ANT203Y5 – BiologicalAnthropology
Lecture 1 – January 7, 2014
The Origin and Evolution of Species
Microevolution vs. Macroevoltuion
- In the previous semester, we focused on micoevolutionary processes:
o Mutation, genetic drift, gene flow and natural selection, and how these
evolutionary factors drive short term changes in gene frequencies.
o The focus was on variation in populations (in particular human populations).
- During this semester, we will focus on evolutionary change over long time periods:
- The word Macroevolution was coined by T. Dobzhansky, referring to evolution at levels
higher than the population (species or higher).
o Change of species through time
- Macroevolution refers to evolutionary change over long periods of time.
- Two key questions to understand macroevolution are:
o What is a species?
o How do new species arise?
The Biological Concept of Species
- There are numerous definitions of “species”. We will use here the biological species
concept, which states that:
- “Species are groups of interbreeding natural populations that are reproductively isolated
from other such groups” - Ernst Mayr
o Individuals from other populations that cannot breed, they are considered to be
from different species (not part of slides)
o Species that can breed and produce fertile offspring, are considered to be the
same species (not part of slides)
- Note that for different populations to belong to the same species, they must
o Be capable of interbreeding under natural conditions, and
o Be capable of producing fertile offspring
- Problem: This concept of species can be applied (with some difficulties) to organisms
with sexual reproduction but not to organisms with asexual reproduction.
- Example: Horse and Donkey
o If they produce a mule; they cannot form viable offspring
o By definition, donkey and horses cannot be classified as the same species
Patterns of Macroevolution
- Macroevolution deals with evolution of species over time. This can happen in two
o Anagenesis: Over time, a species can change in a linear fashion, from one form
to a different form.
Change of species over time; more or less in a linear form
o Cladogenesis: One or more new species branch off an original species.
Cladogenesis involves the formation of new species.
One or more species are going to branch from an original species
The Tempo and Mode of Macrovolution ANT203Y5 – BiologicalAnthropology
Lecture 1 – January 7, 2014
o Macroevolution as a slow and gradual process (Darwin)
o Natural selection primarily responsible for speciation
o Fossil record expected to show smooth species transitions
o Problem: fossil record isn’t complete, and this happens to any organisms
(including human lineage).
- Punctuated equilibrium (Elredge and Gould)
o Long periods of stasis, punctuated by rapid change
Period of where nothing is changes, and then a burst of rapid change
This has more to do with the idea of cladogenesis
o Rapid speciation, happening at the edges of species range.
o Larger role for mutation and drift in small populations
o Predicts long periods of little evolutionary change, with rapid bursts of
o Fewer “transitional” forms in the fossil record (which is a dilemma)
- Both models are not mutually exclusive!
o Some organisms can fit well with the gradualism model (can see the transition
o You can see some example of stratigraphic records, where you can see slow
- Adaptive radiation: formation of many new species following the availability of new
environments or the development of a new adaptation.
o Many adaptive radiations are evident in the fossil record, including the primate
- How do new species form?
- For speciation to occur, a number of factors need to be in place.
o Reduction or elimination of gene flow between populations. Commonly, this
happens through geographic isolation, although other factors may potentially be
involved as well (behavioral isolation).
You need to have enough differentiation so that eventually members of
the different populations do more mate. And if they do, their zygote will
not be viable.
o Genetic divergence. Speciation only happens when, in addition to restricted
gene flow, other evolutionary forces (mutation, genetic drift, natural selection)
act to increase the genetic differences between populations.
Populations can evolve in different, not only because of natural selection.
Genetic drift, in a small population, can cause dramatic changes which
can increase genetic differentiation
o Reproductive isolation. The end result of restricted gene flow and increased
genetic differences will be that populations will become new species. The
populations will no longer