Exceptions to Mendelian Genetics

• Some alleles are neither dominant nor recessive, and some traits are controlled by multiple alleles or multiple genes and may be influenced by the environment

Incomplete dominance

• the heterozygous phenotype is somewhere between (intermediate to) the two homozygous phenotypes.

• We see this often in flowers.

incomplete dominance

• P generation parents are homozygous red and white

• All F1 offspring are pink and heterozygous

• When F1 offspring are allowed to self-pollinate, there is a 1:2:1 phenotypic ratio for the F2 offspring

Codominance

• Both alleles are expressed to some degree in the heterozygous phenotype

• Ex. “Erminette” chickens, roan cattle

Codominance

• In chickens, the allele for black feathers and the allele for white feathers are both being expressed.

• In roan cattle, the allele for red coat color and the allele for white coat color are both being expressed.

Codominance in Cattle

Codominance in Cattle

Multiple alleles• More than two alleles may exist for a given

trait. An example is coat color in rabbits.

• C - full (brown/gray) coat color - dominant to all other alleles

• Cch - “Chinchilla” coat color - all gray - dominant to Ch and c alleles

• Ch - “Himalayan” coat color - white with black features - dominant to c allele

• c - Albino coat color - no pigmentation - recessive to all alleles

Multiple Alleles

• Human blood types are the result of multiple alleles

• Three alleles: IA , IB , and i

• IA and IB are both dominant over i, neither is dominant over the other

• When the IA and IB alleles occur together, they are codominant

Polygenic traits

• Complex traits that are controlled by multiple genes.

• It can be difficult to accurately predict the appearance of some polygenic traits in offspring.

• Ex. human eye color, coat color in labs

Coat color in LabsCoat color in Labs

Qualitative v. Quantitative Traits

• Qualitative traits - have a clear quality, simple inheritance, limited environmental influence

• Ex. attached earlobes

• Quantitative traits - measurable but has many different qualities, complex, large environmental influence

• Ex. milking ability

Charles DarwinCharles Darwin

Introduction to Natural SelectionIntroduction to Natural Selection

Darwin’s Journey

• In 1831, 22-year old Darwin leaves on a 5 year voyage as a naturalist on the HMS Beagle.

• The Beagle travels around the world, with numerous stops along the South American continent.

• As a naturalist, Darwin must make observations of the natural world, specifically plants, animals, geology, and climate of the various regions.

Darwin’s Journey

Darwin’s Journey

• While on his journey, Darwin makes a number of observations:

• Animals living in similar environments exhibit some of the same adaptive features.

• Fossil remains of extinct animals resemble some current species.

Darwin’s Journey

• For example, fossils of the Glyptodon very closely resemble, but are not the same as the armadillo.

• This leads Darwin to believe that species must be changing over time.

Darwin’s Journey

• This notion that species change over time clashes with the beliefs of that time period.

• People are firm believers in divine creation - the belief that a divine being was responsible for the creation of all life forms on Earth.

• This view is static and does not allow for change.

Science in the 1800’s

• At the time, a comprehensive fossil record is being established by geologists.

• The fossil record contradicts the theory of divine creation.

• Several people in the science community begin to discuss species change.

Lamarck• In 1809, Lamarck

theorizes that:

• Physical features change in size based on how much or how little the animal uses them in it’s lifetime. Use and disuse.

• These acquired changes are heritable - are passed from parents to offspring.

Lamarck

Lamarck• Lamarck theorized that these sorts of traits

were heritable. As we will soon see, evolution is something that takes place over generations, not in an individuals lifetime.

• Lamarck did lay the foundation for Darwin’s theory by stating that species change with the “physical conditions of life” - the species environment.

• These changes are evolutionary adaptations.

Darwin’s Journey

• Darwin returns from his journey in 1836.

• He has overwhelming evidence to show that evolution exists.

• He doesn’t publish his theory until 1859.

• Why?

Malthus

• Darwin finds the answer to his problem in an unlikely place - an essay on economics by Thomas Malthus.

• In the paper, Malthus is discussing the growth of human populations and food supply.

Malthus

• Due to different types of growth, populations could easily exceed resources.

• When resources become scarce, war and famine control the population.

• What does this have to do with Darwin and evolution?

Malthus

• To summarize the main point of Malthus’ essay, human populations are limited by their resources.

• Humans are limited by their environment.

• When humans come to conflict over scarce resources, the stronger, better-equipped will win.

• Sound familiar?

Natural Selection

• That’s because it’s the basis for Darwin’s mechanism of evolution!

• Darwin believes that this applies not simply to humans, but to all living species.

• Darwin theorizes that species evolve through the process of natural selection.

Natural Selection

• Natural selection is defined as the differences in survival and reproduction among individuals in a population as a result of their interaction with the environment.

• In other words, some individuals are better able to survive and reproduce because they have traits that make them better at coping with their environment

Natural Selection

• Two major premises for Darwin’s theory:

• Species are not immutable.

• The driving force behind evolutionary change is natural selection or “survival of the fittest”.

Natural Selection

• Within a population, genetic diversity exists.

• In a particular environment, certain individuals in a population are better suited to survive.

• Over time, genes that code for traits that enable individuals to survive and reproduce accumulate (greater allelic frequency).

• Evidence indicates modern species evolved from organisms that are now extinct

Evolution

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Natural Selection

• Beneficial traits are evolutionary adaptations to the environment, and increase the individuals fitness - the relative ability to survive and leave offspring.

Role of mutation• New alleles arise by chance out of random

mutations.

• Some mutations better equip an organism to survive and reproduce (increase fitness), which will lead to those new traits being passed on.

• We consider those traits “beneficial”

• If environmental conditions remain the same, over time the beneficial trait will increase in frequency

Natural Selection

• Darwin presented the following arguments to support evolution:

• Populations have a huge reproductive potential

• Population sizes remain relatively stable.

• Resources are limited.

• Individuals compete for limited resources.

Natural Selection• There is variation among individuals in a

population.

• Most of this variation is heritable.

• Only the most fit individuals survive. Individuals with traits best adapted for survival in the environment are able to outcompete others.

• Over time, traits best adapted for survival in the environment (and the genes behind them) accumulate in the population.

Types of Selection

• Stabilizing Selection

• Directional Selection

• Disruptive Selection

• Sexual Selection

• Artificial Selection

Natural

Artificial

Directional Selection• Case of the

Peppered Moth in England

• Pre-Industrialization = light moth is selected for.

Directional Selection

• After the Industrial Revolution, thick soot covers the trees that the moths live on.

• Now, dark moths are selected for.

African SwallowtailAfrican Swallowtail

Sexual SelectionMale Competition - Brooklyn

Parrots

Male Competition

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Sexual DimorphismPeafowl

Sexual Dimorphism

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Sexual SelectionFemale Choice - Blue-Footed Booby

Female Choice

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Stabilizing Selection

• Eliminates individuals that have extreme or unusual traits.

• Individuals with the most common trait are best adapted.

Height variation in humans

adheres to stabilizing selection

Directional Selection

• Favors traits that are at one extreme of a range of traits.

• Traits at the opposite extreme are selected against.

Example: Industrial Melanism

Disruptive Selection• Selection for

extreme or unusual traits, while selecting against the common traits.

• Example: weeds and disruptive selection for height.

Sexual Selection

• Differential mating of males (sometimes females) in a population.

• Male competition: awards mating opportunities to the strongest males.

• Female choice: leads to traits or behaviors that are attractive to females.

Artificial Selection

Directional selection carried out by humans

Artificial Selection

Artificial Selection

Artificial Selection