Evolution and Speciation Flashcards
Who proposed the theory of evolution
Charles Darwin
In 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection.
Darwin made two main points:
1. Species show evidence of “descent with modification” (evolution) from common ancestors.
2. Natural selection is the mechanism for “descent with modification”.
Natural Selection
The process by which organisms with traits that enhance their chances of survival and reproduction are more likely to pass those traits on to their offspring.
This leads to gradual changes within the population over time.
What did Georges Cuvier contribute to the theory of evolution?
Georges Cuvier observed that fossils found in older strata were different from living organisms and that fossil species varied between strata, indicating extinction.
- Cuvier believed that species do not change over time, i.e. no evolution
- He believed that species were just dying off, then being replaced by others from other geographical areas
What did Jean-Baptiste Lamarck contribute to the theory of evolution?
Lamarck was philosophically opposed the concept of extinction and proposed that species evolved through the inheritance of acquired characteristics, a theory known as “Lamarckism”.
What two observations led Darwin to the theory of evolution?
Observation 1: Individuals in a population vary in their inherited traits
- This variation in populations
(genetic diversity) provides the raw material that natural selection acts upon.
Observation 2: Populations can produce more offspring than the environment can support
- Many offspring fail to survive and reproduce due to limited resources and competition.
- Insufficient resources to support all offspring leads to competition for resources
- This was influenced by Thomas Malthus
What two inferences are made in Darwin’s theory of evolution by natural selection
Inference 1: Individuals with inherited traits that provide an advantage in their environment are more likely to survive and reproduce, leaving more offspring
Inference 2: The unequal ability of individuals to survive and reproduce will lead to the accumulation of advantageous traits in the population over generations
Genotypic Variation
Genotypic (genetic) variation is the difference in DNA among individuals in populations.
- Heriteble
- Contributes to phenotype, but not all genes are expressed
Phenotypic Variation
Phenotype is an organism’s observable characteristics
Phenotypic variation is the variability in phenotypes that exist in a population
- An organism’s phenotype is determined by interaction of environmental factors and its genotype.
- Ex. Hydrangea flowers changing colour based off of soil acidity
True or false
Natural selection causes the rise of new genetic variation
False
Natural selection can only act upon existing genetic variation
Selective Agents
Natural Selection
Environmental factors acting on populations that effect the survival and/or reproduction of individuals in the populations
- When a selective agent consistently causes differences in survival and/or reproduction in a population, it is a selection pressure
True of false
Natural Selection cannot happen without a selection pressure
True
Consistent selection pressure leads to a directional change in the population (natural selection)
- If the selection pressure changes, the direction of natural selection will change (context dependent)
Adaption
Evolution
An inherited characteristic of an organism that improves its chances of survival and reproduction in a specific environment.
True or false
Individuals can evolve
False
Although selection pressures act on individuals, only populations evolve.
- Individual organisms do not change; rather, natural selection alters the proportion of advantageous traits within a population across multiple generations
What are the common evidences for evolution
- Direct observations of evolutionary change
- Homologies
- Fossil record
- Biogeography
Explain
Direct observations of evolutionary change
Recorded and observed changes in populations over time
Examples:
- Galapagos finches (noticible beak size changes due to changing food availibility
- Peppered Moths (change in colour after bark changed colour)
- Antibactirial resistance
- Seletive breeding (dogs, food, cats, ect.)
Homologies
Similarities that arise from a shared evolutionary ancestor
Can be:
- Morphological homologies
- Homologous embryonic structures
- Vestigial structures
- Molecular homologies
Morphological Homologies
Physical resemblances representing variations on a structural theme present in a common ancestor
- Anatomical structures like the tetrapod forelimb (found in mammals, birds, reptiles, and amphibians), exhibit a common underlying structure despite differences in function
- This indicates that these species share a common ancestor, even though the limbs have adapted to different roles, e.g. walking, flying, swimming, or grasping.
Homologous embryonic structures
During the early stages of development, many vertebrates exhibit strikingly similar embryonic features
- Ex. The embryos of fish, birds, and humans all develop gill slits (pharyngeal pouches) and tail-like structures
Vestigial Structures
Remnants of structures that were functional in an organism’s ancestors
- Ex. human appendix, pelvic bones in whales
These structures provide evidence of evolutionary change over time, showing how traits can lose function when no longer advantageous
Molecular Homologies
All living organisms share a universal genetic code (to translate RNA to proteins), strongly suggesting that life evolved from a common ancestor
- Molecular sequences, such as specific protein and DNA similarities, show a hierarchy of relatedness that mirrors the evolutionary tree.
Fossil record: historical evidence of evolution
Although the fossil record is not a complete record of evolutionary history, it provides evidence of:
- Extinction of species.
- Origin of new groups.
- Long-term evolutionary changes within groups.
The dating of sedimentary rock layers (strata) allows fossils to be placed in order of time of origin
- Consistent forms occur in the same aged stratum
Transitional fossils
Fossils document the existence of intermediate forms that appear to be ancestors of living species
- Transitional fossils assist in the reconstruction of evolutionary histories of living taxa, serving as “missing links”
- Transitional fossils document important transitions, e.g. the transition from land to sea in mammalian ancestors of cetaceans
Chronological sequence
Fossil Record
The order in which different groups of taxa appear in the fossil record aligns with their evolutionary relationship
- Data from phylogeny and fossils are often in agreement, providing strong evidence for evolution.
- The branching order of a phylogeny corresponds to the order of appearance of each group in the fossil record.
Biogeography
The study of how species are distributed across the globe
- Provides strong support for evolution by showing how geographic isolation and environmental factors shape species diversity
Isolated populations
Biogeography
Islands such as the Galápagos or Hawaiian archipelagos contain species that are closely related to those on nearby continents but have evolved distinct characteristics due to geographic isolation.
- Remote islands (e.g. Galápagos) often have many endemic species, species that are only found in a specific area and are not found elsewhere on Earth
Endemic Species
Species that are only found in a specific area and are not found elsewhere on Earth
Continental Patterns of Species Distribution
Biogeography (evolutionary histories)
The geographical distribution of living and fossil species assists in the reconstruction of evolutionary histories.
Understanding continental movement and the distribution of living species allows us to predict when and where different groups evolved.
- For example, marsupials are found almost exclusively in Australia, while placental mammals dominate other continents
- This pattern is explained by the historical separation of land masses (continental drift), which isolated South American and Australian populations and allowed them to evolve independently.
Microevolution
The change in allele frequencies in populations over generations
- Individuals represent different combinations of alleles drawn from the gene pool, that is, from all the alleles present in all individuals in the population
How can new genes arise in a population?
Mutation or Gene Duplication
Mutations
Changes in an individual’s DNA sequence
- Occur Randomly
- Creates new alleles
- Causes: i) small-scale (e.g. point mutation) or chromosomal (e.g. insertion/deletion) errors in DNA replication; ii) structural damage to DNA (e.g. radiation)
- Can only be inherited if it impacts gametes
Can be deleterious (‘bad’), neutral, or advantageous (‘good’) in the current environment
Which has the faster mutation rate: eukaryotes or prokaryotes
Mutations occur relatively infrequently in populations
Eukaryotes have much faster evolution rates
Mutation rates are much lower in prokaryotes.
- However, mutations accumulate quickly in prokaryotes because they have very short generation times.
What are the sources of genetic variation
- Mutation
- Sexual Reproduction
Explain the role in genetic variation
Sexual Reproduction
Sexual reproduction amplifies genetic variation by creating new combinations of existing alleles
- Recombination of homologous chromosomes during meiosis shuffles existing genetic material to create new combinations of alleles
- Recombination is often more impactful than mutation in generating genetic variation within sexually reproducing populations over short timescales
What are the factors that alter allele frequencies in populations
Natural selection, genetic drift, and gene flow are the mechanisms that cause changes in allele frequencies in populations
Note: Only natural selection causes adaptive evolution
- Genetic drift and gene flow are random
Relative Fitness
Natural Selection
The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
What are the three types of natural selection?
- Directional selection
- Disruptive selection
- Stabilizing selection
Directional Selection
Natural Selection
Favours individuals that differ from the current mean phenotype of a population in one direction
- Under directional selection, a population’s genetic variance shifts toward a new phenotype with higher relative fitness when exposed to selection pressures
- Occurs in response to consistent selective pressure, i.e. a steady change in the environment
Disruptive Selection
Natural Selection
Favours individuals at both extremes of the phenotypic range
- Intermediate (average) phenotypes have lower fitness than either extreme phenotype
- maintains genetic variation in populations
- Ex. Beak size in birds
- Some may be drastically large or small based on food availibility
Stabilizing Selection
Natural Selection
Favours intermediate or common phenotypes by selecting against extreme phenotypes that deviate from the current population mean
- Stabilizing selection reduces genetic variation in a population and maintains the population’s mean phenotype
- Very Common
- Removes deleterious mutations
Genetic Drift
Random changes in allele frequencies in a population
- More likely in small populations
- Rare alleles are more likely to be lost due to genetic drift
- causes evolutionary change but does not create adaptations (non-adaptive)
Bottleneck Effect
Genetic Drift
The bottleneck effect is a sudden reduction in population size due to a change in the environment
- Population size is reduced within its natural range
Allele frequency in the next generation is different than the previous generation
Examples:
- Cheetas
- Greater Prairie Chickens
Founder Effect
Genetic Drift
Occurs when a few individuals become isolated from a larger population
- Allele frequency in the original large population is unchanged
The small founding population has a small fraction of the total gene pool present in the original population
- Allele frequencies in the small founder population differ from those in the larger originating population
- e.g. increased frequency of Huntington’s Disease in human populations founded by small groups of European migrants
Effects of Genetic Drift
- Genetic drift has the largest impact on small populations.
- Genetic drift causes allele frequencies to change at random.
- Genetic drift can lead to a loss of genetic variation within populations.
- Genetic drift can cause harmful alleles to become fixed in small populations.
Gene Flow
The movement of alleles between populations of a species
- Alleles can be transferred through the movement of fertile individuals (e.g.
dispersal of animals or seeds) or gametes (e.g. pollen)
Gene flow can introduce new variation into the receiving
Gene flow reduces variation between populations over time.
True or False
Gene flow decreases the fitness of the recieving population
Trick Question: It can make it better or worse
Decreasing
- Occurs when the immigration of alleles that decrease fitness is more rapid than natural selection for alleles that increase fitness
- Maladaptive traits are passed along
Increasing
- Examlple: the spread of alleles for resistance to insecticides
What factors create evolutionary change?
3 + 2 bonus ones
- Natural selection (adaptive)
- Genetic drift (non-adaptive)
- Gene flow (non-adaptive)
Other factors that bring about evolutionary change:
- Extirpation of populations reduces a species’ genetic diversity.
- Global extinction of a species (complete loss of genetic diversity).
Neutral Variation
Genetic Variation
Genetic variation that does not confer a selective advantage or disadvantage.
- Natural selection does not affect the frequency of neutral mutations
Maintained by:
- Mutation.
- Recombination (crossing-over of chromosomes during meiosis).
- Independent assortment (of alleles) during meiosis.
- Random mating between individuals (sexual reproduction).
- Random fertilization (sexual reproduction).
- Recessive alleles are hidden from selection in heterozygote individuals.
- Disruptive selection (natural selection).
- Gene flow (between populations).
- Balancing selection.
Balencing Selection
Genetic Diversity
A form of natural selection that maintains genetic diversity by favouring stable frequencies of multiple alleles in the gene pool of a population
Mechanisms:
- Temporal or spatial variation (the need for different alleles at different locations or points in time)
- Heterozygote Advantage (organisms with two dofferent alleles have the advantage, ie. sickle cell disease)
- Frequency-dependent selection (fitness of the alleles depends on lack of others with the same allele ie. scale eating fish)
- Often is a result of interactions between species (predation, parasitism, or competition)
What doesn’t evolution create perfect organisms?
- Adaptations and the genes responsible for them only need to be “good enough” to enable reproduction.
- Natural selection has no goals
- Natural selection only acts on existing variation
- Historical constraints limit natural selection
- Adaptations are compromises
- Chance events (genetic drift, gene flow) and environmental variability limit natural selection
Microevolution vs Macroevolution
The distinction between micro- and macroevolution is arbitrary.
- They are fundamentally identical processes on different time scale
- Between microevolution and macroevolution is speciation, the process by which one species splits into two species over time
Microevolution - evolution at the population level
Macroevolution - broad patterns of evolutionary change above the species level
Macroevolution
Macroevolution - broad patterns of evolutionary change above the species level
- Macroevolution is evolutionary changes occurring on geological time scales.
- Includes
- Novel traits (ex. wings in birds)
- Origin of new groups
- Mass extinctions
Biological Species Concept
(BSC)
A species is a group of individuals that can interbreed, producing viable (healthy and fertile) offspring, and are reproductively isolated from other species
- Members of the same species can exchange genes (through reproduction), which keeps the group unified as a species
- BSC is based on the potential to interbreed rather than physical similarity
Limitations of the Biological Species Concept
(BSC)
Not applicable to fossils or asexual organisms: We can’t use BSC for species that reproduce without mating (like bacteria) or for extinct species that we only know through fossils.
Gene flow between species: While BSC focuses on reproductive isolation (absence of gene flow), sometimes gene flow can occur between different species (hybridization).
What are the common ways of defining species
Name all 4
Biological Species Concept (BSC) - If they can have babies and their babies can have babies
Morphological Species Concept - It they look similar enough
Ecological Species Concept - If they have the same job (niche) and live in the same environments
Phylogenetic Species Concept - The smallest group of individuals with a common ancestor
Morphological Species Concept
identifies a species based on their physical appearance or structural traits.
- Members of a species share a set of characteristics that make them look similar and distinguishable from other species.
- It applies to sexual and asexual organisms but relies on subjective criteria.
Ecological Species Concept
Defines a species by its role in the environment, known as its ecological niche, how the species interacts with other organisms and its habitat
- Members of the same species will have similar preferences and tolerances for environmental conditions, like temperature or food sources.
- This concept also applies to both sexual and asexual species.
- For example, Canada lynx (Lynx canadensis) and bobcat (Lynx rufus) hybridize in the wild, especially in regions where their ranges overlap, but occupy distinct ecological niches that define them as separate species
Phylogenetic Species Concept
Defines a species as the smallest group of individuals that share a common ancestor, based on evolutionary history
- This concept applies to sexual and asexual organisms and is particularly useful for analyzing evolutionary relationships
- For example, giraffes were previously considered a single species, but recent phylogenetic analyses have revealed that giraffes consist of four distinct species
Reproductive isololation
Biological factors (barriers) that prevent two species from interbreeding and producing fertile offspring (hybrids).
- Reproductive isolation keeps species distinct from each other under BSC
Reproductive isolation can happen before or after fertilization
- Prezygotic barriers occur before fertilization and prevent mating or the formation of a zygote (fertilized egg).
- Postzygotic barriers occur after fertilization, where hybrids may be sterile or not survive long enough to reproduce.
How do prezygotic barriers block fertilization?
Name the 5 types
- Impeding different species from attempting to mate.
- Habitat isolation (don’t live in the same space)
- Temporal isolation (don’t mate at the same time)
- Behavioural isolation (mating rituals keep animals of different species from mating - ANIMAL ONLY)
- Preventing the successful completion of mating.
- Mechanical isolation (Square peg, round hole. Don’t think about it)
- Preventing fertilization if mating is successful
- Gametic isolation (Fertilization is unable to happen)
How do postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult?
Reduced hybrid viability: genes of the different parent species may interact to impair the hybrid’s development or survival
Reduced hybrid fertility: even if hybrids are vigorous, they may be sterile (ex. mules)
Hybrid breakdown: some 1st-generation hybrids are vigorous and fertile, but when these hybrids mate with one another or with either parent species, offspring of subsequent generations are feeble or sterile
Hybrid Breakdown
Some 1st-generation hybrids are
vigorous and fertile, but when these hybrids mate with one another or with either parent species, offspring of subsequent generations are feeble or sterile.
- Hybrids are eventually eliminated from the population by natural selection
- Ex. , different strains of cultivated rice have accumulated different mutant recessive alleles at two loci after divergence from a common ancestor
- Hybrids between them are vigorous and fertile, but plants in the subsequent generations carrying too many recessive alleles are small and sterile
Allopatric speciation
The evolution of new species following the geographic isolation of two or more subpopulations of an ancestral species
When a population is geographically divided into two or more isolated groups that are prevented from interbreeding
- Regions with many geographic barriers (e.g. mountains, rivers, etc) typically have more species than regions with fewer barriers
Geographic isolation arises via:
- Dispersal: where a small population becomes isolated at the edge of a larger population.
- Vicariance: the range of a species is split by a change in the environment, creating two subpopulations
True or False
Geographic isolation is a biological barrier to reproduction
False
Barriers to reproduction must be intrinsic; separation itself is not a biological barrier
Sympatric speciation
The evolution of a new species from an ancestral species while both continue to inhabit the same geographic region
- Speciation without geographic separation
- For sympatric speciation to occur the subpopulations occupying the same geographic region must become reproductively isolated from each other
Overlapping subpopulations can become reproductively isolated from each other by:
- Chromosomal errors during meiosis or hybridization of closely related species.
- Polyploidy and hybrid speciation.
- Natural selection for reproductive isolation.
- Habitat differentiation (often by disruptive selection).
- Sexual selection (non-random mating)
Polyploid speciation
When changes in the number of chromosome sets (polyploidy) create genetically distinct descendants that are reproductively isolated from parental forms.
- Occurs infrequently in animals (usually lethal)
- Common among plants
Hybrid Speciation
When interbreeding between two related species creates genetically distinct descendants that are reproductively isolated from the parent species
- Some hybrid species form by polyploidization.
Polyploids
Polyploids have a different number
of chromosome sets than their
parental forms
- Polyploids can only interbreed with individuals of the same ploidy
- Polyploidy can produce new sympatric species within a single generation
Polyploids arise from:
- Hybridization of related species to form allopolyploids (hybrid speciation).
- Chromosomal error during meiosis forming autopolyploids.
Allopolyploids
A species with multiple sets of chromosomes derived from the hybridization of different species
- Allopolyploid hybrids have double the number of chromosome sets than the parent species
- Ex. Wheat
Autopolyploids
An autopolyploid is an individual with more than two chromosome sets, derived from one species
- Can arise spontaneously by genome doubling or by fusion of 2n gametes (failure of cell division during meiosis creates gametes with double the number of chromosomes)
- Example autopolyploids: alfalfa and potato (both tetraploids, 4n)
Hybrid speciation
Sympatric speciation
Hybridization between related species is a trigger for polyploidization, forming allopolyploids
Hybrid speciation can also occur with no change in chromosome number (ploidy), which is known as homoploid hybrid speciation
- Homoploids arise when hybridization produces novel combinations of genes that can adapt hybrids to new habitats that cause them to be reproductively isolated from parental populations
Homoploids
Homoploids arise when hybridization produces novel combinations of genes that can adapt hybrids to new habitats that cause them to be reproductively isolated from parental populations
Habitat differentiation
Sympatric speciation
Natural selection for reproductive isolation between subpopulations can occur after the appearance of new ecological niches.
Example: Apple maggot flies (Rhagoletis pomonella) can live on native hawthorn trees as well
as more recently introduced apple trees.
- Habitat differentiation (apple fruit vs.
hawthorn fruit) increases reproductive isolation between subpopulations
Sexual selection
Sympatric speciation
Natural selection for reproductive isolation between subpopulations can occur under sexual selection.
- Sexual selection occurs when individuals with certain inherited characteristics are more likely than other individuals to obtain mates
- Often results in sexual dimorphism:
marked differences between the sexes in secondary sexual characteristics
Hybrid Zones
Hybridization occurs when species with incomplete reproductive barriers interbreed, resulting in hybrid offspring
- These hybrids may have reduced fertility or survival, or they may be fully viable and fertile, depending on the extent of genetic divergence
Hybrid zones can sometimes form where the ranges of the two species overlap, allowing for limited interbreeding between genetically distinct populations
- A hybrid zone can occur in a single band where adjacent species meet
- Altering environmental conditions can cause the band to shift
What are the outcomes of related species meeting in a hybrid zone?
- Reinforcment (hybrids are less fit, reinforcing reproductive barriers.)
- Fusion (Hybrids are fit for reproduction, species recombine. Weakens reproductive barriers)
-
Stability (continued formation
of hybrid individuals, but the hybrids are not more fit than the parents, so species remain seperate)
How long does it take for new species to form?
Speciation rates vary widely.
- Speciation is generally considered to be a gradual process.
- Estimates of the duration of speciation range from 4,000 years (e.g. cichlid fish) to 40 million years (some beetle taxa).
- The calculated average time for speciation in plants and animals is ~6.5 million years.
Speciation begins only after gene flow between populations is interrupted, not after a given time
True or False
Speciation begins only after gene flow between populations is interrupted, not after agiven time
True
- Once gene flow is interrupted, the populations must diverge genetically to become reproductively isolated.
- This must occur before another event causes gene flow to resume, thus stopping speciation.
- Speciation may occur more rapidly under allopatric speciation, than under sympatric speciation.
