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EVOLUTION: IDEAS ON SPECIES AND DIVERSITY
▶Species
▶Ch.6
▶Ch.7
▶Sociobiology
Evolution is the process of long-term change in biological systems.
Until 1859, most biologists believed in the fixity of species. Lamarck
and Geoffroy thought that species could adapt to their environments by
changes occurring within individual lifetimes. Darwin proposed instead
that natural selection resulted in "descent with modification."
Historical ideas before Darwin:
- Early ideas:
- Local naturalists described the species living around them.
- Most scientists believed in the divine creation of each species.
- John Ray wrote "The Wisdom of God, Manifest in the Works of the Creation" (1691).
- Adaptation was usually attributed to divine benevolence.
(Paley's "Natural Theology" came from this idea, and from John Ray's ideas.)
- Before 1859, most scientists and philosophers insisted that
species were fixed and unchanging;
each was thought to be
an unchanging copy of a heavenly form (=type, idea, eidos).
- To explain diversity, most scientists from
Aristotle until about 1830 believed
in an unchanging, unbroken order of perfection,
the Great Chain of Being (Scala Naturae).
This idea was used to justify:
- Social order (hierarchy) and class structure
- The "divine right of kings"
- "Man's place in nature" (see the quotation above)
- (later on:) racism, sexism, and the search for "missing links"
- Around the time of the French Revolution, social hierarchies (and monarchies) came into question.
New ideas of the Enlightenment included "progress" and environmental determinism:
- Maupertius re-interpreted the Chain of Being as a time sequence;
Lamarck described this unilineal sequence as "La Marche de la Nature"
- Cuvier's break-up of the Chain of Being
- Geoffroy and Lamarck attributed adaptations to environmental influences.
- William Paley ("Natural Theology") used adaptations as proof of God's existence.
Quote: Paley
- J.B. Lamarck tried to explain environmental adaptations.
He claimed that the willful use of a part would strengthen and enlarge
it, while the disuse of a part would cause it to wither (Lamarckism,
or the theory of use and disuse). Lamarck also believed in a single-file
line of progress ("La marche de la Nature").
- E. Geoffroy Saint-Hilaire explained adaptation by direct effects
of the environment (Geoffroyism), a concept which Lamarck rejected.
- Both Lamarck's and Geoffroy's theories rely upon inheritance of
acquired characteristics, changes occurring during an organism's
lifetime. This possibility was later disproved by Weismann and others.
- Charles Darwin explained diversity by branching "descent with modification".
- Darwin explained adaptation by natural selection.
Charles Darwin (1809-1882): A voyage around the world aboard H.M.S.
Beagle convinced him of several facts that earlier theories could
not explain:
On the Origin of Species (1859)
In his book,
On the Origin of Species (1859),
Darwin explained the all of his findings by two new major theories:
- descent with
modification, a branching form of evolution very different from earlier theories.
- natural selection as a
mechanism for evolutionary change.
Evidences for branching evolution ("descent with modification"):
- Patterns of common descent are reflected in classifications, forming "groups within groups".
- Related species share many internal similarities (anatomical,
biochemical, or embryological homologies) despite different
adaptations.
HOMOLOGIES
- These homologies may include vestigial remnants of once-useful parts.
- Similar adaptations often occur under similar circumstances, even
in unrelated species (convergent adaptations).
CONVERGENCE
- Related species often inhabit certain land masses or island groups.
- Fossils can often be arranged in evolutionary sequences.
- Some species vary from place to place, and the differences
are inherited.
Natural selection:
- All living species tend to over-reproduce.
- Most seeds, eggs, or hatchlings die without reproducing.
- All living species are extremely variable.
- Many of these variations are inherited.
- Inherited differences in survival and reproductive ability
(natural selection) bring about change in each generation.
Darwin quotes: selection
"workmanship"
Selection - examples
Evidence for natural selection:
- All living species are highly adapted to their way of life.
- Many adaptations cannot be explain by environmental influence alone.
Examples:
- Unrelated but ecologically equivalent species live on different continents.
- Some embryonic structures (e.g., a flap in the human heart that
seals closed at birth) develop before they become useful
- Some behavior (like bird migration or nest building) occurs
in advance of its usefulness.
- Some adaptations are less than perfect, contrary to an earlier theory
that used perfect adaptation to prove divine creation.
- Natural selection has repeatedly been documented (e.g., among
peppered moths in England), and has resulted in changes over time
in natural populations.
- Artificial selection by animal and plant breeders has produced many new
adaptations, some of them similar to adaptations occurring naturally.
Types of selection: In all types of selection, genotypes contribute genes
unequally to the next generation,
either by differences in mortality and survival, by differences in mating
success, or by differences in fertility and fecundity (leaving offspring).
- Natural selection is differential contribution by natural processes.
The peppered moths of England, selected by predators (birds), are an example.
- Artificial selection is selection of captive species by humans.
- Sexual selection is selection based on success in mating.
- Selection against a dominant trait can eliminate the trait rapidly.
- Selection against a recessive trait works very slowly and becomes
much less effective once the recessive allele becomes rare.
- Selection against heterozygotes can result in either allele becoming
lost and the other taking over 100% of the gene pool.
- Selection favoring heterozygotes over both types of homozygotes
results in balanced polymorphism in which both alleles persist
indefinitely. Sickle-cell anemia is an example of this situation.
- Directional selection shifts the population mean.
- Disruptive selection increases population variance.
- Centripetal or stabilizing selection (very common) reduces variance.
Adaptation
- Structural adaptations
- Biochemical adaptations
- Color & pattern adaptations:
camouflage, industrial melanism, mimicry
Mimicry and camouflage:
Many species gain protection against predators by resembling
their background (camouflage) or by falsely resembling other
species (mimicry).
In Batesian mimicry, a palatable
species resembles a distasteful or harmful one.
Mullerian mimicry
is resemblance among distasteful or harmful species.
- Mimicry works only when certain models are present,
a fact explained easily by natural selection, but not by Lamarckism
or similar theories, nor by theories of special creation.
- Mimicry may vary geographically, with the same mimic species
resembling different models in different places. Natural selection
can explain this; Lamarckism cannot.
Mimicry
Industrial melanism
Evolution Question 3
THE GENETICS OF POPULATIONS
Large, random-mating populations will, under certain assumptions, reach a
genetic equilibrium
in which genotypic proportions tend to remain constant.
Hardy-Weinberg law: In a large,
random-mating population of diploids with no unbalanced mutation, unbalanced
migration, or selection in any form, the genotypic proportions tend to remain
constant. This constancy is called a genetic equilibrium, with
equilibrium frequencies given by the equation
p2 AA + 2pq Aa + q2 aa = 1
or, more simply,
p2 + 2pq + q2 = 1
H-W equilibrium
Sample H-W problem
Microevolution is evolution below the species level.
It results from:
- Variation, brought about by mutation, chromosomal changes, and
genetic recombination through mating.
- Selection, genetic drift, and other forces that act upon variation.
- Gene flow between populations.
- Restricted gene flow, including reproductive isolation.
Selection often reduces variation. Genotypes contribute genes unequally to the next generation.
- Natural selection occurs by natural processes.
- Artificial selection is selection of captive species by humans.
- Sexual selection is selection based on success in mating.
- Selection against a dominant trait can eliminate the trait rapidly.
- Selection against a recessive trait works very slowly and becomes
much less effective once the recessive allele becomes rare.
- Selection against heterozygotes can result in either allele becoming
lost and the other taking over 100% of the gene pool.
- Selection favoring heterozygotes over both types of homozygotes
results in balanced polymorphism in which both alleles persist
indefinitely. Sickle-cell anemia is an example of this situation.
- Directional selection shifts the population mean.
- Disruptive selection increases population variance.
- Centripetal or stabilizing selection reduces variance.
Simulation of Natural Selection
Agents of selection:
- Artificial selection (by human agency)
- Natural selection (by natural agency)
- by predators
- by pathogens, parasites, & other diseases
- by starvation
- by environmental (climatic) extremes (of temperature, etc.)
- other physical agents: fire, landslides, etc.
- sexual selection
Genetic drift ("Sewall Wright effect"): In smaller populations, or for rare alleles,
gene frequencies can fluctuate
randomly in either direction simply by chance.
Important subtypes include:
- "Bottleneck effect" in populations temporarily small
- "Founder effect" among founders of a new population (e.g., Dunkers)
Geographic variation: Natural selection in different environments
causes populations to differ. Gene flow reduces the opportunities
for populations to differ; restricted gene flow allows enhanced differences.
Populations of some geographically widespread species may differ so much
that they may become unable to interbreed.
- If barriers to breeding accompany differences in visible traits,
the species may become divided into subspecies.
- Continuous geographic variation is usually described in terms
of clines (character gradients across a map).
- Geographic variation is usually the first step in species formation.
Illustrations
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SPECIES and SPECIATION
Species are evolutionary units within which gene flow occurs.
Natural populations belong to the same species only if they can
interbreed and leave fertile offspring. Different species are
reproductively isolated from each other. Most new species originate
geographically.
Speciation
More speciation
Speciation PowerPoint slides
Evolution Question 4
Biological species definition: Species are groups of interbreeding
populations that are reproductively isolated from other species.
Quote: Mayr
Reproductive isolating mechanisms that act prior to mating:
- Ecological isolation: Potential mates do not meet because
they live in different habitats or breed at different times or seasons.
- Behavioral isolation: Mating calls or mating rituals differ.
- Mechanical isolation: "Lock and key" mismatch of genitalia.
Reproductive isolating mechanisms that act after mating:
- Gametic mortality: Gametes die before fertilization.
- Zygotic mortality: Fertilized eggs fail to divide properly.
- Embryonic or larval mortality: They die prematurely.
- Hybrid inviability: Hybrids never reach reproductive age.
- Hybrid sterility: Hybrids cannot reproduce, as in mules.
- F2 breakdown: Offspring of hybrids are inviable.
Geographic speciation: Most speciation occurs geographically.
- A species develops geographic variation over its range.
- Geographic barriers prevent contact between populations.
- Reproductive isolation may now evolve.
- Geographic isolation ends, with two possible outcomes:
- No reproductive isolation-- still a single species.
- Reproductive isolation is effective-- two species now exist;
selection will enhance differences between them.
Speciation PowerPoint slides
Lineage: An ancestor-to-descendent sequence of species.
Trend: Continued morphological change within a lineage.
Parallelism: Independent occurrence of the same or similar
trends in different lineages.
Convergence: Similar adaptations in unrelated lineages.
Cladogenesis: The branching of lineages by speciation.
Anagenesis: Evolution within a lineage, between branching points.
Evidence for the adaptiveness of trends:
- Trends often persist for a long time.
- Parallel trends often occur independently.
- Evolutionary rates vary: trends speed up or slow down; they may even
stop altogether or reverse direction.
- Trends in different characters do not always go together but occur
independently (mosaic evolution) and at different rates and times.
For this reason, transitional species like Archaeopteryx are a mosaic
of primitive and advanced features mixed together.
Opportunism: Evolution follows no plan or goal, but
instead takes the path of least resistance.
- Cladogenesis fills the biosphere with more and more species (and niches).
- Diversity among the descendents of a single species (adaptive
radiation) often results.
- Functional problems are often solved differently in different
lineages (multiple solutions, such as diversity among eyes).
- The same trend often occurs repeatedly (iterative evolution).
- Convergence (and its imperfections) show that similar adaptive
opportunities may arise independently more than once.
- Organs that change function usually serve both old and new functions
simultaneously during the transition.
Illustrations
ORIGIN OF LIFE ON EARTH
ORGANIC DIVERSITY AND TAXONOMY
Biological diversity is expressed by arranging organisms into kingdoms,
phyla, classes, orders, families, genera, and species. These groups reflect
evolutionary history and common ancestry as much as possible. Evolutionary
relationships responsible for these arrangements are often depicted in
family trees. The aim of phylogenetics is to reconstruct family trees and base
classifications on them.
Binomial nomenclature: Each species has a two-word name. The first
word (capitalized) is the name of the genus; the second (lower case) is
the name of the species. Example: Homo sapiens.
The Linnaean system: Uses binomial nomenclature throughout.
Species are grouped into genera and genera into higher groups. Any one
of these groups, at any level, is called a taxon (plural, taxa).
The complete Linnaean hierarchy (ranking) of groups is as follows:
Kingdom (the most inclusive group)
Phylum (plural, phyla, sometimes called a "division" in plants)
Class
Order
Family
Genus (plural, genera)
Species (same spelling in both singular and plural)
(Mnemonic: "King Philip Came Over For Good Soup")
Extra ranks are added to this hierarchy as needed, such as subphylum (just
below phylum) or superfamily (just below family).
Example: Humans belong to the Kingdom Animalia, Phylum Chordata,
subphylum Vertebrata, class Mammalia, order Primates, family Hominidae,
genus Homo, species Homo sapiens.
Phylogeny: A family tree of species.
Phylogenetics: The study of family trees.
- Phylogenetic methods use both the fossil record and resemblances among
living species as evidence to reconstruct phylogenies. Species sharing
many similarities are considered to be descendents of a common ancestor
that also shared these similarities. When conflicting evidence arises
from different characters, further study is undertaken to see whether
some of the similarities could have evolved by convergence.
- An important task in phylogenetics is therefore recognizing
homology (resemblance due to common ancestry) and distinguishing
it from analogy or convergence.
- The aim of classification based on phylogenetics is to group together
those species that derive their similarities from a common ancestor.
That means that, insofar as possible, each taxon should be made
monophyletic by including the common ancestor within the taxon.
Taxonomy is the theory behing the making of classifications.
- Phenetic taxonomy: Classifications based on resemblance alone
have long been in disfavor because they do not distinguish convergence
from other causes of resemblance.
- Phylogenetic taxonomy: Modern classifications are based on
phylogenetics, meaning that species that share a common ancestry are
grouped together as much as possible. Strict adherence to this principle
is the basis of cladistics. Cladistic taxonomists construct family
trees first, then base their classifications strictly on the geometry of
branching, ignoring such matters as the diversity or degree of change within
each branch.
Evolutionary (phylogenetic) classification is based on branching descent:
Biological classification
reflects the results of a branching evolutionary process. Insofar as
possible, classifications should be genealogical. Each taxon should
ideally represent one branch of the evolutionary tree, with the smaller
included taxa representing its sub-branches.
Three domains and multiple kingdoms: Most biologists now arrange organisms
into three domains containing multiple kingdoms:
- Domain Archaea contains only the Kingdom Archaebacteria,
a group of oxygen-intolerant procaryotes with RNA sequences different
from those of all other organisms.
- Domain Bacteria (or Eubacteria) contains only a single kingdom
of the same name, including the majority of procaryotes.
- Domain Eucarya contains multiple kingdoms of eucaryotes, including the following:
- (the former "Protista" has been broken up into many kingdoms of eucaryotes that
contain neither tissues nor embryos)
- Kingdom Mycota: Fungi, with cell walls but no plastids.
- Kingdom Plantae: Plants, containing plastids and chlorophyll.
- Kingdom Animalia: Multicellular animals, developing from blastulas.
New Illustrations: PDF
New PowerPoint Illustrations
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HUMAN VARIATION
Human variation
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