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Chapter 15: Evolution

Chapter Worksheet

Ch. 15.1 Darwin's Theory of Evolution by Natural Selection

Developing the Theory of Evolution

At the time of Darwin's voyage, the world believed that the world was only about 6,000 years old and that animals and plants were unchanging.

Darwin on the HMS Beagle

In 1831 the Beagle set out to survey the coast of South America. Darwin's role was as a naturalist and companion to the captain. His job was to collect biological and geological specimens during the ship's travels.

Over the five year voyage, Darwin collected rocks, fossils, plants, and animals. He also read Charles Lyell's Principles of Geology, a book proposing that Earth was millions of years old. This book influenced his thinking as he observed marine fossils in the Andes, dug up giant fossil versions of smaller living mammals, and saw how earthquakes could lift rocks great distances very quickly.

The Galapagos Islands

In 1835 arrived at the Galapagos Islands off the coast of South America. As Darwin collected samples from the four different islands he visited he noticed that the different islands seemed to have their own, slightly different varieties of animals. By observing the appearance of a tortoises' shell, locals could identify the island that it came from.

Studying the specimens Darwin collected, it was learned that almost every specimen from the islands was new to European scientists. These new species most closely resembled species from mainland South America despite the different environments.

Darwin Continued His Studies

Darwin hypothesized that new species could appear gradually through small changes in ancestral species. To learn how such a process could work Darwin worked with animal breeders- pigeon breeders in particular.

A breeder can promote traits by selecting and breeding pigeons that have the most exaggerated traits. Artificial selection- the process of directed breeding to produce offspring with desired traits (selective breeding).

Darwin inferred that if humans could change species by artificial selection, then perhaps the same process could work in nature. Darwin thought, given enough time, perhaps this process could produce new species.

Natural Selection

Darwin read an essay by economist Thomas Malthus that suggested that the human pupulation, if unchecked, eventually would outgrow its food supply. This would lead to a competitive struggle for existence.
Darwin realized that Malthus's ideas could be applied to the natural world. Some competitor would be better equipped for survival than others. Those less equipped would die; the process for natural selection. This provided the framework for a new theory about the origin of species.

Darwin's theory consists of four basic principles:

  1. Variation
  2. Variations can be inherited
  3. Overproduction of offspring
  4. Reproductive advantage- variations that increase reproductive success

Given enough time, natural selection could modify a population enough to produce a new species.

The Origin of Species

Darwin began writing a multivolume book compiled of evidence for evolution and explaining how natural selection might provide a mechanism for the origin of species. He continued to compile evidence in support of his theory for many years.

1858, Alfred Russel Wallace, another English naturalist, proposed a theory similar to Darwin's. Both ideas were presented to the Linnaen Society of London. One year later, Darwin published On the Origin of Species by Means of Natural Selection.

Darwin used the term evolution only on the last page. Evolution- cumulative changes in groups of organisms through time.
Natural selection is not synonymous with evolution; it is a mechanism by which evolution occurs.

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Ch. 15.2 Evidence of Evolution

Multiple lines of evidence support the theory of evolution.

Support for Evolution

A theory provides an explanation for a natural phenomenon based on observations. Theories explain available data and suggest further areas for experimentation. Darwin's book demonstrated how evolution might happen by the process of natural selection, and provided evidence that evolution has occurred on our planet.

The Fossil Record

Fossils provide a record of species that lived long ago and can show how ancient species are similar to current species. Also can show how species have remained unchanged; horseshoe crab.

Darwin acknowledged limitation of the fossil record but also predicted "intermediate fossils" would exist linking species; Archaeopteryx.

Researchers consider two major classes of traits when studying transitional fossils:

  1. Derived traits- newly evolved features that do not appear in the fossils of common ancestors.
  2. Ancestral traits- more primitive features that do appear in ancestral forms.

Transitional fossils provide detailed patterns of evolutionary change for the ancestors of many modern animals.

Comparative Anatomy

Similarities in anatomy suggests that organisms may have shared ancestry.

Homologous Structures

Homologous structures- anatomically similar structures inherited from a common ancestor. Evolution predicts that an organism's body parts are more likely to be modifications of ancestral body parts than they are to be entirely new features.
Bird wings and reptile forelimbs are similar in shape and construction, which indicates that they were inherited from a common ancestor.


Homologous structures are an example for which evolution is the best available explanation for the biological data.

Vestigial Structures

Vestigial structures- are structures that are the reduced forms of functional structures in other organisms. Evolutionary theory predicts that features of ancestors that no longer have a function will become smaller over time until they are lost.

Analogous structures- anatomical structures that can be used for the same purpose and can be superficially similar in construction but are not inherited from a common ancestor.
Do not show close evolutionary relationships, but do show that functionally similar features can evolve independently in similar environments.

Comparative Embryology

Embryo- an early pre-birth stage of an organism's development. Scientists have found that vertebrate embryos exhibit homologous structures during certain phases of development but become totally different structures in the adult forms.
All vertebrate embryos have a tail and paired structures called pharyngeal pouches.

Comparative Biochemistry

Cytochrome c is a enzyme that is essential for respiration. Despite slight variation in its amino acid sequence the molecule has changed very little over time.

Evolutionary theory predicts that the more closely related the species are, the greater the number of of amino acid sequences will be shared.
Scientists have found similar biochemical patterns in other proteins, as well as in DNA and RNA. Comparisons of the similarities in these molecules across specie reflect evolutionary patterns seen in comparative anatomy and in the fossil record.

Geographic Distribution

Distribution of plants and animals that Darwin saw during his travels first suggested evolution to Darwin. Animals on the South American mainland were more similar to other South American animals than they were to animals living in similar environments in Europe.


Patterns of migration were important to Darwin as well. Migration patterns explained why islands had more plant diversity than animal; access. Biogeography- field of study of the distribution of plants and animals around the world.
Evolution is linked with climate and geological forces, especially plate tectonics.

Adaptation

Darwin used four of the five categories above to support his theory of evolution; all but biochemistry which was not well developed during Darwin's time. At the heart of Darwin's theory lies the concept of adaptation.

Types of Adaptation

An adaptation is a trait shaped by natural selection that increases an organism's reproductive success. A way to measure success is by looking at fitness. Fitness- a measure of the relative contribution that an individual trait makes to the next generation; ie. amount of reproductively viable offspring an organism produces.

The better an organism is adapted to its environment, the greater its chances of survival and reproductive success.

Camouflage- morphological adaptations that allow an organism to blend in with their environment.

Mimicry- morphological adaptation in which one species evolves to resemble another species. Harmless & harmful, harmful & harmful.

Antimicrobial Resistance- species of bacteria that originally were killed by a antibiotic have developed drug resistance. Some diseases have re-emerged in more harmful forms.

Consequences of Adaptations

Stephen Gould and Richard Lewontin pointed out that not all features of an organism are adaptive. Some features might be consequences of other evolved characteristics.

Spandrel Example

In architecture a spandrel is a space formed when using arches to support a dome. Spandrels are usually decorative so one might think they exist for decoration. In organisms some features might be prominent but do not increase reproductive success; they are simply unavoidable consequence of prior evolutionary change.

Human Example

Helplessness of infants may be a result of humans walking upright. To walk upright humans need narrow pelvises, which means that babies' heads must be small enough to fit through the pelvic opening.

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Ch. 15.3 Shaping Evolutionary Theory

The theory of evolution continues to be refined as scientists learn new information.

Mechanisms of Evolution

Scientists now know that natural selection is not the only mechanism of evolution. Evolution occurs at the population level, with genes as the raw material.

Population Genetics

At the turn of the twentieth century, genes had not been discovered but the allele was understood to be one form of an inherited character trait, such as eye color. Alleles get passed down from partent to offspring.

In 1908, Godfrey Hardy and German physician Wilhelm Weinberg independently showed that evolution will not occur in a population unless allelic frequencies are acted upon by forces that cause change. In the absence of these forces the allelic frequency remains the same and evolution doesn't occur. Hardy-Weinberg principle- when allelic frequencies remain constant, a population is in genetic equilibrium.

The Hardy-Weinberg principle states that the allele frequencies in populations should be constant.

Conditions

According to the Hardy-Weinberg principle, a population in genetic equilibrium must meet five condition:

  1. no genetic drift
  2. no gene flow
  3. no mutation
  4. mating must be random
  5. no natural selection

If a population is not in genetic equilibrium, at least one of the five conditions has been violated. Hardly any population meets all five conditions for long periods of time.

Genetic Drift

Genetic drift- any change in the allelic frequencies in a population that results from chance. Recall; with simple traits only one allele gets passed by each parent and the allele is selected randomly through independent assortment.
In large populations enough alleles "drift" to ensure the allelic frequency of the population remains relatively constant from one generation to the next. In small populations, the effects of genetic drift are more pronounced, with the chance of losing an allele becoming greater.

Founder Effect- can occur when a small sample of a population settles in a location separated from the rest of the population. Being a smaller population, genetic variations can be larger.

Example is Amish and Mennonite communities in US that display a high-frequency of six-finger dwarfism. Results from members of not marrying outside of the community.

Bottleneck- occurs when a population declines to a very low number and then rebounds. The gene pool is genetically similar to that of the population at its lowest level.

Example is Cheetahs. They are so genetically similar that they appear inbred which decreases fertility and might be a factor in the potential extinction of species.

Gene Flow

Populations in genetic equilibrium experience no gene flow. It is a closed system with no new genes entering or leaving the population.
Few populations are isolated; random movement of individuals increases genetic variation in a population and reduces differences between populations.

Nonrandom Mating

Usually, organisms mate with individuals in close proximity; promoting inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits.

Mutation

The cumulative effect of mutations in a population might cause a change in allelic frequencies, violating genetic equilibrium.
Occasionally a mutation provides an advantage and will be selected for, becoming more common in future generations.
In this way, mutations provide the raw material for natural selection.

Natural Selection

Natural selection acts to select the individuals that are best adapted for survival and reproduction. Natural selection acts on an organisms phenotype and changes allelic frequencies.

Stabilizing Selection- The most common form of natural selection; eliminates extreme expressions of a trait when the average expression leads to higher fitness.

Directional Selection- an extreme version of a trait makes an organism more fit. Industrial melanism and the Peppered Moth.

Peter and Rosemary Grants research of Galapagos Finches; times of drought favored long beak for alternative food choice, rainy favored shorter average beak.

Disruptive Selection- splits a population into two groups, removing individuals with average traits but retaining extreme traits at both ends of continuum.

Sexual Selection- based on ability to attract a mate; males and females usually differ in appearances. Peacocks are example.

Reproductive Isolation

Mechanisms of evolution violate the Hardy-Weinberg principle and can lead to speciation. Most scientists define speciation as the process in which some reproductive members of a population change so much that they can no longer produce fertile offspring with members of the original population. This can happen in two ways:

Prezygotic Isolation (before fertilization occurs)- prevent reproduction by making fertilization unlikely through geographic, ecological, behavioral, or other differences.

Postzygotic Isolation (after fertilization occurs)- when hybrid offspring cannot develop or reproduce. Prevents offspring survival or reproduction; sterile.

Speciation

To occur, a population must diverge and then be reproductively isolated. Two types:

Allopatric Speciation- a physical barrier divides one population into two or more population.
Geographic barriers can include mountain ranges, channels between islands, wide rivers, and lava flows.

Sympatric Speciation- a species evolves into a new species without a physical barrier. The ancestor species and new species live side-by-side during the speciation event.
Polyploidy in plants are a example.

Patterns of Evolution

Speciation is a long process. Hard to witness first hand compared to human life span but we can look at patterns of evolution.

Adaptive Radiation

Also called divergent evolution; can occur in a relatively short time when one species gives rise to many species in response to the creation of a new habitat or another ecological opportunity. Adaptive radiation often follows large-scale extinctions.

Cichlids are thought to have evolved into more than 300 species in less than 14,000 years. Possibly a result of formation of a double jaw.

Coevolution

When a species evolves in close relationship with other species. One form is mutualism in which both species benefit each other. Example: Comet Orchid and the tongue of moths that pollinate them.

Another type occurs when one species can evolve a parasitic dependency on another species. An example is the relationship between a plant and an insect that is dependent on it. Plant has a chemical to deter predators, predators have a tolerance to the chemical.

Convergent Evolution

When unrelated species evolve similar traits even though they live in different parts of the world. Occurs in environments that are geographically far apart but have similar ecology and climate.

Rate of Speciation

Gradualism- theory that evolution occurs in small, gradual steps. Evidence favors this theory.
Punctuated equilibrium- rapid spurts of genetic change cause species to diverge quickly; these period punctuate much longer periods when the species exhibit little change.


 

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Page last updated January 2, 2017.