Biology SN2 Lecture 2: Evolution & Descent

Biology SN2 Lecture 2: Evolution & Descent with Modification

• Darwin's theory argues that survival depends on adaptability, not strength or intelligence
• Species adapt to change over time
• Evolution explains life's unity and diversity
• Natural selection is the driving force of evolution

Before Darwin: Understanding Life

• Aristotle viewed species as fixed and unchanging, organized on a scale of increasing complexity (Scala Naturae)
• The Old Testament describes species as individually designed by God
• Carolus Linnaeus was a founder of taxonomy, grouping species into categories based on similarities, but did not link similarity with evolutionary relationships

Before Darwin: Fossils

• Paleontology (the study of fossils) reveals a succession of organisms throughout Earth's history
• Fossils in layers (strata) show a chronological sequence of life on Earth
• Cuvier proposed catastrophism, suggesting that geological boundaries represented periods of mass extinction rather than gradual change

Before Darwin: Lamarck

• Lamarck proposed that organisms pass traits to offspring based on use and disuse of body parts
• Organisms strive toward perfection, with change in response to environmental pressures, driving adaptations.

Before Darwin: Hutton & Lyell

• Hutton proposed gradualism, the idea that profound changes result from the cumulative effect of slow, continuous processes
• Lyell's work supported and expanded upon gradualism, suggesting Earth's geological processes take place over long periods rather than sudden events

Darwin's Theory

• Darwin's theory rested on Malthus's concept of struggle for existence, Lyell's Principles of Geology, Principles of Artificial Selection, and his observations in the Galapagos, proposing that species adapt over time due to competition and environmental pressures. (i.e. Malthus' essay about struggle for existence).

Darwin's Data from the Galapagos

• Darwin observed diverse adaptations in plants and animals on the Galapagos Islands
• He observed similarities between island species and mainland species, along with distinct differences, indicating that species adapt to local environments

Darwinian Evolution

• Island species derived from mainland species, then diversified
• Species adapt to specific food and environment sources present on those islands

How Does Natural Selection Work?

• Species can produce significantly more offspring than their environment can support

• Environmental resources are scarce, leading to a struggle for existence among individuals of a population

• Individuals in a population vary in their characteristics; variation is inherited

• Individuals with traits best suited to their environment are more likely to survive and reproduce, passing their traits to the next generation

Mechanisms of Variation

• Asexual reproduction results in identical offspring
• Sexual reproduction is responsible for much of the inherited variation present within a population.

Genetic Variation Explained

• Meiosis and fertilization are responsible for genetic variation in sexually reproducing species

How Does Natural Selection Work? Important Notes

• Individuals do not evolve, populations do.
• Natural selection can only act on heritable traits
• Environments change, so traits favorable in one environment may be useless or harmful in another.

Evidence for Darwin's Theory

• Natural Selection in action
• Homology (anatomy, development, and molecular)
• Convergent evolution
• Biogeography
• Fossils

Natural Selection in Action

• Example: peppered moths' adaptation to industrial pollution (black moths became more common due to soot darkening tree bark)

Artificial Selection

• The use of artificial selection in other species, like dogs, leads to diversity of characteristics

Homology: Anatomy

• Common ancestry reveals similar structures in various species (ex: homologous bones in forelimbs of humans, bats, dolphins).
• Vestigial organs are structures that are nonfunctional reminders of ancestral structures.

Homology: Development

• Embryonic development of different species shows great similarities, such as gill slits in vertebrate embryos

Homology: Molecular

• Universal genetic codes and conserved protein sequences indicate a shared ancestry

Convergent Evolution

• Organisms with different ancestries can evolve similar characteristics due to similar environments

Biogeography

• Species distribution patterns suggest common ancestry

Fossil Record

• The fossil record reveals a succession of life forms, consistent with evolutionary relationships
• The oldest known fossils are prokaryotes

Natural Selection, Just A Theory

• The massive amount of data supports Darwin
• Scientists are constantly testing the theory

Neo-Darwinism

• Neo-Darwinism integrates Darwinian ideas with Mendelian genetics. Genes provide basis for adaptations within populations. Mutations drive variation.

Lecture 2: Learning Objectives

• Define evolution, natural selection
• Summarize views prior to Darwin
• Explain natural selection
• Explain the sources of Darwin's theory
• Discuss evidence for natural selection (homology, convergent and divergent evolution, biogeography, and fossils)
• Explain the significance of genes in evolution, as well as sexual reproduction and the role of mutations in driving variations.
• Define scientific terms used in the study of evolution, like: chromosomes, gametes, somatic cells, homologous chromosomes, alleles, recessive/dominant traits, genotype, phenotype, zygote, random fertilization

Sexual Reproduction

• Sexual reproduction produces offspring with unique gene combinations unlike asexual reproduction.
• Two parents combine genetic material, resulting in diversity among offspring.

Sex Cells (Meiosis)

• Sex cells (gametes, e.g., sperm and eggs) are produced through meiosis, a special type of cell division.
• Meiosis reduces chromosome numbers by half, allowing for unique combination of genetic material during fertilization.

Genotype vs Phenotype

• Genotype: genetic makeup (alleles) of an organism
• Phenotype: observable traits (physical appearance, behavior) of an organism.

Dive into the principles of Darwin's theory of evolution, exploring how survival hinges on adaptability over strength. This lecture also examines pre-Darwinian views on species and the critical role of fossils in understanding Earth's history and life's progression. Gain insight into the foundational concepts that shape our understanding of evolution today.