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The Theory of Evolution Charles Darwin’s Life and Work Evidence for Evolution Mechanisms of Evolution Biology I
Charles Darwin’s Life and Work v v v 1809 -1882 English scientist At age 21, Darwin took a job as a naturalist on the English ship HMS Beagle which sailed to South America and the South Pacific on five-year scientific journey around the world. Darwin collected and studied biological specimens at every port along the route, but focused his attention on the unique animals and plants of the Galapagos Islands. His studies provided the foundation for his theory of evolution by natural selection.
The Five-Year Voyage of the HMS Beagle
The Galapagos Islands v v A group of volcanic islands located off the coast of South America Home to a variety of unique species
The Unique Creatures of the Galapagos Islands
Natural Selection v v v Driving force for evolution During the struggle for resources, the strongest survive & reproduce. Idea that at least some of the differences between individuals, which impact their survival and fertility, are inheritable.
Natural Selection and Theory of Evolution v v Natural selection is a mechanism for change in populations that occurs when organisms with certain variations survive, reproduce, and pass their variations to the next generation. Organisms without these variations are less likely to survive and reproduce. As a result, each generation consists largely of offspring from parents with these variations that aid survival. Darwin published the first book about evolution called On the Origin of Species by Natural Selection in 1859. The ideas detailed in Darwin’s book are today’s basic unifying theme of biology.
Four Principles of Natural Selection 1. 2. 3. 4. Variation – individuals in a population differ from one another due to their genetic make-up, whether inherited or from mutations Overproduction – populations produce more offspring than can possibly survive and this results in competition among offspring Adaptation – certain variations allow individuals to survive better their environment better than others and to produce more offspring that share those adaptations for their environment Heritability – as long as environmental conditions remain beneficial for a trait, more individuals will have the trait over time as it is passed from one generation to the next
Fitness n n A measure of the ability to survive and produce more offspring relative to other members of the population in a given environment If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.
Artificial Selection n n The process by which human beings change a species by breeding it for certain traits Humans act as the selective agent by determining which traits are favorable and then breed individual that show those traits In natural selection, the environment creates the selective pressure that determines if a trait is passed on or not Darwin observed the breeding of pigeons to study artificial selection and concluded that the same basic process worked in nature but required more time and could produce new species
Common Descent with Modification v v v Darwin proposed that organisms descended from common ancestors Idea that organisms change with time, diverging from a common form Caused evolution of new species
Phylogeny/Cladistics n n n Phylogeny - the evolutionary history for a group of species Cladistics – classification based on common ancestory Cladogram – evolutionary tree that proposes how species may be related to each other through common ancestors; based on homologous structures
Cladogram of Vertebrate Chordates
Microevolution n n Observable changes in the allele frequencies of a population over time which result in relatively small changes within the species or population; looks at effect of mutations and natural selection on phenotype or form Examples: pesticide resistance, herbicide resistance, bacterial resistance to antibiotics, changes in color or size within a population
http: //www. windows. ucar. edu /tour/link=/earth/Life/geneti cs_microevolution. html&edu=e lem
Macroevolution n n Evolution on a grand scale or above the level of a population or species Looks at the over-arching history of life
Evidence for Evolution Adaptation – any variation that aids an organism’s chances of survival in its environment A. Structural Adaptations 1. Mimicry – a structural adaptation that enables one species to resemble another species Example 1: The Viceroy butterfly has evolved a very similar pattern of coloration to the monarch; it is thought that the Viceroy is acting as a mimic, the similarity providing some protection from predators, such as birds, which often mistake the edible Viceroy for the inedible Monarch.
The Monarch feeds on milkweed and tastes bitter. Potential predators confuse the Viceroy for the Monarch and avoid eating it.
Example 2: The colors and body shape of a yellow jacket wasp and a harmless syrphid fly are similar. Predators avoid both insects. The syrphid fly is on the left, and the yellow jacket wasp is on the right.
2. v v Camouflage – a structural adaptation that enables species to blend with their surroundings; this usually means that they are not easily found by their predators and survive to reproduce The stick insect on the left blends in almost unnoticeably with the branch it sits on. The mottled sand grasshopper on the right is hardly visible on the wood.
England’s Salt and Pepper Moths v v 99+% of population prior to industrial revolution was light colored As coal soot killed lichens on trees light moths stood out to predators Dark form (morph) appeared and by mid 1900 s made up 90+% of population With pollution control, both lichens and light moths are coming back
v v B. Physiological Adaptations 1. Resistance Involve changes in an organism’s metabolic processes May develop in much less time than structural adaptations Examples: v When the antibiotic drug penicillin was discovered about 50 years ago, it was called a wonder drug because it killed many types of disease-causing bacteria and saved many lives. Today, penicillin no longer affects as many species of bacteria because some have evolved a physiological adaptation to prevent being killed by penicillin.
Pesticides are poisons used to kill insects that are pests in crops, swamps, backyards, and homes. Examples are DDT, now banned in many countries, and malathion. These chemical weapons against insects have proved to be doubleedged swords. Natural selection has allowed those insects with genes that somehow enable them to resist the chemical attack to survive. And their offspring inherit the genes for pesticide resistance. DDT was applied worldwide beginning in the mid 1940’s, and by the early 1950’s DDT would not kill house flies.
C. Other Evidence for Evolution 1. Fossils – preserved remnants or impressions left by organisms that lived in the past that help scientists to understand the overall picture of how a species evolved; remnants of animals may be buried, leave impressions, and/or have tissue replaced by harder minerals
2. Anatomy a. Homologous structures – similar in arrangement or structure but not in function; viewed as evidence that organisms evolved from a common ancestor
b. Analogous structures – similar in function but not in structure; do not indicate a common evolutionary ancestor
c. Vestigial structures – body structures that have no function in present-day organisms but were probably useful to an ancestor
3. Embryology – Evolutionary biologists compare structures that appear during the development of different organisms. All of the different classes of vertebrates show a structures called gill slits that appear on the side of the throat and all have a tail as an embryo. As development continues, the differences in the embryos will increase until you can distinguish among them. The similarities among the young embryos suggest evolution from a distant, common ancestor.
4. Biochemistry – Evolutionary relationships among species leave signs in DNA and proteins; in genes and gene products. Scientists compare DNA and RNA of different species and use the results of biochemical studies to help determine the evolutionary relationships of species. One of the most recently developed classification systems for organisms is shown in the phylogenetic tree below and is based on comparisons of DNA and RNA.
Mechanisms for Evolution A. Population Genetics – the studies of the complex behavior of genes in populations of organisms or allele frequencies. v Populations consist of all the members of a species that live in an area with each member of that group possessing genes that characterize the traits of the species, and these genes exist as pairs of alleles. v Evolution occurs as a population’s genes and their frequencies change over time. v Genetic change is the result of a change in the population’s gene pool and the allelic frequency. v A population in which the frequency of alleles remains the same over generations is said to be in genetic equilibrium and is not evolving. v Mechanisms for genetic change include mutations, genetic drift, and gene flow.
v Mutations – may be caused from environmental factors or may be spontaneous; may be lethal or may result in a useful variation allowing the new gene to becomes part of the population’s gene pool by the process of natural selection
Genetic Drift – genes are lost in small populations due to chance events; genes carried by the parents might not be passed on to their offspring; loss of genetic diversity In this example, the parents carry three different genes for a particular trait: a, A and B, but they pass only a and A to their offspring. Gene B is lost due to chance. v
n n Bottleneck Effect: genetic drift that occurs after an event greatly reduces the size of a population Examples: overhunting of a species leads to a great reduction in the number of individuals in the population which reduces the genetic diversity within that population
n n Founder Effect: genetic drift that occurs after a small number of individuals colonize a new area; the genes of these individuals are often very different from those of the larger population; common in communities established by a small number of immigrants Example: Tay-Sachs Disease
The Founder Effect
Sexual Selection n n 1. 2. Reproduction can be costly for females as far as energy is concerned therefore females can be very choosy about mates. Sexual selection occurs when certain traits increase mating success. Intrasexual selection = male competition Intersexual selection = males display traits that attract females; some traits can become very exaggerated such as red air sacs in male frigate birds
v Gene Flow – As individuals move in and out of a population by migrating, the genetic equilibrium is disrupted. When individuals leave a population, genes are lost from the gene pool, and when individuals enter a population, their genes are added to the gene pool.
B. Speciation – The evolution of new species that occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.
Speciation can occur due to a number of factors. 1. Geographic isolation – occurs when a physical barrier divides a population; over time, each small population might adapt to its environment through natural selection and develop its own gene pool and result in new species
2. Reproductive isolation – occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring v Genetic material of the populations becomes so different that fertilization cannot occur. v Behavior may prevent reproduction. Two populations may mate at different times of the year.
Patterns of Evolution v Biologists have observed different patterns of evolution that occur throughout the world in different natural environments. Divergent Evolution – occurs when species that once were similar to an ancestral species diverge, or become increasingly distinct Adaptive radiation – occurs when an ancestral species evolves into an array of species to fit a number of diverse habitats Examples – Darwin’s finches of the Galapagos Islands and Hawaiian Island honeycreeper
v Convergent Evolution – distantly related organisms evolve similar traits and unrelated species occupy similar environments in different parts of the world; they share similar pressures of natural selection due to similar environments Examples – The Senita cactus of Mexico is similar in appearance and adaptations to its environment as the Euphorbia from Madagascar. They both fill a similar niche in the ecosystem of their respective habitats.
Darwin's Theory 1. 2. 3. 4. 5. Individual Organisms In Nature Differ From One Another. Some Of This Variation Is Inherited. Organisms In Nature Produce More Offspring Than Can Survive, And Many Of These Offspring Do Not Reproduce. Because More Organisms Are Produced Than Can Survive, Members Of Each Species Must Compete For Limited Resources. Because Each Organism Is Unique, Each Has Different Advantages & Disadvantages In The Struggle For Existence. Individuals Best Suited To Their Environment Survive & Reproduce Successfully – Passing Their Traits To Their Offspring.
6. 7. 8. Species Change Over Time. Over Long Periods, Natural Selection Causes Changes That May Eventually Lead To New Species Alive Today Have Descended With Modifications From Species That Lived In The Past. The Greatest Potential For Evolutionary Change Occurs When Many Mutations Occur Within A Small Population.