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5.2 Natural Selection - Coggle Diagram
5.2 Natural Selection
Natural selection can only occur if there is variation among members of the same species.
- Within a species, different individuals of that species show genetic variation. Individuals that are best suited for their environment will survive and reproduce.
- If there was no variation within a species, then all individuals would be the same and no individual would be favored over the other and natural selection would not take place.
Charles Darwin
- Charles Darwin developed his understanding of the mechanism that causes evolution over many years, after returning to England from his voyage around the world on HMS Beagle.
- He probably developed the theory of natural selection in the late 1830s, but then worked to accumulate evidence for it.
- Darwin published his great work, The Origin of Species, in 1859. In this book of nearly 500 pages, he explains his theory and presents the evidence for it that he had found over the previous 20 to 30 years.
- One of the observations on which Darwin based the theory of evolution by natural selection is variation.
- Typical populations vary in many respects. Variation in human populations is obvious – height, skin color, blood group and many other features. With other species the variation may not be so immediately obvious but careful observation shows that it is there.
- Natural selection depends on variation within populations – if all individuals in a population were identical, there would be no way of some individuals being favored more than others.
Sources of Variation
- Sexual reproduction can produce variation in a species through fertilization and meiosis.
- Sexual reproduction occurs when two different members of a species create offspring that have a combination of genetic material contributed by both parents.
- During meiosis, 50% of the female's chromosomes will end up in the egg(haploid gamete) and 50% of the male’s chromosomes will end up in the sperm (haploid gamete).
- During meiosis chromosomes will line up or assort independently of each other creating (2n) possible variations of chromosomes in the sex cells.
- During meiosis, specifically, prophase 1, crossing over might occur in homologous chromosomes where parts of each chromosome are exchanged. - - Random fertilization through sexual reproduction gives millions of sperms a chance at fertilizing the egg.
- This allows mutations that have occurred in different individuals to come together in their offspring.
- Lastly, genetic mutations might occur where new alleles are produced. Genetic mutations are the original source of variation within a species.
Adaption
- Adaptations are characteristics that make an individual suited to its environment and way of life.
Overproduction of offspring
Populations tend to produce more offspring than the environment can supporter that could survive in a particular community or ecosystem.
The population density that the environment can support is called the carrying capacity. If there are too many organisms, the demand for resources increases. However, there is a limited supply of resources in an ecosystem. Overpopulation and a limited amount of resources create competition within a population.
Where and how an organism lives is largely due to the specific adaptations that allow it to survive and reproduce in a particular area or habitat
Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring.
Organisms with less desirable traits will die or produce less offspring
Individuals that reproduce pass on characteristics to their offspring
.Much of the variation between individuals can be
passed on to offspring – it is heritable.
Variation in behavior can
be heritable.
Progressive Change
Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
- Since the better-adapted individuals of a species are the ones that survive, reproduce and pass their genes on to the next generation, these alleles will become more frequent within the population. The same would hold true for individuals that are less suited to an environment. These individuals will reproduce less frequently and die more often, thus decreasing the frequency of their alleles within a population These changes happen over many generations
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