Evolution & Natural Selection
1
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾ EVOLUTION AND NATURAL SELECTION
evolution = at the most basic level, evolution is a change in the frequency of alleles in a
population. Individuals do not evolve, only populations. Evolution can refer to all the
changes that transform life on Earth, leading to the diversity of organisms. Evolution
results from genetic change and the consequent phenotypic (expressed traits) change in a
population or species over many generations (often thousands or millions of years =
evolutionary time).
evolutionary adaptation = an inherited characteristic (an allele or combination of alleles) that
enhances an organism's ability to survive and reproduce in a particular environment (or
ecological niche).
reproductive fitness = the contribution an individual makes to the gene pool of the next
generation, relative to the contribution of other individuals in the same population. That
is, how successful an individual is at reproduction (passing on their genes).
Carolus Linnaeus = A Swedish botanist (1707-1778) who introduced a system of classification
of plants based on their sexual organs. In his Systema Naturae (1735), he established the
classification of living things into genus and species and combined genera into classes.
This system was more precise and useful than any previous classification system.
James Hutton = A Scottish philosopher (1726-1797) and the father of geology. In Theory of
the Earth with Proofs and Illustrations (1795), Hutton became the first to show that, in
general, the Earth changes slowly and uniformly by the same processes which are
occurring today. He helped develop the idea of uniformitarianism
Charles Lyell = A British geologist and friend of Darwin's who established the principle of
uniformitarianism in the 1830's (Principles of Geology) and aided Darwin's theory of
gradualism. Lyell's theories assume that the laws of physics and chemistry have remained
the same throughout the history of the earth and that past geological events occurred by
natural processes similar to those which we observe today. Lyell was one of the first
biologists to recognize how fossils formed. Lyell realized that geologic events are not
predetermined and that the geology we see at any one time is the result of the
accumulation of many small changes which occur over time. These ideas of gradualism
and nondirectionalism were adopted by Darwin to help explain biological evolution
Jean Baptiste de Lamarck = A French biol gist who was the first to attempt to completely
explain evolution in 1809 (Philosophie zoologique). Lamarck's theory was based on the
theory of inheritance of acquired characteristics. That is, Lamarck believed that organisms
developed or acquired characteristics that allow them to be successful in a specific
environment. As organisms acquired these new traits, they were passed on to their
offspring. Lamarck's theories have been rejected as they rely on the inheritance of acquired
and not genetically determined traits. To his credit, Lamarck was one the first biologists to
seriously study and classify invertebrates. He was the first to classify crustaceans,
arachnids, and insects separately as well as the first to identify invertebrates as animals
without spinal columns.
Albert Russell Wallace = A young naturalist who worked in the Malay Archipelago who wrote
a letter to Charles Darwin in 1858 asking for advice about his own ideas of natural
selection which spurred Darwin to publish The Origin of Species in 1859. Wallace and
Darwin each independently developed the theory of natural selection.
Evolution & Natural Selection
2
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Charles Darwin = A British naturalist who is credited with establishing the concept of natural
selection and evolution in the early and mid 1800's. Darwin traveled around the world on
a ship (the H.M.S. Beagle) for 5 years (1831-1836) and refined the theory of evolution by
natural selection primarily due to observations he made of closely related organisms that
lived in the Galapagos islands east of South America. In 1859, Darwin published, On the
Origin of Species by Means of Natural Selection, which is the founding text on which the
theory of evolution is based. Darwin's theories differ from Lamarck's in that Darwin
realized that evolutionary change is caused by the differential survival and subsequent
reproduction of organisms that are born with different traits and not by the inheritance of
acquired traits.
population = a group of interacting individuals belonging to one species and living in the same
geographic area.
evolutionary adaptation = an inherited characteristic that enhances an organism's ability to
survive and reproduce in a particular ecological niche.
reproductive fitness = The contribution an individual makes to the gene pool of the next
generation, relative to the contribution of other individuals in the same population. That
is, how successful an individual is at reproduction (passing on their genes).
gene = a sequence of base pairs in DNA that codes for a functional polypeptide.
allele = an alternate form of a gene that codes for the same trait (e.g. brown vs. blue eye color).
genotype = the genetic composition of an organism; the total set of genes in the cells of an
organism
phenotype = the visible or expressed characteristics of an organism controlled by the
genotype; note that not all genes in the genotype are necessarily expressed in the
phenotype
natural selection = differential success in reproduction by different phenotypes resulting from
interactions with the environment. Evolution occurs when natural selection produces
changes in the relative frequencies of alleles in a population's gene pool. That is, alleles for
a gene that increase the chance for the individual to reproduce will become more
widespread in a population of a specific species. Alleles which are detrimental or make it
more difficult for the individual to reproduce will become less common in a given
population.
Darwin’s Theory of Natural Selection is based on the following basic ideas:
1. PERPETUAL CHANGE
2. COMMON DESCENT
3. SPECIATION
4. GRADUALISM
5. NATURAL SELECTION
1. PERPETUAL CHANGE
¾ The living world is always changing. The fossil record shows that there is significant
change in the form and diversity of animals throughout the history of life on Earth.
¾ Species arise and become extinct repeatedly throughout the fossil record.
¾ Often these changes in the phenotypic characteristics of an organism occur in trends. For
instance, the evolution of the horse or horn evolution in the Titanotheres (rhinoceros
relatives).
Evolution & Natural Selection
3
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
2.
¾
¾
¾
COMMON DESCENT
Life has a unified evolutionary history.
Before Darwin, many biologists advocated multiple independent origins of life.
The fossil record together with comparative anatomy and comparative embryology has
allowed for the reconstruction of phylogenetic trees.
¾ The phylogenies of modern species can be traced back as they converge on ancestral
lineages shared with other species.
3. SPECIATION = the evolution (formation) of a new species from a common ancestor
WHAT IS A SPECIES?
Most biologists consider the following as basic criteria for defining a species:
1. Descended from a common ancestor
2. Ability to interbreed
3. Maintenance of genotypic and phenotypic cohesion within species (there are not drastic
differences among populations in allelic frequencies and organismal appearance)
¾
¾
¾
The variation present within a species provides the material from which new species are
produced (speciation = the evolution of new species).
Many species become extinct, but generally over evolutionary time the total number of
species produced by evolution increases through time.
Reproductive barriers prevent species from interbreeding. These barriers evolve gradually
over long periods of time (10’s to 100’s of 1000’s of years or more).
allopatric speciation = speciation that results from the evolution of reproductive barriers
between geographically separated populations.
adaptive radiation = the emergence of numerous species from a common ancestor introduced
to new and diverse environments. The best examples of adaptive radiation come from islands.
Islands are initially colonized by mainland (or other island) plants and animals. The new
island(s) are ideal for evolutionary diversification as many different resources are available that
may have been exploited by other species on the mainland. Darwin’s finches are the classic
example.
4. GRADUALISM
¾ Species do not arise suddenly, but rather as small, continuous changes accumulate in the
phenotypes (and thus the genotypes) of populations
¾ New traits become established in a population by increasing their frequency initially from a
small fraction of the population to a majority of the population.
5. NATURAL SELECTION
¾ Differential success in reproduction by different phenotypes resulting from interactions
with the environment. Evolution occurs when natural selection produces changes in the
relative frequencies of alleles in a population's gene pool. That is, alleles for a gene that
increase the chance for the individual to reproduce will become more widespread in a
population of a specific species. Alleles which are detrimental or make it more difficult for
the individual to reproduce will become less common in a given population.
Evolution & Natural Selection
4
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Thus, Darwin’s train of thought went something like this….
OBSERVATION 1: Organisms have great potential fertility.
OBSERVATION 2: Natural population size normally remains constant.
OBSERVATION 3: Natural resources are limited.
leads to…
INFERENCE 1: There exists a continuing struggle for existence among members of a
population.
OBSERVATION 4: All organisms show variation (no two individuals are exactly alike).
OBSERVATION 5: Some variation is inheritable.
leads to…
INFERENCE 2: There is differential survival and reproduction among varying organisms in a
population.
INFERENCE 3: Over many generations, differential survival and reproduction generates new
adaptations and new species.
fossil = a preserved remnant or impression of an organism that lived in the past.
homologous structures = structures that are similar in different species of common ancestry.
biogeography = the evolution of the geographical distribution of any particular species
determined by their geographical origin, migration patterns, and geologic or climatic events
that would affect their ability to survive in a particular habitat (e.g. continental drift).
artificial selection = selective breeding of domesticated plants and animals to promote the
occurence of desirable inherited traits in the offspring. For instance, if a rancher wants to
raise cows with more meat on them, the rancher will mate large cows with other large cows
over many generations. To get a true breeding variety of white rose, white roses are bred
together over many generations until only the gene for white rose color is found in the
genotypes of all the individuals (then the white rose variety is called a true-breeding
variety).
Scientific Techniques used to provide evidence for the evolution of diverse species from
common ancestors:
1. the fossil record = the chronicle of evolution over millions of years of geological time
engraved in the order in which fossils appear in rock strata.
2. comparative anatomy = the comparison of bodily structures in different species.
Anatomical similarities among many species give signs of common descent.
3. comparative embryology = the study of structures that appear during the development of
different organisms is a major source of evidence for the common descent of organisms.
Closely related species often have similar stages in their embryonic development. (An
embryo is any of the early developmental stages of a multicellular organism).
4. amino acid sequencing = determining the sequence of amino acids in a protein from a
specific organism; this information can be used to compare the amino acid sequences of
proteins from different organisms to determine any evolutionary relationships between the
two organisms.
5. DNA sequencing = determining the sequence of nucleotide bases in DNA from a specific
organism; this information can be used to compare the base sequences (genes) of DNA from
different organisms to determine evolutionary relationships between the two organisms.
6. DNA-DNA hybridization = a method of determining the relatedness between different
species by measuring the extent of hydrogen bonding between single strands of their DNA
molecules.
Evolution & Natural Selection
5
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
¾
MORE ON SPECIATION AND EVOLUTION
gene pool = all the genes in a population at one time.
Hardy-Weinberg equilibrium = the principle that the shuffling of genes that occurs during
sexual reproduction, by itself, cannot change the overall genetic makeup of a population.
Hardy-Weinberg equilibrium will be maintained in nature only if all five of the following
conditions are met:
FIVE CONDITIONS REQUIRED FOR HARDY-WEINBERG =
1. THE POPULATION IS VERY LARGE
2. THE POPULATION IS ISOLATED; THERE IS NO IMIGRATION (GAIN) OR EMIGRATION
(LOSS) OF INDIVIDUALS AND THE ALLELES THEY CARRY INTO OR OUT OF THE
POPULATION.
3. MUTATIONS DO NOT ALTER THE GENE POOL.
4. MATING IS RANDOM (NOT SELECTIVE).
5. ALL INDIVIDUALS ARE EQUAL IN REPRODUCTIVE SUCCESS;
NATURAL SELECTION DOES NOT OCCUR.
genetic drift = a change in the gene pool of a small population due to chance.
bottleneck effect = genetic drift resulting from a drastic reduction in population size (as with
cheetahs).
founder effect = random change in the gene pool that occurs in a small colony of a
population.
gene flow = the gain or loss of alleles from a population by the movement of individuals or
gametes into or out of the population.
nonrandom mating = the selection of mates other than by chance.
random mating = the selection of mates by chance only.
directional selection = natural selection that acts against the relatively rare individuals at one
end of a phenotypic range ( the phenotypes at one end of the bell curve that describe
phenotypes are less likely to be seen in the population).
gradualist model = the view that evolution occurs as a result of populations becoming
isolated from common ancestral stock and gradually becoming genetically unique as they
are adapted by natural selection to their local environments; Darwin's view of species.
punctuated equilibrium model = the idea that speciation occurs in spurts followed by long
period of little change.