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).
