Chapter 13Evolution and Natural Selection

Evolution• Evolution is a change in

the frequency of genetically determined characteristics within a population over time

• Microevolution occurs when there are minor differences in allele frequency between populations of the same species

Evolution

• Macroevolution occurs when there are major differences that have occurred over long periods that have resulted in so much genetic change that new kinds of species are produced

Development of Evolutionary Thought

• For centuries, people believed that the various species of plants and animals were unchanged from the time of their creation– Today we know this is not true, but they knew

nothing of DNA, meiosis, and genetics

Development of Evolutionary Thought

• In the mid-1700’s,Georges-Louis Buffon, a French naturalist wondered if animals underwent change (evolved) over time.

• Buffon had a student, Jean-Baptiste de Lamark who suggested how these changes might occurred. His ideas were known as acquired characteristics.

Development of Evolutionary Thought• Lamark’s theory of

acquired characteristics states that traits gained during an organism’s life and transmitted to the offspring.

• For example, giraffes had short necks, but in order to reach the leaves, their necks stretching. When these giraffes reproduced their offspring acquired their parent’s longer necks.

Theory of Natural Selection• Charles Darwin (1809-1882)

was the son of a physician and member of a family with a history scientific curiosity.

• After briefly studying medicine, he enrolled at Cambridge University, receiving his B.A. degree in 1831.

• After graduating, Darwin left on a multi-year expedition (1831-1836) on the HMS Beagle, primarily as a social companion to the captain.

Natural Selection • The purpose of the trip was to chart poorly known coastal waters off South America.

• While the crew were surveying, Darwin went on many expeditions to collect plants and animals on mainland South America and the islands which the ship visited.

• Darwin noted that the temperate species in South America looked much more like tropical South American species than they did temperate species from Europe.

Natural Selection and Charles Darwin• When the Beagle arrived at

the Galapagos Islands, 900km west of South America, he observed many new species, yet similar to those on the mainland.

• This included 13 species of finches, with dramatically different beak morphologies.

• Some were very “unfinchlike” with bills like insect-eating warblers.

• Some Galapagos finch species were found only on one island, others had more than one.

Natural Selection• After his return to England, Darwin

developed his ideas on evolution and natural selection by 1844, but delayed publishing.

• He spent 20 more years collecting more observations to support his theory.

• In 1858, Alfred Wallace, a naturalist working in Indonesia, sent Darwin a manuscript with essentially the same theory as Darwin

• This prompted Darwin to finish his book, On the Origin of Species by Means of Natural Selection in 1859.

Origin of Species• The first point of the

Origin of Species is that all organisms are related through descent from some unknown ancestor that lived in the past.

• Over time, species changed via adaptations which fit them to specific ways of life.

• These ideas, he called descent with modification.

Origin of Species• The second point in the Origin of

Species is that the dominant mechanism for evolution of species is natural selection.

• Darwin argued that evolution of species in the natural world was analogous to artificial selection of domesticated animals and plants.

• Individuals with desired characteristics are breed with other individuals with desired characteristics.

• This can produce tremendous changes in the phenotype of a species in a relatively short period of time.

Origin of Species• The theory of natural selection is based on the

following assumptions about the nature of living things:1. All organisms produce more offspring than survive.2. No two organisms are exactly alike.3. Among organisms, there is a constant struggle for

survival.4. Individuals that possess favorable characteristics for

their environment have a higher rate of survival and produce more offspring.

5. Favorable characteristics become more common in the species, and unfavorable characteristics are lost.

Natural Selection - Giraffes• Using Darwin-Wallace theory with

giraffes1. In each generation more giraffes would

be born than the food supply could support.

2. In each generation, some giraffes would inherit longer necks, and some would inherit shorter necks.

3. All giraffes would compete for the same food source.

4. Giraffes with longer necks would obtain more food, have a higher survival rate, and produce some offspring.

5. As a result, succeeding generations would show an increase in the number of individuals with longer necks.

Natural Selection and Evolution• The gene pool of a species is modified by:– chance events in small populations (founder effect,

genetic drift, genetic bottlenecks)– immigration and emigration– Mutations– Natural selection

• As a result, the gene pool in the next generation differs from that of the previous generation - microevolution.

Natural Selection – Darwin’s Finches• Darwin hypothesized that differences in bill-size lead

to differences in the success of Galapagos finches when feeding.

• Detailed research over the last 30+ years have shown that bill size varies among finches and is inheritable (genetically controlled).

• During drought years in the Galapagos, the local plants produce fewer seeds than during normal, wet years.

• All the finches prefer to eat the smaller, easier to handle seeds first but soon they are all gone.

• All that remain are the larger seeds, which are harder to open.

• Larger billed birds can open these larger seeds, but smaller billed birds cannot.

Natural Selection – Darwin’s Finches• Larger billed birds survive drought better and

leave more offspring - more fit.• The alleles that favor large bills become more

common among the next generation.• The average size of birds and their bills increases

in the population.• Larger billed birds are less fit during wet years

when small seeds are abundant.• Larger billed birds are less

efficient when feeding on small seeds.

Hardy-Weinberg Equilibrium• The Hardy-Weinberg equilibrium allows us to predict

the frequencies of alleles in the offspring if we know the frequency of alleles in the parental population.

• If a population demonstrates genotype frequencies from generation to generation that are consistent with Hardy-Weinberg, then it is NOT evolving.

Hardy-Weinberg Equation

• The Hardy-Weinberg Equation is p2 + 2pq +q2 = 1

• p2 stands for the frequency of homozygous dominant alleles

• 2pq stands for the frequency of heterozygous alleles• q2 stands for the frequency of homozygous recessive

alleles

Hardy-Weinberg Equilibrium

• Conditions necessary for gene frequencies to remain constant are the following:1. Mating must be completely random2. Mutations must not occur3. The migration of individual organisms into and

out of the population must not occur.4. The population must be very large5. All genes must have an equal chance of being

passed onto the next generation

Effects of Natural Selection• In natural selection,

particular phenotypes match (or don’t) the local environmental conditions.

• Some phenotypes do well, live long, and produce abundant offspring (and other don’t).

• If these phenotypic characters have a genetic basis, individuals that have favored alleles will contribute more alleles to the next generation.

Effects of Natural Selection• In stabilizing selection,

the environment favors individuals at the middle of the range of phenotypes, reducing phenotypic variation.

• With the range of color possibilities, the ones that do not blend into the environment, will be more likely to be killed by predators.

Effects of Natural Selection

• In directional selection, the environment favors individuals at the one end of the range of phenotypes, shifting phenotypic variation.

• For example, periods of drought favor larger individuals among Darwin’s finches in the Galapagos.

• Bigger birds compete better for scarce food, survive better, and leave more offspring.

• During wetter periods, the opposite is true.

Effects of Natural Selection• In diversifying selection,

the environment selects against average individuals and favors the extremes, increasing phenotypic variation.

• Melanistic moths are either light colored or dark colored to match light or dark tree bark.

• Intermediate colors match neither background and increase predation by birds.