Descent with Modification & Darwin’s Discoveries
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Know who Darwin is and his accomplishments around evolution. Also be familiar with what he discovered on his journey to the Galapagos Islands
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Wrote the Origin of Species
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Came up with the concept of descent with modification, which describes evolution
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Understand the theories of evolution, and which one is more likely:
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Catastrophism
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Uniformitarianism
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Lamark vs. Darwin: how did their theories of evolution differ?
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Lamark believed that acquired traits could be passed on through use and disuse. However, as we know today, that is not how evolution works
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Darwin believed that natural selection was the driving force behind evolution, which is correct. This process of natural selection leads to adaptations in populations
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Adaptations = inherited traits that increase an individual’s chances of survival and ability to pass on their genes
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Observations that support Darwin’s idea of natural selection:
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Artificial selection. Know what this is, and how it’s different from natural selection
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Variation in a population
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Overproduction of offspring
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Key points about natural selection:
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It’s a process in which organisms with specific traits are able to survive better and pass on their genes
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Natural selection can increase the number of organisms with favorable adaptations, which are suited for their specific environment or niche
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Sometimes natural selection can result in new species
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Also remember: populations evolve, individuals do not evolve! (common trick question)
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Evidence of evolution: (know what each of these are, and examples of each)
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Vestigial structures (e.g., our wisdom teeth and appendix)
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Divergent vs. convergent evolution:
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What are the differences? Which one would involve homologous structures, and which would involve analogous structures?
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What are some examples of each?
Populations, not individuals, evolve over time. Image courtesy of Petr Ganaj from Pexels
The Evolution of Populations
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Evolution is possible because of the genetic variation that exists in populations. There will always be some variation within a population, as well as across different populations
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Sources of genetic variation:
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New alleles arising through random mutations
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Genes may change locations on a chromosome
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Rapid reproduction cycles. A great example is bacteria
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Sexual reproduction, because of crossing over and independent assortment in meiosis
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Population genetics:
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Population = group of individuals from the same species that lives in the same area
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Gene pool = the combined genes within a population, including all alleles
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Hardy-Weinberg Principle
If a population is at Hardy-Weinberg equilibrium, then the gene pool is staying constant. For this to be true, the following conditions must be met: (important, must remember)
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No mutations in the population
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No migration in/out of the population
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Random mating
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No natural selection
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Very large population size
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These conditions are not very realistic, but we use them when solving Hardy-Weinberg equilibrium (H-W) problems.
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There are two important equations to remember for H-W problems:
p + q = 1
p^2 + 2pq + q^2 = 1
Where p = frequency of dominant allele, q = frequency of recessive allele
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Genetic Drift:
This is when a gene pool changes over time. It occurs more significantly in small populations. Two key examples to know are:
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Founder effect
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Bottleneck effect
You should know the difference between the two and a couple of examples of each. You can refer to this article for a review of each of these effects
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Then there is the concept of gene flow. How is this different from genetic drift?
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Types of natural selection:
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Directional
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Disruptive
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Stabilizing
*Review these three types with this article from Khan Academy and understand the differences, especially relating to the changes in allele frequencies
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If there are some unfavorable alleles in a population, why don’t all of those get weeded out (die off) over several generations? Because of the following naturally-occurring mechanisms:
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Diploidy -- having two sets of chromosomes (and alleles) instead of only one
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Balancing selection, such as the heterozygote advantage (EX: gene affecting the individual’s response to sickle-cell anemia) and frequency-dependent selection. Know how to explain these two types of balancing selection
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Reasons why natural selection cannot produce perfect organisms:
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An organism or population cannot will a favorable allele to appear; they pop up randomly through mutations
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Evolution is limited by what has already happened in a lineage/evolutionary path, and the species that are currently living on Earth (i.e., cannot fashion a brand-new organism from scratch)
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Many traits have tradeoffs, so they will not be perfect
​Image courtesy of Thameur Dahmani from Pexels
Speciation
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The biological definition of speciation is that it occurs when a group of individuals can no longer breed with members of its original population (they’ve evolved to be different enough to have this happen)
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There are many factors that can contribute to reproductive isolation, which can ultimately lead to formation of a new species:
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Prezygotic barriers such as habitat, temporal, behavioral, mechanical, and gametic isolation
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Postzygotic barriers such as reduced hybrid survival or fertility
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Allopatric speciation: New species is geographically separated from the original. Allopatric means “other country”
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Sympatric speciation: Interestingly, new species can also form without geographic separation from the original population. That is called sympatric speciation
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Hybrid zone = an area where individuals from different species can meet and mate. This is usually due to two different species occupying adjacent areas of land
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There are two way that speciation can occur over time:
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Punctuated equilibrium = Has periods of rapid/sudden changes leading to a new species
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Gradualism = species form slowly and gradually over time
The Earth’s History and Early Earth Conditions
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When Earth first formed, the environment was not ideal for life and there was nothing living. Over time, scientists believe the following steps led to the first life forms:
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Small organic molecules joining together over time
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Further joining into larger macromolecules, such as protein
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These molecules get packaged into protocells, which are enclosed within a membrane
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Self-replicating molecules eventually arose, allowing genes to be passed on to the next generation
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Scientists use methods such as radiometric dating and the half-life of a particular isotope in order to determine fossil ages and piece together the evolutionary history of past species
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The geologic record
There are three eras:
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Cenozoic -- several ice ages; many primate species arose; the genus Homo first arose
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Mesozoic -- includes the Jurassic and Triassic ages; angiosperm diversity increases
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Paleozoic -- includes the Cambrian explosion, which was a sudden boom of diversity across all kinds of groups of animals, such as bony fish, reptiles, and vascular plants
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How did the first eukaryotes form on Earth?
This is where the endosymbiotic theory comes in. Know what it is, and what evidence points to it
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Evidence for the endosymbiotic theory:
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The inner membranes of chloroplasts and mitochondria are very similar to those of prokaryotes
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These organelles also have their own small bits of DNA inside
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They replicate by splitting, a process similar to reproduction of some prokaryotes
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