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Wild cat has adapted to its environment over time

Descent with Modification & Darwin’s Discoveries

Know who Darwin is and his accomplishments around evolution.  Also be familiar with what he discovered on his journey to the Galapagos Islands

  • Wrote the Origin of Species

  • Came up with the concept of descent with modification, which describes evolution


Understand the theories of evolution, and which one is more likely:

  • Catastrophism

  • Uniformitarianism


Lamark vs. Darwin: how did their theories of evolution differ?

  • Lamark believed that acquired traits could be passed on through use and disuse.  However, as we know today, that is not how evolution works

  • Darwin believed that natural selection was the driving force behind evolution, which is correct.  This process of natural selection leads to adaptations in populations

    • Adaptations = inherited traits that increase an individual’s chances of survival and ability to pass on their genes


Observations that support Darwin’s idea of natural selection:

  • Artificial selection.  Know what this is, and how it’s different from natural selection

  • Variation in a population

  • Overproduction of offspring


Key points about natural selection:

  • It’s a process in which organisms with specific traits are able to survive better and pass on their genes

  • Natural selection can increase the number of organisms with favorable adaptations, which are suited for their specific environment or niche

  • Sometimes natural selection can result in new species


Also remember: populations evolve, individuals do not evolve! (common trick question)


Evidence of evolution: (know what each of these are, and examples of each)


Divergent vs. convergent evolution:

  • What are the differences?  Which one would involve homologous structures, and which would involve analogous structures?

  • What are some examples of each?


Populations, not individuals, evolve over time.  Image courtesy of Petr Ganaj from Pexels

The Evolution of Populations

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


Sources of genetic variation:

  • New alleles arising through random mutations

  • Genes may change locations on a chromosome

  • Rapid reproduction cycles.  A great example is bacteria

  • Sexual reproduction, because of crossing over and independent assortment in meiosis


Population genetics:

  • Population = group of individuals from the same species that lives in the same area

  • Gene pool = the combined genes within a population, including all alleles


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)

  1. No mutations in the population

  2. No migration in/out of the population

  3. Random mating

  4. No natural selection

  5. Very large population size


These conditions are not very realistic, but we use them when solving Hardy-Weinberg equilibrium (H-W) problems.


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


Genetic Drift:

This is when a gene pool changes over time.  It occurs more significantly in small populations.  Two key examples to know are:

  1. Founder effect

  2. 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


Then there is the concept of gene flow.  How is this different from genetic drift?


Types of natural selection:

  • Directional

  • Disruptive

  • Stabilizing

*Review these three types with this article from Khan Academy and understand the differences, especially relating to the changes in allele frequencies



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:

  • Diploidy -- having two sets of chromosomes (and alleles)  instead of only one

  • 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


Reasons why natural selection cannot produce perfect organisms:

  • An organism or population cannot will a favorable allele to appear; they pop up randomly through mutations

  • 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)

  • Many traits have tradeoffs, so they will not be perfect


Image courtesy of Thameur Dahmani from Pexels


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)


There are many factors that can contribute to reproductive isolation, which can ultimately lead to formation of a new species:

  • Prezygotic barriers such as habitat, temporal, behavioral, mechanical, and gametic isolation

  • Postzygotic barriers such as reduced hybrid survival or fertility


Two types of speciation:

  1. Allopatric speciation:  New species is geographically separated from the original.  Allopatric means “other country”

  2. Sympatric speciation:  Interestingly, new species can also form without geographic separation from the original population.  That is called sympatric speciation


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


There are two way that speciation can occur over time:

  • Punctuated equilibrium = Has periods of rapid/sudden changes leading to a new species

  • Gradualism = species form slowly and gradually over time

The Earth’s History and Early Earth Conditions

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:

  1. Small organic molecules joining together over time

  2. Further joining into larger macromolecules, such as protein

  3. These molecules get packaged into protocells, which are enclosed within a membrane

  4. Self-replicating molecules eventually arose, allowing genes to be passed on to the next generation


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


The geologic record

There are three eras:

  1. Cenozoic -- several ice ages; many primate species arose; the genus Homo first arose

  2. Mesozoic -- includes the Jurassic and Triassic ages; angiosperm diversity increases

  3. 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


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


Evidence for the endosymbiotic theory:

  • The inner membranes of chloroplasts and mitochondria are very similar to those of prokaryotes

  • These organelles also have their own small bits of DNA inside

  • They replicate by splitting, a process similar to reproduction of some prokaryotes

In addition to knowing the concepts and terminology, it's also crucial to apply them by doing plenty of practice problems.  If you're struggling to find good problems to work through, check out our popular Practice Portal!  

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