Question

Explain In the early 1800 s, a naturalist proposed that the giraffe species has a long neck as a result of years of stretching their necks to reach leaves in tall trees. Why isn't this true?

Short answer

Giraffes' long necks are a result of natural selection, not acquired traits.

Step-by-step solution

Introduction to the Theory

In the early 1800s, the naturalist Lamarck proposed that characteristics acquired during an organism's lifetime can be passed on to their offspring. This means he believed giraffes' long necks developed from repeatedly stretching to reach high leaves, and these acquired traits were inherited.

Understanding Modern Evolutionary Theory

Modern evolutionary biology, based on Darwin's theory, explains natural selection as the process by which traits that enhance survival and reproduction become more common in a population over generations. Stretching a neck during a lifetime wouldn't change the genetic material passed to offspring.

Genetic Basis of Traits

Traits like neck length result from the genetic information in DNA. These traits can only be passed to the next generation through reproduction if there is genetic variation favoring longer necks, which gives individuals survival advantages in their environment.

Evidence from Observations and Genetics

Studies show that giraffes born with naturally longer necks may access more food and survive better during food scarcity, leading to more offspring. This selective reproductive advantage spreads the trait through the population over generations, as seen with giraffes.

Key concepts

Natural Selection

Natural selection is a core concept in evolutionary biology, explaining how certain traits become prevalent within a population over time. It was Charles Darwin who first introduced this idea, transforming our understanding of evolution. The theory of natural selection posits that individuals within a species exhibit variations in their traits. These variations can influence an individual's ability to survive and reproduce within their environment.

For example, if a giraffe with a slightly longer neck can reach more food than others, this advantage may lead to better survival chances during scarce periods. This giraffe is more likely to reproduce and pass down the advantageous neck length to its offspring. Over many generations, this trait becomes more common, showing how natural selection can drive evolutionary change.

This process does not occur randomly. It is often a response to specific environmental pressures, ensuring that only the fittest individuals pass their genes to the next generation. Understanding natural selection is essential for grasping how species adapt over time.

Genetic Variation

Genetic variation is the diversity in gene frequencies within a population, a fundamental aspect necessary for evolution by natural selection. It ensures that no two individuals are genetically identical, except for identical twins. This variability arises from mutations, gene shuffling during sexual reproduction, and other genetic mechanisms.

This diversity is crucial because it provides the raw material for evolution. Without genetic variation, all giraffes would have the same neck length, making it impossible for natural selection to favor those with longer necks. Only with this variation can natural selection "choose" the individuals best adapted to their environment, allowing them to have more offspring.

Genetic variation is often visible in traits like neck length among giraffes, providing different survival advantages. It's important to note that genetic variation is inherently a result of the existing genetic make-up, not from traits acquired during an individual's lifetime.

Inheritance of Acquired Traits

The idea of inheritance of acquired traits was proposed by Jean-Baptiste Lamarck, which is not consistent with modern understanding of genetics. Lamarck theorized that if an organism changed during its life to adapt to its environment, those changes would be passed to its offspring. For instance, he suggested giraffes got their long necks by reaching for leaves and then passed this trait on.

However, this theory doesn't align with what we know about genetics today. Traits acquired during an organism's life, such as muscles built from exercise or a stretched neck, are not encoded in an organism's DNA and thus cannot be passed to the next generation.

Our modern understanding is that traits must be encoded in genetic material, such as DNA, to be inheritable. This misunderstanding underscores why Lamarck's theory has been replaced by Darwinian theory, which correctly emphasizes genetic inheritance.

Darwin's Theory

Charles Darwin's theory of evolution by natural selection provides a framework for understanding how species evolve over time. Introduced in his seminal work, "On the Origin of Species," it suggested that species evolve through a process where advantageous traits are preserved while disadvantageous ones are eliminated.

Darwin's theory differed fundamentally from Lamarck's by emphasizing the role of genetic inheritance in evolution. According to Darwin, over many generations, genetic variations that enhance survival and reproduction become more common within a population. This is distinctly different from acquired traits, which Darwin contested could not be inherited.

Darwin's observations and studies on various species, including finches and the diversification in their beaks, provided empirical evidence supporting his theory. This revolutionary idea has since formed the basis of modern evolutionary biology, offering insights into the mechanisms that drive the diversity of life on Earth.