CH: 22-24 (CH:24 The Origin of Species (The biological species concept …
CH:24 The Origin of Species
The biological species concept
emphasizes reproductive isolation
is Latin for “kind” or “appearance
we commonly distinguish between various “kinds” of organisms based on differences in their appearance
EX dogs and cats
a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
but do not produce viable, fertile offspring with
members of other such groups
the process by which one species splits into two or more species.
changes over time in allele frequencies in a population,
the broad pattern of evolution above the species level.
Because biological species are defined in terms of reproductive compatibility, the formation of a new species hinges on r
the existence of biological factors that impede members of two species from interbreeding and producing viable, fertile offspring
Such barriers block gene flow between the species and limit the formation of
offspring that result from an interspecific mating
based off habitat preference
based off time of day
based off courtship rituals
based off mating attempt
based off sperm not being able to fertilize egg
reduced hybrid viability
genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment
reduced hybrid fertility
Hybrid may be sterile
Some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next generation are feeble or sterile.
block fertilization from occurring
may contribute to reproductive isolation after the hybrid zygote is formed
ecological species concept
defines a species in terms of its ecological niche
morphological species concept
distinguishes a species by body shape and other structural features
Speciation can take place with or
without geographic separation
gene flow is reduced when two populations of one species become geographically separated from each other.
One or both populations may undergo evolutionary change during the period of separation, resulting in the establishment of barriers to reproduction
Hybrid zones reveal factors that
cause reproductive isolation
Many groups of organisms form
members of different species meet and mate, producing at least some offspring of mixed ancestry
Many hybrid zones are stable, in that hybrid offspring continue to be produced over time
In others, reinforcement strengthens prezygotic barriers to reproduction, thus decreasing the formation of unfit hybrids
Speciation can occur rapidly or slowly and can result from changes in few or many genes
New species can form rapidly once divergence begins—but it can take millions of years for that to happen
The time interval between speciation events varies considerably, from a few thousand years to tens of millions of years
Researchers have identified particular genes involved in some
cases of speciation
Speciation can be driven by few or many genes.
CH:22 Descent with Modification:
A Darwinian View of Life
The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species
Ideas About Change over Time
Among other sources of information, Darwin drew from the
work of scientists studying
the remains or traces of
organisms from the past
New layers of sediment cover older ones and compress them into superimposed layers of rock
The fossils in a particular stratum provide a glimpse of some of the organisms that populated Earth at the time that layer formed
the study of fossils
Lamarck’s Hypothesis of Evolution
Theory of Inheritance of Acquired Characteristic
If an organism changes during life in order to adapt to its environment, those changes are passed on to its offspring.
Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life
Darwin’s Focus on Adaptation
During the voyage of the Beagle, Darwin observed many examples of
inherited characteristics of organisms that enhance their survival and reproduction in specific environments
he reassessed his observations,
he began to perceive adaptation to the environment
Could a new species arise from an ancestral form
From studies made years after Darwin’s voyage, biologists have concluded that this is indeed what happened to a diverse group of finches found on the Galápagos Islands
Darwin realized that explaining such
adaptations was essential to understanding evolution
His explanation of how adaptations arise centered on
individuals that have certain inherited traits tend to survive and reproduce at higher rates than do other individuals because of those traits
Charles Darwin (1809–1882)
born in Shrewsbury, in western England
He quit medical school and enrolled at Cambridge University, intending to become a clergyman.
At Cambridge, Darwin became the protégé of John Henslow, a botany professor
The Voyage of the Beagle
Darwin embarked from England on the Beagle in December
The primary mission of the voyage was to chart poorly
known stretches of the South American coastline
Darwin, however, spent most of his time on shore, observing and collecting thousands of plants and animals
He also noted that the plants and animals in temperate regions of South America more closely resembled species living in the South American tropics than species living in temperate regions of Europe
Darwin also spent much time thinking about geology
He experienced geologic change firsthand when a violent earthquake shook the coast of Chile
he observed afterward that rocks along the coast had been thrust upward by several meters
Finding fossils of ocean organisms high in the Andes
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Evolution is supported by an overwhelming amount of scientific evidence
the state of having the same or similar relation, relative position, or structure
variations on a structural theme that was present in their
EX: human arm comparable to cat leg, whale fin, or bat wing
“leftover” structures of marginal, if any, importance to the organism
EX: skeletons of some snakes retain vestiges of the pelvis and
leg bones of walking ancestors.
a diagram that reflects evolutionary relationships among groups of organisms.
the independent evolution of similar features in different lineages.
In such examples in which species share features because of convergent evolution, the resemblance is said to be
features share similar function, but not common ancestry
the scientific study of the geographic distributions of species
all of Earth’s landmasses into a single large continent due to continental drift
found nowhere else in the world
CH:23 The Evolution of Populations
Genetic variation makes evolution possible
In The Origin of Species, Darwin provided abundant evidence that life on Earth has evolved over time
he proposed natural selection as the primary mechanism for that change.
Mendel proposed a model of inheritance in which organisms transmit discrete heritable genes to their offspring
differences among individuals in the composition of their genes or other DNA sequences
Individuals within all species vary in their phenotypic traits
EX: in humans you can easily observe phenotypic variation in facial features, height, and voice
this and many other molecular traits also vary extensively among individuals
Sources of Genetic Variation
mutation, gene duplication, or other processes produce new alleles and new genes
can be produced rapidly in organisms with short generation times.
Sexual reproduction can also result in genetic variation as existing genes are arranged in new ways
New alleles can arise by
change in the nucleotide
sequence of an organism’s DNA.
While many mutations are harmful,
many others are not
much of the DNA in eukaryotic genomes does not encode proteins
Point mutations in these noncoding regions generally result in neutral variation
differences in DNA sequence that do not confer a selective advantage or disadvantage
The Hardy-Weinberg equation can be used to test whether a population is evolving
the presence of genetic variation
does not guarantee that a population will evolve
group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
We can characterize a population’s genetic makeup by describing its
consists of all copies of every type of allele at every locus in all members of the population
this picture shows gene pool for dogs
named for the British mathematician and German physician, respectively, who independently developed this idea in 1908.
allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences
p2 + 2pq + q2 = 1
p is the frequency of the "A" allele and q is the frequency of the "a" allele in the population
p2 represents the frequency of the homozygous genotype AA, q2 represents the frequency of the homozygous genotype aa, and 2pq represents the frequency of the heterozygous genotype Aa.
Sum has to equal 1
To determine whether a population is in Hardy-Weinberg equilibrium, it is helpful to think about genetic crosses in a new way.
consider the combination of alleles in all
of the crosses in a population
conditions for Hardy-Weinberg Equilibrium
The gene pool is modified if mutations occur
or if entire genes are deleted or duplicated.
If individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing of gametes does not occur and genotype frequencies change.
3.No natural selection
Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success.
4.Extremely large population size
In small populations, allele frequencies fluctuate by chance over time (a process called genetic drift).
5.No gene flow
By moving alleles into or out of populations, gene flow can alter allele frequencies.
Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Chance events can also cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations
In genetic drift, chance fluctuations in allele frequencies over generations tend to reduce genetic variation
process by which heritable traits increase an organism's chances of survival and reproduction
individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits.
transfer of genetic variation from one population to another
Gene flow, the transfer of alleles between populations, tends to reduce genetic differences between populations over time.