SPECIATION
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A species is defined as being reproductively isolated from other species. How are
they isolated?
Prezygotic & Postzygotic Barriers
- Prezygotic Barriers:
- Impede mating between species or hinder the fertilization of eggs if members
of different species attempt to mate.
- Habitat Isolation: Two species living in different habitats within the same
area. This could refer to two species of snakes; one living in/around water but
the other being strictly terrestrial. This also could apply to parasites that have
different hosts.
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Temporal Isolation: Two species that breed at different times of the day,
different seasons, or years. Ex: rainbow trout & brown trout.
Behavioral Isolation: Species-specific signals & elaborate behavior to attract
mates are important to attract the same species. Ex: fireflies, bird songs
Mechanical isolation: Being anatomically incompatible. Ex: insects,
flowering plants.
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Gametic Isolation: Even if all else fails and a mating does occur, the eggs &
sperm can rarely fuse to form a zygote.
- Gamete recognition is based on the presence of specific molecules located on
the surface of the egg as well as specific enzymes in acrosomes on the sperm.
- Fig. 13.5
- Some researchers argue that the subspecies concept is sufficiently flawed as to
be irrelevant
- If you really want a good cry, try doing a "subspecies and extinct" search on
the web; you will find things like, "three tiger subspecies are now extinct:
Caspian tiger (P.T. virgata), Javan tiger (P.T. sondaica), Bali tiger (P.T.
balica)."
Postzygotic Barriers:
- When prezygotic barriers are crossed and a hybrid zygote forms, one of
several postzygotic barriers may prevent development of a viable, fertile
hybrid.
- Reduced Hybrid Viability: Genetic incompatibility between the 2 species may
abort development of the hybrid at some embryonic stage.
- Reduced Hybrid Fertility: Even if 2 species mate and produce a robust
offspring, the offspring could be completely or largely sterile. Ex: crossing a
horse & a donkey to obtain a mule.
- Hybrid Breakdown: When some species cross-mate, the first generation of
hybrids are fertile, but when these hybrids mate with one another or with
either parent species, offspring of the next generation are weak or sterile.
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Punctuated Equilibrium vs. Gradualism
- Punctuated Equilibrium instead of a slow, continuous movement, evolution tends
to be characterized by long periods of virtual standstill ("equilibrium"),
“punctuated" by episodes of very fast development of new forms.
- According to the gradualist model, species descended from a common ancestor
diverge more and more in morphology as they acquire unique adaptations
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Systematics:
- The evolutionary history of a species or a group of related species is called
phylogeny.
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Reconstructing a phylogenetic tree is part of the scope of systematics; the study of
biological diversity in an evolutionary context.
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Classification & Taxonomy
(Linnaeus)
- Kingdom
- Phylum (Division)
- Class
- Order
- Family
- Genus
- Species
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Taxons:
- A taxon is said to be monophyletic if a single ancestor gave rise to all species in
that taxon and to no species placed in any other taxon.
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A taxon is polyphyletic if its members are derived from two or more ancestral
forms not common to all members.
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A paraphyletic taxon is one that excludes species that share a common ancestor
that gave rise to the species included in the taxon. Ex: Class Reptilia excludes the
Class Aves (birds) although a reptilian ancestor common to all reptiles is shared.
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Homology:
- Likeness attributed to shared ancestry
- The forelimbs of mammals are homologous; they share a similarity in the skeletal
support that has a genealogical basis.
- Homology must be distinguished from analogy in evolutionary tress!
- Analogy: Similarities due to convergent evolution, not common ancestry.
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Homology must be sorted from analogy to reconstruct phylogenetic tress on the
basis of homologous similarities.
- Generally, the more homology, the more closely related 2 species are. This needs
to be reflected in their classification.
3 Types of Systematics:
- Phenetics:
- Also known as numerical taxonomy, is an attempt to classify organisms based
on similar morphology, usually by observable traits, regardless of their
evolutionary relation. Phenetics is widely disregarded these days.
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Classical Evolutionary Systematics:
- Attempts to balance the extent of divergence & the branching sequence.
Cladistics:
- Classifies organisms according to the order in time that branches arise along a
phylogenic tree, without considering the degree of divergence.
There are many ways to classify organisms. Understand that taxonomy constantly
changes due to new evidence. We are going to use the 6 Kingdom systems.
Kingdom:
- Archeabacteria: ancient bacteria
- Eubacteria: recent bacteria
- Protista: Eukaryotic singled celled, i.e. Euglena, Paramecium, Amoeba
- Fungi: mushrooms, etc
- Plantae: vascular & non-vascular
- Animalia: from sponges to humans