4.2 Evolution by Means of Natural Selection Flashcards
Conditions for natural selection
- Variation in traits of individuals within a population
- Variation in traits is heritable
- More offspring are produced than can survive due to limited resources
- Some individuals are better adapted for survival and reproduction in their environment that others based on trait differences
Limiting resources of natural selection
Food, suitable habitat, and mates to reproduce with
Malthusian principle
Populations increase at an exponential rate while availability of resources increases linearly
Intraspecific competition
Competition between members of the same species
Interspecific competition
Competition against other species
Natural outcomes of human overpopulation
Poverty, famine, and disease
Extant
living (species)
Macroevolution
Big-picture idea that species diversify over time, eventually forming clusters of closely related groups that biologists classify into families and phyla
Microevolution
Concerned with the details of how evolution occurs
Focuses on change in allele frequencies
Population genetics
Studies prevalence of alleles in a population and how populations differ genetically
Recombination
Produces new DNA sequences often from homologous chromosomes during crossover
Mutations
Changes to nucleotide sequence of a genome
Point mutation effects
May be beneficial if cause small change in protein structure that increases ability of protein to perform function
May cause no change if in non-coding region of DNA
Effects of mutation on duplicate gene
Mutation in a duplicate copy may cause minimal harm due to copy of un-mutated gene still existing
Translocation or duplication mutation effects
Can be beneficial by causing higher gene expression whether due to extra gene or translocation to area with more active promoter region
Polymorphism
Two or more alleles for the same trait
Limits to evolution
Two or more alleles for the same trait
Limits to evolution
- Mutation rate
- Rate of mutation spread
Gene
Unit of heritability
Single-gene trait
An individual gene controlling one trait
Polygenic trait
Many genes acting together to produce a single trait
Pleiotropy
A single gene influencing more than one trait
Negative frequency-dependent selection
Rare phenotypes have an advantage in a specific environment and natural selection makes them more common
Positive frequency-dependent selection
Rare phenotypes have a disadvantage and become even less favorable in a specific environment
Heterozygous advantage
Having a higher fitness than a homozygous individual. Eg sickle cell anemia
Directional selection
(Of polygenic traits)
Favors one end of extreme phenotype, population average shifts to the left or right of a bell curve
Stabilizing selection
(Of polygenic traits)
Favors intermediate phenotypes, narrowing the distribution of the bell curve
Disruptive selection
(Of polygenic traits)
Favors extreme phenotypes on both ends and average is unfavorable
Over time different morphs, subspecies, and even species may form
Genetic drift
Random change in allele frequencies over time
More prevalent in smaller populations with less genetic variation where alleles can disappear from the population due to few deaths
Population bottleneck
Population is dramatically reduced causing survivors to have a disproportionately high contribution to gene frequency of descendants
Founder effect
A type of population bottleneck where few individuals containing only a small amount of genetic variation from their original population form a new population in an isolated area
G.H. Hardy and Wilhelm Weinberg
In 1908, they independently explained how allele frequencies remained constant over time
Hardy-Weinberg Principle
Allele frequencies in a population stay constant over time if five conditions met:
- No mutations
- Population size large, not affected by genetic drift
- Population is isolated from other populations, no immigration, emigration, or other types of gene flow
- Mating occurs randomly, without inbreeding or positive assortative mating
- No natural selection, including sexual selection, occurring
Gene flow
Genetic info transferred between populations
Sexual selection
Members of one sex choose mates or members of on sex compete with each other for mates
Hardy-Weinberg equation
P^2 + 2pq +q^2 = 1
(Dominant homozygous + heterozygous + recessive homozygous = 1)
Reproductive isolation
Inability of a species to reproduce with a similar species
Prezygotic factors
- Habitat isolation
- Temporal isolation
- Behavioral isolation
- Mechanical isolation
- Gametic isolation
Habitat isolation
Geography separated species or species find mates in different habitats
Temporal isolation
Species mate during different seasons or different times of the day
Behavioral isolation
Organisms are not attracted to each other (such as differing mating rituals)
Mechanical isolation
Species physiologically different enough to not be able to mate
(Such as distinctly shaped genitals)
Gametic isolation
Gametes from two species are unable to fuse to form zygote
Postzygotic Factors
- Zygote mortality
- Hybrid inviability
- Hybrid sterility
Zygote mortality
Egg fertilized but zygote can’t mature to form offspring
Hybrid inviability
High mortality in hybrid species
Hybrid sterility
Hybrid unable to reproduce
Allopathic speciation
Geographically isolated populations become different species
(May be driven my natural selection)
Peripatric speciation
Type of allopathic speciation
Subpopulation established on the edge of the main population’s habitat becomes a new species over many generations
Parapatric speciation
Subpopulations o a species have different ranges but overlap with each other at least somewhat
Individuals don’t mate randomly, rather they mate with geographic neighbors
Sympatric speciation
Formation of new species without geographic isolation
Rare especially in animals
Adaptive radiation
Evolution of a single species into diverse species adapted to specific niches
Mass extinction
Large number of living species goes extinct in a relatively short time
Often followed by periods of high adaptive radiation and speciation
Catastrophic species selection
Phenomenon where some species survive a catastrophic event that leads to mass extinction and others don’t
The end-Ordovician extinction
444 MYA
Extinction of 85% of species including trilobites, corals, and brachiopods
Advancement and subsequent melting of ice sheets
Cooling of Earth
Drastic drop in ocean levels
The end-Devonian extinction
380-359 MYA
Loss of about 75% of species including trilobites, corals, and placoderms
Major changes in climate, ocean oxygen levels and other environmental shifts
Volcanic activity and asteroid impact may have contributed
The end-Permian extinction
252 MYA
Largest mass extinction event in history
Loss of about 90% of species including many invertebrates
Only mass extinction to have caused loss of large number of insect species
Thought to have taken place in several waves
Likely cause by volcanic activity in Siberia leading to global warming, toxic gasses, massive wildfires, ocean acidification, and destruction of ozone layer
The end-Triassic extinction
201 MYA
Loss of nearly 80% of species including many reptiles, but dinosaurs fared well
Caused by volcanic eruptions
The end-Cretaceous extinction
65.5 MYA
Loss of about 75% of species including dinosaurs, marine invertebrates, and numerous small reptiles
Small mammals survived by burrowing and storing food underground allowing them to become more dominant during the Cenozoic era
Species Survival Plan
Programs put in place by the Association of Zoos and Aquariums
Involves breeding endangered and threatened species with the ultimate goal of being able to reintroduce them to their native habitats
