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Explore the principles of descent with modification and natural selection, as well as the evidence for evolution, including fossil records, anatomy, embryology, biogeography, and molecular biology. Understand the process of how species evolve and adapt over time.
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History of Life Biology for Majors
Descent with Modification Natural selection, Darwin argued, was the 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 thought offspring with inherited characteristics that help them compete 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.
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 of 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 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.
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 Biogeography 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.
Species A species is a group of individual organisms that interbreed and produce fertile, viable offspring. Populations of species share a gene pool: a collection of all the variants of genes in the species. While dogs look different from one another they are one species because they can interbreed.
Speciation Speciation is the formation of two species from one original species. Two new populations must be formed from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Allopatric speciation results from geographic separation of populations from a parent species Sympatric speciation occurs within a parent species without a geographic separation.
Speciation As one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism goes extinct. Darwin illustrates this in a. A modern taxonomy of elephants is shown in b.
Allopatric Speciation Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms. The further the distance between the two groups, the more likely it is that speciation will occur since they face different environmental conditions.
Adaptive Radiation From one original species of bird, multiple others evolved, each with its own distinctive characteristics.
Autopolyploidy Autopolyploidy results when mitosis is not followed by cytokinesis.
What does this have to do with speciation? These new gametes will be incompatible with the normal gametes produced by this plant species. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.
Alloploidy Alloploidy results when two species mate to produce viable offspring. A normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.
Polyploidy in Plants Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.
Reproductive Isolation Many types of diverging characters may affect the reproductive isolation, the ability to interbreed, of the two populations. • Prezygotic barrier blocks reproduction from taking place and include differences such asgameticbarriers and incompatible reproductive organs • Postzygotic barrier occurs after zygote formation • Temporal isolation – differences in breeding schedules • Habitat isolation – physical separation • Behavioral isolation – the presence or absence of a specific behavior prevents mating
Hybrid Zone A hybrid zone is an area where two closely related species continue to interact and reproduce, forming hybrids.
Rates of Speciation: Gradual In gradual speciation, species diverge at a slow, steady pace as traits change incrementally.
Rates of Speciation: Punctuated Equilibrium In punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.
Population Genetics and Allele Frequency Population genetics studies how selective forces change a population through changes in allele and genotypic frequencies. The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population. Until now we have discussed evolution as a change in the characteristics of a population of organisms, but behind that phenotypic change is genetic change. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. The gene pool is the sum of all the alleles in a population.
Hardy-Weinberg Principle of Equilibrium Hardy-Weinberg Principle of Equilibrium states that a population’s allele and genotype frequencies are inherently stable— unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy-Weinberg principle assumes conditions with no mutations, immigration, emigration, or selective pressure for or against a genotype, plus an infinite population. While no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes.
Heritability and Genetic Variance Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. The diversity of alleles and genotypes within a population is called genetic variance. Inbreeding, the mating of closely related individuals, can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease.
Causes of Change in Genotype Frequencies A population’s allele and genotype frequencies can change because of: • selection pressure, or driving selective force • genetic drift or the effect of chance
Genetic Drift Genetic drift in a population can lead to the elimination of an allele from a population by chance.
Bottleneck Effect Small populations are more susceptible to the forces of genetic drift. The bottleneck effect is when a chance event or catastrophe can reduce the genetic variability within a population.
Founder Effect Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers.
Gene Flow Gene flow is the flow of alleles in and out of a population due to the migration of individuals or gametes.
Mutations Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance.
Nonrandom Mating If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location.
Geographic Variation Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient. If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.
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.
Adaptive Evolution 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.
