Evolutionary Anthropology Lecture 2: Selection

Selection

Week 2 Assigned Readings

• Excerpts from Chapter III of Evolution by Futuyma and Kirkpatrick (2017).

Darwin, 1837

• The Great Chain of Being was a prevalent concept.

• Current understanding:

  • Earth is approximately 4.6 billion years old.

  • Life on Earth is approximately 3.5 billion years old.

  • All organisms share common ancestry and undergo change through time, which is evolution.

Evolution in the 1700s

• Natural scientists in Europe began questioning the fixity of species.

• Comte de Buffon (1707-1788):

  • Recognized unique regional plants and animals.

  • Observed that migrating animals changed in response to new environments.

• Georges Cuvier (1769-1832):

  • Introduced the concept of extinction.

  • Explained dinosaur fossils through extinction events.

• Charles Lyell (1797-1875):

  • Convinced the scientific world that the Earth was very old through stratigraphy.

  • Proposed the idea of Uniformitarianism.

Early 19th Century Understanding

• Deep time and geological processes were recognized.

• Relationship between environment and variation was noted.

• Change occurred between generations.

• However, there was no valid mechanism for change between generations.

Explaining Evolution

• Scientists sought to explain how evolution occurred.

• Change is the pattern, and the mechanism is the process.

• Jean-Baptiste Lamarck (1744-1829) proposed Lamarckian Inheritance.

• Lamarckian Inheritance: Inheritance of acquired characteristics.

Charles Darwin

• Darwin's background:

  • Came from a wealthy family.

  • Studied Medicine at the University of Edinburgh.

  • Studied Theology at the University of Cambridge.

  • Served as a resident naturalist and gentleman’s companion on the HMS Beagle (1831-1836).

  • Documented natural features.

Darwin's Observations

• Fossils bore similarities to living species.

• There was huge diversity in living forms across many parts of the world.

• Galápagos Islands:

  • Some variation in traits among otherwise similar animals.

  • Darwin’s finches: Each island had its own type of finch with varied beak size, shape, and diet.

  • Different beak shapes facilitate resource exploitation in different niches.

  • This minimized competition for resources and resulted in different forms on different islands.

Darwin's Integration of Ideas

• Darwin integrated several ideas to theorize Natural Selection as a mechanism for evolution:

  1. Ideas of evolution and diversity of forms.

  2. Importance of sexual reproduction.

  3. Ideas of Thomas Malthus about competition for resources and the struggle for existence.

  4. Observations in the Galapagos.

• Thomas Malthus: Improvements in conditions lead to increase in population, resulting in competition for resources.

Darwin's Publication Delay

• Darwin returned from his voyage in 1836 but didn’t publish until 1859.

• He spent the intervening years developing his ideas:

  • 1837: Writes down ideas on the “transmutation of species” in a notebook.

  • 1841: Writes to a friend that he will work on a book titled “Varieties & Species”.

  • 1842: Makes a pencil outline of his theory of “descent with modification”.

  • 1844: Expands sketch into essay on the origin of species and natural selection.

  • Rejected fixity of species & noted relationship between the origin of new species and environmental adaptation.

Alfred Russell Wallace

• In 1857, Alfred Russell Wallace sends “On the Tendency of Varieties to Depart Indefinitely from the Original Type” to Darwin.

• Darwin worried about his life’s work being "scooped."

• Darwin to Lyell: “I never saw a more striking coincidence. ..If Wallace had my MS sketch written out in 1842 he could not have made a better short abstract!”

• Wallace's background:

  • A naturalist in the Malay Archipelago (SE Asia).

  • Little formal education, leaving school at 14.

  • Previous occupations included surveyor, school master, civil engineer, and insect collector.

  • Independently came to similar conclusions about evolution as a response to environmental pressures.

Joint Publication

• 1858: Papers by Darwin and Wallace were presented at the Linnean Society (1858).

• Neither author attended.

• Established that Darwin’s ideas on species were consistent from 1837 until the present.

• 1859: Darwin published his ideas about natural selection in a book titled ‘The Origin of Species by Means of Natural Selection’.

  • Immediate bestseller & controversial.

Natural Selection

• An explanation for how evolution occurs.

• An evolutionary process that occurs when certain phenotypes confer an advantage or disadvantage in survival and/or reproductive success.

• Beneficial traits increase in frequency over time in a population because of increased survival and reproduction of individuals with those traits.

Requirements for Natural Selection

1. There must be variation in the population.

2. A trait must be heritable for selection to act on it.

3. Differential survival due to competition for resources.

Examples of Natural Selection

• Giraffe Neck Length:

  1. Variation: Some giraffes had slightly longer necks than others.

  2. Heritability: Neck length is passed from parent to offspring.

  3. Competition: Giraffes with longer necks can access more food and produce more offspring.

  • Result: Adaptation to environment, descent with modification, change in inherited traits of a population through successive generations.

• "Survival of the fittest": Fitness is the number of offspring an individual leaves in the next generation; reproductive success is more important than only survival.

• Environmental Changes: If long-neck giraffes no longer have a selective advantage and the long neck becomes an energetic burden, we would expect to see changes over evolutionary time.

  • Not goal driven!

Further Examples of Natural Selection

• Evolution by natural selection can occur gradually or rapidly, depending on environmental pressures and other factors like genetic variation in the population.

• Overfished Atlantic Cod:

  • Population numbers declined, leading to evolutionary changes.

  • Earlier maturation: Individuals that matured earlier had a selective advantage because they were more likely to reproduce or to produce more offspring than individuals that took longer to mature.

• Darwin’s Finches:

  • Selection pressures acted to drive beak evolution.

  • Birds with deeper beaks are more successful when there are droughts.

  • Birds with shallower beaks are more successful when the climate is wet.

  • Fluctuations in mean beak depth of mediumground finches are correlated with fluctuations in weather over a period of eight years.

• Peppered Moth:

  • Originally, peppered moth populations were primarily white with black speckles, with a minority of individuals being darker.

  • During the industrial revolution, pollution darkened the trees.

  • Darker moths were more camouflaged, and this coloration increased in frequency over time.

Common Thread

• In all examples:

  • A change in the environment occurs.

  • Some individuals have phenotypic variants that allow them to have higher fitness in this new environment.

  • Over time, that phenotypic variant becomes more common in the population = evolution.

Evolutionary Constraints

• Natural selection can only work with the genetic and phenotypic material already in a population.

• Constraints exist.

  • E.g., land mammals can only get so large.

  • Some structures are imperfectly adapted for their current function.

• Exaptations are features that are co-opted and modified for new functions during evolution.

Types of Selection

• Depending on the environment, natural selection can act in any direction:

  • Can select for or against a trait, for the intermediate form of a trait, or for either extreme of a trait.

  • Types of selection:

    • Directional

    • Stabilizing

    • Disruptive

Directional Selection

• Selection for one extreme or the other.

• Examples:

  • Moth coloration

  • Beak size in finches

  • Arthropod resistance to insecticides

Stabilizing Selection

• Selection for an intermediate/average form.

• Reduces phenotypic variance.

• Examples:

  • Birth weight in humans

  • Stasis in tarsiers

• Natural selection is a force of evolution but can also act to keep a population the same over time.

  • Stasis

  • Stabilizing selection is likely to play a role.

Disruptive Selection

• Selection for both extremes; intermediate forms are selected against.

• Rare.

• Examples:

  • Red crossbill beak size

Balancing Selection

• Selection that maintains genetic variation within a population.

• E.g., multiple alleles

• Example:

  • Sickle cell anemia

Sickle-Cell Anemia

• Caused by a point mutation on the hemoglobin allele.

• Less efficient hemoglobin.

• Two alleles:

  • A allele – normal hemoglobin

  • S allele – sickle cell hemoglobin

• Sickle cell anemia causes approximately 500,000 deaths per year.

• Why hasn’t natural selection removed this trait?

• AA

  • Normal blood cells

  • May die from malaria

• SS

  • Sickled cells

  • Severe anemia

  • May die from sickle cell anemia (but less likely to die from malaria)

• AS

  • Some sickled cells

  • Mild to no anemia

  • Less likely to die from malaria

• Where malaria risk is high, the AS genotype is favored.

• Heterozygote is more fit than either homozygote.

• Result = Both alleles are maintained.

Variants of Natural Selection

• Sexual selection

• Kin selection

• Artificial selection

Sexual Selection

• How can we explain the evolution of sexual dimorphism or secondary sexual traits?

• Darwin made 3 observations in Descent of Man and Selection in Relation to Sex (1871):

  • Traits with extreme sexual dimorphism tend to be exaggerated much more often in males.

  • Not expressed in immature males.

  • These traits are used by males in mating.

• Sexual selection: Selection caused by competition for mates; selection for traits (within one sex) that aid in mate acquisition.

• Sexual selection can cause the evolution of traits that decrease survival if the reproductive advantage they produce compensates for that cost.

• Why males?

  • Fundamental differences in reproductive biology.

  • Males produce more gametes.

  • Reproductive success is constrained by access to females.

  • More variation in reproductive success.

  • Compete for access.

• How might a male increase its mating success?

  • Increase attractiveness to attract females.

  • Increase fighting ability.

  • Results in male competition (intrasexual selection) and female choice (intersexual selection).

• Intrasexual selection is strongest and most intense among primates.

  • Leads to sexual dimorphism, primarily in body size and canine size.

• Infanticide is the killing of infants by other members of their species.

  • Found in primates living in one male-multi female groups.

  • Females cannot conceive if they are lactating (Lactational amenorrhea).

  • Males that display this behavior have higher reproductive success.

Kin Selection

• Natural selection seeks to maximize fitness.

• Adaptations must contribute to reproductive success/fitness.

• This view emphasizes individual reproductive success; each organism should act selfishly to increase reproductive success.

• However, primates and many other animals appear to act altruistically.

• Altruistic behaviors are those that benefit others at a cost to the actor.

• Examples of altruistic behaviors:

  • Parental care

  • Many primates groom each other.

  • Many primates give alarm calls to warn each other.

  • Some primates help take care of other infants (alloparenting).

• Thus, many animals, including primates, live in groups and do not appear to only act selfishly.

• How could altruistic behaviors evolve if there is a cost to the actor?

• Kin selection: Any behavior that benefits yourself or your kin improves your reproductive success.

• “Socially valuable but individually disadvantageous characters.” –Haldane

• Because kin are related, they share many of the same genes.

• Increase your own fitness in terms of your genes by increasing your kin’s reproductive success (indirect fitness).

Hamilton's Rule

• Altruistic behaviors will be favored by selection if the costs of performing the behavior are less than the benefits multiplied by the coefficient of relatedness between the actor and recipient.

• r \times B > C

  • r = coefficient of relatedness

  • B = benefit to the recipient

  • C = cost to the actor

Artificial Selection

• The ability of humans to shape plants and animals for our own uses was important to Darwin’s development of natural selection.

• Artificial selection: the selective breeding by humans of animals and plants.

• Can happen very rapidly because the selective force is strong.

• Silver fox domestication experiments in Russia.

• Experiments have been ongoing for ~60 years.

• Saw changes in behavior and phenotype in just a few years.

Up Next

• Microevolution, other forces of evolution.