A number of processes affect evolution in populations, some directly by
changing the gene frequencies, some indirectly by increasing the variation in
the population, which is then affected by the other processes which change the
gene frequencies directly.
Processes which Produce/Increase Genetic Variation with Populations
Those processes which do not change gene frequencies in the population are:
exchange of genetic information between members of a homologous pair of
chromosomes at meiosis. Results in new combinations of alleles on
Law of Independent Assortment. Results in new combinations of chromosomes in
every gamete (egg or sperm).
combination of parents for most alleles. Results in new combinations of
chromosomes in every child. Every child is a unique combination with 50% of
genes coming from each parent.
Those processes which do change gene frequencies in the population (i.e. they
cause evolution directly) are:
Mutation. A change in genetic code known
as a mutation consists of changes in the hereditary instructions contained
within a gene. A mutation, the ultimate source of all genetic variation,
must occur in reproductive cells to cause genetic change in offspring. The
process of mutation involves random genetic change primarily due to chemical
factors and ionizing radiation
Makes a better allele (rare).
Does not affect functioning of allele (common).
Disrupts functioning of allele (most common). To have significance in the
long run, the mutation must occur in an egg or a sperm, and so be
inherited by next generation. Mutations are the ultimate source of all new
information, and new alleles.
Gene Flow (immigration). Gene flow, the movement of genes from one
population to another, describes the process of the loss or gain of genes in
a population due to the emigration or immigration of fertile individuals, or
the transfer of gametes, between populations. This process introduces new
genes into a population resulting in populations becoming more similar to
Processes which reduce genetic variation with populations (cause evolution
variety" natural selection - selective reduction in genetic
variation, causes changes in gene frequencies through time as individuals
carrying alleles not well-adapted to that particular environment reproduce
less successfully than individuals carrying alleles well-suited to that
particular environment. Reduces variation by reducing or removing all the
alleles carried by poorly adapted individuals. Results in population
changing to become better and better adapted to their environment, as long
as the environment remains the same. If the environment changes, the game
begins anew. Differential net Reproductive success is measured by the
number of offspring who survive to reproduce. Even small differences in
reproductive success can lead to major changes in the gene frequencies
over time. [Under certain circumstances, natural selection can lead to an
increase in variation within a population, as when an adaptive mutant
allele appears and is selected for, but this is the only way natural
selection can increase variation within a population.
Selection. Where natural humans are the agents; the source of all
domesticated plants and animals, including such wonders as the modern
strawberry and Chihuahuas.
Selection. Natural selection with female choice of males for mating
partners (based on genetically derived traits) as the agents; the source
of most sexually dimorphic features such as lions' manes, antlers in deer,
canines in baboons, and the like.
Genetic Drift. The process of genetic drift,
dealing with chance preservation or extinction of particular genes, may be
defined as a variation in the gene pool, a change in allele frequency, in a
small population due to chance. Random reduction in genetic variation,
causes changes in gene frequencies through time as individuals die or fail
to reproduce as successfully as others because of non-genetic factors: the
operation of chance factors in evolution. Most important in small
death not related to genetic makeup -- bad luck.
successful at reproduction due to non-genetic factors -- become a nun,
priest, don't like kids, radiation makes you infertile, gonorrhea,
syphilis, schistosomiasis, and the like,. make you infertile.
factors in the production of gametes -- some alleles are not passed on by
chance, e.g., if both my children inherit my O rather than my A allele for
Effect. A small group of people originated the population, did not carry
all the variation present in parent population, e.g., group stranded on
desert isle, Pitcairn's Island.
These evolutionary processes
operate continuously, both individually and in various combinations, to change
the gene frequencies in a population over time -- that is, to cause evolution.
Evolution occurs by any and all of these processes. Charles Darwin's
contribution was in recognizing the important role of natural selection in
Evolution, but evolution also occurs through these other processes, which may be
as important, or even more important, than natural selection in many instances.
01/25/06 06:55 PM