Evolution: Microevolution and Macroevolution

Overview of Evolution

• Evolution involves heritable changes in one or more characteristics of a population or species across generations
• Microevolution refers to changes in a single gene within a population over time
• Macroevolution involves the formation of new species or groups of species

Species and Populations

• Species are groups of related organisms sharing a distinctive form
• Sexually reproducing species are capable of interbreeding to produce viable and fertile offspring
• Populations consist of members of the same species likely to encounter each other, thus having the opportunity to interbreed

History of Evolutionary Theory

• Empirical thought relies on observation to form ideas or hypotheses, rather than non-physical or spiritual views
• In the 1600s, there was a shift towards empirical thought to find a rationale behind processes and phenomena
• By the late 1700s, certain European scientists suggested that life forms are not fixed
• George Buffon proposed that life forms change over time
• Jean-Baptiste Lamarck noted that while some animals remained the same, others changed; he believed living things evolved upward toward human "perfection" and suggested the inheritance of acquired characteristics like giraffe necks
• The Uniformitarianism Hypothesis suggests slow geological processes lead to substantial change and that the Earth is much older than 6,000 years
• Thomas Malthus, an economist, posited that only a fraction of any population will survive and reproduce

Charles Darwin

• Charles Darwin, a British naturalist born in 1809, was influenced by his observations
• He noticed that island species possessed traits allowing them to exploit their environments
• He observed Galapagos Island finches he saw similarities in species but noted differences that provided specialized feeding strategies
• He formulated his theory of evolution by the mid-1840s
• Darwin spent several years studying barnacles
• In 1856, Darwin began writing his book
• In 1858, Alfred Wallace sent Darwin an unpublished manuscript with similar ideas
• Papers by Darwin and Wallace were published together
• Darwin's "On the Origin of Species" was published in 1859, detailing his observational support for the theory

Descent with Modification

• Evolution is based on Variation within a given species

Components of Variation

• Traits passed from parent to offspring
• Genetic basis for traits, though the specific mechanism remained unknown at the time
• Natural Selection is a way for evolution to occur
• More offspring are produced than can survive, so the better adapted survive
• Competition exists for limited resources in the environment
• Individuals with better traits flourish and reproduce

Evidence of Evolutionary Change

• The fossil record shows successive evolutionary change when fossils are compared by age
• Unique species on islands and remote areas have arisen in isolation through biogeography
• Convergent evolution results in anatomically similar species from different lineages occupying similar environments
• Selective breeding profoundly modifies domesticated species through artificial selection

Fossils and Transitional Forms

• Fishapod (Tiktaalik rosae): Illuminates steps leading to the evolution of tetrapods
• The transitional form has traits of both ancestor and descendant species
• Fishapod had a broad skull, flexible neck, eyes on top of head, primitive wrist, and five fingers
• It could peek above water and look for prey
• Paleontological finds involving horses reveal evolution involves adaptation to changing environments

Fossil Evidence in Horses

• Fossil records revealed adaptive changes in size, foot anatomy, and tooth morphology
• Changes are attributed to natural selection adaptations to changing global climates
• Dense forests were replaced with grassland
• Horses adapted to run faster and eat tougher food

Biogeography

• Biogeography studies the geographical distribution of extinct and modern species
• Isolated continents and islands evolved distinct plant and animal communities
• Endemic species are naturally found only in a particular location
• Island fox (Urocyon littoralis) evolved from the mainland gray fox (Urocyon cinereoargenteus)

Convergent Evolution

• Two unrelated species from different lineages showing similar characteristics (adaptations) because they occupy similar environments
• The giant anteater & echidna both have long snouts and tongues
• English ivy and wintercreeper both have aerial rootlets for clinging

Selective Breeding

• Artificial selection involves programs designed to modify traits in domesticated species
• Darwin was influenced by pigeon breeders
• Nature chooses parents in natural selection, while breeders choose in artificial selection
• It is made possible by genetic variation
• Breeders choose desirable phenotypes
• Dog breeds, Brassica plants, and corn are all examples of selective breeding

Homology

• Fundamental similarity due to descent from a common ancestor can be anatomical, developmental, or molecular
• Homologous structures are anatomically similar structures because they evolved from a structure in a common ancestor

Anatomical Homology

• Modern vertebrates have the same set of bones that have undergone evolutionary change for different purposes
• Homologous structures are derived from a common ancestor
• Vestigial structures are anatomical structures with no apparent function but resemble structures of presumed ancestors
• The human coccyx and ear wiggling muscles are examples of vestigial structures

Developmental Homology

• Species differing as adults often bear similarities during embryonic stages
• The presence of gill ridges in human embryos indicates humans evolved from an aquatic animal with gill slits (epithelial slits)
• Human embryos have long bony tails

Molecular Homology

• Similarities in cells at the molecular level show living species evolved from a common ancestor
• All living species use DNA to store information
• The same type of gene is often found in diverse organisms such as biochemical pathways
• The p53 gene prevents cancer and is found in diverse species
• Sequences of closely related species tend to be more similar to each other
• Molecular characteristics are found in nearly all species

Genes in Populations

• A population is a group of individuals of the same species occupying the same environment and able to interbreed
• Population genetics studies genes and genotypes in a population
• It aims to understand genetic variation, its maintenance, and changes to it over generations
• It helps understand how genetic variation is related to phenotypic variation
• A gene pool consists of all alleles for every gene in a given population
• Genetic variation and how variation changes from one generation to the next is studied
• There is emphasis on allele variation

Genes and Polymorphism

• Polymorphism means traits display variation within a population
• This is due to two or more alleles influencing phenotype
• Polymorphic genes contain two or more alleles
• A monomorphic gene is predominantly a single allele
• Single Nucleotide Polymorphisms (SNPs) are the smallest type of genetic change in a gene
• SNPs are the most common type of genetic variation; 90% of variation in human gene
• Raw material for evolution and healthy populations exhibit a high level of SNPs

Allele and Genotype Frequencies

• Calculations for allele and genotype frequencies are related but distinct
  • Allele frequency = (Number of copies of a specific allele in a population) / (Total number of all alleles for that gene in a population)
  • Genotype frequency = (Number of individuals with a particular genotype in a population) / (Total number of individuals in a population)

Four O'Clock Plants

• 49 red-flowered plants with the genotype CRCR
• 42 pink-flowered plants with the genotype CRCW
• 9 white-flowered plants with the genotype CWCW

Hardy-Weinberg Equilibrium

• Conditions for Hardy-Weinberg Equilibrium:
  • No new mutations occur
  • No natural selection occurs
  • The population is large enough that allele frequencies do not change due to random sampling error
  • No migration occurs between different populations
  • Random mating occurs
• Real population meets these conditions

Microevolution

• Changes in a population's gene pool from generation to generation
• Change occurs because new genetic variation is introduced through:
  • Mutations, gene duplication, exon shuffling, or horizontal gene transfer
• Evolutionary mechanisms alter the prevalence of an allele or genotype
  • Natural selection, random genetic drift, migration, or nonrandom mating leads to widespread genetic change

Factors Governing Microevolution

• Sources of new genetic variation
  • Random mutations within pre-existing genes introduce new alleles into populations but at a very low rate
  • Abnormal crossover events and transposable elements increase gene copy #
  • Genes from one species may be introduced into another species

Evolutionary Mechanisms Altering Genetic Variation

• Natural selection; certain traits increases survival and reproduction
• Genetic drift; random change in genetic variation from generation to generation
• Migration; migration occurs between populations that have different allele frequencies
• Nonrandom mating; individuals select mates based on their phenotypes or genetic lineage

Natural Selection

• Beneficial heritable traits become more common in successive generations through natural selection
• Over time, this leads to adaptations that promote survival and reproduction in a specific environment
• Reproductive success is the likelihood of an individual contributing fertile offspring to the next generation
• Reproductive success is attributed to an organisms to be better adapted or traits directly associated with reproduction

Modern Description of Natural Selection

• Within a population, allelic variation arises from random mutations that cause differences in DNA sequences
• Some alleles encode proteins that enhance an individual's survival or reproductive capability compared to other population members
• Individuals with beneficial alleles are more likely to survive and contribute their alleles in the next generation's gene pool
• Over generations, these altered allele frequencies can significantly change population characteristics
• Fitness is a measure of the relative likelihood that a genotype will contribute to the gene pool of the next generation compared to other genotypes

Natural Selection Patterns

• Patterns include:
  • Directional selection
  • Stabilizing selection
  • Diversifying selection
  • Balancing selection

Directional Selection

• Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment
• Initiators; new allele with higher fitness introduced and/or prolonged environmental change

Stabilizing Selection

• Stabilizing selection favors survival of individuals with intermediate phenotypes
• Extreme values of a trait are selected against
• Example: Optimal clutch size balances offspring survival with parental care

Disruptive/Diversifying Selection

• Diversifying selection favors the survival of two or more different genotypes that produce different phenotypes
• It is likely to occur in populations occupying heterogeneous environments
• Members of the populations can freely interbreed

Balancing Selection

• Balancing selection maintains genetic diversity
• Alleles are kept in balance, and therefore maintained in a population over many generations
• Two common ways this happens is, for a single gene, an advantage for the heterozygote and negative frequency-dependent selection in which rates individuals have a higher fitness

Grants' Observations in Galápagos Finches

• A specific example of Natural Selection, The Grants focused on Daphne Major, an isolated, undisturbed habitat of resident finches
• They measured and compared beak sizes of parents and offspring over many years
• During drought years, birds with larger beaks had a better survival rate
• In the year after drought conditions, the average beak depth increased

Sexual Selection

• Sexual selection is a form of natural selection that affects the traits of sexually reproducing species to make finding a mate/engaging in mating more likely
• Often affects male characteristics more intensely than for females
• Explains traits that may hurt chances of survival, but increase odds of reproduction
• Intrasexual Selection occurs Between members of the same sex
  • Males directly compete for mating opportunities or territories
• Intersexual Selection is “Female's Choice" and occurs Between members of opposite sex
  • Results in showy characteristics for males, cryptic female choice and also inhibits inbreeding

Genetic Drift

• Genetic drift changes allelic frequency due to random chance
• It involves random events unrelated to fitness
• Favors either the loss or fixation of an allele
• Frequency reaches 0% or 100%
• Happens faster in smaller populations

Genetic Drift: Bottleneck Effect

• Genetic bottlenecks occur when a population is reduced dramatically and then rebuilds
• Members are randomly eliminated without regard to genotype
• Surviving members may have allele frequencies different from the original population
• Allele frequencies can drift substantially when the population is small
• The new population is likely to have less genetic variation

Genetic Drift: Founder Effect

• The founder effect occurs when a small group of individuals separates from a larger population and establishes a new colony
• The founding population has less genetic variation than the original
• Allele frequencies in the founding population may differ markedly from the original population

Neutral Theory of Evolution

• Neutral Variation is When variation in natural populations is caused by genetic drift
• It does not preferentially select for any particular allele
• Most genetic variation is due to neutral mutation accumulations with high frequencies due to genetic drift
• Neutral mutations do not affect phenotype so they are not acted upon by natural selection
• Much of the modern variation in gene sequences is explained by neutral variation rather than adaptive variation
• Sequencing data supports this idea
• Nucleotide substitutions are more likely in 3rd base of codon (usually doesn't change amino acid) than 1st or 2nd (usually changes a.a.)
• Changing the amino acid is usually harmful to the encoded protein

Migration and Nonrandom Mating

• Gene flow occurs when individuals migrate between populations with different allele frequencies
• Migration decreases differences in allele frequencies between populations
• Migration tends to enhance genetic diversity within a population

Nonrandom Mating

• One of the conditions required to establish Hardy-Weinberg equilibrium is random mating
• Random mating occurs when Individuals choose their mates irrespective of their genotypes and phenotypes
• Assortative Mating means that individuals of similar phenotypes are more likely to mate
• It Increases the proportion of homozygotes
• Ex: Jumping spiders; large females prefer large males (defeat small ones)
• Disassortative Mating means that dissimilar phenotypes mate preferentially and it favors heterozygosity
• Inbreeding (inbreeding depression) When choice of mate is based on genetic history
• It does not favor any particular allele, but DOES increase likelihood of homozygous individual
• May have negative consequences for recessive alleles
• Lower fitness is caused in a population if homozygous offspring have a lower fitness value