Evolution and Natural Selection

• Evolution and natural selection explain how species change and diverge over time

What is Evolution

• Organic or biological evolution refers to the change in one or more inherited traits over time
• This change occurs within a population of organisms
• Traits can be anatomical, biochemical, or behavioral

Types of Evolution

• Macroevolution is a long-term phenomenon that leads to major changes
• Macroevolution includes changing from one species to another (speciation)
• Microevolution is a short-term phenomenon that leads to minor changes
• Microevolution results in changes within a population over time

History of Evolutionary Thought

• Ancient Greek and Roman texts touched on evolution
• Middle Ages to Modern Period: Western thought revolved around creationism and God
• Late 18th Century: Rise of Paleontology, Jean Baptiste Lamarck developed an ultimately false theory of evolution
• Mid 19th Century: Charles Darwin and Alfred Wallace published their theory on evolution through natural selection

Theory of Transmutation of Species

• Species change over time to better adapt to the environment by acquiring characteristics
• Characteristics acquired during an individual's lifetime passed to offspring

Difference Between Evolution and Natural Selection

• Evolution predates Darwin
• Evolution constitutes the change in organisms over time
• Natural selection serves as a mechanism or a way for evolution to occur

Inheritable Traits

• Evolution occurs through inheritable traits, so what traits are inheritable and how are they inherited?

Genotype

• Genotype refers to the complete set of genes in an organism
• Genes control the inheritable traits of an individual
• Individuals in a population share the same genotype but exhibit slight variations
• Genotypes are inheritable

Phenotype

• Phenotype refers to the set of observable traits that make up an organism’s structure or behavior
• Phenotype is determined by genotype along with environmental factors
• Phenotypes are not inheritable

Difference Between Theory of Transmutation and Theory of Natural Selection

• Theory of Transmutation is based on passing phenotype
• Theory of Natural Selection bases on the inheritance of genes/genotype

Genes

• Genes are made of molecules of DNA
• The order of the bases in the DNA molecules determines the information encoded in the gene
• The information in genes are instructions for creating proteins
• DNA is copied and passed down to subsequent generations
• DNA varies between individuals

Gene Sequences

• How do gene sequences vary between individuals?

Alleles

• Alleles refer to different forms of the same gene
• Varying base pairs between genes produces different results, such as different eye colors
• Changing base pairs changes what genes create, resulting in a different outcome
• Alleles indicate genetic variation in populations

Allele Frequency

• Allele frequency permits variation, which permits evolution
• Evolution occurs when allele frequency changes over time
• Genotype change equals phenotype change

How Does Evolution Occur?

• Evolution depends on variation, selection, and inheritance

Variation

• Variation comes from genetic mutation in DNA
• Changes in base pairs lead to change in the gene, which creates new alleles
• Genetic change results in genetic diversity

Selection

• Selection gives rise to an increased rate of reproduction in some individuals relative to others
• Natural selection allows individuals to reproduce more based on adaptability to their environment

Evolution as Survival of the Fittest

• Fitness in evolutionary biology is determined by an individual's capability to pass on genes to the next generation
• This occurs relative to competition between other individuals, as seen in humans vs. insects

Fitness

• Fitness begs the question of how many offspring individuals have and how many survive to reproductive age
• Fittest individuals produce more offspring that survive, leading to more genes being passed on over time
• Evolution occurs because fitness varies among individuals in a population

What Determines Fitness

• Genetic variation/different alleles produce more or less fit individuals

Why This Causes Evolution

• More fit individuals can produce more offspring compared to less fit individuals
• This can change the average genotype in the individuals of a population over time and becomes the most prevalent type of variation

Directional Selection

• Natural selection selects individuals at one extreme end of the distribution of variation
• An example is giraffe necks, longer necks lead to more food and more offspring, leading to more longer neck individuals

Disruptive Selection

• Natural selection selects individuals at both extremes of the distribution of variation
• An example is finch beaks: large beaks and small beaks are idea for big nuts and small nuts or insects
• Medium sized beaks are not as useful, leading to most finches having large or small beaks

Stabilizing Selection

• Natural selection selects individuals in the middle of the distribution of variation, and against both extreme ends
• An example is birth weights in babies, high and low birth weights more likely to die in infancy, leading to babies in the middle to grow up and pass on their genes more

Recap of Natural Selection

• Natural selection selection stems from the natural environment, i.e. weather, climate, predators
• Differences in phenotype lead to differential success in survival and reproduction
• Alleles that lead to better survival become more frequent due to improving ability to pass on genes

Other Mechanisms That Lead to Evolution

• Besides natural selection, other mechanisms can lead to evolution

Artificial Selection

• Human selection decides which individuals with what traits can breed and therefore create a population with a specific trait
• Examples: livestock and plant crops

Biased Mutation

• Some mutations are more likely to occur than others
• The genetic variation in a population is biased to a certain mutation
• This means that more of the population will have that certain mutation over time

Sexual Selection

• The environment not does determine who gets to reproduce more often
• One of the sexes develops a preference for mating
• Generally, the females are the ones who have these preferences

Genetic Drift

• Genetic drift occurs due to change in allele frequency due to random change or sampling error
• This is more relevant to small populations

Gene Flow

• Gene flow refers to changes in allele frequency by migration into or out of a population