Unit 7: Natural Selection Flashcards
1
Q
Fitness
A
- Individuals whose inherited traits confer an advantage have a better chance of surviving in a given environment and will leave more offspring
- Unequal fitness will lead to gradual changes in a population, with favorable traits accumulating over generations
- Evolutionary fitness is measured by reproductive success
- Fitness does not equal strength. Fitness depends upon the environment
- Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction
- Different genetic variations can be selected in each generation
- These gradual changes from unequal fitness may accumulate over time resulting in a new species.
2
Q
Adaptions
A
- Higher fitness may be the result of favorable adaptations to an environment
- An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment
3
Q
Natural Selection
A
- Variation exists in natural populations
- Many more offspring are born each season than can possibly survive to maturity.
- As a result, there is a struggle for existence (competition)
- Characteristics beneficial in the struggle for existence tend to become more common in the population, changing the average characteristics of the population → adaptations
- Over long periods of time, and given steady input of new variation into a population, these processes lead to the emergence of new species
- The combination of alleles that provide fitness increase in the population as organisms with the highest fitness are better able to survive and reproduce
- Over time, natural selection reduces the variation in a population as the alleles for advantageous traits increase in frequency in the gene pool
- Which traits are favored depends on the environment
- When the environment changes, which traits are advantageous can change as well
- Decreases variation
4
Q
What causes an increase in variation in the population
A
- Mutation
- Gene flow
- Sexual reproduction
5
Q
What causes an decrease in variation in the population
A
- Non-random mating
- Genetic drift
- Natural selection
6
Q
Mutations
A
- Change in DNA sequence
- Might change the amino acid sequence → change the protein structure & function
- Mutations create variation
- Can make new phenotypes (ex. Dark fur in rock pocket mice)
- Changes in protein may change an organism’s phenotype and therefore its fitness
- Ex. dark fur in rock pocket mice was advantageous after volcanic eruption but not before
- Increases variation
7
Q
Gene Flow
A
- Movement of individuals and alleles in and out of population
- Ex. seeds and pollen distributed by wind and insects
- Ex. migration of animals
- Causes genetic mixing across regions
- Can introduce new variation to a population
- Increases variation
8
Q
Sexual Reproduction
A
- Crossing over and independent assortment during meiosis results in the production genetically unique gametes
- Random fertilization of these unique gametes result in variation among offspring
- Increases variation
8
Q
Non-Random Mating
A
- Traits that attract mates increase reproductive success and get passed on
- Reduces the variation in population - alleles for traits that attract mates become more common in the gene pool over time
- Decreases variation
9
Q
Genetic Drift
A
- A chance event causes a change in the population
- Founder effect - a small group leaves and starts a new colony
- Bottleneck effect - a disaster reduces a population to a small number
- Decreases genetic variation: some rare alleles may be at high frequencies and others may be missing, not due to fitness (remember the blue Fugates?)
- This is a big problem - makes it more likely the entire population will die due to an environmental change
- Ex. a bottleneck event and inbreeding from a single surviving litter has caused cheetahs to be nearly genetically identical
- Decreases variation
10
Q
Hardy Weinberg Equilibrium
A
- Hypothetical model to measure change in allele frequency in a population
- It represents a population that is non evolving - gene frequencies will not change over time if there is no evolution!
- We expect to see NO change or no difference between expected and real gene frequencies over time
- Conditions for a population or an allele to be in H-W equilibrium (AKA for there to be NO evolution):
- Very large population (no genetic drift)
- No migration (movement in or out)
- No mutation (no genetic change)
- Random mating (no sexual selection)
- No natural selection - The law tells us that populations maintain a reservoir of variability so that if future conditions require it, the gene pool can change
- If recessive alleles were continually tending to disappear, the population would soon become homozygous
- Under Hardy-Weinberg conditions, genes that have no present selective value will nonetheless be retained.
11
Q
Allele Frequency Equation
A
- p is the symbol for the frequency of the dominant allele
- q is the symbol for the frequency of the recessive allele
- p + q = 1
12
Q
Genotype/phenotype Frequency Equation
A
p^2 + 2pq + q^2 = 1
13
Q
How to Find Allele Frequencies when it Doesn’t say H-W
A
- If it doesn’t say it is H-W equilibrium then just count the alleles to find allele frequencies (it will give you the number of organisms with a specific alleles like AA or Bb)
- There are always twice the number of alleles as there are individuals in the population (every individual has two alleles for a gene)
14
Q
Evidence for Evolution: Fossil Record
A
Evidence of extinct life preserved in rock, ice, amber, wood, etc
15
Q
Evidence for Evolution: Anatomical Record
A
- Homologous structures - similarities in characteristics resulting from common ancestry
- Analogous structures - different internal structures but similar functions due to adaptation to similar environments
- Vestigial structures - remnants of structures that were functional in an ancestral species but have little or no function in modern species
16
Q
Evidence for Evolution: Embrology
A
Closely related species have similar embryological development
17
Q
Evidence for Evolution: Molecular Record
A
- All cells use ribosomes to produce proteins
- There are also similarities in biochemical processes
- Because the genetic code is universal, we can compare DNA and protein structure - closely related species have sequences that are more similar than distantly related species
- The more amino acid differences in a protein shared by different species, the more time has passed since their divergence
17
Q
Selective Breeding/Artificial Selection
A
- Occurs when humans affect variation in other species by determining which traits should be passed on
- Contrary to natural selection when the environment influences which traits are passed on based on survival
- Ex. breeding of dogs, livestock, and crops
18
Q
Selection Graphs: Disruptive Selection
A
- Natural selection favors the extreme phenotypes
- Can lead to speciation
- Ex: frogs that breed during different times of year
19
Q
Selection Graphs: Stabilizing selection
A
- Natural selection favors the intermediate phenotype
- Ex: human birth weight
19
Q
Speciation
A
- These gradual changes from unequal fitness may accumulate over time resulting in a new species. New species arise when populations do not reproduce together (no gene flow) and evolve separately over time
- Speciation occurs when populations are reproductively isolated from each other
- Reproductive barriers will prevent interbreeding
Types of speciation: - Allopatric speciation - geographic separation causes reproductive isolation
- Sympatric speciation - species live in the same area but are reproductively isolated for another reason (could be due to genetic mutation)
20
Q
Selection Graphs: Directional Selection
A
- Natural selection favors one of the extreme phenotypes
- Happens mostly when environment changes
- Ex: peppered moth
21
Q
Postzygotic Barriers (occur after a zygote forms)
A
- After a zygote forms from gametes of two different species, the zygote is unable to develop into a viable, fertile adult
- Reduced hybrid viability - genes of different parent species may interact and impair the hybrid’s development
- Reduced hybrid fertility - chromosomes of parents may differ in number or structure such that meiosis in hybrids may fail to produce normal gametes
- Ex. horses have 64 chromosomes, donkeys have 62, and mules have 63
- Hybrid breakdown - hybrids may be fertile and viable in first generation, but when they mate, offspring are feeble or sterile
- Ex: certain rice hybrids
21
Prezygotic Barriers (prevent a zygote from forming)
- Geographic (habitat) isolation - a geographic barrier (such as mountain range) prevents species from breeding with each other
- Ecological isolation - species live in the same area but in different habitats so they rarely encounter each other
- Ex. lions live in grasslands and tigers in rainforests
- Temporal isolation - species breed at different times and therefore not with each other
- Behavioral isolation - differences in courtship rituals prevent species from breeding with each other
- Mechanical isolation - differences in body structure prevent breeding (ostrich and hummingbird)
- Gametic isolation - sperm from one species may not be able to fertilize an egg of another species
22
Rates of Speciation
- Gradualism - slow changes over a long period of time. Ecological conditions change very gradually over a long period of time
- Punctuated equilibrium - rapid burst of change and long periods of little or no change. Changing ecological conditions are the stimulus for evolution to take place
22
Adaptive Radiation
- Evolution of many species when introduced to new environmental challenges - allows empty ecological roles (niches) to be filled. Single species or small group of species rapidly diversifies into many new species
- Ex. Darwin’s finches - quickly adapted to different islands, leading to multiple species
23
Divergent Evolution
- Species with a common ancestor evolve differences over time
- Occurs when adaptation to new habitats results in phenotypic diversification
- Will have homologous structures
24
Different Origins of Life Theories
- Organic molecules could have been synthesized from inorganic molecules - Urey-Miller experiment showed that inorganic molecules that would have been present in the early atmosphere could form organic molecules
- Organic molecules could have been transported to Earth by a meteor
- Inorganic precursors on primitive Earth used available free energy to react in the absence of oxygen to form organic precursors (contain C, H, and O)
- RNA World Hypothesis - RNA was most likely the first genetic material
- Prokaryotes were the first living organisms, led to eukaryotic cells (endosymbiotic theory)
24
Convergent Evolution
- Though not closely related, species evolve similar solutions to similar environmental pressures
- Will have analogous structures
25
Coevolution
- A species evolves in response to the evolution of another
- Ex: Predator-prey relationship, parasite-host relationships, and flowers & pollinators
26
Cladogram
Charts that predict evolutionary relationships between different extant or extinct species or groups of organisms
26
Cladistics & Phylogenetics
- Classification based on common ancestry
- Phylogeny - the evolutionary history for a group of species based on evidence from living species, fossil record, and molecular data
- Clade - a group of organisms that share a common ancestor
- Derived characters - shared traits
- Outgroup - least closely related to the remainder of the organisms
- Node - the most recent common ancestor of a clade
- Nodes indicate speciation has occurred
- Species that are more closely related share the greatest number of common ancestors
- Remember that the organisms/species can be turned/flipped around by the node and they are still equally related to the other species who don't share that node
27
Intersexual vs Intrasexual selection
Intersexual selection, or mate choice, involves one sex choosing mates based on specific traits, while intrasexual selection, or mate competition, involves members of the same sex competing for access to mates
28
what is a hybrid zone?
typically form when populations that have been isolated and evolved independently come back into contact
29
Outcomes of Hybrid Zones
- Reinforcement: If hybrids are less fit, natural selection strengthens reproductive barriers (like to genetic incompatibilities, reduced survival, or infertility), leading to further speciation
- Fusion: If hybrids are equally or more fit, the two species may merge into one. Reproductive barriers break down, leading to increased interbreeding
- Stability: If hybrids continue to be produced but remain limited to the zone. hybrids survive but remain restricted to the hybrid zone without merging the species
