13.1 Evolution and Natural Selection
1. being able to recognize evolution when you see one
2. Explain how natural selection acts on individuals to shape the characteristics of
3. identify, describe and understand different mechanisms underlying evolution
Readings: Chapter 10: 265-273
Reminder: Please evaluate your instructor’s teaching effectiveness in this course
The Geek philosopher Plato thought that living organisms were fixed. Linnaeus collected “type”
specimens and classified organisms in the hierarchy of kingdoms, phyla, classes, orders, families,
genera, and species. They ignored variation among individuals in a species and believe that
organisms were unchanged and considered variations in individuals that deviate from the
perfect “type” were due to imperfect environments and conditions of this world. Many
scientists in early 19 century did not believe that species would go extinct. They thought
organisms not found but whose fossils exist were in unexplored regions of the world.
Commonly thoughts regarding organisms in the 19 Century: Organisms were fixed and
unchanged; the existing ones do not disappear and no new ones appear.
What do you make of the following observations?
1. Fossil records show striking resemblances between the fossils unearthed and the living
organisms found in the same geographic area. Many generations have lived in the same
place for a long time. The species has adapted to the land and has not needed to
change drastically over time. The living organism came from the older species; the
species changed over time.
2. Vestigial traits (reduced or incompletely developed structure without function) in
organisms are similar to functioning structures in closely related species (e.g., some
whales and boas have tiny hip and leg bones; blind cave-dwelling fish have eye sockets).
We have all derived from a common ancestor. Organisms do change over time; the
traits that may have been used and useful earlier become smaller and unused over time.
3. Mocking birds found on neighbouring islands are all distinct different species (each
island has a species that is different from that of other islands) but all are similar.
In 1809, Jean-Baptiste de Lamarck proposed spontaneous generation of living organisms which
gradually evolved toward perfection. Lamarck proposed inheritance of acquired characters. If
you worked out and developed strong muscles, your children would also have strong muscles
(thinking from the perspective of individuals). Lamarck was the first one to propose a
mechanism for changes in organisms. Lamarck came up with an idea about plausible mechanisms for why species change over time; however the acquired traits he talks about are
Charles Darwin and Alfred Wallace noted variation among individuals in a population and
proposed that variation was the key to understanding the nature of species - this is population
thinking: thinking from the perspective of population as opposed to individuals.
Darwin and Wallace proposed Natural selection as one mechanism that underlies evolution.
Evolution: a change of gene (allele) frequencies in a population over generations.
You can view gene frequencies in a population as percentages of different genotypes in a
population. For example, in a population of 5 people, 2 people have brown eyes (heterozygous
Bb) and 3 people have blue eyes (homozygous bb); the frequency (percentage) of B allele in the
population is 20% (2 out of 10 alleles). If in the next generation, for whatever reason, the same
population of 5 individuals, two people has 2 homozygous brown eyes (BB) and 3 people have
blue eyes (bb); the frequency of B allele in the population has now increased to 40% and
evolution has occurred.
Which of the following are examples of evolution?
1. A species of bi-annual grass (grass that lives for 2 growing seasons) was found to be common
on an island without herbivores. Most grasses had soft and juicy leaves but a few had tough
and spiny leaves. A small group of herbivores was introduced on to the island and began to
feed voraciously on this species of grass. Twenty years later, this species of grass is as common
as before, but all grasses now have tough and spiny leaves. yes
2. The tree Calvaria major on Mauritius Island has not been able to reproduce in the last 400
years, due to the extinction of its seed disperser Dodo. This species of tree is extremely long-
lived and its population size has essentially remained the same in the past four centuries.
However, examining well preserved specimens, biologists found that the leaves of these tress
have gradually become smaller and thicker in the past two hundreds years. No(it has not gone
3. An island once had vegetation that supported a population of 1000 wild, flightless geese (the
life span of geese is 5 years). Due to the global warming and soil erosion in the past 50 years,
the biomass of vegetation (and therefore the quantity of food available to the geese) on the
island has decreased by 45%. However, there are still 1000 flight-less geese living on the island
today and biologists discovered that these living geese digest food much more efficiently than
their ancestors 50 years ago. Yes (their digestion has changed over 10 generations)
4. On an island, one finds an annual herbaceous plant that has both yellow and pink flower
varieties (a plant has either yellow of pink flower but not both). The flower color is passed on
from parents to their propagules (young). Initially only 30% of the population have pink flowers. Biologists found that the color of the flower does not affect the reproductive success or the