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Theory of Evolution. Biology for Majors. Descent with Modification. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. Most characteristics of organisms are inherited, or passed from parent to offspring.
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Theory of Evolution Biology for Majors
Descent with Modification Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. • Most characteristics of organisms are inherited, or passed from parent to offspring. • More offspring are produced than are able to survive, so resources for survival and reproduction are limited. Thus, there is competition for those resources in each generation. • Offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification.
Natural Selection Ultimately, natural selection leads to greater adaptation of the population to its environment as in the beaks of the finches which were adapted to different food sources.
Evolution Natural selection, the driving force behind evolution, can only work if variation exists among organisms. Variation arises ultimately from genetic mutations. Diversity is further encouraged through sexual reproduction. As environments change, selective pressures shift and favor different adaptations. In this way, given thousands or millions of years, species evolve.
Physical Evidence of Evolution: Fossils Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution. For example, scientists have recovered highly detailed records showing the evolution of humans and horses.
Physical Evidence for Evolution: Anatomy Homologous structures and vestigial structures across diverse groups of related organisms, such as leg bones at right, provide support for the theory of evolution.
Physical Evidence for Evolution: Convergent Evolution More evidence of evolution is the convergence of form in organisms that share similar environments. The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations to their environments.
Divergent Evolution When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; but look very different as a result adaptation to different kinds of pollinators.
Convergent Evolution In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestor.
Physical Evidence for Evolution: Embryology Embryology provides evidence of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have such magnified consequences in the adult that embryo formation tends to be conserved. As a result, structures that are absent in some groups often appear in their embryonic forms and disappear by the time the adult or juvenile form is reached. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth.
Evidence for Evolution from Biogeograpy Biogeography offers further clues about evolutionary relationships. The presence of related organisms across continents indicates when these organisms may have evolved. For example, some flora and fauna of the northern continents are similar across these landmasses but distinct from that of the southern continents. Islands such as Australia and the Galapagos chain often have unique species that evolved after these landmasses separated from the mainland.
Evidence for Evolution from Molecular Biology Molecular biology provides data supporting the theory of evolution. In particular, the universality of DNA and near universality of the genetic code for proteins shows that all life once shared a common ancestor. DNA also provides clues into how evolution may have happened. Gene duplications allow one copy to undergo mutational events without harming an organism, as one copy continues to produce functional protein.
Not “just a theory” Many misconceptions exist about the theory of evolution—including some perpetuated by critics of the theory. First, evolution as a scientific theory means that it has years of observation and accumulated data supporting it. It is not “just a theory” as a person may say in common vernacular.
Misconceptions of Evolution Another misconception is that individuals evolve, though in fact it is populations that evolve over time. Individuals simply carry mutations. Furthermore, these mutations neither arise on purpose nor do they arise in response to an environmental pressure. Instead, mutations in DNA happen spontaneously and are already present in individuals of a population when a selective pressure occurs. Once the environment begins to favor a particular trait, then those individuals already carrying that mutation will have a selective advantage and are likely to survive better and outproduce others without the adaptation.
Misconceptions of Evolution Finally, the theory of evolution does not in fact address the origins of life on this planet. Scientists believe that we cannot, in fact, repeat the circumstances that led to life on Earth because at this time life already exists. The presence of life has so dramatically changed the environment that the origins cannot be totally produced for study.
Microevolution and Population Genetics Microevolution, or evolution on a small scale, is defined as a change in the frequency of gene variants, alleles, in a population over generations. The field of biology that studies allele frequencies in populations and how they change over time is called population genetics. Microevolution is sometimes contrasted with macroevolution, evolution that involves large changes, such as formation of new groups or species, and happens over long time periods. However, most biologists view microevolution and macroevolution as the same process happening on different timescales. Microevolution adds up gradually, over long periods of time to produce macroevolutionary changes.
Populations A population is a group of organisms of the same species that are found in the same area and can interbreed. A population is the smallest unit that can evolve—in other words, an individual can’t evolve.
Alleles An allele is a version of a gene, a heritable unit that controls a particular feature of an organism. When the alleles are different, one—the dominant allele, W—may hide the other—the recessive allele, w. A plant’s set of alleles, called its genotype, determines its phenotype, or observable features, in this case flower color.
Allele Frequency Allele frequency refers to how frequently a particular allele appears in a population. In general, we can define allele frequency as: Frequency of allele A= If there are more than two alleles in a population, add up all of the different alleles to get the denominator. It’s also possible to calculate genotype frequencies—the fraction of individuals with a given genotype—and phenotype frequencies—the fraction of individuals with a given phenotype. These are different concepts from allele frequency.
Natural Selection and Fitness Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness. Relative fitness measures which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.
Frequency-Dependent Selection In frequency-dependent selection individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) phenotypes are selected for.
Sexual Selection Sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two (below).
Phylogenetic Tree A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships.
Rooted and Unrooted Phylogenetic Trees Both of these phylogenetic trees shows the relationship of the three domains of life—Bacteria, Archaea, and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor while the (b) unrooted tree does not.
UnderstandingPhylogeneticTrees The root of a phylogenetic tree indicates that an ancestral lineage gave rise to all organisms on the tree. A branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is a basal taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy.
Limitations of Phylogenetic Trees Groups that are not as closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a closer relative. For example, the phylogenetic tree in Figure 1 shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs appear more similar than lizards and rabbits.
Things to Remember about Phylogenetic Trees • Unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. • Any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms on the previous slide, just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed.
Taxonomy The taxonomic classification system uses a hierarchical model to organize living organisms into increasingly specific categories.
Taxonomy At each sublevel in the taxonomic classification system, organisms become more similar.
Genetics and Taxonomy Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, changes and updates must be made as new discoveries occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available.
Practice Question Natural selection acts on individuals, not alleles. Explain the significance of this statement.
Quick Review • What was Charles Darwin’s theory of natural selection? • What evidence supports the theory of evolution by natural selection? • What role do mutations play in microevolution? How do adaptations enhance the survival and reproduction of individuals in a population? • How do phylogenetic tree document evolutionary relationships?
