Evolution Flashcards
Define descent with modification
All organisms share common ancestor
Descendants have accumulated modifications that help them better adapt to environment
Define microevolution
Change in allele frequency of a gene pool form one generation to the next (within a population)
How can microevolution lead to macroevolution ?
If genetic changes result in reproductive isolation from ancestral population
Define macroevolution
Evolution above species level ; descent of different species from common ancestor
Define population
A summation of all the organisms of the same species, who live in the same geographical area and have the capability of inbreeding
Define gene pool
Collection of all genes and various allelic forms of those genes within a particular population
Define allele frequency
Frequency or proportion of a particular allele in the gene pool
Define variation ( phenotypic )
The differences in characteristic between individual organisms belonging to same natural population
What is variation caused by ?
Genetic variation
Effect of the environment
What does variation result in ?
Different fitness and reproductive success between individuals in same population, under particular conditions (selective pressures) of their natural habitat
Is effects of environment that result in variation, heritable ?
It cannot be inherited
What leads to genetic variation ?
- meiosis and random fusion of gametes
- mutations : gene mutation & chromosomal aberration
How does meiosis and random fusion of gametes lead to variation ?
Reshuffling of existing alleles give rise to new combinations of alleles and genotypes in individuals = different phenotypes
How does gene mutation lead to variation ?
Results in new alleles (same gene but diff form) and new characteristics, increasing gene pool for natural selection to work on
How does chromosomal aberration lead to variation ?
Results in new genotypes with new combinations of alleles, affecting characteristic of individuals
Are mutations disadvantageous ?
Mostly
However when there is increased genetic variation, beneficial mutations may arise
When env change, mutations once unfavourable become favourable
Are mutations passed down / inheritable ?
Only those that occur in gametes are passed down to next generation
Define gene flow
Movement of alleles from one population to another due to migration of individuals
How else can genetic variation in population be increased ?
Gene flow from different population
Do all variations results in differential reproductive success ?
No
- some genetic variations are not expressed (eg. intron mutation)
- phenotype expressed may not be significant to reproductive success
- env conditions may not be electing for the particular phenotype
What is Charles Darwin’s definition of evolution ?
Descent with modification
- all organisms are related through descent, share a common ancestor
- descents have accumulated modifications or adaptations that fit them into their environments
What are the two inferences that Charles Darwin made from his observations of nature ?
- Selective agents / pressures present
- Survival of the fittest
What are the observations that Charles Darwin had for him to propose natural selection as mechanism for evolution ?
- Overproduction of offspring
- Constancy in number
- Variation among offspring
- Like produces like
Which group does natural selection operate on ?
Populations
What is natural selection ?
Process in which individuals that has certain heritable characteristics reproduce at a higher rate than other individuals
What three conditions must be present for natural selection to occur ?
- Variation must exist amount individuals in a population
- There must be differential reproductive success - some individuals are more likely to survive and reproduce, thus leaving behind more offspring under particular selection pressure
- Variation that confers advantage to these individuals must be inheritable
What are the five steps of natural selection ?
- Pre-existing variation between individuals in a population
- Selective pressures
- Differential reproductive success
- those with certain variations that allow them to exploit the environment better will be selected for / have selective advantage : survive longer and reproduce more - Passing down advantageous alleles
- traits are not passed down but the favourable form of genes (alleles) - Change in allele frequency over time
- frequency of alleles that are selectively favourable and passed down will increase in the next generation while those not favoured will decrease
- overtime leads to change in allele frequency in gene pool overtime = microevolution
Does selective advantage for one environment confer the same advantages in anther environment?
No
Selective advantage of an allele relates to one specific environment and one specific time only
Do organisms undergo mutation so that they are able to adapt to the environment ?
No
- genetic variation existed in the population due to mutation, prior to the introduction of selective pressure
- mutation occurs randomly and cannot be controlled : environmental conditions / selective pressures select against individuals not so well adapted to environment
What is the smallest unit of evolution ?
A population
Why is population the smallest unit of evolution ?
Evolution involves changes in allele frequency in the gene pool of a population and can only be measured by changes in the relative proportions of heritable variations in a population in successive generations
What are the three different types of selection ?
- Directional selection
- Stabilising selection
- Disruptive selection
What is directional selection ?
Selects for one extreme of the phenotypic range
Frequency curve of the phenotype shifts towards one direction
- eg. Industrial melanism in peppered moths
What is stabilising selection ?
Selects for the intermediate phenotypes but not both extremes
Reduces variation and maintains a certain phenotype
- eg. Birth weight in humans, stem height in plants
What is disruptive selection ?
Selects for both extremes of phenotypic range over intermediates
What happened to the peppered moths during industrialisation ?
Before industrialisation
- trees and rocks that moths rest on had a light background and the black form of moths were subjected to more predation
During industrialisation
- walls, rocks and trees blackened by soot
After industrialisation
- dark form moths became better camouflaged against darker background and were selected for
Natural selection of antibiotic-resistant bacteria
In presence of antibiotics
- bacteria that confer antibiotic resistance will be at selective advantage while those without will be killed off
- resistant bacteria will survive, reproduce and pass down antibiotic resistant genes to offspring
Natural selection of sickle cell anemia and malaria in humans
Individuals with sickle cell trait ( HbA HbS ) do not develop malaria - infected RBCs sickle and die, preventing reproduction of parasite
In places with high incidence of malaria
- sickle cell trait is selected for
- HbA HbA genotype susceptible to malaria thus selected against
- HbS HbS selected against as they develop sickle cell anemia
- therefore HbS allele frequency higher in places with high incidence of malaria
What are the three mechanisms that preserve genetic variation ?
- Heterozygote protection
- Balancing selection
- heterozygote advantage
- frequency-dependent selection - Neutral variation
Why is there need to preserve genetic variation ?
Prevents natural selection from decreasing the gene pool by culling all unfavourable genotypes
What is heterozygote protection ?
Unfavourable alleles are shielded from natural selection as recessive alleles
- recessive alleles are masked by dominant allele and dominant phenotype is expressed
- if dominant phenotype is favourable and the individual survives and reproduces, the recessive allele can be passed down and persists in the gene pool
When can recessive alleles under heterozygote protection be beneficial ?
When environment changes
When does balancing selection occur ?
When natural selection maintains two or more alleles (two or more phenotypic forms) in the gene pool of the population
What is heterozygote advantage ?
Individuals who are heterozygous at a particular locus having greater fitness over both kinds of homozygotes
- both alleles will be maintained by natural selection
What is frequency-dependent selection ?
Fitness of a specific phenotype is dependent on its frequency relative to other phenotypes in a population : fitness of a particular phenotype declines if it becomes too common in a population
- frequencies of the different phenotypes oscillate over time
Eg. Scale eating fish
- two types : left-mouthed and right-mouthed
- prey guards against the most common scale eating fish thus selection favours the least common mouth phenotype
- frequency of phenotype oscillate overtime and selection keeps frequency of each phenotype close to 50%
What is neutral variation ?
Most of genetic variation in populations have little or no impact on reproductive success and thus natural selections does not affect these variations
Why does neutral variation occur ?
- Mutations that do not result in changes to proteins thus no changes to phenotype do not have impact on reproductive success
- in humans, most genetic variation found in introns (non-coding sequences) appear to confer no selective advantage or disadvantage
- silent mutation : different codon sequences code for the same AA due to degenerate code - Mutations that result in changes to proteins can also be neutral
- AA that are changed have similar properties or are not in critical position - Selective pressures may not be selecting for the particular phenotype / phenotype may not be significant to reproductive success
- eg. human eye colour
What are some variations found in non-coding sequences that are not neutral ( impacts organisms phenotype and reproductive success) ?
Mutation to splice sites at the start and end of introns
- affects splicing thus changes mature mRNA sequence
- changes AA sequence of proteins thus affects protein function
Mutation to control elements ( promoters, enhancers, silencers )
- affects control/rate of gene expression
- changes amount/timing of proteins synthesised
What are the evidences that support Darwin’s theory of evolution ?
- Anatomical homology
- Pentadactyl limb
- vestigial structures
- fossil records - Molecular homology
- Biogeography
- island biogeography
- the marsupial-placental convergence
What is homology ?
The study of similar characteristics (anatomical or molecular) found in different species of organisms, inherited from a common ancestor
What does being more closely related means ?
Organisms are derives from a more recent common ancestor thus there is less time to accumulate differences
What is anatomical homology ?
Physical structure found across different species and fossil records that are derived from a common ancestor
How does the pentadactyl limb show evidence of Darwin’s theory of evolution ?
Share common ancestor : the forelimbs of all mammals show the same basic plan in terms of relative position of limb and arrangement of bones in the limb
Descent with modification : the basic structural plan has been modified which allows the forelimb to adapt to a certain method of locomotion in a particular environment
What type of evolution is the diversification of the forelimb an example of ?
Divergent evolution
What is divergent evolution ?
Process in which a trait from a common ancestor evolves into different variations over time in different species
What is speciation ?
A consequence of divergent evolution - the divergence of one species into two or more descendant species
What is adaptive radiation ?
An example of divergent evolution except that the process of divergence occurs to give a rise in multiple species over a relatively short period of time
- eg. Darwin’s finches
What are vestigial structures ?
Anatomical structures that have been reduced and may be non-functional in a particular species but are homologous to a functional structure in other species
- remnants of features that once served important functions in the organisms ancestors
- the structure lost its function as the selective pressure that selects for it no longer present, thus the structure undergoes degeneration
What are fossils ?
Preserved remains of past organisms
How can fossils serve as direct evidences for evolution ?
Studies of fossil records show that past organisms differed from present day organisms and that many species have become extinct - suggest that ancestors of present organism have changed through time as they adapted to environmental conditions
What does the array of which fossils appear show ?
Shows that organisms have appeared in a historical sequence
Shows evolutionary changes that have occured over time which presents a succession of life forms form simple to more complex
How does molecular homology show evolutionary relationship ?
Degree of similarity between the sequences of homologous genes / proteins in different organisms show their evolutionary relationship
- more similar the sequences, more closely related the organism
What are the two ways to measure molecular homology?
DNA sequence
AA sequence
Is DNA sequence or AA sequence more accurate in measuring molecular homology ?
DNA sequence
- AA degeneracy where different amino acids code for the same codons
Is mutation in the coding or non-coding regions of DNA sequences more accurate for tracking of evolutionary relationships ?
Non-coding (introns, telomeres, centromere)
- mutation in coding regions are expressed, those selected against are not passed down thus cannot be tracked
- non coding mutations remains and are easily tracked since they accumulate
What is biogeography?
The study of geographic distribution of both extinct and living organisms
What does biogeographic distribution demonstrate ?
- Closely related species tend to be found in the same geographical region
- Similar ecological niches in distant regions are occupied by unrelated species that are similar looking or have similar characteristic due to similar selective pressures
What is an ecological niche ?
A position or role taken by a kind of organism in its community ( like job openings )
What is convergent evolution ?
Adaptations of unrelated species to a similar environment due to similar selective pressures
How does island biogeography show evidence of Darwin’s theory of evolution ?
Descent with modification : ancestral species from mainland migrate to islands and succeeded in adapting to the various niches in the island with different environmental conditions and selective pressures thus resulting in different traits
Darwin’s finches on the Galápagos Islands
14 species of finches found on the islands are similar to each other and to the species on the mainland
Selective pressures on the various islands differ from mainland resulting in formation of 14 species with distinctive beaks specialised for different diets on the various islands
What is adaptive radiation ?
Process which one species gives rise to multiple species exploiting different niches in a relatively short time
How is adaptive radiation and divergent evolution different ?
Adaptive radiation occurs in a short amount of time with multiple species formed form a common ancestor
Divergent evolution is when a new species is formed from a common ancestor
What are analogous structures ?
Similar characteristics with similar functions between different species not derived from a common ancestor
What is the marsupial-placental convergence ?
Two major group of mammals, marsupials and placentals, have independently evolved similar characteristics although they are separated on different continents
What are the 5 species concepts ?
- Biological species concept
- Ecological species concept
- Morphological species concept
- Genetic species concept
- Phylogenetic species concept
What is the biological species concept?
A species is a group of populations whose members have the potential to interbreed in nature and produce viable and fertile offspring but do not produce viable, fertile offspring with members of other species
- members of same species are reproductively compatible while different species are reproductively isolated
What are the limitations of biological species concept?
- not applicable to asexually reproducing species
- not applicable to self-fertilising species
- there are exceptions in which the hybrid individuals are both viable and fertile
What is the ecological species concept?
A species is a group of organisms exploiting the same ecological niche, each species has its own niche which includes type of habitat it lives in, feeding relationships and other behaviours
If two organisms have very similar needs it will result in competition
What are the advantages of ecological species concept?
- it can be applied to asexual and sexual organisms
- emphasises the role of disruptive natural selection as organisms adapt to different environmental conditions
- useful in distinguishing bacterial species because bacteria cells of the same species are likely to use the same types of nutrients and grow under same conditions (niches)
What are the limitations of ecological species concept ?
- difficult to determine degree of competition between different members of the same species
- it does not consider morphology and reproductive compatibility
What is the morphological species concept ?
A species is a group of organisms that are similar in morphology / anatomy
What are the advantages of morphological species concept ?
- can be applied to asexual and sexual organisms
- can be useful even without information on the extent of gene flow
What are the limitations of morphological species concept ?
- morphology of members of the same species sometimes look very different while members of different species may look similar
- subjectivity in deciding the number and type of morphological traits to compare (amount of differences necessary to determine if two populations are considered different species)
- subjectivity in determining differences in morphological trait that exhibits continuous variation (how different is different enough)
What is the genetic species concept ?
A species is a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups
- using coding and non coding data from nuclear and mitochondrial genomes to identify species
What are the advantages of genetic species concept ?
- applies to individuals from (known) different species which are unable to interbreed and yet produce viable and fertile offspring (genetically isolated but not productively isolated)
What are the limitations of genetic species concept ?
- difficulty in determining degree of genetic differences required to indicate separate species (how different is different enough)
- the value between 2-10% to determine genetic species is subjective
What is phylogenetic species concept ?
A species is the smallest group of organisms that share a common ancestor, forming one branch on the phylogenetic tree
- phylogenetic history is traced through comparing characteristics (morphology and genetic sequence) to distinguish groups of individuals that are sufficiently different to be a different species
What are the advantages of phylogenetic species concept ?
- it applies to asexual and sexual organisms
What are the limitations of phylogenetic species concept ?
- difficulty in determining degree of differences required to indicate separate species
Which species concept would be most useful identifying species in the field ?
Morphological species concept
- based only on appearance, can be used immediately
What is speciation ?
Process which one species splits into two or more species, the formation of new species
When does new species form ?
When gene pools become separated, no interbreeding between different populations of the same species
Occurs over a relatively long time
What is the process of speciation ?
- Existence of biological factors ( reproductive barriers) impede gene flow between two populations
- Different mutations arise independently and change accumulates in each isolated population
- Over a period of time (many generations), genetic variation between non-interbreeding groups is magnified to such an extent that individuals become reproductively isolated (no longer able to produce viable and fertile offspring)
What are the two different types of speciation ?
Allopatric speciation
Sympatric speciation
What is allopatric speciation ? Isolation mechanism ?
Resulting from existence of physical or geographical barriers
Mechanism : geographical
What is sympatric speciation? Isolation mechanism ?
Occurs for populations that live in the same geographical location where there are no physical barriers, speciation forms through instances of physiological or behavioural differences
Mechanism : physiological, behavioural
What is geographical isolation ?
Different populations of the same species are separated from each other due to physical barriers or over long distances
How does geographical isolation lead to allopatric speciation ? ( ANS SCHEME )
- Different populations of the same species are separated from each other due to physical barrier or over long distances
- Fertile individuals from different populations are no longer able to mate with each other ( no gene flow ) 3.thus results in two district gene pools
- As each environment has different selection pressures on the organisms, forces of evolution such as natural selection acts on the separated organisms. Accumulations of independent mutations occur in each population. As different mutations arise, genetic drift also alters allele frequencies.
- Overtime, genetic differences between the two populations increase, they are then unable to make to form fertile viable offspring
What is behavioural isolation ?
Courtship rituals that attract mates and other sexual display or behaviours unique to the species which enables mate recognition does not attract mates thus no gene flow
What are the four types of physiological isolation ?
- Temporal isolation
- Mechanical isolation
- Gametic isolation
- Onset of polyploidy
- autopolyploidy
- allopolyploidy
What is temporal isolation ?
Individuals from different populations reach sexual maturation at different times or have different mating seasons
What is mechanical isolation ?
Individuals from different populations have morphological ( physical ) differences in their sex grams that hinder successful mating (incompatibility of sex organs )
What is gametic isolation ?
Individuals of different populations are able to mate but the sperms are not able to survive in the reproductive tract or other biochemical mechanisms that prevent fusion of sperm and egg
What is polyploidy ?
Heritable condition of possessing more than 2 sets of chromosomes
Based on number of chromosomes, how to tell is an individual is fertile or not ?
Inherited an odd number of chromosome sets (3n, 5n etc) :
- usually infertile
- chromosomes cannot pair up correctly ( homologous set ) during meiosis thus no functional gametes can be produced
Inherited an even number of chromosome sets :
- usually fertile
- chromosomes can still pair up during meiosis and produce functional gametes
What is autopolyploidy ?
Polyploid individuals who have more than 2 chromosome sets that are all derived from a single species
Eg.
- non-disjunction (failure of cell to divide) during mitosis resulting in tetraploid individual (4n)
- if 4n individual and 2n individual interbreed, offspring is 3n (infertile) thus 4n and 2n are reproductively isolated and 4n forms new species
What is allopolyploidy ?
Polyploid individuals whose chromosomes are all derived from two different species
Eg.
- 3n sterile offspring from cross of two different species are able to propagate asexually
- develop into fertile individuals (6n) due to non-disjunction event during mitotic cell division, becomes news species
What is phonology ?
Organisation of species according to particular characteristics which takes into consideration the evolutionary relationship between species
What is classification ?
Organisation of species according to particular shared characteristics
May not take into account evolutionary relationship
What is binomial nomenclature ?
Two part format of the scientific name
What are the two parts of binomial nomenclature?
First part : name of the genus to which the species belongs to
- first letter capitalised
Second part ( specific epithet ) : one species within the genus
- italicised (underline in writing)
Eg. Homo sapiens
- genus : homo
- species : sapiens
What is a taxonomy ?
Scientific discipline to name and classify the diverse life forms
How are organisms grouped into taxonomic levels ?
Based on their similarities (morphological)
- more similarities = more taxonomic levels shared
What are the taxonomic categories ?
Kingdom (KING)
Phylum (PHILIP)
Class (CAME)
Order (OVER)
Family (FOR)
Genus (GOOD)
Species (SOUP)
What is a taxon?
A named taxonomic unit at any level of the hierarchy (name of —)
What are the limitations of taxonomic hierarchy ?
- Grouping on similarities alone can be misleading due to presence of analogous structures
- Related organisms can look very different as a result of adaptation to different environments
- Taxonomic ranks are not equivalent across kingdoms (families have diff number of diff species)
- Does not take into account evolutionary relationship
What does phylogenetic classification group organisms based on ?
Shared morphological characteristics
Anatomical features of fossils
Existing organisms
Molecular data
etc.
What is a phylogenetic tree ?
Evolutionary history of a group of organisms represented in a two way branching diagram
What does the phylogenetic tree represent ?
A hypothesis about evolutionary relationships
What does a branch on the phylogenetic tree represent ?
Evolution of taxonomic unit (eg species) overtime
What does a branch point on phylogenetic tree represent ?
Divergence of two evolutionary lineages from a common ancestor
What does the tip of the phylogenetic tree represent ?
Modern taxonomic units
What is a clade on the phylogenetic tree ?
Group of organisms consisting of the most recent ancestral species and all of its descendants ( any ancestor but just must include all descendants )
What does branch length on phylogenetic tree represent ?
Relative amount of genetic changes
- longer branch = more time to accumulate mutations = more genetic changes
Time
Extinct species
- branches that end before present day : extinct
Organisms that are more 1. ____ related would be expected to accumulate 2. ____ number of evolutionary differences
Organisms more 3. ___ related should share a 4.___ proportion of their DNA
- Distantly
- Greater
- Closely
- Greater
How does DNA sequence show evolutionary relationship ?
DNA sequence differ at many sites : distantly related
DNA sequence differ at few sites : closely related
Which region of DNA ( coding or non-coding ) can be used for comparison?
Both
What are the advantages of molecular methods ?
- Methods are objective as molecular characters are unambiguous whereas morphological features can be subjective and difficult to distinguish
- Molecular data are quantitative and can be easily converted to numerical form for mathematical and statistical analysis
- AA and DNA sequences can be easily obtained / assessed from electronic databanks for easy comparative study and classification
- Each nucleotide position can be regarded as a character, therefore offers limitless set of characters to be studied
- Studies involving DNA sequences data of all organisms can be conducted as all known life is based on nucleotides
- Some molecular differences may not be expressed in phenotype
- Can evaluate analogous structures arising from convergent evolution by studying their molecular differences
