Darwin and Evolution of Populations

What is Evolution?

Evolution is the gradual change of a species over time to better suit their environment.

• There have been many explanations for how evolution occurs over the last few hundred years.

  • EX: Jean-Baptiste Lamarck (1744-1829) proposed that an organism can pass on characteristics that it has acquired during its lifetime to its offspring. 

• The current accepted theory for evolution was created by Charles Darwin during the 1800s.

Voyage Aboard the HMS Beagle

• In the 1830s, naturalist Charles Darwin went on a five-year 

voyage aboard the HMS Beagle, which was sent to survey 

the land in South America and offshore islands. 

• Observations Darwin made during the voyage:

  • Organisms had features that made them well suited to very diverse environments such as Brazil’s humid jungles, Argentina’s broad grasslands, and the Andes’ mountain tops.

  • Plants and animals in temperate regions of South America more closely resembled species living in the South American tropics than species living in temperate regions in Europe.

  • Fossils he found, though clearly different from living species, distinctly resembled the living organisms located where they were found. 

Voyage Aboard the HMS Beagle

• One of the most influential stops for Darwin while aboard the Beagle was at the Galapagos Islands.

  • The animals on the Galapagos resembled species that lived on the South American mainland.

  • EX: There were a large variety of finches on the Galapagos Islands that were similar to each other but appeared to be different species. The finches were also different, but still very similar, to a species of finches on the South American mainland.

  • Darwin hypothesized that the Galapagos had been colonized by organisms that had strayed from the mainland and then diversified, giving rise to new species on the various islands. 

Darwin’s Finches

• Darwin concluded that a common ancestor from the South American mainland first colonized the islands.

• Over time, the birds developed adaptations for specific niches, such as the food they ate. 

• An adaptation is an inherited trait that makes an organism more fit for its environment. 

  • In biology, “fitness” refers to an organism's ability to survive and reproduce. 

  • EX: large beaks are an adaptation of some finches, which allows them to crack open seeds.

• A niche refers to the role or function of an organism or species in an ecosystem.

  • EX: an organism’s food source, where it lives, what services it provides to an ecosystem, etc. 

  • Organisms or species with overlapping niches leads to competition.

Darwin’s Finches: Adaptations and Niches

On the Origin of Species

• In 1859, Darwin compiled evidence he had been gathering for years, including from his voyage on the Beagle, and published a book titled, On the Origin of Species. 

• Darwin’s book introduced the scientific theory that populations evolve over the course of generations through a process called natural selection.

  • Natural selection is a process in which individuals with favorable inherited traits are more likely to survive and reproduce.

  • Natural selection is the mechanism, or driving force, of evolution.

• Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection.

• Darwin argued that a similar process occurs in nature.

• Darwin never used the word evolution in the first edition of On the Origin of Species.

• Instead he used the phrase “descent with modification,” which summarized Darwin’s perception of the unity of life

  • The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past.

• In the Darwinian view, the history of life is like a tree with branches representing life’s diversity

• Fossils of extinct species help to “fill in” the morphological gaps between present-day groups.

Key Features of Natural Selection

• Members of a population often vary in their inherited traits.

• All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce.

• Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to have more offspring than other individuals.

• This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

• Note that populations, not individuals, evolve over time

• Natural selection can only increase or decrease existing heritable traits that vary in a population. It does not create new traits.

• The traits that are adaptive will vary with different environments.

Requirements for Natural Selection to Occur

• Variation: Offspring must be produced with traits that differ from one another.

• Adaptation: Certain variations must be more advantageous than other variations. Some individuals must be more likely to survive and reproduce. 

• Heritability: The favorable trait, or adaptation, must be genetic and capable of being inherited by the next generation.

• Competition: More offspring must be produced than can be supported by the environment.

• In a nutshell, organisms that possess genetic variations that allow them to outcompete others for resources will survive and pass their genes on to the next generation, thus shifting allele frequency to their traits. 

Microevolution

• Microevolution is the change in allele frequencies in a population over many generations. 

• There are a few main factors that can lead to changes in the allele frequency in a population….

5 Fingers of Evolution

• Mnemonic device to help you remember the 5 mechanisms that can cause evolution in a population. 

  • Genetic Drift (Pinky)

  • Sexual Selection (Ring)

  • Mutations (Middle)

  • Gene Flow (Pointer)

  • Natural Selection (Thumb)

Genetic Drift

• Genetic drift occurs when allele frequencies in a population change by random chance, not because of natural selection.

• The smaller a population, the more likely it is that chance alone will cause deviation from a predicted result.

• Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations. 

• Two examples of genetic drift:

  • The founder effect

  • The bottleneck effect

Genetic Drift: The Founder Effect

• The founder effect occurs when a few individuals become isolated from a larger population and start a new population.

• Allele frequencies in the small founder population can be different from those in the larger parent population due to chance.

Genetic Drift: The Bottleneck Effect

• The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change.

  • Such as an earthquake, flood, hurricane, forest fire, etc.

• The natural disaster kills individuals in a population entirely by random chance; it has nothing to do with their traits or adaptations.

• By chance, the resulting gene pool may no longer be reflective of the original population’s gene pool.

• Understanding the bottleneck effect can increase understanding of how human activity affects other species.

• EX: Greater Prairie Chickens in Illinois

  • Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois 

  • The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

  • Genetic drift during the bottleneck may have led to a loss of genetic variation and an increase in the frequency of harmful alleles

Genetic Drift

• Key points of genetic drift:

• Genetic drift is significant in small populations

• Genetic drift can cause allele frequencies to change at random

• Genetic drift can lead to a loss of genetic variation within populations

• Genetic drift can cause harmful alleles to become fixed

Gene Flow

• Gene flow, or gene migration, consists of the movement of alleles into (immigration) and out of (emigration) populations. This occurs when:

  • Gametes or seeds (in plants) are carried into another population

  • Breeding individuals migrate into or out of population

• Gene flow leads to the frequencies of alleles changing in a population. 

Sources of Heritable Variation

• Two sources of heritable variation:

1. Mutations in gametes (sex cells) introduce new types of alleles into a population. 

2. Genetic shuffling during sexual reproduction

• Crossing over and independent assortment of alleles during gamete formation leads to different combinations of alleles in sperm and egg cells.

• The random combination of egg and sperm cells during fertilization leads to offspring that have different combinations of traits. 

Sexual Selection

• Sexual selection occurs when certain adaptations increase an individual's ability to secure a mate. 

• Males can father many offspring because they continuously produce sperm in great quantity, whereas females produce few eggs.

  • Reproduction is thus typically the result of female choice. Males often must compete with one another for a mate. 

• Sexual selection can lead to sexual dimorphism: Males and females who differ in size and other traits.

  • Female choice can explain why male birds are more ornate than females.

Natural Selection

• Evolution by natural selection involves both chance and “sorting”

  • New genetic variations arise by chance

  • Beneficial alleles are “sorted” and favored by natural selection

• Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness. This is not necessarily true for random events such as genetic drift and gene flow. 

• Natural selection acts directly on an organism's phenotype, which will have an indirect effect on the allele and genotype frequencies found in a population. 

• Many phenotypic differences are influenced by multiple genes (polygenetic) and vary along a continuum. This creates a range of phenotypes. 

  • EX: height, skin color, birth weight, etc. 

• Heritable variation is essential for evolution by natural selection to occur!

Types of Natural Selection

• There are three modes in which natural selection can act on a population:

  • Directional selection: occurs when conditions favor individuals at one end of the phenotypic range

  • Disruptive selection: occurs when conditions favor individuals at both extremes of the phenotypic range

  • Stabilizing selection: occurs when conditions favor intermediate variants and act against extreme phenotypes

Heterozygous Advantage

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes

  • As a result, natural selection will tend to maintain two or more alleles at that locus.

• EX: The Sickle-Cell Allele

  • The sickle-cell allele causes deleterious mutations in hemoglobin but also grants resistance to malaria.

  • People who are heterozygous don’t die of sickle cell disease and are also more likely to survive malaria. 

  • This keeps the sickle-cell allele from being “weeded-out” by natural selection.