Chapter 10 – Introduction to Dinosaur Evolution
Introduction
To Reiterate
Evolution is a FACT and a THEORY
Basic Concepts in Evolutionary
Theory
Part I: Genetics and Natural Selection
Terminology
Evolution – change in a
population between generations, referred to as “descent with
modification”
Changes to an individual during its life are not evolution
Population – group of interbreeding organisms
Lineage – an evolving population through time, from ancestral to descendant forms
Speciation – evolution of one
species into another
Results from reproductive isolation, where neither ancestral or descendant species can
reproduce with the other to form offspring that can reproduce
Hybrid – sterile
offspring of two different species, like
mules from donkeys and horses
A number of speciation events have been observed beginning early in
the 20th Century
Pharmaceutical and chemical companies continually update antibiotic
and pesticide formulas as bacteria and insects and weeds evolve resistant forms
Gene – a nucleotide sequence in
a DNA molecule that provides a code for all or part of a protein
Chromosome – assemblages of genes
Locus – location of a gene on a chromosome
Allele – variation of a gene at any locus
Genotype – a pair of alleles at a locus
Genome – sum total of all the genes in an individual
Gene pool – sum total of all the genes in a population at any given time
Phenotype – physical appearance
and behavior of an organism
Interaction of genotype with environment – “Nature versus Nurture”
Theropod trackway with three toes on one foot and two toes on the other
Adaptation –a physical attribute
of an organism that can help it to survive at least long enough to reproduce
successfully
To affect evolution, an adaptation must be heritable
Inheritance of acquired adaptations, or acquired characteristics, hypothesized by Jean-Baptiste Lamark, does not occur
Natural Selection
Key concept of evolution co-proposed
by Charles Darwin ("The Origin of Species by Means of Natural
Selection"; 1859) and Alfred Russell
Wallace
Based on following tenets:
1. there is natural heritable variation among the individuals of a given species
2. organisms produce many offspring that die before reproductive age
3. some variations are more favorable than other variations in the struggle for existence
4. individuals surviving to reproduce must be those with the most successful combinations of variable characteristics, which will be passed on to their offspring, thus changing the population over time
“Survival of the fittest”
Fittest means “better adapted” or refers to the number of
offspring produced by an individual, and
has little or nothing to do with physical strength
Neo-Darwinism – modification of
Darwin’s original proposal that takes modern genetics, which Darwin and
Wallace knew nothing about, into account
Gregor Mendel discovered many of the basic factors of genetics at the
same time that Darwin was writing, but
his results were not widely recognized until the early 20th Century
Genetics
Terminology
Dominant
and recessive genes
Homozygous
and heterozygous
Gene
frequency – the
frequency of one allele to another allele at their locus
1.0 for homozygous; 0.5 for heterozygous
Punnet
square – diagram
used by geneticists to calculate the probability of genotypes and phenotypes
for offspring
Genotype
frequency – the
probabilities of genotypes resulting from a Punnet square
Hardy-Weinberg
ratio – the
expected genotype frequencies assuming random mating and no natural selection
Population
genetics – the study
of the factors that affect gene frequencies
Observed
gene frequencies can be different from expected gene frequencies as a result of
natural selection
Application to dinosaurs – Centrosaurus
apertus
Horn length alleles – short is
dominant; long is recessive
See
Figure 10.2 for genotype frequencies starting with a homozygous dominant male
crossed with a homozygous recessive female
Natural selection scenarios that would
reduce short horn length genotype frequencies
Females
preferentially mating with long horned males
Reduced
parasitism for long horned individuals as birds that removed horn parasites
noticed long horns more often
Short
horn less effective during intraspecific competition for mates
Short horn
less effective against theropod predators
Microevolution versus
Macroevolution
Microevolution – changes in gene
frequency in a population (at the species level)
Macroevolution – larger-scale
transitions, such as the evolution of amphibians to amniotes or dinosaurs to
birds
Macroevolution
is the cumulative effect of microevolution
Directional selection and
Cope’s rule
Directional selection –
consistent change in a population though time in a particular direction
Cope’s (proposed by Edward
Drinker Cope) Rule – organisms show a directional trend toward larger
body size in their lineages through time
Numerous
exceptions
Marsh
illustrated horse evolution from Hyracotherium to Equus as following Cope’s Rule;
George Gaylord Simpson showed that horse evolution was more complicated, like a bush, with many separate horse species
being part of lineages where body size decreased
Variation also provided by
recombination and Mutation
Recombination – exchange of
genes between pairs of chromosomes during meiosis
Mutation – alteration in the
structure of genes themselves generally caused by environmental factors, like
radiation or mutagenic chemicals
Part II: Mechanisms for Macroevolution
Transitional fossils
Technically any fossil because all
organisms are in transition between generations
Notable transitional fossils are
pointed to as examples of macroevolution
Pikaia, Acanthostega, Archaeopteryx, Basilosaurus
Speciation and the rate of evolution
Allopatric Speciation - new species from isolation of a small population that becomes genetically distinct from its parent species; gene flow is halted
Consensus
mechanism by which speciation occurs
Well
documented from islands
Debate as
to rate that new species evolve
Phyletic Gradualism - slow accumulation of minor
changes bringing about a transition from a parent species to its descendant
species
Darwin’s
proposal
Punctuated Equilibrium - rapid bursts of change
followed by long periods of little or no change
Stephen
Jay Gould and Niles Eldredge
there are examples of both Phyletic Gradualism
& Punctuated Equilibrium, which are end members of a spectrum (see Figure
10.3)
Genetic drift – random change in gene
frequencies in small, nonrepresentative subpopulations of a larger population
Adaptive radiation
a few species adapted to a limited environment evolve to many species adapted to many different environments
Galapagos finches; Mesozoic reptiles; Cenozoic mammals
occurs when new environments or niches become
available
Vicariance biogeography
Vicariance is an increase in biodiversity caused by dividing environments rather than by dispersal of populations
Caused by large scale plate tectonics activity as part of Wilson Cycles
Very clear in dinosaur evolution
Low diversity during the Late Triassic and Early Jurassic prior to significant splitting of Pangea; increasing diversity from the Middle Jurassic through the Cretaceous as Pangea split into increasingly separated continents
Sympatric speciation
Result from intraspecific, rather than environmental, factors
Results in closely related species that can have overlapping geographic ranges
Red Queen hypothesis
Coevolution of two or more species
Two species continually match one another’s defenses only to maintain the status quo
Exaptation
Sometimes called preadaptation
A species possesses a heritable trait that is
favored prior to application of the selective pressure
Tetrapod limbs apparently evolved in fully aquatic species, prior to movement of tetrapods onto land
Marine arthropods had exoskeletons and snails had shells, both useful in preventing drying out prior to movement of arthropods and snails onto land
Independent testing of macroevolutionary
relationships based on fossils
Molecular phylogeny – relative differences in biomolecules between living organisms
Time element introduced by calculating rates of change in these biomolecules, called molecular clocks, assuming rates don’t change
Evolutionary Origin of Dinosaurs
Amniote Evolution and Diversification
before the Dinosaurs
First amniotes
Evolved the amniote egg, which protects the developing embryo from drying out, so that amniotes are not dependent on an aqueous environment, as are amphibians
Occurred during the Carboniferous based on certain skeletal features (lightened skull, reinforced pelvis, flexible ankle) that reflect adaptations to a terrestrial lifestyle
No eggs
or nest found, due either
to non-mineralization or
non-recognition
Some modern amphibians exhibit behaviors that probably occurred in the first amniote, including: internal fertilization, retention of eggs in the body (to cover with a membrane) and embryo development within the egg
Clade Amniota is synonymous with Class Reptilia
Amniota
is monophyletic because it includes all descendents of the first amniote, including reptiles (lizards, snakes, turtles, crocodilians, plus
pterosaurs, extinct marine reptiles and dinosaurs), birds and
mammals; Reptilia is
paraphyletic because it excludes birds and mammals
Amniota divided into 3 major clades based on
skull structure
Anapsida, Synapsida and Diapsida
The first
amniotes were anapsids, which
lack temporal fenestrae; turtles
are modern anapsids
Synapsids
and diapsids evolved from the common ancestor of anapsids during the late
Carboniferous
(310-320 million years ago)
Diapsids and anapsids are placed together in Clade Eureptilia
Synapsida
Mammalia & extinct “mammal-like reptiles”
Characterized
by a skull with a lower temporal fenestra
“Mammal-like reptiles” include
sail-backed forms like Dimetrodon and therapsids and were the dominant
land vertebrate during the Permian & Early and Middle Triassic periods
Therapsids gave rise to mammals in the Late Triassic, about the same time as dinosaurs appear
Diapsida divided into 2 clades, Lepidosauria
and Archosauria, during the Permian period
Lepidosauria – modern lizards &and
snakes and tuatara and extinct Euryapsids (mosasaurs, ichthyosaurs and
Plesiosaurs)
Archosauria – modern crocodiles and birds
and extinct dinosaurs, pterosaurs
and thecodonts (a polyphyletic grouping of non-dinosaur/pterosaur archosaurs)
Archosaur Evolution and
Diversification
Archosaurs have specific skeletal traits
including antorbital and dentary fenestrae
Archosauria includes 2 important clades
Crurotarsi = Modern crocodilians & extinct
relatives
Hinge
formed between ankle bones and tarsals
Middle
and Late Triassic representatives are phytosaurs, rauisuchians and aetosaurs
Ornithodira (bird neck) = Modern birds &
extinct dinosaurs and pterosaurs
Pterosauria = active
flying reptiles with maneuvering ability
Wings were skin stretched between elongated 4th finger, sides of the body & rear limbs
Pterosaurs were warm-blooded - fine hair found covering well-preserved pterosaurs
active flyers must maintain constant high internal temperature
Sparrow-sized
to members with wingspans as large as 15.5 m!
Dinosaur Ancestors and the Origin of
Dinosaurs
Precursors
Small but long-limbed reptiles from the Middle Triassic of Argentina
Marasuchus and Lagerpeton
First Dinosaurs
Eoraptor lunensis, Herrerasaurus ischigualastensis and Staurikosaurus pricei
Saurischians
from Argentina and Brazil
Ischagualasto
Formation of Argentina is at least 227.8 ± 0.3 my old
Pisanosaurus mertii is the oldest ornithischian, also from Ischagualasto
There are a number of fragmentary specimens as well from the Late Triassic of North America and Africa
Dinosaurs rapidly diversified following
extinction of large archosaurs and most therapsids (see table 10.2 for list of Late Triassic
dinosaurs)
Maybe from an asteroid impact or from upright stance