(300s BCE) organisms are organized on a "Scala Naturae" from most primitive to most complex. Also, organisms seek to "move towards perfection"
Leonardo da Vinci
(late 1400s) fossils ("figured stones") are the remains of extinct organisms
Georges Louis Leclerc de Buffon
(late 1700s) species were created, but were not perfeectly adapted. They then gradually changed to their present condition by "degeneration" from the forms created by God. But Buffon suggested no mechanism for this change.
Georges Leopold Cuvier
(early 1800s) believed in fixity of species, but suggested repeated mass extinctions that have thinned out initial full group of fixed species and created exotic fossils.
(middle 1800s) species don't evolve, and fossils can be explained by the theory that there have been 50-80 TOTAL extinctions of life, followed by a new creation after each one.
(early 1800s) species aren't fixed, but change due to "use and disuse of parts." This implied that acquired characteristics could be inherited and could be the basis of evolution. Also, each species has an unconscious striving to rise on the Scala Naturae.
(1830) ordinary processes operating at today's rates could account for the geology of the earth (geological gradualism).
(1798) populations tend to increase until they run out of resources. This implied that competition for resources was the rule in nature.
Darwin's Theory of Evolution
- 1.) Organisms have many more offspring than the environment can support.
- 2.) There is severe competition between offspring.
- 3.) Offspring vary from one another, and some of them have heritable traits that give them a competitive advantage
- 4.) Offspring having advantageous traits survive at higher rate and make up a large proportion of the next generation.
- 5.) Over time, the advantageous trait becomes predominant and any disadvantageous trait becomes predominant and any disadvantageous traits disappear from the population.
Parsimony (Occam's Razor)
a scientific preference to explain data with the simplest possible explanation or with the explanation requiring the fewest assumptions. This leads to a preference for explanations that rely on the orginary operation of natural law, not supernatural intervention.
an explanation of a process that implies that the process is attempting to attain a goal or fufill a purpose. E.g. "The earth turns so that the benefits of sunlight will be available to all living things"
an explanation of a process that implies that the process is purposeless, and is merely responding blinding to forces acting on it. E.g. "The earth turns because of conservation of angular momentum"
the belief that the ultimate test of any assertion is observation and the evidence of our senses.
Aristotle's idea that species were fixed (non-evolving) and arranged in order from the lowest and simpliest to the highest and most complex (man). By extension, it is also the idea that it is natural that man should dominate the earth and all other species.
in biology, the view that each species has an ideal type, and individual differences are imperfections and deviations from the ideal. BUT A more modern view is that there is no ideal type, just an average with variation around it.
Fixity of species
the view that each species has certain unchacnging characteristics, that one species cannot change into another, and that new species cannot arise.
the view that most geological changes came about suddenly from forces far more powerful than the forces operating today
the view that the catastrophic forces above shaped the earth in less than 1 million years.
the view that geological changes can be explained by ordinary forces operating over vast periods of time, implying that the earth is much older than 1 million years.
Evidence for Evolution:
- 1. Large changes in domestic animals and plants brought about by selective breeding.
- 2. Many examples of induced natural selection (e.g., development of antibiotic resistance, change in life history characteristics by manipulation of predation pressure).
- 3. Convergences--sharks and dolphins, bats and birds, and penguins and seals look very much alike despite the fact that they are different kinds of animals. (analogous structures, as opposed to homologous structures)
- 4. Similarities of comparative anatomy suggest that certain groups of organisms had a common ancestor and other groups had another ancestor.
- 5. Strong similarities in embryonic development in diverse animals
- 6. Presence of vestigial structures
- 7. Fossils of extinct animals that seem related to animals of today.
- 8. Biogeography --animals form similar habitats across the world are not related to each other, but appear to be related to neighboring animals in very different habitats (e.g., animals on the arid Cape Verde Islands off Africa are related to the rainforest animals of Africa, and animals on the arid Galapagos Islands off South America are related to the rainforest animals of South America).
- 9. Coevolution-- change of two related species such that the changes in one species match or respond to the changes in the other species (e.g., coevolution of very deep flowers with very long tongues of their moth pollinators).
- 10. Extensive molecular similarities.
the theory that traits of parents are blended in the offspring, and therefore the parental forms of the traits can never be recovered again.
the theory that parental traits are carried on particles (genes) that can be shuffled and hidden, but which retain the ability to be fully expressed in future generations.
Fitness of a genotype
the extent to which a set of alleles making up a genotype is propagated(reproduced) into future generations. There is no such thing as the fitness of an allele because an allele cannot be propagated by itself. It must be in a "package" with other alleles.
differential survival and reproduction of different phenotypes. This determines the fitness of the alleles causing the phenotypes.
the proportion of the alleles at a locus in a population that are of a particular type (e.g., the proportion of alleles at the A/a locus that are A in the whole population).
a change (not necessarily an adaptive one) in allelic and/or genotypic frequencies. There are several forces that can change allelic and genotypic frequencies, but usually natural selection is the only one of these that consistently increases fitness.
- (since 1940's)
- 1.) Evolution is a change in allelic (and genotypic) frequencies.
- 2.) The frequencies of alleles conferring a reproductive (fitness) advantage increase over time and become predominant in a population.
The "More Modern" Synthesis?
- (since about 2000)
- 1.) change in allelic frequencies is only part of the story.
- 2.) There are physical limits on embryonic development (stickiness between cells, tissue elasticity, etc.) that constrain the course of evolution.
- 3.) Much is decided not by changes in genes coding for protein, but by the regulation of those genes. This could easily cause many changes to occur in concert (e.g., bigger jaws, bigger teeth, and a more powerful bite).
- 4.) Epigenetics (modification of DNA by methylation of other chemical changes) can change the expression of genes and their mutation frequency without changing their base sequence.
all the genes of a group of interbreeding organisms
a group of organisms that can potentially interbreed and produce fertile offspring
a group of actually interbreeding organisms within a species
the study of the frequencies, distributions and inheritance of alleles in a population.
Frequency of allele A:
- the fraction of all the alleles at the A/a locus in a gene pool that are A.
- Given: population has 30% aa, 60% Aa, and 10% AA
- Frequency of A = (0)(0.3) + (0.5)(0.6)( + (1.0)(0.1) = 0.4
- if mating is random, the population is large, and there is no mutation, selection, or migration, then:
- (a) Allelic frequencies will never change
- (b) After one generation, gentoypic frequencies will never change
- (c) Genotypic frequencies may be computed from allelic frequencies. Assume a locus has only two alleles, A and a. Where p is the allelic frequency of the A allele and q is the allelic freuency of the a allele:
- proportion of AA in population = p2
- proportion of aa in the population = q2
- proportion of Aa in the population = 2pq
These result from random mating
Genetic (or Hardy-Weinberg) equilibrium (HWE)
allelic frequencies are not changing because the conditions of the Hardy-Weinberg Principle are satisified. It is possible for a population to be at genetic equilibrium at some loci and not at others.
different phenotypes experience different mortality or reproductive rates
the allelic frequency of thsoe entering or leaving differs from that of the population
some subgroups within the population either preferentially mate with each other or refrain from mating with each other
mating based on preferrred phenotypes rather than on random choices
alleles are changes in the gametes of individual organisms
in small, isolated populations, alleles fluctuate randomly in frequency due to chance events like random mating outcomes and accidents. Alleles can be permanetly lost if the frequency fluctuates to zero. Then the alternative allele is said to be "fixed" E.g. in a Bb x Bb mating that only produces 2 offspring, there is a 1/16 chance that both offspring will be bb. If this is the whole population, B has been lost forever. The larger the population is, the less chance that all offspring in a generation will be bb, so drift is more severe in small populations.
following population reduction, drift can cause loss of alleles. Genetic diversity of the population is drastically reduced.
isolated populations may have an atypical gene pool because they are sometimes founded by just a few individiuals.
different phenotypes experience different mortality or reproductive rates. Alleles conferring a higher birthrate and/or lower deathrate should become more common with each generation.
extreme phenotypes are selected against, so allelic frequencies tend to stay the same
Coadapted gene complex
a group of alleles that confers superior fitness, but only when they are present together (e.g., having strong muscles is only an advantage if the organism also has strong bones, camouflage only works if the organism remains absolutely still)
one of the extreme phenotypes is selected for, so allelic frequencies tend to move steadily in one direction
intermediate phenotypes are selected against, so the population tends to divide into two groups with different allelic frequencies
selection for strongly dimorphic sexes (such as male birds with elaborate plumages, male lions with big manes) because organisms without these exaggerated secondary sexual characteristics do not get to mate.
selection for behavior that favors relatives, perhaps even at the expense of the individual
evolutionary change within a species (development of anitbiotic resistance)
evolutionary change that produces large-scale change and produces or eliminates taxa at the species level or higher (species, genera, families, orders, etc.)
Biological species concept
a species is a group of organisms that can interbreed and produce fertile offspring
Morphological species concept
a species is a group of organisms that have a distinctive morphology (anatomy and appearance)
Cohesion species concept
a species is a group of organisms that share the same coadapted combinations of alleles. Any organism that gets too far from the correct combination of alleles suffers refuced fitness.
Evolutionary species concept
a species is a single evolutionary lineage that maintains a distinctive identity when compared to other such lineages
the formation of a reproductively isolated population (a new biological species)
speciation that occurs when two populations that can interbreed are geographically isolated from one another and subsequently lose the ability to interbreed
speciation in populations that live in the same area
formation of a zygote from the gametes of two different species, followed by a doubling of its chromosome number by miotic nondisjunction. Allopolypoid individuals can mate with each other but not with either of their "parent" species.
formation of a 4n zygote by union of two 2n gametes from the same species. can not mate with parent species, only 4n.
an area where two species ranges overlap and interbreeding has produced a hybrid between the two species
hybrids are not as fit as the parent species so there is selection in each parent species to mate with the other species rather than with their own species. The hybrid zone expands and both the parent species may disappear.
increases the difference between a group of organisms and an offshoot group (e.g. between two sibling species or between repiles and mammals)
maintains size of difference between two groups
makes groups of organisms with dissimilar ancestry look more alike (sharks and dolphins)
speciation that takes place gradually by the accumulation of small differences between the "parent" and "offspring" species. Because both species are continually changing, they cease to exist in their original forms.
speciation that takes place because of a sudden, dramatic divergence in offspring species from parent species. both continue to exist.