Presentation on theme: "Natural Selection and the Evidence for Evolution"— Presentation transcript:
1Natural Selection and the Evidence for Evolution Biological ChangeNatural Selection and the Evidence for Evolution
2What is evolution?The modern theory of evolution is a fundamental concept in biologyEvolution – change in populations over timeCharles Darwin was the first to publish his ideas of how species evolve
3Common descentThe 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 descentThe fossil record also shows cases in which one plant or animal type evolved into different types over time.
4Charles DarwinBegan his work in 1831 (age 21) as the naturalist on the HMS BeagleAs the ship’s naturalist, it was his job to study and collect biological specimens at each port along the routOn 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
5Charles Darwin 1859 published On the Origin of Species his theory of natural selection to explain how organisms evolveDarwin developed his ideas while sailing on the Beagle
6Adaptations: Evidence for Evolution Structural adaptations arise over timeMimicry – where one species resembles another speciesCamouflage – where a species blends with their surroundingsPhysiological adaptations arise over timeAntibiotic resistance of bacteria
7Other Evidence for Evolution FossilsAnatomyHomologous structures – structures that are similar in anatomy but have different functionCommon evolutionary originAnalogous structures – body parts similar in function but have different structureNo common evolutionary originVestigial structures – body structure that has no function in present-day organisms but was probably useful to an ancestorBiochemistryDNA, RNA
9More Evidence for Evolution EmbryologyAs 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
13Principles of Darwin’s Theory of Natural Selection VariationHeritabilityOverproductionReproductive Advantage
14Remember… The Principles of Darwin’s Theory Overproduction of offspringVariationHeritabiltiyReproductive Advantage (preferential selection of traits)
15Species ChangeOrganisms that are best matched to their environment are more likely to survive and reproduceAdaptation – a trait that improves an organism’s change for survival and reproduction
16AdaptationIndividual members of a single species exhibit differences in their appearance and functionDifference result from random changes in genetic material from sexual reproduction and mutations
17Natural SelectionOrganisms with traits that help them survive and reproduce pass their characteristics to their offspring.Helpful traits survive and spread through the populationHarmful traits disappear over timeAs a result, a population may evolve into a new species
18Mismatch with environment Natural SelectionreproductionInherited traitsRandom mutationsVariationMismatch with environmentAdaptationsSurvivalDeath; no offspring
19Populations evolve, not individuals! Variation exists among individuals within a populationAn individual with variations that make them poorly adapted to the environment will not survive and reproduceRemember genotypes (genes) and phenotypes (expression of genes)Evolution occurs as a population’s genes and their frequencies change over time
20Natural Selection Acts on Variation Some variations increase or decrease an organism’s chance of survival in a particular environmentThree typesStabilizing selectionDirectional selectionDisruptive selection
21Types of Natural Selection Directional selectionOccurs when individuals at one end on the frequency distribution are better adapted to the environment than those in the middleDisruptive selectionOccurs when individuals near the upper and lower ends of the distribution are better adapted than those in the middleStabilizing selectionOccurs when individuals near the center of the distribution are more fit than individuals at either end
23Determine 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 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.
24QuestionA 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.
25Classification of organisms Organisms are classified based on internal and external characteristicsSpecies – most specific unit of classification
26The Evolution of Species Speciation – the process of the evolution of a new speciesOccurs when members of similar populations no longer interbreed and produce fertile offspring
27Evolutionary HistoryPhylogeny describes the evolutionary history of a related group of speciesAll organisms on Earth evolved from a single ancestorLife on earth began about 3.5 billion years ago; since that time, new species have emerged, lived and died outNew species evolve from pre-existing species
28Fossil EvidenceThrough 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.
31ExtinctionOrganisms try to survive in their environments through adaptation, however many species become extinctExtinction can occur because of environmental changes, human interference or as a result of failure to adapt to new conditions
32Extinction Cont.Extinction is a natural and important part of evolutionIt is estimated that 999 of every 1,000 species that have ever lived on Earth have become extinctThe average species survives between 2 and 10 million yearsEven the most highly adapted species become extinct.
33Mass ExtinctionsThere have been many mass extinctions during Earth’s historyMass extinction = when more than 50% of species were wiped outMass extinctions make it possible for new species to developSurviving species are able to diversify
34Identifying change in species Biological ChangeIdentifying change in species
35Changes in genetic equilibrium Mutations cause genetic changeCaused by environmental factors such as radiation, chemicals, or can simply occur by changeIf a mutation is useful, it persists and becomes part of the gene poolLethal mutations cause death and are quickly eliminated from the gene pool
36Mutations 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
37Types of EvolutionMacroevolution: The generation of major change in the assemblage of organisms: speciationMicroevolution: Changes in the gene pool of a population that result in changes in allele frequencies; they arise without the influence of selection pressure
38Mechanisms of Evolution Populations evolve, not individuals
39Population GeneticsWhen Charles Darwin developed his theory of natural selection in the 1800’s, he did so without knowing about genesThe 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
40Genes and PopulationsGene pool: The collection of genes in a populationBecause diploids have only two versions of each gene, each has only a small fraction of possible alleles in a populationGenotype: The genetic makeup of an individual at a given locus, taking into account the two possible allelesGenotype frequency is the proportion of a given genotype in the populationAllele frequency refers to the proportion of a particular allele, such as A or aPhenotype: the traits of an individualPhenotype frequency is the proportion of a given phenotype in the populationPhenotype frequency is influenced by the dominance characteristic of an allele
41Disruption to Genetic Equilibrium Genetic Drift – alteration of allelic frequencies by chance eventsCan greatly affect small populationsEx. Amish of Lancaster County Pennsylvania – 6 fingers and toesIndividuals in this community have a 1 in 14 chance of having this mutationIndividuals in the larger population of the United States only have a 1 in 1,000 chance of having the mutation
42Disruption to Genetic Equilibrium Gene flow – the transport of genes by migrating individualsWhen an individual leaves a population, it takes its genes with itWhen an individual enters a population, it introduces new genes
43Alleles 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 generationsChanges in the gene pool refer to changes in the frequency of the allelesIf 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.
44Population StabilityThe conventional view might be that dominant alleles would eventually come to dominate the gene pool, and the recessives disappearThey 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.
45Hardy Weinberg Principle This condition can be modeled as the Hardy-Weinberg Equilibrium, which requires:Large population sizeNo migrationRandom matingNo net mutationsAll genotypes have similar selective valueThis is idealized and rarely actually occurs, but is a useful tool
46The Ps and Qs of H&W Imagine 2 alleles, A and a p is the frequency of Aq the frequency of aSo, p + q = 1The mathematical equivalent of a random mating can be given by multiplying this relationship by itselfTherefore, (p + q)2 = 1 = p2 + 2pq + q2p2 = frequency of AA2pq = frequency of Aaq2 = frequency of aaGiven this condition, we can always work out the frequencies of each allele in a sexual population that is not evolving.
47H-W: Example Remember, since (p + q)2 = 1 p2 + 2pq + q2 = 1 Let’s say that a population has the following genotypic and allelic frequenciesNote how all frequencies add up to 1.0
48A 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 class1. Find the frequency of homozygous recessives (q2)in the classCan you roll your tongue? If so you are either TT or TtNote how many can roll tongues ________If not, you are ttNote 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 = pTake the root of q2 from above. ___________We can now calculate p. p = 1 - q
495 Factors Upset Genetic Equilibrium MutationNonrandom matingGenetic DriftGene FlowNatural SelectionAll of these are conditions that were required by the H-W equilibrium to NOT occurThey cause changes in allelic frequency, and result in microevolutionThey all occur routinely
50Population Genetics Populations evolve not individuals! All changes in a population occur at the gene levelVariations lead to adaptationsAdaptations that are beneficial become more common in the populationGenotype frequency is the proportion of a given genotype in the population
51Things that lead to change in populations Reproductive BarriersSexual selection - male/female choice in matesReproductive isolationPrezygotic isolationGeographic Isolation – when a physical barrier divides a populationEcological and behavioral barriersPostzygotic isolationHybrid offspring cannot develop or reproduce
52SpeciationSpeciation occurs when a population diverges, and can no longer breed and produce viable offspring, often due to reproductive isolationAllopatric speciation – when a physical barrier divides one population into two or more populations. The separate populations will eventually contain organisms that, over time will no longer be able to breed successfullySympatric speciation – when a species evolves into a new species without a physical barrier. Sometimes based on resource utilization, sometimes caused by polyploidy in plants.
53Patterns of Evolution Adaptive radiation (aka: divergent evolution) results when populations adapting to different environmental conditions change , becoming less alike as they adapt, resulting in new species.In other words, when an ancestral species evolves into an array of species to fit a number of diverse habitatsEx. Hawaiian Honeycreepers, Galapagos finches, African cichlidsConvergent Evolution – when distantly related organisms evolve similar traitsOccurs when unrelated species occupy similar environmentsSimilar environmental pressure Similar pressures of natural selection
54Patterns of EvolutionCoevolution – When the evolution of one species affects the evolution of another species.Mutualism is a form of coevolutionHummingbirds and flowersPoisonous newts and the predators that can eat them
55Speciation can occur quickly or slowly… Gradualism – the idea that species originate through a gradual change of adaptationsPunctuated Equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in betweenEnvironmental changes, or introduction of competitive species can lead to rapid changesHappens quickly – in about 10,000 years or lessBoth gradualism and punctuated equilibrium are supported by fossil evidence