Chapter 19 – Evolution of Life I: How Populations Change from Generation to Generation - Comprehensive Notes

Chapter 19 – Evolution of Life I: How Populations Change from Generation to Generation

Overview of Evolution

• Evolution is a change in one or more heritable characteristics of a population from one generation to the next.
• Evolution can lead to the formation of new species.
• Selective breeding: a human-driven process of changing the genetic information and characteristics of a population over time.
• Empirical thought relies on observation to form an idea or hypothesis.
• In the 1600s, a shift toward empirical thought encouraged scholars to rely on observation to understand life and the natural world.

The Work of Several Scientists Set the Stage for the Ideas of Darwin and Wallace

• In the late 1700s, some European scientists challenged the belief that life-forms are fixed and unchanging.
• George Buffon (French zoologist) proposed that populations of living things change over time.
• Jean-Baptiste Lamarck (French naturalist) examined fossils and realized that some animals remained the same while others changed over time.
  • Hypothesized species change over generations by adapting to new environments.
  • The mechanisms proposed by Lamarck were inaccurate, however, the work was important in promoting the idea of evolutionary change.
• Erasmus Darwin (grandfather of Charles Darwin) was a contemporary of Buffon and Lamarck and an advocate for evolutionary change; he noted how breeders changed the traits of domesticated plants and animals.

Darwin and Wallace Suggested That Existing Species Are Derived from Pre-existing Species

• Charles Darwin's thinking was influenced by work in other fields and his own observations on the Beagle's roughly 5-year journey.
• James Hutton and Charles Lyell (Scottish geologists) supported the uniformitarianism hypothesis.
  • Slow geological processes (ex: erosion) lead to substantial change over time, which implied that the earth was much older than 6,000 years.
• Thomas Malthus (English economist) wrote about limits to population growth and that not all members of a population will survive and reproduce.
• Charles Darwin noted distinctive traits of island species that allowed them to better exploit their environment.
  • Ex: Galapagos island finches exhibited differences that provided them with specialized feeding strategies.
• Similarly, Alfred Wallace was also involved with specimen collection activities in Indonesia, Malaysia, and Singapore.

Darwin and Wallace Arrived at the Same Conclusions

• In 1856, Charles Darwin began writing a book to explain his ideas regarding evolution.
• In 1858, Alfred Wallace (British naturalist) sent Darwin an unpublished manuscript proposing many of the same ideas about evolution.
• Two papers, one by Wallace and one by Darwin, were published together in the Proceedings of the Linnean Society of London.
• In 1859, Darwin's book On the Origin of Species was published.
• Darwin and Wallace both suggested that existing species are derived from pre-existing species.

Natural Selection Changes Populations from Generation to Generation

• Darwin expressed his ideas about evolution as “the theory of descent with modification through variation and natural selection”
• Evolution refers to a process of change; according to Darwin’s ideas, evolution occurs from generation to generation due to 2 interacting factors: genetic variation and natural selection.
• Variations in traits may occur among individuals of a given species; variations are based on genetic differences and are heritable (passed from parents to offspring).
• In each generation, more offspring are usually produced than will survive and reproduce (often due to resource limitations).
• During the process of natural selection, individuals with heritable traits that make them better suited to their native environment tend to flourish and reproduce whereas other individuals are less likely to survive and reproduce.
• As a result of natural selection, certain traits that favor reproductive success become more prevalent in a population over time.

Evidence of Evolutionary Change

• Evidence that reflects the process of evolution has been obtained from many sources.

Fossils Show Successive Evolutionary Change

• Fossils are the preserved remains of past life; the fossil record provides evidence of the history of life on Earth.
• Fossils show evolutionary change in Tiktaalik roseae (nicknamed fishapod).
  • It is a transitional form because it displays an intermediate state between an ancestral form (fish) and the form of its descendants (tetrapods).
  • Unlike a true fish, T. roseae had a broad skull, flexible neck, eyes on top of the head, primitive wrist and five finger-like bones.
• The changes observed in the fossil record of whales reveal a progression over the past 50 million years from a terrestrial tetrapod to aquatic animals that lack hind limbs and have many adaptations that are beneficial in an aquatic environment.
• Genus Pakicetus comprised the earliest known whales – wolflike meat eaters that spent time in fresh water, have a long skull, have a bony wall around middle ear (found in modern whales but not in other mammals).
• Genus Ambulocetus – semiaquatic whales, inhabited brackish water, large tail vertebrae, possibly used tail for swimming, eyes toward sides.
• Genus Remingtonocetus – lived in saltwater, fat pad in jaw that aided underwater hearing
• Genus Rodocetus – nasal opening shifting away from tip of snout, only 4 toes on hind limb (suggesting hind limb degeneration).
• Genus Dorudon – completely aquatic whales, had a blowhole, forelimbs became flippers, tiny hindlimbs, tail modified to form fluke.
• Odontoceti and Mysticeti are suborders of Cetacea (includes many extinct species and all modern species of whales, dolphins, and porpoises) – complete loss of hind limb, blowhole is nasal opening.

Biogeography Indicates That Species in a Given Area Have Evolved from Pre-existing Species

• Biogeography is the study of the geographic distribution of extinct and living species.
• Isolated continents and island groups have evolved their own distinct plant and animal communities.
• Islands often contain many endemic species, species naturally found only in a particular location.
• Ex: the island fox (Urocyon littoralis) which lives on the Channel Islands evolved from the mainland gray fox (Urocyon cinereoargenteus).

Convergent Evolution Suggests Adaptation to the Environment

• Convergent evolution occurs when two species from different lineages have independently evolved similar characteristics because they occupy similar environments.
• Similar characteristics due to convergent evolution are called analogous structures.

Selective Breeding Is a Human-Driven Form of Selection

• Selective breeding refers to programs and procedures designed to modify traits in domesticates species.
• Also called artificial selection; human breeders select which individuals will reproduce based on desirable traits (whereas nature “chooses” parents through natural selection).
• Genetic variation makes selective breeding possible; humans have employed selective breeding for centuries.

A Comparison of Homologies Shows Evolution of Related Species from a Common Ancestor

• Homology refers to a similarity that occurs due to descent from a common ancestor; homologies may involve anatomical, developmental, or molecular features.
• The limb bones of modern vertebrates are an example of anatomical homology.
  • The same set of bones has undergone evolutionary changes, becoming modified to perform different functions in different species.
• Vestigial structures are anatomical features that have no current function but resemble structures of presumed ancestors.
• Developmental homology refers to similarities that occur during development; species that differ as adults are often similar during embryonic stages.
  • During human development, there are several features of embryos that are not present at birth.
  • Human embryos have gill ridges, like a fish embryo, even though oxygen is supplied via the umbilical cord.
  • Human embryos have a bony tail
  • Closely related species share similar developmental pathways.
• Molecular homology refers to similarities that occur at the molecular level.
  • The same gene is often found in diverse organisms.
  • The degree of similarity between genetic sequences from different species reflects the evolutionary relatedness of those species.

Genes in Populations

• The gene pool includes all of the alleles for every gene in a population.
• Individuals that reproduce contribute to the gene pool of the next generation.
• Population genetics is the study of the genetic variation within a gene pool and how variation changes from one generation to the next.
• Emphasis is often on variation in alleles among members of a population.

Populations are Dynamic Units

• A population is a group of individuals of the same species that occupy the same environment at the same time and can interbreed with one another.
• Some species occupy a wide geographic range and are divided into discrete populations.
• Populations may change in size and location from one generation to the next; genetic composition usually changes as well.
• Genetic changes may involve adaptation, in which a population becomes better suited to its environment.

Genes Are Usually Polymorphic

• Polymorphism refers to the presence of two or more variants for a given character within a population (ex: two color variations).
• A polymorphic gene commonly exists as two or more alleles in a population (each allele occurs at a frequency greater than 1%).
• A monomorphic gene exists predominantly as a single allele in a population (99% or more alleles of a given gene are identical).
• How can genes become polymorphic?
  • deletion, duplication, or change in a single nucleotide
  • A single-nucleotide polymorphism (SNP; pronounced “snip”) is the smallest type of genetic variation (i.e., a point mutation) that can occur within a gene and is the most common
  • SNPs represent 99% of all variation in human DNA sequences among different people
  • A human gene of 2,000 to 3,000 bp in length has an average of 10 SNPs
  • Large, healthy populations exhibit a high level of genetic diversity

Population Genetics Is Concerned with Allele and Genotype Frequencies

• Genetic variation can be analyzed quantitatively; calculations of allele frequency and genotype frequency are fundamental when analyzing population genetics.

The Hardy-Weinberg Equation Relates Allele and Genotype Frequencies in a Population

• The Hardy-Weinberg equation describes the relationship between allele and genotype frequencies when a population is not evolving.
• For genes that exist in 2 alleles, p + q = 1
  • p is the allele frequency of one allele (ex: CR)
  • q is the frequency of the second allele (ex: CW)
• The Hardy-Weinberg equation predicts that allele and genotype frequencies will remain the same, generation after generation, for a population in equilibrium.
• If p + q = 1, then (p + q)^2 = 1^2
• The Hardy-Weinberg equation states that p^2 + 2pq + q^2 = 1
  • p^2 = the genotype frequency of homozygotes
  • 2pq = the genotype frequency of heterozygotes
  • q^2 = the genotype frequency of homozygotes
• In a population, the frequency of a gamete carrying a particular allele is equal to the allele frequency in that population
• To be in equilibrium, evolutionary mechanisms that can change allele and genotype frequencies are not acting on a population

Conditions for Hardy-Weinberg Equilibrium:

• No new mutations occur to alter allele frequencies
• No natural selection occurs
• The population is so large that allele frequencies do not change due to chance
• No migration occurs between different populations
• Random mating does occur
• In reality, populations rarely reach equilibrium
• If frequencies are not in equilibrium, an evolutionary mechanism is at work

Microevolution Involves Changes in Allele Frequencies from One Generation to the Next

• The term microevolution is used to describe changes in a population’s gene pool (such as changes in allele frequencies) from generation to generation.
• Microevolution happens due to the introduction of new genetic variation and evolutionary mechanisms that alter the frequencies of existing genetic variation.

Natural Selection

• Natural selection is the process by which individuals with certain heritable traits tend to survive and reproduce at higher rates than those without those traits.
• Over time, natural selection may result in adaptations, changes in populations of living organisms that increase their ability to survive and reproduce in a particular environment.
• As a result of natural selection, certain traits that favor reproductive success become more prevalent in a population over time.

Natural Selection Favors Individuals with Greater Reproductive Success

• Reproductive success is the likelihood of an individual contributing fertile offspring to the next generation.
• Natural selection occurs because some individuals in a population have greater reproductive success than others.
• Reproductive success is commonly attributed to 2 categories of traits:
  • Characteristics that make organisms better adapted to their environment (and therefore more likely to survive and reproduce)
  • Traits that are directly associated with reproduction (like finding a mate)

Fitness is a Quantitative Measure of Reproductive Success

• Fitness (w) is the relative likelihood that one genotype will contribute to the gene pool of the next generation compared with other genotypes; fitness is a measure of reproductive success.
• By convention, the genotype with the highest reproductive success is assigned a fitness value of 1.0; fitness values of other genotypes are assigned relative to this value

Natural Selections Follows Different Patterns

• Natural selection can occur in several ways
• Patterns include directional selection, stabilizing selection, diversifying/disruptive selection, and balancing selection
• During directional selection, individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment
• Stabilizing selection favors the survival of individuals with intermediate phenotypes and selects against those with extreme phenotypes
• Diversifying (disruptive) selection favors the survival of two or more different genotypes that produce different phenotypes
• Balancing selection maintains genetic diversity in a population. Over many generations, balancing selection results in balanced polymorphism, where 2 or more alleles are kept in balance and maintained in a population
  • Balancing selection may involve:
    • Heterozygote advantage, when heterozygotes have the highest fitness
    • Negative frequency-dependent selection where common individuals have a lower fitness, and rare individuals have a higher fitness (important among certain prey populations)

The Grants Observed Natural Selection in Galapagos Finches

• Evolutionary biologists Rosemary Grant and Peter Grant have studied natural selection in finches found on the Galapagos Islands for decades
• Among other traits, the Grants have measured the beak sizes of parents and offspring

Sexual Selection is a Type of Natural Selection Pertaining to Traits That Are Directly Involved with Reproduction

• Sexual selection is a form of natural selection by which individuals with certain traits are more likely than others to engage in successful mating
• In intrasexual selection, members of one sex (usually males) directly compete with each other for the opportunity to mate with individuals of the opposite sex
• In intersexual selection, members of one sex (usually females) choose their mates on the basis of certain desirable characteristics

Genetic Drift

• Genetic drift refers to changes in allele frequencies due to random chance; changes in allele frequencies due to genetic drift happen regardless of the fitness of the individuals that carry those alleles.
• Over many generations, genetic drift favors either the elimination or the fixation of an allele.
• Changes in allele frequency due to genetic drift are faster in small populations.
• Genetic drift can rapidly alter allele frequencies when the size of a population dramatically decreases (bottleneck effect, founder effect).
• Populations may experience a bottleneck, a drastic reduction in size where members are eliminated without regard to their genetic composition (ex: flood, drought, earthquake).
  • The bottleneck effect refers to the change in allele frequencies of the resulting population due to genetic drift.
  • New population is likely to have less genetic variation.
• The founder effect occurs when a small group of individuals separates from a larger population and establishes a new colony in a new location
• Japanese evolutionary biologist Motoo Kimura proposed the neutral theory of evolution, in which genetic drift promotes the accumulation of neutral genetic changes that do not affect reproductive success
• Kimura agreed with Darwin that natural selection is responsible for adaptive changes in a species during evolution
  • His main ideas was that much of the variation in DNA sequences is explained by neutral variation rather than adaptive variation à not all genetic changes are adaptive

Migration and Nonrandom Mating

• In addition to natural selection and genetic drift, migration between populations and nonrandom mating may influence genetic variation and the relative proportions of genotypes
• Gene flow is the transfer of alleles into or out of a population; it occurs whenever individuals move between populations having different allele frequencies
• Migration and gene flow tend to enhance the genetic diversity within a population
• Nonrandom mating can take different forms:
  • Assortative mating occurs when individuals with similar phenotypes are more likely to mate à decreases heterozygotes
  • Disassortative mating occurs when individuals with dissimilar phenotypes are more likely to mate à increases heterozygotes
  • Inbreeding occurs when individuals choose a mate that is part of the same genetic lineage à increases proportion of homozygotes and decreases proportion of heterozygotes
  • Rare recessive diseases are more common when inbreeding occurs