CH 23 ORIGIN OF SPECIES & MACROEVOLUTION
1. Species Concepts
2. Mechanisms of Speciation
3. Pace of Speciation
4. Evolutionary Developmental Biology and the Form of New Species
- Species: a group of organisms that maintains a distinctive set of attributes in nature.
- Speciation: mechanisms that promote the formation of new species.
- Macroevolution: evolutionary changes that create new species and groups of species.
23.1 Species Concepts
- True number of species on Earth is between 10 million and 100 million.
- It’s difficult to decide whether the two populations truly represent separate species because they might display one or more
traits that are somewhat different but not enough to warrant their placement into a different species – classified as
The Members of a Species have a common set of characteristics that distinguish them from other species
- Species concept: a way to define the concept of a species and provide an approach to distinguish one species from
- There are 2 most commonly applied concepts:
1. Morphological Species Concept
- Species are categorized based on physical characteristics.
- Some drawbacks:
o It’s difficult to decide how many traits to consider when characterizing individuals,
o it’s difficult to analyze quantitative traits that vary in a continuous way among members of the same species
o Degree of dissimilarity that separates different species may not be easy to decide on.
o Members of the same species sometimes look very different and vice versa.
2. Biological Species Concept
- A second way to define a species is by their ability to interbreed with one another in nature but cannot do so with
members of other species.
- Reproductive isolation: prevents one species from successfully interbreeding with other species. Mar/19/10
- Three drawbacks:
o Difficult to determine if 2 populations are reproductively isolated, especially if they’re large populations with
overlapping geographical ranges.
o Many cases in which 2 different species can interbreed yet consistently maintain themselves as separate
species. (Ex. Yucca plants)
o Cannot be applied to asexual species – bacteria, fungi, or extinct species.
Reproductive isolating mechanisms help to maintain distinctiveness of each species
- Reproductive isolating mechanisms: mechanisms that prevent interbreeding between different species, such as
genetic changes that occur because a species becomes adapted to its own particular environment.
o Prezygotic isolating mechanisms: prevent the formation of a zygote
o Postzygotic isolating mechanisms: block the development of a viable and fertile individual after fertilization
has taken place.
- Interspecies hybrid: when two species do produce offspring
Prezygotic Isolating Mechanisms
- There are 5 types of prezygotic mechanisms:
1. Geographic isolation: occurs when 2 different species live in geographically separate areas and do not have the
opportunity to come together for mating.
2. Temporal isolation: species reproduce at different times of the day or year.
3. Behavioural isolation: e.g. differences in songs of birds
4. Mechanical isolation: morphological features such as size or incompatible genitalia.
5. Gametic isolation: when gametes fail to unite in a successful fertilization (ex. In aquatic animals that release sperm
and egg into the water, gametic isolation prevents interspecies hybrids)
Postzygotic Reproductive Isolating Mechanisms
- Hybrid inviability: an egg of one species is fertilized by a sperm from another species but the fertilized egg cannot
develop past the early embryonic stages.
- Hybrid sterility: an interspecies hybrid is viable but sterile.
- Hybrid breakdown: interspecies hybrids are viable and fertile, but subsequent generations harbour genetic
abnormalities that are detrimental.
23.2 Mechanisms of Speciation
- Two main explanations have been proposed for why life diversifies:
1. Species are a consequence of adaptation to different ecological niches
2. Species of sexually reproducing organisms arise via reproductive isolation.
Speciation can occur as a linear or a branching process Mar/19/10
- During anagenesis, a single species is transformed into a different species over many generations due to evolutionary
mechanisms like food source.
- Cladogenesis: division of a species into 2 or more different species, usually due to geographic isolation.
Geographic Isolation Can Promote Allopatric Speciation
- Sexual reproduction is not a barrier to anagenesis but is a barrier to cladogenesis.
- A population will evolve as a single unit if the members can successfully breed with one another thereby preventing its
divergence into two or more discrete species.
- Allopatric speciation: most prevalent way for caldogenesis to occur, when members of a species becomes
geographically separated from the other members.
o Ex. Founder effect population on new land will evolve into different species.
- Adaptive radiation: a single ancestral species has evolved into a wide array of descendant species that differ greatly in
their habitat, form or behaviour.
Hybridization can occur when reproductive Isolation Between 2 populations is incomplete
- Speciation can occur when members of a species are only partially separated or when a species lives in one small area
or moves about very little.
o Ex. A mountain range may divide a species but there may be breaks in the range where the 2 groups are
- Hybrid zones are zones where 2 populations can interbreed.
- For speciation to occur, the amount of gene flow within hybrid zones must be very limited.
Sympatric Speciation Occurs when populations in direct contact become reproductively isolated from each other
- Sympatric speciation occurs when members of a species that initially occupy the same habitat within the same range
diverge into 2 or more different species.
- Involves abrupt genetic changes that quickly lead to the reproductive isolation of a group of individuals.
- Change in chromosome number, such as polyploidy in plants.
- Complete nondisjunction of chromosomes during gamete formation can increase number of chromosome sets in the
same species (autopolyploidy)
o Fig. 23.12: Galeopsis tetrahit is thought to be an allotetraploid derived from 2 diploid species – G. pubescens
and G. speciosa
o These 2 diploids contain 16 chromosomes each and G. tetrahit has 32 chromosomes
nondisjunctionallotetraploid with 4 sets of chromosomes.
- Sympatric speciation can also occur when genetic changes enable members of a species to occupy a new niche within
the same geographic range as the original species.
o Ex. Colonial bentgrass growing on contaminated soil. Mar/19/10
23.3 The Pace of Speciation
- Gradualism: each new species evolves continuously over long spans of time.
- Punctuated equilibrium: the tempo of evolution is more sporadic, so species exist relatively unchanged for many generations
but these periods are punctuated with short periods during which evolution occurs far more rapidly, because of environmental
changes via the founder effect or new mutations that are quickly acted on by natural selection.
- Environmental shifts may drive gene pool in a particular direction via natural selection.
- Species of large animals with long generation times tend to evolve slowly.
23.4 Evolutionary Developmental Biology
- This is a field that compares the development of different organisms in an attempt to understand ancestral relationships between
organisms and the developmental mechanisms that bring about evolutionary change.
The Spatial Expression of genes that affect development can have a dramatic effect on phenotype
o Ex. BMP4 gene (bone morphogenetic protein 4) causes cells to u