Unit 7 - Evolution

7.1 - Definition of Evolution

• Evolution descent with modification.
• Descent can happen only when one group of organisms gives rise to another.
• Evolution describes change in allele frequencies in populations over time.
• When one generation of organisms reproduces and creates the next, the frequencies of the alleles for the various genes represented in the population may be different
• from what they were in the parent generation.
• Frequencies can change so much that certain alleles are lost or others become fixed—all individuals have the same allele for that character.
• Over many generations, the species can change so much that it becomes quite different from the ancestral species, or a part of the population can branch off and become a new species (speciation).
• Gene flow - the change in allele frequencies as genes from one population incorporates into another.
• Mutation always random with respect to which genes are affected, although the changes in allele frequencies that occur as a result of the mutation may not be.
• Random mating - organisms participate in intrasexual selection, which represent competitive interactions between the same sex (male-to-male or female-to-female) and intersexual selection, which represents the selection of reproductive partners of the opposite sex.
• This leads to the evolution of secondary sexual characteristics for organisms to persuade members of the opposite sex.

7.2 - Natural Selection

• Based on 3 conditions:
  • Variation: for natural selection to occur, a population must exhibit phenotypic variance— in other words, differences must exist between individuals, even if they are slight.
  • Heritability: parents must be able to pass on the traits that are under natural selection. If a trait cannot be inherited, it cannot be selected for or against.
  • Differential reproductive success: measures how many offspring you produce that survive relative to how many the other individuals in your population produce. The condition simply states that there must be variation between parents in how many offspring they produce as a result of the different traits that the parents have.

7.3 - Lamarck and Darwin

• Lamarck proposed the idea that evolution occurs by the inheritance of acquired characters.
• Why he’s wrong: The changed character cannot be passed onto the offspring—the instructions in the sex chromosomes that direct the production of offspring cannot be changed after they are created at the birth of an organism.
• Lamarck confused genetic and environmental (postconceptive) change, which is not surprising because no one had discovered genes yet.
• Darwin - suggested the idea of natural selection and coined the phrase “survival of the fittest.”
• Although he didn’t call them genes, he proposed a hypothetical unit of heredity that passed from parent to offspring.

7.4 - Adaptations

• Adaptation a trait that if altered, affects the fitness of the organism.
• The result of natural selection and can include not only physical traits such as eyes, fingernails, and livers but also the intangible traits of organisms.

7.5 - Types of Selection

• Directional selection
  • This occurs when members of a population at one end of a spectrum are selected against, while those at the other end are selected for.
  • For example, imagine a population of elephants with various-sized trunks.
  • In this particular environment, much more food is available in the very tall trees than in the shorter trees.
  • Those with the longest trunks survive and reproduce most successfully.
• Stabilizing selection
  • This describes selection for the mean of a population for a given allele.
  • A real example of this is human infant birth weight—it is a disadvantage to be really small or really big, and it is best to be somewhere in between.
  • Stabilizing selection has the effect of reducing variation in a population.
• Disruptive selection
  • Also known as diversifying selection.
  • This process can be regarded as being the opposite of stabilizing selection.
  • Selection is disruptive when individuals at the two extremes of a spectrum of variation do better than the more common forms in the middle.
  • Snail shell color is an example of disruptive selection.
  • The three processes above describe the way in which allele frequencies can change as a result of the forces of natural selection.
• Sexual selection
  • Occurs because individuals differ in mating success.
  • Sexual selection is purely about access to mating opportunities.
  • Within-sex competition - in mammals, the males usually compete since females are a limiting source.
  • Choice - in mammals, the females usually choose since they invest a lot in each reproductive effort.
• Artificial selection: when humans become the agents of natural selection.

7.6: Evolution Patterns

Coevolution

• The mutual evolution between two species, which is exemplified by predator– prey relationships.

• The prey evolves in such a way that those remaining are able to escape predator attack.

• Eventually, some of the predators survive that can overcome this evolutionary adaptation in the prey population. This goes back and forth, over and over.

  Convergent evolution

• Two unrelated species evolve in a way that makes them more similar.

• They are both responding in the same way to some environmental challenge, and this brings them closer together.

• We call two characters convergent characters if they are similar in two species, even though the species do not share a common ancestor.

  Divergent evolution

• Two related species evolve in a way that makes them less similar.

• Divergent evolution can lead to speciation (allopatric or sympatric).

  Parallel evolution

• Similar evolutionary changes occurring in two species that can be related or unrelated.

• They are simply responding in a similar manner to a similar environmental condition.

7.7 - Sources of Variation

• Mutation Random changes in the DNA of an individual can introduce new alleles into a population.
• Sexual reproduction - The three main sources of variation in sexual reproduction are crossing over which occurs in prophase 1 of meiosis 1, random assortment during metaphase 1, and the random fertlization of gametes.
• Balanced polymorphism - Some characters are fixed, meaning that all individuals in a species or population have them.

7.8 - Speciation

• Species a group of interbreeding (or potentially interbreeding) organisms.
• Speciation the process by which new species evolve.
• Main forms of speciation:
• Allopatric speciation - Interbreeding ceases because some sort of barrier separates a single population into two.
• The two populations evolve independently, and if they change enough, then even if the barrier is removed, they cannot interbreed.
• Sympatric speciation - Interbreeding ceases even though no physical barrier prevents it.
• Other important terms:
• Polyploidy - A condition in which an individual has more than the normal number of sets of chromosomes.
• Although the individual may be healthy, it cannot reproduce with nonpolyploidic members of its species.
• This is unusual, but in some plants, it has resulted in new species because polyploidic individuals are only able to mate with each other.
• Balanced polymorphism - This condition can also lead to speciation if two variants diverge enough to no longer be able to interbreed (if, e.g., potential mates no longer recognize each other as possible partners).
• Adaptive radiation - a rapid series of speciation events that occur when one or more ancestral species invades a new environment.

7.9 - When Evolution Is Not Occurring: Hardy-Weinberg Equilibrium

• Hardy-Weinberg equilibrium - theoretical concept to describe those special cases where a population is in stasis, or not evolving.
• This can be an excellent tool to determine whether a population is evolving or not; if we find that the allele frequencies do not add up to one, then we need to look for the reasons for this
• Therefore, although the Hardy-Weinberg equilibrium is largely theoretical, it does have some important uses in evolutionary biology.
• Only if the following conditions are met can a population be in Hardy-Weinberg equilibrium:
  • No mutations
  • No gene flow
  • No genetic drift
  • No natural selection
  • Random mating
  • Determining whether a population is in Hardy-Weinberg Equilibrium
  • Formula: p + q = 1
  • P: the frequency of allele 1, often the dominant allele.
  • Q: the frequency of allele 2, often the recessive allele.
  • If the population is in Hardy-Weinberg Equilibrium, the frequency of the two alleles always adds up to 1.
  • p² + 2pq + q² = 1, where p² and q² represent the frequency of the two homozygous conditions (AA and aa).
  • The frequency of the heterozygotes is pq plus qp or 2pq (Aa and aA).
  • Since p represents the dominant allele, it makes sense that p² represents the homozygous dominant condition.
  • By the same logic, q² represents the homozygous recessive condition.

7.10: The Evidence for Evolution

Homologous characters

• Traits are said to be homologous if they are similar because their host organisms arose from a common ancestor (which implies that they have evolved).

• For example, the bone structure in bird wings is homologous in all bird species.

  Embryology

• The study of embryos reveals remarkable similarities between organisms at the earliest stages of life, although as adults (or even at birth) the species look completely different.

• Human embryos, for example, actually have gills for a short time during early development, hinting at our aquatic ancestry.

• Darwin used embryology as an important piece of evidence for the process of evolution.

  Vestigial characters

• Most organisms carry characters that are no longer useful, although they once were.

• Sometimes an environment changes so much that a trait is no longer needed, but is not deleterious enough to actually be selected against and eliminated.

• Darwin used vestigial characters as evidence in his original formulation of the process of evolution, listing the human appendix as an example.

• Fossil record—the physical manifestation of species that have gone extinct.

7.11 - Phylogeny

• Systematics the study of evolutionary relationships and looks at the similarities and differences between species.
• With the similarities and differences established, you can construct phylogenetic trees and cladograms that show evolutionary relationships among lineages.
• While both show relationships between lineages, phylogenetic trees show the amount of change over time calibrated by fossils or a molecular clock.
• Phylogenetic trees and cladograms - used to represent evolutionary relationships among organisms as well as track traits that are either lost or gained over time.
• Shared characteristics will be found in multiple lineages.
• Shared, derived characteristics are shared only by the subset of the species and are not inherited from the most recent common ancestor.

7.12 - Key Ideas

• Nodes or branching points represent the most recent common ancestor of any two groups or lineage.
• Morphological similarities of living or fossil species can be used to construct phylogenetic trees or cladograms.
• Molecular data such as DNA or protein sequences can be used to construct phylogenetic trees or cladograms.

7.13 - Macroevolution

• Microevolution - evolution at the level of species and populations.

• Macroevolution includes the study of evolution of groups of species over very long periods of time.

• Patterns of macroevolution:

• Gradualism evolutionary change is a steady, slow process.

• Punctuated equilibria model - change occurs in rapid bursts separated by large periods of stasis.

  

7.14: Origins of Life on Earth

• Heterotrophs organisms that cannot make their own food.

• Geological evidence provides support for the models of the origin of life on Earth.

• Earth formed about 4.6 billion years ago with no signs of life due a hostile environment until 3.9 billion years ago.

• The Miller-Urey Experiment demonstrated that several organic compounds could be formed spontaneously by simulating the conditions of Earth’s early atmosphere.

• Another theory - RNA world hypothesis - states that RNA could have been the earliest genetic material on Earth.

• A simple RNA molecule could copy itself without other molecules to help, drive chemical reactions like proteins, and be able to store genetic information just like DNA.