Chapter 17 Microevolution

Designer Pets

• The many varieties or breeds have been produced through artificial selection by humans

• Some breeds would have been selected against by nature had it not been for human protection

The Great Chain of Being

• Many of the ancient Greeks, including Aristotle, attempted to explain the natural world by making direct observable

• By the 14th century, the ancient view of gradual levels of organization from lifeless matter to the most complex organisms had been formalized into the great Chain of Being

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• The chain extended from lowest level forms to spiritual beings

• Each being (species) had its fixed place in the divine order—unchanged and unchanging since creation

Biogeography

• Is the geographic distribution of life forms on Earth.

Questions from Biogeography• During the global voyages of the 16th

century scientist asked the following questions:– Where do all these species “fit” into the great

“Chain”?– Why were certain species found in only some

parts of the world but not others?– How did so many species get from the center

of creation to islands and isolated places?

Comparative Morphology

• Studies of the comparative morphology of seemingly unrelated animals led to questions of why certain structures should be so similar

• Example: Pelvic Girdle in snakes and tail bones in humans

Questions About Fossils• Studies of sedimentary beds revealed that

deposits had been laid down slowly, one above the other– Fossils: layers held recognizable remains or

impression of an organisms– The arrangement of the layers suggested that

different organisms had lived at different times

• De Buffon’s explanation: perhaps species originated in more than one place, and perhaps species became modified over time -- EVOLUTION

New Evidence into Old Beliefs • Georges Cuvier believed in an original

creation of all species• Cuvier suggested that the abrupt changes

in the fossil record in different rock strata reflected the concept of catastrophism– After each catastrophe, fewer species

remained.– The survivors were not new species, it was

just that their ancestors’ fossils had not been found

Lamarck

• Theory of acquired characteristics is the idea that simple forms had changed into more complex ones by a built-in drive for perfection up the Chain of Being.– Example: Giraffe stretching its neck to reach

higher branches would result in offspring with longer necks

• Experiments could not uphold his thoughts on acquired traits

Larmarck’s and Darwin &Wallace’s View

Charles Darwin

• Father of Evolution

Voyage on the Beagle• Darwin traveled on the Beagle for five

years as naturalist.– He collected and studied variety of plants and

animals – While on the trip he read Lyell’s book

Principles of Geology, which proposed the theory of uniformity – the notion of gradual, lengthy molding of the earth’s geologic structure

– Thus, the earth was millions of years old and not thousands of years – evidence of Evolution

Darwin’s theory takes form

• Darwin returned after five years at sea and began pondering the “species problem” what could explain the remarkable diversity among organisms?

• In Argentina, Darwin had observed extinct glyptodonts that bore suspicious resemblance to living armadillos; Darwin wondered if the present species had evolved from the extinct one?

Variation in Traits• Thomas Malthus had suggested that as a

population outgrows its resources, its members must compete for what is available.

• Darwin felt that if some normally variant members of a population bore traits that increased their survival, then nature would select those same individuals to survive, reproduce, and possibly change future populations traits.

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• On the Galapagos Islands, the dozen or so species of finches all varied from one another to some extent but resembled the mainland finches to some degree also; perhaps they ha descended from common ancestor

Darwin’s Finches• 1) Finches on the Galápagos Islands resembled a

mainland finch but there were more types.2) Galápagos finch species varied by nesting site, beak size, and eating habits.3) One unusual finch used a twig or thorn to pry out insects, a job normally done by (missing) woodpeckers (Darwin never witnessed this finch behavior).4) The variation in finches posed questions to Darwin: did they descend from one mainland ancestor or did islands allow isolated populations to evolve independently, and could present-day species have resulted from changes occurring in each isolated population?

Example of Darwin’s Finches

Continue…• Darwin reasoned that a population is

evolving when it heritable traits are changing through successive generations.

• 1858, Darwin received a paper from Alfred Wallace, who had developed much the same theory on natural selection but independently of Darwin

• Darwin and Wallace presented a joint paper but Darwin published (alone) his ideas in book form in 1859

Individuals Don’t Evolve– Populations Do

• Populations evolve, not individuals

• A population is a group of individuals belonging to the same species, occupying the same given area

• A population exhibits variation among the individual members, but they also hold certain morphological, physiological, and behavior traits in common.

The Gene Pool

• Individuals of the same populations generally have the same number and kinds of genes – All of the genes in the entire population

constitute the gene pool– Each gene exists in two or more slightly

different molecular forms called alleles, which offspring inherit and express as phenotype

• Each particular mix of alleles depend on these five factors:– Gene mutations create new alleles– Crossing over and genetic recombination are

normal results of meiosis – Independent assortment of chromosomes

occur in meiosis – Fertilization between genetically varied

gametes produces “new” combinations of genes

– Abnormal changes in chromosomes or number can occur.

Stability and Change in Allele Frequencies

• Allele frequencies are a measure of the abundance of each kind of allele in the entire in the entire population

• Evolution can be detected by a change in allele frequencies from the genetic equilibrium as established by the Hardy-Weinberg rule

Hardy Weinberg: five conditions necessary for stable population

• No mutations occurring

• Population is very very large

• Population is isolated from other populations of the same species

• All members survive, mate, and reproduce (no selection)

• Mating is random

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• Because of these five conditions are rarely fulfilled in natural populations, any derivation from the reference point established by the “rule” will indicate evolution

• Microevolution is the change in allele frequencies brought about by mutation, genetic drift, gene flow, and natural selection

Mutation Revisited • Mutations are heritable changes in DNA

that can alter gene expression

• Mutations are random and the phenotypic outcome may be neutral, beneficial, harmful, or even lethal to the individual depending on other interactions

• Mutations are the only source of new alleles – the genetic foundation for biological diversity

Three types of mutations

• Lethal Mutations: is expression of a gene that results in death

• Neutral Mutations: where or not they are expressed in phenotype, have no effect on survival and reproduction

• Beneficial Mutations: are those that bestow survival advantages

Section 17-5

• Read Hardy-Weinberg on your own

Natural Selection Revisited

• Natural Selection probably accounts for more changes in allele frequencies than any other microevolutionary process

Major Points to Natural Selection

• Observation: All populations have the reproductive capacity to increase in numbers over generation

• Observation: No population is able to increase indefinitely, for its individuals will run out of food, living space, and other resources

• Inference: Because more individuals are produced than can survive to reproductive age, the members of a population must compete for the available resources

• Observation: All the individuals have the same genes, which represent a pool of heritable information

• Observation: Most genes occur in different molecular forms (alleles), which give rise to differences in phenotypic details

• Inferences: Because adaptive traits promote survival and reproduction, they must increase frequency over the generation and less adaptive traits must decrease in frequency or disappear

• Conclusions: A population can evolve by natural selection, that is the traits characterizing the population can change over time when its individuals differ in one or more heritable traits that are responsible for differences in survival and reproduction

Natural Selection: Adaptation

• Adaptation: Evolutionary term that describe an organisms better suited to their environment.

• Organisms that are adapted to their environment will have the ability to survive and reproduce.

• Extinction occurs when an organisms is not adapted to their environment and will not be able to reproduce

Organisms have Variations

• Variations that make adaptation possible are those that are passed on from generation to generation.

• Darwin could not state the cause of variations because genetics was not yet established.

Organisms Struggle to Exist

• Malthus proposed that human populations outgrow food supply and death and famine were inevitable.

• Darwin applied this to all organisms; resources were not sufficient for all members to survive.

• Therefore, there is a constant struggle for existence; only certain members survive and reproduce.

Organisms Differ in Fitness• 1. Organisms whose traits enable them to reproduce

to a greater degree have a greater fitness.2. Darwin noted that humans carry out artificial selection. a. Early humans likely selected wolf variants; produced the varieties of domestic dogs.b. Many crop plant varieties can be traced to a single ancestor.d. Evolution by artificial or natural selection occurs when more fit organisms reproduce and leave more offspring

On the Origin of Species

• Darwin’s publish book: On the Origin of Species

• It waited 20 years to publish his book

• He used this time to complete experiments to prove his theory of Natural Selection

What is Directional Selection?

• Allele Frequencies in a consistent direction

• Shifts may be in response to environmental pressures or occur as a new mutation appears and proved adaptive

EXAMPLES

• Peppered Moths:

• Pesticide Resistance

• Antibiotic Resistance

Stabilizing Selection• Favors the most common phenotype in the

population

• It counters the effects of mutations, genetic drift, and gene flow

Disruptive Selection

• Favors forms at the extremes of the phenotypic range of variation and selects against the intermediate forms

Sexual Selection

• Most species have distinctively male and female phenotypes – sexual dimorphisms

• Sexual selection is based on any trait that gives the individual a competitive edge in mating and producing offspring.

• Usually it is the female that are the agents of selection when they pick their mates

Maintaining Two or More Alleles

• A variation on the stabilizing theme in which two or more forms of a trait are maintained in fairly stable proportions depending on survival value in the environment

• Population is in balanced polymorphism when nonidentical alleles for a trait are being maintained at frequencies greater than 1%

Sickle Cell Anemia

• Humans that are homozygous for sickle-cell anemia develop the disease and die at early age

• BUT– Individuals with alleles for both normal

hemoglobin and sickle-cell hemoglobin (heterozygous) have the greatest chances of surviving malaria

Gene Flow

• Genes move with individuals when they move out of (emigration), or into (immigration), a population

• The physical flow (a result of shuffling) tends to minimize genetic variation between population

Genetic Drift

• Is the random fluctuation in allele frequencies over time, due to chance occurrences alone

• Affects: Small Populations

Bottlenecks and Founder Effect

• Bottleneck: Some stressful situations greatly reduces the size of a population leaving a few individuals to reestablish the population

• Founder Effect: A few individuals leave the original population to establish a new one

Genetic Drift and Inbred Populations

• Inbreeding refers to nonrandom mating among closely related individuals

• It tends to increase the homozygous condition, thus leading to lower fitness and survival rates