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By the end of this section, you will be able to:
  • Describe the different types of variation in a population
  • Explain why only heritable variation can be acted upon by natural selection
  • Describe genetic drift and the bottleneck effect
  • Explain how each evolutionary force can influence the allele frequencies of a population

Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. The distribution of phenotypes among individuals, known as the population variation    , is influenced by a number of factors, including the population’s genetic structure and the environment ( [link] ). Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures.

 This photo shows four kittens in a basket: two are gray, black, orange, and white, the third cat is orange and white, and the fourth cat is black.
The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)

Genetic variance

Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child.

Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck theorized that acquired traits could, in fact, be inherited; while this hypothesis has largely been unsupported, scientists have recently begun to realize that Lamarck was not completely wrong. Visit this site to learn more.

Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation.

The diversity of alleles and genotypes within a population is called genetic variance    . When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding    , the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression    .

Questions & Answers

which element is the largest
Haja Reply
francium
Izzati
how many groups do we have in the periodic table
Theola Reply
how many times can a man release before the lady get pregnant
Smart Reply
nearly a hundred million
Izzati
one
Moses
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Izzati
two times
Theola
what is biology
Darkoa Reply
it's study of all living organisms and their characteristics
Ice
by what proces do ameaba reproduce?
Okello Reply
hi dear
Janet
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Okello
Hi
Muah
hello
Bob
yes
Mathew
hi
promise
what is biology
promise
amoeba reproduce asexually
Chidi
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Mary Reply
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Mary
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Mary
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Rachel
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Izzati
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Izzati
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Izzati
what is active transport
Amona Reply
is the movement's of water & mineral salts from the reason of highly concentrated to the reason of low concentration across semi permeable membrane & it take place in plant
Okello
in what situation would the use of a scanning microscope be ideal and why?
Anthony Reply
what is euglina
Ruth Reply
no idea
Enock
Which of the following statements regarding membrane transport is false? 1. Glucose is transported only by facilitated diffusion 2.Each protein carrier will only bind and b transport one type of solute.
#2. Each protein carrier will only bind and b transport one type of soluble
Only
#2 is false for the regarding membrane transport.
Only
Tanks a lot
Thanks a lot
what is gene
Okello Reply
what is somatic cell
garaadmaxamed Reply
what is Biology?
garaadmaxamed
is the study of living things
garaadmaxamed
what is organ
Chan
what is biology
Isiaka Reply
is the study of all living things
Motinga
and their interactions with each other and the environment
Angela
it is the study of all living organisms and their characteristics
ketchem
the study of living things and their surroundings
Ade
what's is biology
Mohamed Reply
what's is dna
Mohamed
deoxybonucliec acid
Gibril
explain the osomor regulations in amoeba and paramecium
Adannaya Reply
explain the osimoregulation in man
Adannaya
who can explain the osmoregulation in amoeba and in man
Adannaya

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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