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14.1 Darwin’s Voyage on HMS Beagle
Charles Darwin’s work on evolution challenged established worldviews.
 Darwin proposed a mechanism for evolutionary change called natural
selection (自然選擇;物競天擇).
 Darwin’s hypothesis for evolutionary change, after much testing, eventually
became accepted as theory.
Darwin voyaged from 1831 - 1836 on the HMS Beagle, a ship mapping the world’s
Darwin observed firsthand different plants and animals in various locales.
These observations played an important role in the development of his thoughts about
the nature of life on earth.
14.2 Darwin’s Evidence
Darwin made several observations that helped lead him to believe that species evolve
rather than remain fixed.
1) fossils of extinct (絕種的) organisms resembled those of living organisms.
2) geographical patterns suggested that organismal lineages (後裔) change
gradually as individuals move into new habitats.
3) islands have diverse animals and plants that are related to yet different from
their mainland sources.
14.3 The Theory of Natural Selection
Thomas Malthus’ Essay on the Principle of Population (1798) provided Darwin
with a key insight.
 while human populations tend to increase geometrically (幾何級數), the
capacity for humans to feed this population only grows arithmetically (算術
Darwin expanded Malthus’ view to include every organism.
 all organisms have the capacity to over-reproduce (過度繁殖).
 only a limited number of these offspring survive and produce the next
Darwin associated survivors with having certain physical, behavioral, or other
attributes that help them to live in their environment.
 by surviving, they can pass their favorable characteristics on to their own
Darwin envisioned (想像) the frequency of favorable characteristics
increasing in a population through a process called natural selection.
 favorable characteristics are specific to an environment; they may be
favored in one but not in another.
 organisms whose characteristics are best suited to their particular
environment survive more often and leave more offspring.
Darwin’s selection concept is often referred to as the “survival of the
fittest” (適者生存)
 Darwinian fitness does not refer always to the biggest or the strongest.
 fitness, in evolutionary theory, refers to organisms who, due to their
characteristics, survive more often and leave more offspring.
Darwin drafted his ideas in 1842 but hesitated to publish them for 16 years.
Another researcher, Alfred Russel Wallace, sent an essay to Darwin outlining a
theory of evolution by natural selection.
 Wallace had independently arrived at the same mechanism for
evolution that Darwin had.
 Darwin arranged for a joint presentation of their ideas in London.
Darwin finally published On the Origin of Species in 1859.
14.4 The Beaks of Darwin’s Finches
Darwin’s finches are a closely related group of distinct species.
 all the birds are similar to each other except for the shape of their
 genetic differences account for the physical differences in the beaks.
 birds with larger beaks make more of a protein called BMP4.
Darwin supposed that the birds evolved from a single ancestor to become
individual species who specialized in particular foods.
Peter and Rosemary Grant studied the medium ground finch on the island of
Daphne Major in the Galápagos (加拉巴哥群島) .
 they measured beak shape over many years and recorded feeding
 the finches preferred to feed on small, tender seeds.
 the finches switched to larger, harder-to-crack seeds when the
small seeds became hard to find.
 beak depth increased when only large, tough seeds were
The Grants’ work with the medium ground finch is an example of evolution
in action.
 average beak depth increased after a drought.
 only large-beaked birds were able to crush the bigger seeds and
survive to make the next generation.
 when wet periods returned, smaller beaks prevailed at handling the
then more plentiful small seeds.
14.8 Genetic Change in Populations: The Hardy-Weinberg Rule
G.H. Hardy and W. Weinberg, in 1908, studied allele frequencies in a gene
 in a large population in which there is random mating, and in the
absence of forces that change allele frequencies, the original genotype
proportions remain constant from generation to generation.
 because the proportions do not change, the genotypes are said to be in
Hardy-Weinberg equilibrium (哈溫平衡).
 if the allele frequencies are not changing, the population is not
Hardy and Weinberg arrived at their conclusion by analyzing the frequencies
of alleles in successive generations.
 frequency is the proportion of individuals with a certain characteristic
compared to an entire population.
 knowing the frequency of the phenotype, one can calculate the
frequency of the genotypes and alleles in the population.
By convention, the frequency of the more common of the two alleles is
designated by the letter p and that of the less common allele by the letter q.
Because there are only two alleles, the sum of p and q must always equal 1.
In algebraic terms, the Hardy-Weinberg equilibrium is written as an
The Hardy-Weinberg equilibrium only works if the following five
assumptions are met:
1) The size of the population is very large or effectively infinite.
2) Individuals mate with one another at random.
3) There is no mutation.
4) There is no immigration or emigration (移居).
5) All alleles are replaced equally from generation to generation (natural
selection is not occurring).
Most human populations are large and randomly mating with respect to most
traits and thus are similar to an ideal population envisioned by Hardy and
1) for example, the frequency of heterozygote carriers for recessive
genetic disorders can be estimated using the Hardy-Weinberg
14.9 Agents of Evolution
Five factors can alter the proportions of homozygotes and heterozygotes
enough to produce significant deviations from Hardy-Weinberg predictions:
genetic drift
nonrandom mating
Mutation is a change in a nucleotide sequence in DNA.
mutation rates are generally too low to significantly alter
Hardy-Weinberg proportions.
mutations must affect the DNA of the germ cells or the mutation will
not be passed on to offspring.
however, no matter how rare, mutation is the ultimate source of genetic
variation in a population
Migration is the movement of individuals between populations.
the movement of individuals can be a powerful force upsetting the
genetic stability of natural populations.
the magnitude of the effects of migration is based on two
the proportion of migrants in the population.
the difference in allele frequencies between the
migrants and the original population.
Genetic drift (遺傳漂變:在經過短短幾個世代之後,部分基因在族群中消
失,或整個基因都變為同基因型的情形,稱之) describes random changes in
allele frequencies.
in small populations, the frequencies of particular alleles may be
changed drastically by chance alone.
in extreme cases, individual alleles of a given gene may be.
all represented in few individuals.
accidentally lost if individuals fail to reproduce or die.
A series of small populations that are isolated from one another may come to
differ strongly as the result of genetic drift.
founder effect (奠基者效應) occurs when one of a few individuals
migrate and become the founders of a new, isolated population at some
distance from their place of origin.
the alleles that they carry will become a significant fraction of
the new population’s genetic endowment.
the founder effect is important in the evolution of organisms on
oceanic islands.
Nonrandom mating (非隨機配對) occurs when individuals with certain
genotypes mate with one another either more or less commonly than would be
expected by chance.
sexual selection (性擇) is choosing a mate often based on physical
nonrandom mating alters genotype frequencies but not allele
Disruptive selection (分裂選擇) is a form of selection in which the two
extremes in an array of phenotypes become more common in the population.
selection acts to eliminate the intermediate phenotypes.
for example, beak size in African black-bellied seedcracker finches is
under disruptive selection because the available seeds are only large or
Directional selection (定向選擇) is a form of selection that occurs when
selection acts to eliminate one extreme from an array of phenotypes.
for example, in Drosophila, files that fly toward light can be selected
against and those that avoid light selectively bred, producing a
population of flies with a greater tendency to avoid light.
14.10 Sickle-Cell Disease
Sickle-cell disease (鐮形細胞貧血症) is a hereditary disease affecting
hemoglobin molecules in the blood.
 the disorder results from a single nucleotide change in the gene
encoding beta-hemoglobin.
 this causes the sixth amino acid in the chain to change from
glutamic acid (very polar) to valine (nonpolar).
 as a result, the hemoglobin molecules clump together and
deform the red blood cell into “sickle-shape”.
Persons homozygous for the sickle-cell genetic mutation frequently have a reduced
 red blood cells that are sickled do not flow smoothly through capillaries.
 oxygen transport is affected.
Heterozygous (異型合子:指生物的單一性狀由兩個不同的對偶基因控制而成)
individuals make enough function hemoglobin to keep their red blood cells healthy.
The frequency of the sickle-cell allele is about 0.12 in Central Africa.
 one in 100 people is homozygous for the defective allele and develops the
fatal disorder.
 sickle-cell anemia strikes roughly two African Americans out of every
If natural selection drives evolution, why has natural selection not acted against the
defective allele in Africa and eliminated it from the population there?
The defective allele has not been eliminated from Central Africa because people who
are heterozygous are much less susceptible to malaria (瘧疾).
 the payoff in survival of heterozygotes makes up for the price in death of
 this is called heterozygote advantage.
 stabilizing selection occurs because malarial resistance
counterbalances lethal anemia
14.13 The Biological Species Concept
Speciation (物種形成) is the macroevolutionary process of forming new species
from pre-existing species.
 it involves successive change:
 first, local populations become increasingly specialized.
 then, if they become different enough, natural selection may act to
keep them that way.
Ernst Mayr (演化生物學家) coined the biological species concept (生物種概念*),
which defines species as:
“groups of actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups”
Populations whose members do not mate with each other and cannot produce fertile
offspring are said to be reproductively isolated and, thus, members of different
Barriers called reproductive isolating mechanisms (生殖隔離機制) cause
reproductive isolation by preventing genetic exchange between species.
 prezygotic isolating mechanisms (合子形成前的隔離機制)
 prevent the formation of zygotes.
 postzygotic isolating mechanisms (合子形成後的隔離機制)
 prevent the proper functioning of zygotes once they have
14.14 Isolating Mechanisms
There are six different prezygotic isolating mechanisms:
 geographical isolation (地理隔離)
 ecological isolation (生態隔離)
temporal isolation (時間隔離)
behavioral isolation (行為隔離)
mechanical isolation (機械隔離)
prevention of gamete fusion (交配後隔離)
Geographical isolation occurs simply in cases when species exist in different
areas and are not able to interbreed.
Ecological isolation results from two species who occur in the same area but
utilize different portions of the environment and are unlikely to hybridize.
Temporal isolation results from two species having different reproductive
periods, or breeding seasons, that preclude (杜絕) hybridization.
Behavioral isolation refers to the often elaborate courtship (求偶) and mating
rituals (習慣) of some groups of animals, which tend to keep these species
distinct in nature even if they inhabit the same places.
Mechanical isolation results from structural differences that prevent mating
between related species of animals and plants.
Prevention of gamete fusion blocks the union of gametes even following
successful mating.
If hybrid matings do occur, and zygotes are produced, many postzygotic
factors may prevent those zygotes from developing into normal individuals.
 in hybrids, the genetic complements of two species may be so different
that they cannot function together normally in embryonic development.
 even if hybrids survive the embryo stage, they may not develop
 finally, many hybrids are sterile (不生育的).
哈-溫理論(Hardy-Weinberg Theory
溫伯格(W. Weinberg, 1862-1937)和哈帝(G. Hardy, 1877-1947)各自發表論文,認
(Hardy-Weinberg genetic equilibrium),並基於假想情況下,一個族群不會發生演
一位患者的機率來看,表示該隱性基因的出現頻率是 1/100,相對的顯性基因為
性狀不會有太大的改變(founder effect)。與其他族群的交流也會改變基因比例
(genetic flow),甚至族群固定選擇配偶的方式與突變等等,都會影響基因的比例。