Natural Selection and Evolution

Natural Selection and Evolution Overview

• Evolution refers to biological changes in populations over time, resulting in descendants that are genetically different from their ancestors.
• Microevolution involves small-scale changes affecting a single population.
• Macroevolution involves large-scale changes affecting species across populations.

Charles Darwin and Natural Selection

• Charles Darwin was an English naturalist who traveled to the Galápagos Islands.
• Darwin observed different species of finches and tortoises on different islands.
• Each species had unique adaptations suited for their specific environment.
• Darwin developed the theory of natural selection as the mechanism for evolution.

Principles of Natural Selection

• Natural selection occurs when organisms with the best traits (adaptations) survive, live longer, and reproduce more.
• Fitness measures an organism’s ability to survive and reproduce in its environment.
• More offspring are produced than can survive, leading to competition for limited resources.
• Variation refers to differences in physical traits among individuals within a population.
• Mutations are random changes in DNA and serve as the ultimate source of variation.
• Genetic recombination during meiosis (crossing over) also contributes to variation.
• Migration, or gene flow between populations, introduces new genetic variation.

Adaptation and Evolution

• Adaptation is a trait that allows an organism to survive better in its environment.
• Beneficial traits become more common over time because they improve survival and reproduction.
• This process changes the gene pool, which represents the combined alleles of a population.
• Descent with modification describes how populations change over time as gene frequencies shift.
• Traits are inherited from ancestors, and beneficial traits become more common.
• Individuals do not evolve; populations do.

Modes of Selection

• Directional selection favors one extreme version of a trait.
• For example, dark-colored moths survived better than light-colored moths after industrial pollution darkened trees.
• Disruptive selection favors two extreme traits, removing the average.
• For example, snakes on rocks are gray, and those in grass are green, while intermediate colors are less fit because predators can see them easily.
• Stabilizing selection eliminates extreme traits and favors the average.
• For example, human babies with very low or very high birth weights have lower survival rates, while average-sized babies survive best.

Speciation

• Speciation is the formation of a new species through evolution from a pre-existing species.
• A species is a group of organisms that can successfully interbreed and produce fertile offspring.
• Isolation must occur for speciation to happen.
• Over time, gene pools become different, preventing interbreeding and leading to the development of different species.

Extinction

• Extinction is the elimination of a species.
• Gradual extinction happens slowly due to changes in climate or natural disasters.
• Mass extinction happens when a catastrophic event suddenly changes the environment.
• Examples of catastrophic events include volcanic eruptions, meteors, and tsunamis.

Evolutionary Rates

• Gradualism involves slow, constant changes over a long period of time.
• The evolution of peppered moths during the Industrial Revolution is an example of gradualism.
• Punctuated equilibrium involves bursts of rapid change followed by periods of stability.
• Mammal population changes during the Mesozoic and Paleozoic eras are examples of punctuated equilibrium.

Types of Evolution

• Divergent evolution occurs when different species evolve from a common ancestor.
• Different environments lead to changes in populations.
• Adaptive radiation is a type of divergent evolution where a species rapidly evolves over a short time.
• Mammals evolving into bears, elephants, and koalas is an example.
• Convergent evolution occurs when unrelated species evolve similar characteristics due to similar environments.
• Certain animals look alike, even though they are not closely related.
• Coevolution occurs when two populations of organisms form a specialized relationship and evolve in response to each other.
• Flowers and the insects that pollinate them is an example of coevolution.

Key Concepts Summary

• Speciation occurs when populations become isolated and evolve into different species.
• Extinction can be gradual or occur suddenly due to catastrophic events.
• Evolution happens through gradualism (slow changes) or punctuated equilibrium (rapid bursts).
• Divergent evolution creates new species from a common ancestor, while convergent evolution leads to similar traits in unrelated species.
• Coevolution describes how species evolve together due to mutual influence.

Taxonomy Overview

• Taxonomy is the field of biology that classifies organisms.
• The purpose of taxonomy is to organize organisms based on shared characteristics.

Domains of Life

• Eubacteria are prokaryotic and include "true" bacteria (e.g., pathogens).
• Archaebacteria are prokaryotic and found in extreme environments.
• Eukarya are eukaryotic organisms.
• Domains are subdivided into phylum, class, order, family, genus, and species.

Carolus Linnaeus and Binomial Nomenclature

• Carolus Linnaeus is known as the father of taxonomy.
• Binomial nomenclature involves using the genus and species (italicized or underlined) to name organisms.
• An example of binomial nomenclature is Homo sapiens.

Phylogeny (Evolutionary History)

• All organisms share a common ancestor.
• Phylogeny is based on shared inherited characteristics.

Key Evolutionary Milestones

• 3.6 billion years ago (BYA): First life form – Prokaryotes.
• 2.1 BYA: First eukaryotes evolved via endosymbiosis (one prokaryote engulfed another).
• The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living bacteria.

Phylogenetic Trees

• Phylogenetic trees are diagrams that show evolutionary relationships among species.
• Branches indicate new species divergence.
• Hypotheses are based on shared traits (DNA, proteins, behavior) as well as genetics and inheritance patterns.

Key Concepts in Phylogenetics

• Maximum parsimony suggests that the simplest explanation is most likely correct.
• Taxa are major groups classified by evolutionary relationships.

Interpreting a Phylogenetic Tree

• Groups most closely related share a recent common ancestor.
• First-diverging groups evolved longest ago.
• Start from the root (common ancestor) when reading a phylogenetic tree.
• Move forward in time as you go up the tree.
• Branching points indicate speciation.
• Loss of a branch represents extinction.

Common Misconceptions

• Evolution is not a ladder.
• Humans are not the pinnacle of evolution.

What We Learn from Phylogenetic Trees

• Identifying common ancestors of organisms.
• Understanding evolutionary history and divergence.