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Explore the principles of Darwin's theory of natural selection and the evidence that supports the idea of biological change over time. Learn about adaptations, fossil records, anatomy, biochemistry, and more. Discover how species evolve through variation, heritability, overproduction, and reproductive advantage.
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Biological Change Natural Selection and the Evidence for Evolution
What is evolution? • The modern theory of evolution is a fundamental concept in biology • Evolution – change in populations over time • Charles Darwin was the first to publish his ideas of how species evolve
Common descent • The scientific theory that all living organisms on Earth descended from a common ancestor. • The structures and functions of all living organisms are encoded in the same basic nucleic molecules, DNA and RNA. • Similarities in amino acid sequences between various organisms also suggest common descent • The fossil record also shows cases in which one plant or animal type evolved into different types over time.
Charles Darwin • Began his work in 1831 (age 21) as the naturalist on the HMS Beagle • As the ship’s naturalist, it was his job to study and collect biological specimens at each port along the rout • On the Galapagos Islands, Darwin studied many species that were unique to the islands, but similar to species elsewhere. These observations led him to consider the possibility that species can change over time. • It took him 22 years to find an explanation for how species change over time
Charles Darwin • 1859 published On the Origin of Species • his theory of natural selection to explain how organisms evolve • Darwin developed his ideas while sailing on the Beagle
Adaptations: Evidence for Evolution • Structural adaptations arise over time • Mimicry – where one species resembles another species • Camouflage – where a species blends with their surroundings • Physiological adaptations arise over time • Antibiotic resistance of bacteria
Other Evidence for Evolution • Fossils • Anatomy • Homologous structures – structures that are similar in anatomy but have different function • Common evolutionary origin • Analogous structures – body parts similar in function but have different structure • No common evolutionary origin • Vestigial structures – body structure that has no function in present-day organisms but was probably useful to an ancestor • Biochemistry • DNA, RNA
More Evidence for Evolution • Embryology As development continues from embryo to a more mature organism, the differences increase, however, in the earliest stages of growth and development, many vertebrate embryos are remarkably similar
Biological Change Species Change
Principles of Darwin’s Theory of Natural Selection • Variation • Heritability • Overproduction • Reproductive Advantage
Remember…The Principles of Darwin’s Theory • Overproduction of offspring • Variation • Heritabiltiy • Reproductive Advantage (preferential selection of traits)
Species Change • Organisms that are best matched to their environment are more likely to survive and reproduce • Adaptation – a trait that improves an organism’s change for survival and reproduction
Adaptation • Individual members of a single species exhibit differences in their appearance and function • Difference result from random changes in genetic material from sexual reproduction and mutations
Natural Selection • Organisms with traits that help them survive and reproduce pass their characteristics to their offspring. • Helpful traits survive and spread through the population • Harmful traits disappear over time • As a result, a population may evolve into a new species
Natural Selection reproduction Inherited traits Random mutations Variation Mismatch with environment Adaptations Survival Death; no offspring
Populations evolve, not individuals! • Variation exists among individuals within a population • An individual with variations that make them poorly adapted to the environment will not survive and reproduce • Remember genotypes (genes) and phenotypes (expression of genes) • Evolution occurs as a population’s genes and their frequencies change over time
Natural Selection Acts on Variation • Some variations increase or decrease an organism’s chance of survival in a particular environment • Three types • Stabilizing selection • Directional selection • Disruptive selection
Types of Natural Selection • Directional selection • Occurs when individuals at one end on the frequency distribution are better adapted to the environment than those in the middle • Disruptive selection • Occurs when individuals near the upper and lower ends of the distribution are better adapted than those in the middle • Stabilizing selection • Occurs when individuals near the center of the distribution are more fit than individuals at either end
Disruptive Selection Stabilizing Selection Directional Selection
Determine the type of natural selection indicated by the following examples. • Members of a population of Amazon tree frogs hop from tree to tree searching for food in the rain forest. They vary in leg length. Events result in massive destruction of the forest’s trees. After several generations, only long-legged tree frogs remain alive. • Different grass plants in a population range in length from 8cm to 28 cm. The 8-10 cm grass blades receive little sunlight, and the 25-28 cm grass blades are eaten quickly by grazing animals. • The spines of sea urchin population’s members vary in length. The short-spined sea urchins are camouflaged easily on the seafloor. However, long-spined sea urchins are well defended against predators.
Question • A population of woodpeckers have beak lengths ranging from 2cm-4cm. The woodpeckers with 3 cm beaks are able to reach more insects in the trees in which they feed. Explain the following: A.Is longer beak length an advantageous adaptation? B. How would you expect the population of woodpeckers to evolve after a very long period of time? C. What type of selection would this be an example of? Explain your answer.
Classification of organisms • Organisms are classified based on internal and external characteristics • Species – most specific unit of classification
The Evolution of Species • Speciation – the process of the evolution of a new species • Occurs when members of similar populations no longer interbreed and produce fertile offspring
Evolutionary History • Phylogeny describes the evolutionary history of a related group of species • All organisms on Earth evolved from a single ancestor • Life on earth began about 3.5 billion years ago; since that time, new species have emerged, lived and died out • New species evolve from pre-existing species
Fossil Evidence • Through fossil evidence, physical and molecular similarities between ancient species and modern species have been found. • Physical and molecular similarities between diverse species has also been found.
Extinction • Organisms try to survive in their environments through adaptation, however many species become extinct • Extinction can occur because of environmental changes, human interference or as a result of failure to adapt to new conditions
Extinction Cont. • Extinction is a natural and important part of evolution • It is estimated that 999 of every 1,000 species that have ever lived on Earth have become extinct • The average species survives between 2 and 10 million years • Even the most highly adapted species become extinct.
Mass Extinctions • There have been many mass extinctions during Earth’s history • Mass extinction = when more than 50% of species were wiped out • Mass extinctions make it possible for new species to develop • Surviving species are able to diversify
Biological Change Identifying change in species
Changes in genetic equilibrium • Mutations cause genetic change • Caused by environmental factors such as radiation, chemicals, or can simply occur by change • If a mutation is useful, it persists and becomes part of the gene pool • Lethal mutations cause death and are quickly eliminated from the gene pool
Mutations • Natural Mutations occur at a regular rate. • The number of differences between the genetic material and different species estimates how long ago two species share a common ancestry
Types of Evolution • Macroevolution: The generation of major change in the assemblage of organisms: speciation • Microevolution: Changes in the gene pool of a population that result in changes in allele frequencies; they arise without the influence of selection pressure
Mechanisms of Evolution Populations evolve, not individuals
Population Genetics • When Charles Darwin developed his theory of natural selection in the 1800’s, he did so without knowing about genes • The principles of today’s modern theory of evolution are rooted in population genetics. • Today we refer to the Synthetic Theory of Evolution, which included the principled of genetics
Genes and Populations • Gene pool: The collection of genes in a population • Because diploids have only two versions of each gene, each has only a small fraction of possible alleles in a population • Genotype: The genetic makeup of an individual at a given locus, taking into account the two possible alleles • Genotype frequency is the proportion of a given genotype in the population • Allele frequency refers to the proportion of a particular allele, such as A or a • Phenotype: the traits of an individual • Phenotype frequency is the proportion of a given phenotype in the population • Phenotype frequency is influenced by the dominance characteristic of an allele
Disruption to Genetic Equilibrium • Genetic Drift – alteration of allelic frequencies by chance events • Can greatly affect small populations • Ex. Amish of Lancaster County Pennsylvania – 6 fingers and toes • Individuals in this community have a 1 in 14 chance of having this mutation • Individuals in the larger population of the United States only have a 1 in 1,000 chance of having the mutation
Disruption to Genetic Equilibrium • Gene flow – the transport of genes by migrating individuals • When an individual leaves a population, it takes its genes with it • When an individual enters a population, it introduces new genes
Alleles and Population Genetics • Although individuals are affected by the process of natural selection, it is the makeup of the population that is critical for determining the subsequent generations • Changes in the gene pool refer to changes in the frequency of the alleles • If the allele frequencies in a population do not undergo change over time, we say that the population is in genetic equilibrium, the population is not evolving.
Population Stability • The conventional view might be that dominant alleles would eventually come to dominate the gene pool, and the recessives disappear • They do not necessarily do so; in fact, allele frequencies change only when influenced by other factors. • The stability of populations over time is explained by the Hardy-Weinberg Equilibrium.
Hardy Weinberg Principle • This condition can be modeled as the Hardy-Weinberg Equilibrium, which requires: • Large population size • No migration • Random mating • No net mutations • All genotypes have similar selective value • This is idealized and rarely actually occurs, but is a useful tool
The Ps and Qs of H&W • Imagine 2 alleles, A and a • p is the frequency of A • q the frequency of a • So, p + q = 1 • The mathematical equivalent of a random mating can be given by multiplying this relationship by itself • Therefore, (p + q)2 = 1 = p2 + 2pq + q2 • p2 = frequency of AA • 2pq = frequency of Aa • q2 = frequency of aa • Given this condition, we can always work out the frequencies of each allele in a sexual population that is not evolving.
H-W: Example • Remember, since (p + q)2 = 1 • p2 + 2pq + q2 = 1 • Let’s say that a population has the following genotypic and allelic frequencies • Note how all frequencies add up to 1.0
A Fun Experiment in Class • Tongue rolling is described by a simple dominant character, T and we can study the HW equilibrium using this trait, in this class 1. Find the frequency of homozygous recessives (q2)in the class • Can you roll your tongue? If so you are either TT or Tt • Note how many can roll tongues ________ • If not, you are tt • Note how many cannot roll tongues ________ • Take this number and divide by the class total: ______; this is the frequency of homozygous recessives (q2). 2. What is the frequency of p? • Since p + q = 1, then 1 - q = p • Take the root of q2 from above. ___________ • We can now calculate p. p = 1 - q
5 Factors Upset Genetic Equilibrium • Mutation • Nonrandom mating • Genetic Drift • Gene Flow • Natural Selection • All of these are conditions that were required by the H-W equilibrium to NOT occur • They cause changes in allelic frequency, and result in microevolution • They all occur routinely
Population Genetics • Populations evolve not individuals! • All changes in a population occur at the gene level • Variations lead to adaptations • Adaptations that are beneficial become more common in the population • Genotype frequencyis the proportion of a given genotype in the population