulfe Village, Yatenga, Burkina Faso.

Photo 22.19 Teachers and native children; Hachipancha Village, Andean foothills, Peru.

Photo 22.20 Modern Mexican family; Janitzio Island, Patzcuaro, Mexico.

Photo 22.21 Family portrait: parents and children of European ancestry; United States.

Photo 22.22 Harbor, San Cristobal Is.

Photo 22.23 Shoreline, Darwin Bay, Tower Is.

Photo 22.24 Volcanic features, Bartolome Is.

Photo 22.25 Punta Moreno lava field, Isabella Is.

Photo 22.26 Crater of Vulcan Alcedo, Isabella Is.

Photo 22.27 "Pit Crater," sinkhole caused by receding lava, Highlands, Santa Cruz Is.

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The Mechanisms of Evolution