Biology SN2 Lecture 2: Evolution Overview

Biology SN2 Lecture 2: Evolution & Descent with Modification

• Darwin's theory of evolution posits that species that are more adaptable to change are more likely to survive
• Species evolve over time through adaptations to their environments. This occurs due to natural selection, where organisms with favorable traits are more likely to survive and reproduce.
• The evolution of species is closely tied to their environment and related processes
• Natural selection is a mechanism of evolutionary change. Natural selection leads to adaptive evolution, where populations change over time. Individuals with traits that allow for survival and reproduction in a specific environment are more likely to pass these traits to their offspring
• Darwin developed his theory based on various factors: Malthus' essay on struggle for existence, Lyell's Principles of Geology, principles of artificial selection, and his observations in the Galapagos.
• Species are capable of producing more offspring than the environment can support, leading to competition.
• Populations tend to be stable in size, because environmental resources are limited. Competition for resources, predation, disease, and environmental conditions limit population growth
• Members within a population vary extensively. No two individuals are exactly alike, and these variations are heritable. Survival often depends on inherited traits.
• Genetic variation is increased through sexual reproduction and mutation.
• Differential reproductive success (survival of the fittest) occurs due to variation in inherited traits, which provide advantages in survival and reproduction. This gradual change in traits within a population, which leads to adaptations.

Before Darwin: Understanding Life

• Aristotle viewed species as fixed and unchanging, arranging life on a scale of increasing complexity (scala naturae).
• The Old Testament of the Bible posits that species were individually designed by God
• Carolus Linnaeus founded taxonomy, grouping similar species into categories. He did not link resemblances to evolutionary kinship
• Paleontology (the study of fossils) provided evidence of the succession of organisms that populated Earth. Older strata showed greater differences, in comparison to modern life

Before Darwin: Fossils

• Fossils are the remains or traces of organisms from the past found in sedimentary rock. Layers (strata) reveal the sequence of events in geological history.
• Fossils demonstrate the succession of organisms over geological time
• Cuvier noted that older strata showed greater differences in fossil organisms compared to modern life, he proposed the idea of "catastrophism" to explain these differences
• Cuvier did not believe in evolution. Instead, he proposed alternative explanations like floods or droughts to explain evolutionary differences.

Before Darwin: Lamarck

• Lamarck proposed a theory of evolution based on "use and disuse," arguing that traits acquired during an organism's lifetime could be passed on to offspring.
• An example is a giraffe's neck stretching to reach higher leaves, resulting in an elongated neck, which is then passed to offspring.
• Lamarck did not explain the inheritance of acquired traits

Before Darwin: Hutton and Lyell

• Hutton proposed the theory of gradualism; profound changes take place over time due to small, continuous geological processes
• Lyell proposed that changes in Earth's surface result from slow, continuous actions. This suggests that the Earth is much older than previously believed.

Darwin and Evolution

• Malthus' essay on the struggle for existence in human populations influenced Darwin.
• Lyell's principles of geology suggested that the Earth is very old and has been changing gradually.
• Artificial selection also influenced Darwin; changes in organisms can occur over short periods.
• Darwin observed various adaptations in plants and animals in the Galapagos as diverse environments.

Darwin: The Origin of Species (2 Main Ideas):

• Today's organisms are descendants from ancestor species that differ from modern ones. This illustrates evolutionary history
• Natural selection is the mechanism for evolutionary change. Natural selection leads to environmental adaptations in a population over time.

Darwinian Evolution

• Darwin was fascinated by unusual organisms found in various environments, particularly in the isolated island of Galapagos
• He hypothesized that these organisms evolved from mainland species, that had then become diversified over time
• The origin of new species and the adaptation to their environment are related processes. This includes clear differences in the characteristics of the organisms.

How Does Natural Selection Work?

• Species can produce more offspring that the environment supports, leading to competition
• Environmental resources are limited and competition results, with limited resources only a fraction of an organisms offspring survive
• There are variations in individuals, that are heritable
• Different reproductive success exists, that leads to traits which give advantages in the environment
• Populations evolve, individuals do not
• Natural selection can only act on heritable traits

Genetic Variation Explained

• Sexual reproduction mixes alleles during meiosis and fertilization. The offspring inherits a combination of genes from their parents
• Asexual reproduction creates identical copies, as the offspring inherits all genes from their parent.
• Mutations (changes in DNA) are the original source of different alleles

How Does Natural Selection Work? (Important notes)

• Natural selection only acts on heritable traits, which are passed down between generations
• Environmental factors vary from time to time and location, so a favorable trait can be non-useful in another environment
• Natural selection does not create perfect organisms and does not create perfect traits; it instead "edits" existing variations to adapt to the environment
• Chance and natural selection interact
• The environment selects for traits based on their fitness in a specific or particular environment

Evidence for Darwin's Theory

• Many different sources of evidence support Darwin's theory: Natural selection in action, homology (anatomy, development, and molecular), convergent evolution, biogeography and the fossil record

Natural Selection in Action (Peppered Moth):

• In pre-industrial Britain, peppered moths were primarily light colored. These moths blend in with light-colored tree bark. This allowed them to hide from predators.
• The industrial revolution darkened tree bark due to soot. Dark-colored moths became more prevalent, as they were better camouflaged.

Natural Selection in Action (Drug Resistance):

• Use of drugs can select for resistant pathogens. Such resistance can lead to adaptive evolution in pathogens
• Researchers have developed numerous drugs to combat HIV, for example. But these drugs can select for viruses that are resistant to them.

Homology: Anatomy

• Homologous structures are similar in anatomy because of shared ancestry. Even though they have divergent functionalities. Examples include the human arm, cat forelimb, whale flipper and bat wing. These structures exhibit similarities in bone arrangement, muscles, and nerves
• Vestigial organs are remnants of structures that served functions in an organism's ancestors, but are no longer useful
• Natural selection will not act on vestigial organs to eliminate them

Homology: Development

• Embryos of different vertebrates show similarities in early development. For example, pharyngeal pouches (gill slits) appear in embryos, but develop into different structures in adults (e.g. gills in fish and parts of the ear in mammals). Homologous structures present in the embryos show the common ancestry between organisms.

Homology: Molecular

• The universal genetic code is evidence of common ancestry. The same codons in DNA and mRNA translate similar amino acids in diverse organisms.
• Similarities in proteins (e.g., hemoglobin) between organisms reflect evolutionary relationships and are observed to be inversely proportional to the closeness of kinship between different species

Convergent Evolution

• Convergent evolution occurs when organisms with separate ancestors develop similar traits due to similar environmental pressures.
• For example, similar environments can have independent evolutions like long sharp claws and elongated snouts.

Biogeography

• Species tend to be more closely related to other species in the same geographical area, as opposed to species in different geographical locations. This suggests a common ancestor.
• Continental drift (plate tectonics) and the formation and isolation of populations are related to geographic events and their impact on adaptations

Fossil Record

• The fossil record provides a chronicle of life's history, showing a succession of forms. The succession in the fossil record is consistent with other evidence of descent with modification.
• The fossil record demonstrates that prokaryotes are older than eukaryotes. The oldest fossils are prokaryotes, indicating the precedence of prokaryotic life on earth.

Natural Selection …Just a Theory?

• A scientific theory accounts for many observations and data, attempting to explain and integrate these phenomena.
• Darwin's theory is supported by massive amounts of data from different sources.
• Neo-Darwinism (Modern synthesis) incorporates Mendelian genetics into Darwin's theory of evolution. Specifically, it states that natural selection influences heritable (genetic) variations and that mutations (especially random copying errors) provide the main source of these genetic variations.

Lecture 2: Learning Objectives

• Defines evolution and natural selection
• Understands pre-Darwinian views of life, which include Aristotle, Old Testament, Linnaeus, fossils, Hutton, Lyell, and Lamarck.
• Explains how natural selection works
• Identifies the source of Darwin's ideas (4 main sources)
• Provides evidence for natural selection for example using convergence, divergence, homology, biogeography, fossils
• Understands the importance of genes, evolution, and sexual reproduction
• Know the meaning of scientific terms, such including chromosomes, gametes, somatic cells, homologous chromosomes, alleles, recessive, dominant, genotype, phenotype, zygote, random fertilization

Summary Slides

• Asexual reproduction produces offspring identical to the parent, as the parent donates its genes to their offspring. The parent is the only single donor
• Sexual reproduction produces offspring with unique combinations of genes from two parents, leading to genetic variation.

Sex Cells (Produced By Meiosis)

• Meiosis produces haploid sex cells (gametes), with half the number of chromosomes compared to somatic cells. They contain DNA from both parents.
• The fertilized egg (zygote) contains a complete set of chromosomes (diploid)

Genotype VS Phenotype

• Genotype is the genetic makeup, involving the combination of alleles
• Phenotype is the observable characteristic of an individual determined through the genotype.

The Behavior of Recessive Alleles

• Recessively inherited trait/disorders occurs in homozygous recessive individuals.
• Carriers of recessive alleles are heterozygous for a particular trait, but they do not express the recessive characteristic, and only pass the recessive trait down to their offspring
• Relatives to carriers can have the same recessive traits

Principles of Inheritance: Examples

• Explains different traits inherited through simple mechanisms like genotypes.

The Evolutionary Significance of Genetic Variability

• Populations evolve through differential reproductive success
• Natural selection results in the accumulation of favorable genetic variations
• Different allele combinations can offer advantages in certain environments, or a variety of specific environments

Natural Selection in Action (Introduced Species)

• Soapberry bugs use their beaks to feed on seeds in fruits. Bugs with beaks that match the depth of seeds have a higher reproductive success, as this increases their chances of eating seeds
• Natural selection impacts beak lengths when food sources change.

Homology: Anatomy (Vestigial Organs)

• Vestigial organs are remnants of structures that served a function in the organism's ancestors. An example includes vestigial hind limb bones in snakes and whales