Gender: The sexual identity of an
individual, e.g., male, female,
hermaphrodite.
Kin selection: The selection of
individual behavior that benefits
relatives, even at a personal disadvantage, since relatives share
genes with the individual. The genes ultimately benefit - are
passed
on to future generations and may increase in frequency - even
though the individual might suffer.
Genotype: The group of genes that an
individual possesses. In
sexually reproducing species, all individuals except identical
twins
have different genotypes. All members of a clone have the same
genotype.
Phenotype: The combination of characters
expressed by the
individual. The phenotype results from the interaction between the
genotype and the environment. The same genotype in different
environments may express different phenotypes.
Optimization: A compromise in which
advantages compensate for
disadvantages and produce an overall best phenotype in a certain
kind of organism. For example, a horse can run fast but its hind
legs cannot scratch away insects. Its long tail, though, can brush
away flies that lay parasitic eggs on its skin.
ESS: An Evolutionary Stable Strategy, that
is, a strategy that gives
a higher reproductive fitness than any alternative strategy.
Therefore, the ESS is stable and cannot be replaced by one of the
alternatives. For example, a 50-50 sex ratio is the most fit
"strategy" in an outcrossing population. It will not be replaced by
a biased sex ratio in favor of either males or females.
Cost of sex: In a sexual species where
only females raise offspring,
the necessity of producing males for fertilization even though they
do not contribute to raising offspring means less fitness compared
to an asexual species which produces only females.
Recombination: The production of gametes
with new combinations of
alleles (other than what the parents had). Thus, recombination
includes new combinations of whole chromosomes, one from each
parent. It also includes new combinations of chromosomal segments
resulting from crossing-over.
Sex: The condition of a species that has
1) two sexes which are
different from one another; 2) gametes of unequal sizes
(anisogamy);
3) outcrossing; 4) recombination.
DNA repair: Damaged DNA has an altered
sequence. Before cell
division, the chromosomes are replicated and then consist of double
strands. The (new) replicated strand of DNA has a gap opposite the
damaged part of the original DNA strand. In meiosis, the gap
promotes recombination, with the result that the recombined DNA
strand is like the previously undamaged strand - the chromosome has
been repaired.
Outcrossing: Breeding with unrelated
individuals - the opposite of
inbreeding. Outcrossing produces the lowest incidence of identical
alleles on both chromosomes, therefore promoting a high degree of
heterozygosity.
Epigenetic defects: Errors in gene
expression that arise during
embryological development of the individual. For example, certain
sequences of DNA on one chromosome may be inactivated by
methylation
and the altered DNA inherited by all descendant cells.
Methylation: The attachment of a methyl
group (-CH3) to cytosine in
the DNA molecule. Methylation is thought to result in the
inactivation of the gene. Perhaps, methylation is the mechanism
that
inactivates one of the 2 X-chromosomes in each cell of the female
mammal.
Independent assortment: The production of
a new genotype by
fertilization by 2 gametes formed by the independent segregation of
non-homologous chromosomes during meiosis. In other words, during
the first cell division of meiosis the separation of the members of
a pair of homologous chromosomes occurs independently of the
separation of all other pairs. Whether the male member of the pair
goes to one daughter cell or the other does not influence the
destination to which any other male member of a different pair
goes.
Red Queen Hypothesis: Any evolutionary
change in a species is
experienced by coexisting species as a change of their
environmental
conditions. If a species is to continue to survive in that habitat,
it must constantly evolve as rapidly as possible. (Attributed to
Van
Valen - 1973, Evolutionary Theory 1,1-30 - from the statement of
the
Red Queen in Through the Looking Glass "that it takes all the
running you can do, to keep in the same place. If you want to get
somewhere else, you must run at least twice as fast as that!").
Muller's ratchet: An asexual population
can never contain, in any
of
its lines, a load of mutations smaller than that already existing
in
its presently least-loaded lines. However, recombination in
sexually
reproducing populations can form a mutant-free haploid genome from
2
haploid genomes that contain different mutants. In other words,
without sex deleterious mutations will accumulate in the
population.
(From the idea of a ratchet that allows movement in only 1
direction.)
Sex ratio: The ratio of males to females
in a litter of offspring.
The primary sex ratio occurs at conception. The secondary sex ratio
occurs at birth.
Sex ratio hypothesis: Where parental
investment affects the breeding
success of offspring of one sex to a greater extent than that of
another, parents should bias the sex ratio of their offspring
towards the sex whose breeding success is most variable. In
vertebrate polygynous species male breeding success is much more
variable than female success - some males breed but many do not,
most females breed.
Cost of recombination: Sexual
reproduction produces an individual
with pairs of chromosomes, one from the male parent and one from
the
female parent. Only half of the chromosomes of one very well
adapted
parent will be inherited by its offspring, the other half is
inherited from the other parent. The breaking-up of well adapted
combinations of parental genes during sexual reproduction is the
cost of recombination.
Mutation: A change in the gene that
produces a different expression
of a character from that expressed with the normal form of the
gene.
In general, new mutations are considered harmful to the individual,
i.e., it is considered deleterious. For example, the mutation for
wingless produces flies without wings.
Pleiotropic genes: Genes which have more
than one effect on the
phenotype. For example, the sickle-cell gene produces hemoglobin
with an abnormal shape, with an altered oxygen-carrying ability,
and
an increased resistance to falciparum malaria.
Hermaphrodite: An individual with both
male and female reproductive
organs. Also called monoecious, especially in plants which produce
both pollen and seeds.
Sequential hermaphroditism: An individual
which is first one sex,
usually male, and then the other. Although both reproductive organs
occur in the same individual, they do not occur there
simultaneously
and self-fertilization is impossible.
Genetic drift: In populations of a few
individuals, those that
happen to breed can have a major influence on subsequent gene
frequencies. In other words, chance events can significantly alter
the gene pool in small populations.
Frequency-dependent selection: The rarest
alleles have highest
fitness. Rare alleles tend to increase in frequency. As they become
more common, they become less fit. For example, mimics are largely
immume to predation as long as they are rare relative to their
distasteful models. As the tasteful mimics increase in density,
there are not sufficient numbers of distasteful models relative to
the more numerous mimics to teach predators to avoid eating both
the
models and the mimics.
Tangled Bank theory: When population
density is high - the
environment is saturated - individuals which are different have
higher fitness than those which are similar to one another.
Parthenogenesis: Development of an
individual in a sexual species
without the male contributing genes to the new individual. For
example, one of the 3 discarded meiotic female cells may reunite
with the egg cell and produce a new individual.
Incompatability system: Genes in plants
that prevent or hinder
pollen from a plant successfully fertilizing an ovule from the same
plant.
Nondisjunction: The failure of chromosome
strands to separate at
cell division. The result is that one daughter cell receives both
strands and the other receives none. The resulting cells have an
abnormal number of chromosomes, and usually die.
Euploidy: Having twice the haploid set of
chromosomes, 2N.
Aneuploidy: Having almost twice the
haploid set of chromosomes,
e.g., 2N+1 or 2N-1 chromosomes.
Isogamy: Having gametes of equal size and
appearance, such as in the
green alga Chlamadomonas.
Haploid: A set of only one member of each
pair of chromosomes, the
set carried by the gametes, 1N.
Diploid: A set of both members of each
pair of chromosomes, the 2N
set carried by the cell after fertilization.
Synapsis: The close apposition of
homologous chromosomes during the
first meiotic cell division. The chromosomes actually touch at
several points and crossing-over may occur.
Tetrad: The group of 4 chromosomal
strands formed at synapsis.
Crossing-over: The breaking of maternal
and paternal chromosomal
strands and rejoining of the broken segments. A resulting
chromosome
may contain different parts of the original male and female
parental
chromosomes.
Mitosis: Cell division producing 2
daughter cells, each with exactly
the same chromosomes that the parent cell contained. Asexual
reproduction.
Meiosis: One cell dividing twice and
producing 4 daughter cells,
each with 1/2 the number of chromosomes contained in the parent
cell. Meiosis produces the 1N gametes in sexual reproduction.
Testis-determining factor: A gene on the
Y chromosome that causes
maleness. It is probably a major controling gene that turns on a
number of other genes responsible for male fertility and secondary
sexual characteristics.
ZFY: A "zinc-finger" protein, studied in
sex-reversed human beings,
that was thought to be coded for by the testis-determining factor.
The ZFY was present in XX males and absent in XY females.
Sex-determination: The process by which
the sex of the individual is
determined during its lifetime. In some species genes on
chromosomes
determine the sex of the individual regardless of environmental
conditions. In other species, environmental conditions such as
temperature determine whether the genes for maleness or for female-
ness are expressed in the individual. Some species possess both
male
and female gonads simultaneously. Other species possess male and
female gonads sequentially, thus experiencing sex reversal.
Restriction enzyme: An enzyme that cuts
a DNA strand at a specific
location related to the molecular sequence of DNA.
Electrophoresis: A technique in which
proteins of differing electric
charge and mass are separated from one another when subjected to an
electric current for a period of time.
Conserved DNA sequence: A strand of DNA
which shows relatively few
differences among different species, suggesting that few mutations
occurred during the evolution of these species from their common
ancestor. The similarity of DNA implies that most mutations that
did
occur within this sequence of DNA resulted in a significant
decrease
in fitness.
Chromosome: A threadlike structure, in
the form of a double helix,
which carries genetic information. The genetic information is coded
by a linear sequence of 4 different DNA molecules. Chromosomes
which
carry the sex-determining genes are called sex chromosomes. Other
chromosomes are called autosomes.
Homologous chromosomes: Chromosomes that
occur in pairs, one from the
male parent and one from the female parent. Except for the sex
chromosomes, they are similar in size and shape and contain genes
that code for the same characters.
Organizational concept: The idea that the
sex of an animal's gonads are determined by its chromosomes at the
time of conception. The gonads subsequently produce sex steroid
hormones that control further sexual development of the individual.
Part of this concept is the notion that the vertebrate individual
will become female unless exposed to testosterone during
development.
Sex hormones: Androgens, including
testosterone, are the male sex hormones. Estrogens and progestins
are the female hormones. Both male and female sex hormones are
present in a single adult vertebrate individual, but their
concentrations differ between the sexes.
Masculinization: The development of male
sexual character under the influence of androgens.
Hormonal controls: The chemical
environment within the body that determines the sexual development
of the individual.
Homotypical sexual behavior: Adult mating
behavior consistent with the individual's gonadal sex.
Nongenetic trigger: A factor other than
an individual's own chromosomes that determines the sex of the
adult. Such factors are environmental and include temperature,
nutrient condition of the habitat, social interactions, and
others.
Evolutionary precursor: An adaptation in
ancestors or relatives that is modified in a species considered to
be more highly evolved.
Hypothalamus: A part of the vertebrate
brain closely associated with the control of sex and its
expression.
Evolutionary view of sexuality: An
understanding of the biology of sex that explains maleness and
femaleness as adaptations. The control of sex development in the
individual has changed during evolution.
Niche: The ranges of conditions and
resources within which an organism can survive and reproduce.
Dioecious (Greek. dis, two +
oikos, house): Producing male and female
structures on separate individuals.
Monoecious (Greek. monos, one +
oikos, house): Bearing reproductive structures of both
sexes on the same plant. In angiosperms, the two sexes are not in the
same flower.
Last updated on January 30, 2004
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