Theme 1: Topics 5-6

Descent with modification

Species share common traits because they are descended from common ancestors (unity). Differences between species arise when species accumulate heritable changes or adaptations after diverging from their common ancestor (diversity).

Natural Selection

The process is a drawn-out, complex process involving multiple interconnected causes through which populations of living organisms adapt and change. It is the mechanism for “descent with modification” (evolution).

The Theory of Evolution

The theory of evolution, proposed by Charles Darwin, explains the gradual change and diversification of species over time, including the appearance of new species, because all species share a common ancestor.

Selective Agents

Environmental factors acting on populations that affect the survival and/or reproduction of individuals in the populations → Biotic (predators, disease, competitors), and Abiotic (spce, light, temperature, water)

Why is natural selection context-dependent?

What direction change or changes a population will experience depends on whether or not selection pressure is consistent

Why is variation in populations essential for natural selection?

Genetic diversity in individuals in populations provides the raw material that natural selection acts upon

Direct observations of evolutionary change

Speciation (Galapagos finches, artificial selection, drug resistance in virus and bacteria)

Homologies

SImilarities that arise from a shared evolutionary ancestor

The fossil record

Provides evidence of the extinction of species, the origin and diversification of new groups, and transitional fossils.

Biogeography

The study of how species are distributed across the globe → continental patterns of species distribution reflect evolutionary histories.

Morphological homologies

Physical resemblances like anatomical structures eg. the forelimb which is found in mammals, birds, reptiles, and amphibians

Homologous embryonic structures

During the early stages of development, many vertebrates exhibit strikingly similar embryonic features eg. embryos of fish, birds, and humans all develop gills and tail-like structures

Vestigial structures

Remnants of structures that were functional in an organism’s ancestors eg. pelvic bones in whales, and human appendix

Molecular homologies

Molecular sequences, such as specific protein and DNA similarities, show a hierarchy of relatedness that mirrors the evolutionary tree.

Transitional fossils

Serve as missing links, and are evidence of important transitions, eg. the transition from land to sea in mammalian ancestors of cetacean

Chronological sequence (fossil record)

The order in which different groups of taxa appear in the fossil record aligns with their evolutionary relationships.

Genotypic variation vs. Phenotypic variation

Genotypic → Variation in the genetic makeup of an organism (heritable)

Phenotypic → Variation in an organism’s observable characteristics