ch 26 Flashcards
Evolution Occurs Through Genetic Change Within Populations
1.Biological evolution includes genetic change
only.
2.Biological evolution takes place in groups of
organisms.
-An individual organism does not evolve; what evolves is the gene pool common to a group of organisms.
First step of evolution
Genetic variation arises
– Variation originates as mutation, which produces new alleles, and recombination.
– Mutation and recombination are random and produce genetic variation continually, regardless of evolution’s
requirement for it
Second step of evolution
Change in the frequencies of genetic variants.
– Shifts in the allele frequencies (due to things such as migration, drift, natural selection) shift the composition of the gene pool common to a group of organisms and constitutes evolutionary change
Types of evolution: Anagenesis
evolution taking place in a single lineage with the passage of time
Types of evolution: Cladogenesis
splitting of one lineage into two; new species arise
Evolution in Bighorn Sheep
Numbers decimated due to loss of habitat, competition from livestock, diseases carried by domesticated sheep, and hunting
– hunting regulations increased minimum size of the horns of rams (males) that could be shot.
Hunters shot rams with large horns, before they were able to reproduce, producing artificial selection
for smaller horns in rams.
Advantages of Molecular Data
− Molecular methods used with all organisms.
− Molecular methods
applied to a huge amount of genetic variation.
− All organisms compared with use of some molecular data.
− Molecular data are quantifiable.
− Molecular data often provide information about process of evolution.
− Database of molecular information is large and
growing
Population variation
most populations of organisms possess large amounts of variation in their protein and DNA sequences.
- Analysis of proteins in a population allows for
discovery of evolutionary relationships.
Neutral-mutation hypothesis
proposes that much molecular variation is adaptively neutral; individuals with different
molecular variants have equal fitness at realistic population size.
− Suggests variants in DNA and protein sequences are
functionally equivalent – therefore natural selection does not differentiate between them.
− Their evolution is shaped largely by genetic drift and
mutation!
Some genetic variation maintained by natural selection
In these cases, genetic variants are not functionally equivalent,
instead, they result in phenotypic effects that cause differences in reproduction (fitness differences)
Overdominance
in which the heterozygote has higher fitness than either homozygote.
An example of genetic variation maintained by natural selection
heterozygote advantage
Ex: In tropical Africa, where malaria is common:
- Homozygous dominant (normal cells): HAHA (Die or reduced reproduction from malaria)
- Homozygous recessive: HaHa (Die or reduced reproduction from sickle cell anemia)
- Heterozygote carriers: HAHa
(Relatively free of both conditions; Survive and reproduce more; more common in the population)
Cladogenesis occurs though speciation which is
–the process by which one population separates into two distinct evolutionary
groups.
biological species
concept (Ernst Mayer, 1942).
- Defined a species as a group of organisms whose
members are capable of interbreeding with one another but are reproductively isolated from the members of other species
Several associated problems with biological species concept
− Reproductive isolation cannot be determined from
fossils, and often difficult to determine whether even
living species are capable of exchanging genes.
− There are organisms that are accepted as different
species but exchange genes.
− Biological species concept can not be applied to
asexually reproducing organisms (such as bacteria).
alternative definitions for a
species: Morphospecies concept
- defines a species based
entirely on phenotypic (morphological) similarities and differences.
alternative definitions for a
species: Phylogenetic species concept
– defines a species as
the smallest recognizable group that has a unique
evolutionary history.
Reproductive isolation
the existence of
biological characteristics that prevent genes from
being exchanged between different species.
Reproductive isolating mechanism
- any biological factor or mechanism that prevents gene exchange
Prezygotic reproductive isolating mechanisms
mechanisms that prevent gametes from two different species from fusing and
forming a hybrid zygote
Prezygotic reproductive isolating mechanisms:
Ecological
members of the two species do not encounter one another and therefore do not reproduce
Prezygotic reproductive isolating mechanisms:
Behavioral
differences in behavior prevent interbreeding
Prezygotic reproductive isolating mechanisms
Gametic
mating between individuals of different species may take place but the gametes do not form zygotes
Prezygotic reproductive isolating mechanisms
Temporal
reproduction in different species takes place at different times
of the year
Prezygotic reproductive isolating mechanisms
Mechanical
anatomical differences that prevent successful copulation
Postzygotic reproductive isolating mechanisms
gametes of
two species may fuse and form a zygote, but there is no gene flow between the two species.
If prezygotic reproductive isolating mechanisms fail (or have not yet evolved), mating between two organisms of different
species may take place, and a hybrid zygote containing genes from two species may be formed.
In many cases, the species are still separated by:
– Hybrid inviability
– Hybrid sterility
– Hybrid breakdown
Hybrid inviability
– incompatibility between the genomes of the two
species prevents the hybrid zygote from developing.
Hybrid sterility
hybrid embryos complete development, but are sterile,
so that genes are not passed between the two parental species.
Hybrid breakdown
further crossing of the hybrids produces inviable or
sterile offspring.
Speciation
process by which new species arise
Allopatric speciation
– When a geographic barrier splits a
population into two or more groups and prevents gene flow between the
isolated groups
Sympatric speciation
– Arises in the absence of any geographic barrier to gene flow; reproductive isolation mechanisms
evolve within a single interbreeding population
Allopolyploidy often arises when
two diploid species hybridize
Tetraploid contains
two copies of each
chromosome, is usually fertile and is reproductively isolated
Genetic differentiation leads to the evolution of
reproductive isolating mechanisms, which restrict
gene flow between populations and leads to
speciation.
Phylogeny
- the evolutionary relationships among a group of organisms
Most evolution takes place over long periods!
Biologists reconstruct phylogenies by inferring the evolutionary relationships among present-day organisms.
Past and Current phylogenies
P-phylogenies were reconstructed on phenotype
C – molecular data, protein/DNA sequences are
used
