evolution Flashcards
Lamarck’s theory
- Life had originated many times
- Over time, lineages become more complex
- Thus, at any one time, there are recently originated simple forms of life, and older more complex forms of life
Darwin and Wallace: Similarities
- Evolution as a “fact”
- Common descent
- Gradualism
- Population change
- Natural selection
Darwin and Wallace: Differences
*Darwin emphasized competition between individuals of the same species to survive and reproduce
*Wallace emphasized environmental pressures on varieties and species forcing them to become adapted to their local environment.
Synonymous mutation
the AA does not change
produced amino acid sequence is not modified. This is possible because the genetic code is “degenerate”
Nonsynonymous mutation
the AA changes
there is usually an insertion or deletion of a single nucleotide in the sequence during transcription
causes a frameshift mutation
How often mutations occur varies:
- Among regions of the genome
- Base composition (transitions and transversions occur at different rates)
- Position in the genome (centromeres vs chromosome tips)
- Presence of epigenetic modifications
selection
- Occurs when there is a difference in the survival and/or reproduction of individuals based on their phenotype
- Survival and reproduction can be summarized as an individual’s fitness
Selection increases the frequency of the favoured trait in the next generation
- Survival and reproduction can be summarized as an individual’s fitness
natural selection
When the difference in fitness occurs due to conditions in the biotic or abiotic environment
artificial selection
When the difference in fitness occurs due to human preference for traits (i.e., selective breeding)
sexual selection
when individuals mate preferentially with particular individuals rather than at random
- Important because individuals that survive but fail to reproduce make no contribution to the next generation = no fitness
- Thus, sexual selection has the power to favour traits that enhance chances of reproduction, even if those traits reduce chances of survival
- Limited by the need to balance survival and reproduction
adaptation
Characteristic that enhances the survival or reproduction of organisms that bear it relative to alternative character states
Process by which members of a population become better adapted to their environment over time
migration
The movement of individuals or gametes
If migrants survive and reproduce, gene flow occurs
gene flow
Gene flow changes the allele frequencies in populations in proportion to the rate and direction of migration
genetic drift
Random changes in allele frequencies from one generation to the next
* Driven by chance, not selection
Mechanisms in stable populations: Random survival
Genetic drift has larger impacts in small populations
population bottleneck
when a population is dramatically reduced in size, by chance the small number of survivors may not be representative of the original population
founder effect
when a population is started (“founded”) by a small number of individuals, they are unlikely to possess all of the alleles found in the gene pool of the source population
* Founder populations will be less variable than original
* Rare alleles are likely to be lost
three conditions for selection
- Variation in a trait
- Heritability of a trait
- Differential fitness conferred by a trait
Qualitative versus Quantitative Traits
- Qualitative traits are those that have discrete qualities often controlled by alleles at a single locus
Quantitative traits show continuous variation can be influenced by genes at many loci as well as by the environment
Directional selection
favours one extreme value
Stabilizing selection
favours average values
Disruptive selection
favours both extreme values
Intra-sexually
competition between members of the same sex
Inter-sexually
mate choice
Pre-copulatory
acquiring mating opportunities
Post-copulatory
events that occur during and after mating
Frequency-dependent selection
the fitness of a given phenotype depends on its frequency in a population
negative Frequency-dependent selection
The less common a phenotype is, the higher its fitness
Heterozygote advantage
When different alleles are favoured under different environmental conditions heterozygous individuals may outperform homozygotes
Certain variants of the PGI gene are able better able to fly in cold conditions, while others are better able to fly in hot weather
Heterozygous individual can fly over a greater range of temperatures than either type of homozygote
Continental-Island model
One-way gene flow from a large continental mainland population to a smaller island population
Allele frequency on the island changes at a rate that depends upon the
* rate of gene flow
* difference in allele frequency between the island and the mainland
island model
Describes gene flow among many populations exchanging immigrants with one another
stepping stone model
Allow subpopulations to exchange individuals only with adjacent subpopulations in one or two or more dimensions
Two subpopulations that are far apart will experience little gene flow and will be more different that two subpopulations closer together
Probability of mating decreases with distance
* Individuals chosen from two proximate locations are more closely related then locations distributed further apart
types of trade offs
- allocation constraints
- functional conflicts
- shared biochemical pathways
- ecological circumstances
- sexual v. natural selection
Macroevolution
Zooming out on the tree of life from populations and species to look at grand trends in evolution
patterns in macroevolution
Stasis Lineages don’t change much in outward appearance for long periods of time
phylogeny
A diagram that depicts the lines of evolutionary descent of different lineages
Shows divergence events as dichotomous branching
Taxon (plural ‘taxa’)
a group of individuals that are related at any scale
tips of the tree
terminal taxa or sometimes “leaves”
nodes
branching points on the phylogenic tree
clade
a taxon that consists of all evolutionary descendants of a common ancestor
Clades can form nested sets of taxa on a tree
sister species
Two species that are each other’s closest relatives
sister clades
Two clades that are each other’s closest relatives
ancestorial traits
characters inherited from the common ancestor of a group
derived traits
characters that are unique to specific subgroups
homologous traits
A feature shared by taxa that are descended from a common ancestor
Synapomorphy
Derived traits shared across all members of a group
Evidence of common ancestry
Autapomorphy
A derived trait that is unique and defines that taxon
Homoplasies
traits that are superficially similar but do not arise from shared ancestry
A trait that appears homologous may actually be homoplastic
Homoplasies can arise through
- Convergent Evolution
- Independently evolved features subjected to similar selective pressures become superficially similar
- Evolutionary Reversal
A character reverts from a derived state back to an ancestral one
nomenclature
The first name identifies the genus, the second name identifies the species
The genus name is capitalized, the specific name is not Both are always italicized NOTE: higher classifications like order or phylum are not written in italics
Do Keep Pond Clean Or Frog Gets Sick
Domain
Kingdom
Phylum
Clade
Order
Family
Genus
Species
Monophyletic groups (or clades)
contain all the descendants of a particular ancestor and no other organisms
Polyphyletic groups
contain members of more than one recent common ancestor
Paraphyletic groups
contain some, but not all, the descendants of an ancestor
Species concept: Morphological
- If two organisms look different, they are different species
- In practice, take a selection of specimens, examine their morphology, and group specimens together into species
Uses the judgement of the Taxonomist to decide where species boundaries lie
- In practice, take a selection of specimens, examine their morphology, and group specimens together into species
Species concept: Morphological pros
- Requires only a physical description (can be applied to fossils)
- Can use statistical approaches to cluster individuals based on measurement data
Species concept: Morphological cons
- Members of a species don’t always look alike
- Sexual dimorphism
- Life stage
- Different species can look really similar
Cryptic species
Two or more species are morphologically indistinguishable but do not interbreed
Species concept: Biological
- Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups
- Proposed by Ernst Mayr (1940)
- Implies that individuals in different species cannot successfully interbreed
e.g. female horse Equus caballus x male donkey E. africanus = mule (sterile hybrid)
Species concept: Biological pros
- Identifies evolutionarily independent groups
Species concept: Biological cons
- What about asexually reproducing or hermaphroditic species?
- What about geographically isolated individuals that could reproduce, but never do because of their isolation?
- Reproductive compatibility is an ancestral character, reproductive isolation is derived
- Can interbreed
- Reproductively isolated
Species concept: Ecological
- Each species has a discrete ecological niche (way of living) defined in many different ways (e.g. climate, habitat, food), and different from all other species
- The ecological niche of a species can be represented as an n-dimensional hyperspace
Overlap in two niche dimensions resolved by a third, etc.
- The ecological niche of a species can be represented as an n-dimensional hyperspace
Species concept: Ecological pros
- Ecological data can help tease apart cryptic species
e.g., based on their song or the time of day that they are active
Species concept: Ecological cons
- Ecological niches are extremely complicated with many possible ways that niches can differ between species
- Gathering all the data to truly understand a given individual’s ecological niche is so much effort that it is essentially impossible
Species concept: Phylogenetic
- A group of organisms that share an ancestor (monophyly); a lineage that maintains its integrity with respect to other lineages through both time and space.
- A species is a “tip” on a phylogeny, that is, the smallest set of organisms that share an ancestor and can be distinguished from other such sets.
Identifies populations with no gene flow as species
- A species is a “tip” on a phylogeny, that is, the smallest set of organisms that share an ancestor and can be distinguished from other such sets.
Species concept: Phylogenetic pros
- Applies to all organisms (e.g., bacteria, cryptic)
- Readily assessed using DNA
Species concept: Phylogenetic cons
- How many diagnostic characters are enough? How many genes do you have to sequence before it’s enough like
- Application of this concept results in a LOT of species (overdiagnosis?)
Prezygotic barrier
preventing mating
Postzygotic barrier
preventing the development of viable offspring
Habitat isolation
Populations that live in different habitats and do not meet.
Temporal isolation
Mating or flowering occurs at different seasons or times of the day
Behavioral isolation
There is little or no sexual attraction between females and males.
Gametic isolation
Female and male gametes fail to attract each other or are unviable
Mechanical isolation
Structural differences in genitalia or flowers prevent copulation or pollen transfer.
Hybrid inviability
zygote fails to develop or fail to reach sexual maturity
Hybrid sterility
Hybrids fail to produce functional gametes
Hybrid breakdown
the offspring of hybrids have reduced viability or fertility
How many genes need to be different for speciation to occur?
– as few as five and as many as 200 genes can contribute to hybrid inviability
– far fewer gene differences have been found to be sufficient in and of themselves to confer postzygotic isolation (n =4)
Mode of Speciation: Allopatric
evolution of genetic reproductive barriers between populations that are geographically isolated
- Some members of a population are separated from others by a geographic barrier.
- Gene flow between the two populations is greatly reduced (barrier is almost impervious to gene flow)
- The two populations evolve separately in their respective habitats
- If the two populations come together sympatrically, they can no longer interbreed
Mode of Speciation: Peripatric
new species are formed from isolated peripheral populations
* form of allopatric speciation
* linked to founder effect
* potentially more common than standard allopatric speciation.
Mode of Speciation: Parapatric
occurs due to variations in mating frequency of a population within a continuous geographical area
- Population is not split, but is large and extends over a large range.
- Some members of the population may never interact; or change in environment across range causes differential selection pressures.
- Gene flow between members of the population is restricted; much weaker than divergent selection.
- Two groups evolve separately; can no longer interbreed.
Limited interaction among individuals occupying a large range can reduce gene flow
Mode of Speciation: Sympatric
genetic divergence of multiple populations (from a single parent species) inhabiting the same geographic region
- Gene flow between individuals is not restricted due to geographic barriers
- Population may be divided by the following mechanisms
- New niche exploitation
- Disruptive selection
- Polyploidy
- New niche exploitation
- Disruptive selection, favouring extreme phenotypes
Autopolyploidy
chromosomes derived from the same species
Allopolyploidy
chromosomes derived from different species
Adaptive Radiation
- Often occurs in island archipelagoes
- Individuals of a population colonize new habitat
- Gene flow restricted between isolated population and parent population
- Resource environment and predator pressures different in new habitat
- New niches available
- Individuals in new habitat adapt accordingly, occupying new niches
- Adaptation to the new niche drives rapid speciation
Species Radiation
- Often occurs in island archipelagoes
- Individuals of a population colonize new habitat
- Gene flow restricted between isolated population and parent population
- Genetic drift and lack of geneflow result in the accumulation of differences
- More slowly results in the formation of allopatric species
- Adaptation is STILL HAPPENING, but not the driving force in speciation
What is a life history?
An individual’s pattern of allocation, throughout life, of time and energy to various fundamental activities, such as growth, repair of cell and tissue damage, and reproduction
Lack’s Dilemma
David Lack (1947) predicted that parents should be selected to attempt to raise the number of offspring that would maximize offspring production
However, it is not unusual to find birds laying fewer eggs than the Lack clutch! (tested by brood augmentation) 67% of 77 studies
Optimizing fitness in a variable and unpredictable environment
The paradox:
* Bet-hedging
* Longevity trade-off
- The hard work of raising offspring can reduce lifespan
- Parental birds foraging for food to supply offspring operate at about 4X basal metabolic rate
- Birds may be laying fewer eggs to increase odds of surviving to succeeding years
* Reproductive value
Considering not just present, but also future reproductive success
Having too many offspring at once may lead to:
* Reduced survival in the nest
* Reduced survival from fledging to the next breeding season
* Reduced probability that the parents would lay a second clutch of eggs the same year
