Biology and the Tree of Life
Biological science was founded partially with the development of the cell theory and the theory of evolution by natural
A phylogenetic tree is a diagram that may be interpreted as a graphical representation of inferred evolutionary relationship[s
among species. Phylogenies can be established by analyzing similarities (and differences in traits
Biologists ask questions, generate hypotheses to answer them, and design experiments that test the predictions made by
Most scientific theories comprise of two components:
o Patterns observed in the natural world
o Processes identified that produce the patterns
The Cell Theory
In the late 1660s, Robert Hooke and Anton van Leeuwenhoek were the first scientists to observe cells
Cell: a highly organized compartment bounded by a plasma membrane that contain concentrated chemicals in an aqueous
Cell Theory: states that all organisms are made of cells and all cells come from preexisting cells.
Louis Pasteur’s Experiment
Hypothesis: a proposal
Prediction: something that can be measured and must be correct if a hypothesis is valid
Louis Pasteur: demonstrated that cells arise from cells and not by spontaneous generation
o Pasteur achieved this via an effective experimental setup
Implications of the Cell Theory
Because all cells come from preexisting cells, all individuals in a population of single-celled organisms are related by common
In a multicellular organism, all of the cells present descend from preexisting cells and are connected by common ancestry
Orgins of Life
Current ideas for the orgin of life include models wherein cell based life emerged from matter (molecules)
The Theory of Evolution by Natural Selection
In 1858, Darwin and Wallace proposed that all species are related by common ancestry.
Darwin and Wallace also propsosed that the characteristics of species can be modified from generation to generation.
Evolution: entails that species are related to one another and can change through time.
Natural selection: a process that explains how evolution occurs.
Fitness: may be concetualised as the ability of a typical individual (i.e within a group) to survive and produce offspring.
Adaption: (at the microscopic scale) may refer to a trait that increases the fitness of a typical individual in a particular
Natural Selection and Populations
Population: a group of indivuduals of the same species living in the same area.
Two conditions must be met for natural selection to occur in a population:
o Individuals in the population vary among themselves in characteristics that are heritable
o In a particualr environment, certain versions of these heritable traits help individuals survive better or reproduce
more than do other versions.
If certain heritable traits lead to increased success in surviving and producing offspring, then these tratis become more
common in the population over time.
Natural selection acts on individuals, but evolutionary change affects only populations.
Evolution occurs when heritable variation leads to diffe