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Gene flow

Another important evolutionary force is gene flow    : the flow of alleles in and out of a population due to the migration of individuals or gametes ( [link] ). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population, but it can also introduce new genetic variation to populations in different geological locations and habitats.

This illustration shows an individual from a population of brown insects traveling toward a population of green insects.
Gene flow can occur when an individual travels from one geographic location to another.

Mutation

Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype.

Nonrandom mating

If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating    occurs. One reason is simple mate choice; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selected for by natural selection. One common form of mate choice, called assortative mating    , is an individual’s preference to mate with partners who are phenotypically similar to themselves.

Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.

Environmental variance

Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment ( [link] ). A beachgoer is likely to have darker skin than a city dweller, for example, due to regular exposure to the sun, an environmental factor. Some major characteristics, such as gender, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range.

This photo shows a person holding a baby alligator.
The sex of the American alligator ( Alligator mississippiensis ) is determined by the temperature at which the eggs are incubated. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males. (credit: Steve Hillebrand, USFWS)

Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation    is seen between most populations and can be significant. One type of geographic variation, called a cline    , can be seen as populations of a given species vary gradually across an ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline.

If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.

Section summary

Both genetic and environmental factors can cause phenotypic variation in a population. Different alleles can confer different phenotypes, and different environments can also cause individuals to look or act differently. Only those differences encoded in an individual’s genes, however, can be passed to its offspring and, thus, be a target of natural selection. Natural selection works by selecting for alleles that confer beneficial traits or behaviors, while selecting against those for deleterious qualities. Genetic drift stems from the chance occurrence that some individuals in the germ line have more offspring than others. When individuals leave or join the population, allele frequencies can change as a result of gene flow. Mutations to an individual’s DNA may introduce new variation into a population. Allele frequencies can also be altered when individuals do not randomly mate with others in the group.

Art connections

[link] Do you think genetic drift would happen more quickly on an island or on the mainland?

[link] Genetic drift is likely to occur more rapidly on an island where smaller populations are expected to occur.

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what is metabolism
Mlungisi Reply
pls what is the chemical symbol for Methane
Afia Reply
CH4
Francis
ch4
Paolo
what is life?
Josephus
What do you think is life?
Isala
Hi
Isaac
hey
Isala
what is DNA
Machuol
all living things have certain characteristics in common which are referred to as dash
IBUKUN Reply
classifying living things in the world in two major groups
IBUKUN
mention the seven characteristics that distinguish living things from nonliving things
IBUKUN
which of the characteristics of living things involved taking in the use of seed by animals as well as the taking in of mineral sentences and their uses by plants
IBUKUN
Life process
Afia
Growth, Respiration, Excretion, Movement, Sensitivity, Nutrition,and Reproductive
Afia
Nutrition
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nutrition
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Diffusion
Afia
Is it a serious question?
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what's the question
Joseph
how many days do a bean seed take to germinate
Nando
no idea
Afia
what is DNA
Yahaya Reply
genetic information
MG
Deoxyribonucliec acid (DNA) is the cell's hereditary material that contains instructions for growth, development and reproduction.
Joseph
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oly
what's different between sex and gender
oly
Are there differences between sex and gender?
Theo
lol
Andrew
hi Yahaya its Deoxyribonucleic acid
Neimar
describe an experiment to show that plants require light for photosynthesis
Diyara Reply
uuh... putting a plant in a dark closet... and another in a light enviroment?
Anastasiya
wow! awesome explanation Anastasiya.
Joseph
please can you explain why air is homogenous
Joseph
Because each layer of the Earth's atmosphere has a different density, each layer of air is it's own homogenous.
MG
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Mustafa
what's DNA
Mustafa
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Bigger Reply
An amoeba is a cell or an organism that has the ability to ulter it's shape.
Joseph
An amoeba has an irregular shape and it changes constantly,it's a unicellular organism belong to a group called protists it has a pseudopodia used for locomotion and ingestion...
Emmanuel
amoeba is an organism that has an inregular shape which changes constantly
Cashizz
amoeba is an unicellular organism that uses pseudopodia,it does not have a constant shape
Alohan
what is the difference between DNA and RNA
Alohan
DNA with oxygenated but RNA without oxygenated
qax
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Survive
asexual
qax
amoeba are protozoa with one cell and no fixed shape
James
heterotrophic and outrotrophic
qax
what is the between arteries and capillary
qax
arteries>arterioles>capillaries. decrease in size and thus pressure
Anastasiya
DNA deossoribonucleic acid. RNA ribonucleic acid. The difference between the two lies in a lack of one oxygen on the sugar in DNA. Also: in DNA the azotate bases are Guanine, Citosine, Adenine and TIMINE base, the latter is replaced by URACILE in RNA. DNA formes a double helix structure...
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... while RNA form is usually a single stand line, but it can form loops etc
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what is a zygote?
Darius Reply
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MG
A zygote is an organism formed after fertilization
Neimar
how do plants form a zygote
Paclina Reply
What is zygote
Van
what is zygote
Darius
Zygote is located inside the ovule, which is present in the ovary.
MG
zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual.
MG
Okay
Van
what is biology?
Aadan Reply
biology is the study of living n non living organism
Kelsia
what is procotist?
Kelsia
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Tamala Reply

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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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