Genetic variation in a population is caused by mutations.

• In a population, these genetic variances result in a variety of phenotypes.

• Different kinds of selection operate on these phenotypic variances, resulting in variable reproductive success.

• Populations develop when some members of the population outperform others in terms of reproductive success.

When comparing the DNA sequences of genes shared by various animals, the closer the DNA sequences are, the more recently the organisms had a common ancestor.

• For example, the GAPDH (glyceraldehyde-3-phosphate dehydrogenase) gene in humans and chimps is about 95% identical in sequence, yet the GAPDH gene in humans and dogs is only approximately 91% similar.

• This suggests that humans and chimps have a more similar genome.

Humans and chimps have a more recent common ancestor than humans and dogs.

• Because environmental variables do not normally modify an organism's DNA sequence, molecular evidence is considered very strong proof.

• Morphology—Homologous features with same origins but distinct functions are also indications of evolution.

The number and arrangement of bones in human hands, bat wings, and whale fins, for example, are strikingly similar, demonstrating shared ancestry and evidence of evolution.

Fossils—

• The presence of fossils from animals that no longer exist on Earth is another proof of evolution.

  • Transitional fossils represent transitional phases of evolution between ancestral and contemporary species.

  • The age of fossils can be determined by examining the age of the rock strata in which they are discovered or by utilizing radioactive isotopes to date them.

Vestigial structures—Some creatures have anatomical characteristics that do not appear to have a purpose in the contemporary organism but may have served a purpose in an ancestor organism.

• The tailbone, which today serves no function in humans but may have helped our tree-dwelling ancestors balance on or traverse between branches, is an example of this.

• Convergent evolution occurs when species that dwell in comparable habitats gain similar traits despite the absence of a recent common ancestor.

• Sharks (cartilaginous fish) and dolphins (mammals) do not have a common ancestor, yet due to their comparable surroundings, they have developed similar body forms.

Biogeographical evidence—

• Biogeography is the study of species distribution.

• Species found on South American islands are more similar.

• Species found in South American islands are more similar to those found in South America than to species found in North America.

• With prolonged drug exposure, only antibiotic-resistant bacteria will survive and be prevalent in the community over time.

It is important to note that bacteria do not evolve or "learn" to be resistant to antibiotics; some bacteria already have the mutation that gives them resistance.

• These antibiotic-resistant bacteria are initially less numerous in the community, but they are more likely to survive and reproduce in an antibiotic-containing environment than non-antibiotic resistant bacteria.

• As these bacteria multiply in the presence of the antibiotic, the frequency of bacteria with antibiotic resistance will rise over time.

• Another example is the peppered moth, which was discovered in England in the 1800s.

• The wing color of peppered moths varies; some moths have darker wings, while others have lighter wings.

The color of a moth's wings is inherited. Birds eat moths, and moths with wing colors that blends in with their surroundings are more difficult for birds to perceive, hence they are less likely to be eaten by birds.

• Prior to the Industrial Revolution, trees in the moths' habitat in England were coated with a light-colored lichen, which allowed the lighter-colored moths to blend in with the trees.

• Lighter-colored moths became more difficult for birds to discover and consume as a result, and these lighter-colored moths predominated in the moth population.

However, during England's Industrial Revolution in the 1800s, sulfur dioxide emissions from factories destroyed most of the light-colored lichen on the trees.

• Lighter-colored moths lost their advantage, while darker-winged moths were able to blend in with the darker tree bark.

• Darker moths become more difficult for birds to discover and feed as a result.

• According to Darwin's theory of evolution by natural selection, there were natural variances in the moth population, with some moths having darker wings and others having lighter wings.

Moths with darker wings had a survival benefit throughout the Industrial Revolution, and as a result of natural selection, they grew more common in the population.

• According to natural selection theory, the environment selects for people that have phenotypes that provide a survival advantage in that environment.

• Different phenotypes may confer an advantage if the environment changes, and the changing environment can influence the trajectory of evolution, as demonstrated in the peppered moths example.

Natural selection can also result in stabilizing selection, in which the intermediate phenotype is favored while extreme phenotypes are rejected.

• Clutch size (the number of eggs produced by birds every reproductive cycle) is subject to stabilizing selection.

• A bird that lays a high number of eggs may have too many young to care for and feed, resulting in low reproductive success.

• However, if the bird only lays one or two eggs every reproductive cycle, there will be no offspring.

• The typical clutch size in robins is five to six eggs per nest.

• Natural selection can sometimes result in disruptive selection, in which individuals at both ends of the phenotypic spectrum are more likely to survive and reproduce than those with an intermediate phenotype.

• Consider an ecosystem that has light-colored sandy soil with dark, rocky areas intermingled.

• Mice with light-colored fur may blend in with the sandy soil and be less conspicuous to predators in this habitat.

• Dark-furred mice might hide in the dark-colored rocks. Mice with a middle-of-the-road fur hue couldn't blend in with either the dirt or the grass.

Humans choose which individuals in a community survive and reproduce, rather than the environment choosing for those with good traits.

• Brassica oleracea wild cabbage is an example of artificial selection in plants.

• Farmers have deliberately selected wild cabbage for desired qualities throughout the years.

• Farmers produced kale by deliberately selecting plants with larger leaves.

• Cauliflower resulted by selecting plants with bigger blossom clusters.

• Brussels sprouts were created by crossbreeding plants with lateral buds.

• Broccoli was created by deliberately breeding plants with more strong stalks and more blooms.

• Sexual selection happens when some qualities make people more likely to attract mates than others.

Individuals with higher virtuous attributes will, over time, outnumber those with less virtuous traits.

• Individuals with features that make them more likely to attract mates grow increasingly frequent in the population over time.

• Individuals of one sex are selective in choosing mates from the opposite sex in intersexual selection.

• Mate selection may be influenced by the perceived fitness of the other sex's members, with individuals who appear stronger or healthier being more likely to have offspring that will survive.

• Many bird activities are centered on mate selection.

• Coloration, bird songs, mating dances, and nesting behaviors can all be used by birds to select mates.

• Pavo cristatus is one example of this (the peacock).

• Peacocks with more vibrant plumage are more likely to attract mates than those with duller plumage.

• Sula nebouxii, the blue-footed booby bird, females choose mates with the males.