Offspring resemble their parents because genetic information is passed on, but they are not identical to them. That variation is what makes evolution possible.
Heredity is the passing of genetic information from one generation to the next. Offspring therefore resemble their parents without being identical to them.
Information comes from both parents, so a child may resemble one parent in some characteristics and the other parent in others.
The nucleus of a cell contains chromosomes, which are made of a molecule called DNA. A gene is a section of DNA that contains genetic information. Genes influence characteristics, often by providing the instructions for making proteins. Most human body cells contain 46 chromosomes arranged in 23 pairs.
The structure of DNA, a double helix, was worked out in the 1950s. Rosalind Franklin and Maurice Wilkins carried out important X-ray diffraction work on DNA. Data from that work contributed to the double-helix model built by James Watson and Francis Crick.
One common biological definition of a species is a group of organisms that can reproduce with one another to produce fertile offspring.
That is the test, rather than appearance. A horse and a donkey can breed, but the mule they produce is infertile, so they are different species.
Continuous variation can take any value in a range, such as height, mass or hand span. Measured across a large group it usually gives a bell-shaped distribution, often displayed using a histogram.
Discontinuous variation falls into distinct categories with nothing in between, such as human blood group or whether a pea plant produces round or wrinkled peas. It is best shown on a bar chart.
Variation has two sources: inherited from parents, and environmental, caused by surroundings such as diet or sunlight. Many characteristics are affected by both.
Because individuals vary, some have inherited characteristics better suited to their environment than others. Those individuals are more likely to survive and reproduce, so the alleles associated with those characteristics are passed on more often.
Over many generations the helpful characteristic becomes more common in the population. This is natural selection, and it is how species change over time.
If the environment changes, because the climate shifts, a new predator or disease arrives or food becomes scarce, individuals that were well adapted may no longer be.
If no individuals of a species have the characteristics needed to survive the change, and the species cannot move elsewhere, it may become extinct. Extinction means no members of that species are left anywhere.
Biodiversity is the variety of living things in an area. High biodiversity can make an ecosystem more resilient to change, because ecological roles are shared among a wider range of species. It is also a source of medicines and of crop varieties.
Gene banks and seed banks store seeds, eggs or sperm from many varieties and species. If a species becomes rare or extinct in the wild, the stored material keeps its genetic variety available for restoring populations or for breeding disease resistance into crops.
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